We show that the correspondence between quantum and classical mechanics can be tuned by varying the coupling strength between an atom or a molecule and the modes of a cavity. In the acceleration gauge (AG) representation, the cavity-matter system is described by an effective Hamiltonian, with a non-trivial coupling appearing in the potential, and with renormalized masses. Importantly, and counterintuitively, the AG coupling changes non-monotonically with the strength of the cavity-matter interaction. As a result, one obtains an effective (approximately decoupled) cavity-matter dynamics both for the case of weak and strong interactions. In the weak coupling regime, the effective mass parameters essentially coincide with their standard interaction free counterparts. In contrast, the renormalized atomic/molecular mass increases as the cavity-matter inter-action is increased. This results in AG dynamics of matter governed by a conventionally looking atomic/molecular Hamiltonian, whose effective Planck constant is reduced when the cavity-matter interaction is increased.

Let H be a frustration-free Hamiltonian describing a 2D grid of qudits with local interactions, a unique ground state, and local spectral gap lower bounded by a positive constant. For any bipartition defined by a vertical cut of length L running from top to bottom of the grid, we prove that the corresponding entanglement entropy of the ground state of H is upper bounded by \({\tilde{O}}(L^{5/3})\). For the special case of a 1D chain, our result provides a new area law which improves upon prior work, in terms of the scaling with qudit dimension and spectral gap. In addition, for any bipartition of the grid into a rectangular region A and its complement, we show that the entanglement entropy is upper bounded as \({\tilde{O}}(|\partial A|^{5/3})\) where \(\partial A\) is the boundary of A. This represents a subvolume bound on entanglement in frustration-free 2D systems. In contrast with previous work, our bounds depend on the local (rather than global) spectral gap of the Hamiltonian. We prove our results using a known method which bounds the entanglement entropy of the ground state in terms of certain properties of an approximate ground state projector (AGSP). To this end, we construct a new AGSP which is based on a robust polynomial approximation of the AND function and we show that it achieves an improved trade-off between approximation error and entanglement.

We study the effect of acoustic phonons on the quantum phase transition in the O($N$) model. We develop a renormalization group analysis near (3+1) space-time dimensions and derive the RG equations using an $\epsilon$-expansion. Our results indicate that when the number of flavors of the underlying O($N$) model exceeds a critical number $N_c=4$, the quantum transition remains second-order of the Wilson-Fisher type while, for $N\le 4$, it is a weakly first-order transition. We characterize this weakly first-order transition by a length-scale $\xi^*$, below which the behavior appears to be critical. At finite temperatures for $N\le 4$, a tricritical point separates the weakly first-order and second-order transitions.

Two-dimensional periodically driven systems can host an unconventional topological phase unattainable for equilibrium systems, termed the Anomalous Floquet-Anderson insulator (AFAI). The AFAI features a quasi-energy spectrum with chiral edge modes and a fully localized bulk, leading to non-adiabatic but quantized charge pumping. Here, we show how such a Floquet phase can be realized in a driven, disordered Quantum Anomalous Hall insulator, which is assumed to have two critical energies where the localization length diverges, carrying states with opposite Chern numbers. Driving the system at a frequency close to resonance between these two energies localizes the critical states and annihilates the Chern bands, giving rise to an AFAI phase. We exemplify this principle by studying a model for a driven, magnetically doped topological insulator film, where the annihilation of the Chern bands and the formation of the AFAI phase is demonstrated using the rotating wave approximation. This is complemented by a scaling analysis of the localization length for two copies of a quantum Hall network model with a tunable coupling between them. We find that by tuning the frequency of the driving close to resonance, the driving strength required to stabilize the AFAI phase can be made arbitrarily small.

Josephson junctions and superconducting quantum interference devices (SQUID) are important electronic elements, which are based on normal conductor sandwiched between two superconductors. These junctions are produced by evaporation techniques, and once they are embedded in an electronic circuit, their properties are fixed. Using SQUIDs as a tunable component requires the ability to generate Josephson junctions in situ in a reversible controllable manner. In this work we demonstrated how a normal (metallic) region along a line traversing a superconductor can be turned on and off externally thus potentially generating a controllable Josephson junction or a SQUID. The concept is based on a long, current-carrying excitation coil, piercing a ring shaped superconductor with nucleation points. The vector potential produced by this coil generates a circular current that destroys superconductivity along a radial line starting at the nucleation point. Unlike the destruction of superconductivity with magnetic field, the vector potential method is reversible and reproducible; full superconductivity is recovered upon removing the current from the coil and different cool-downs yield the same normal lines.

We show via a combination of material realistic quantum-kinetic theory and experimental differential pump-probe results, that performance issues in tunnel-injection QD lasers are caused by a filtering effect, resulting from the hybridization of different QD shells with the injector quantum well. The real footprint of applicability in optical communication system is the large signal modulation response, which, on the other hand is much less often investigated.

We describe the fabrication process and properties of an InP based quantum dot laser structure grown on a 5° off-cut silicon substrate. Several layers of quantum dot based dislocation filters embedded in GaAs and InP were used to minimize the defect density in the quantum dot active region which comprised eight emitting dot layers. The structure was analyzed using high resolution transmission electron microscopy, atomic force microscopy and photoluminescence. The epitaxial stack was used to fabricate optical amplifiers which exhibit electroluminescence spectra that are typical of conventional InAs quantum dot amplifiers grown on InP substrates. The amplifiers avail up to 20 dB of optical gain, which is equivalent to a modal gain of 46 cm^-1.

Revisiting the intensity-dependent quantum efficiency (IDQE) technique in the context of non-fullerene acceptors, we find that at forward-bias conditions, the response exhibits what seems to be anomalous behavior that is not consistent with light excitation induced trap filling. Analysis based on the Shockley–Read–Hall model leads to the conclusion that the contacts cause the traps to be completely full in the dark. The role of the light excitation is to half-empty the traps, and thus, the “anomalous” behavior is created. By fitting the IDQE at several bias levels, we find that the trapping is consistent with multiphonon capture by a state close to the middle of the gap. As trap-assisted recombination is a significant loss mechanism, it is essential to fully monitor it for indoor applications as well as to cross the single junction 20% power conversion efficiency limit.

Size-dependent change of the electronic band structure is one of the key features of nanoparticles in the quantum confinement region. CuFeS₂ nanoparticles have a strong absorption feature in the visible region that has, controversially, been described as neither an excitonic transition nor a free carrier plasmon oscillation. Instead, the absorption feature in CuFeS₂ nanoparticles has been attributed to quasi-static optical resonances from inter-band transitions between the valence band (VB), intermediate band (IB), and conduction band (CB). As such, we hypothesized that the feature should be subject to quantum confinement effects through modification of the electronic bands. In this paper, we show experimentally that the optical resonance absorption peak red-shifts and the optical band gap blue-shifts as the particle size decreases. Through density functional theory (DFT) and the tight binding (TB) modeling, we elucidate the size dependence of the band structure, especially focusing on the change in the IB. Using a Lorentzian oscillator optical model to simulate the absorption spectrum with inputs from the DFT-calculated band structure and band shifts from the TB model, we find that the size-dependent shifts of the optical resonance peak position in CuFeS₂ are due to a tri-band quantum confinement effect that results in both the VB to IB and IB to CB gap expansion that accompanies a decrease in particle size. We also find that the transitions between the IB and CB play only a minor role in the optical spectrum. Moreover, the linear optical Lorentzian model predicts that the optical resonance peak is tunable across the visible range by partially filling the IB, lowering the CB, or expanding the IB.

Room-temperature strong coupling between plasmonic nanocavities and monolayer semiconductors is a prominent path towards efficient, integrated light-matter interactions. However, designing such systems is challenging due to the nontrivial dependence of the strong coupling on various properties of the cavity and emitter, as well as the subwavelength scale of the interaction. In this work, we develop a methodology for obtaining hybrid nanostructures consisting of plasmonic metasurfaces coupled to atomically thin WS2 layers, exhibiting extreme values of Rabi splitting, by inverse design of the near-field plasmonic response. Contrary to common measures such as the quality factor or the mode volume, our method relies on an overlap-integral-based metric. We experimentally measure large values of Rabi splitting for our nanoantenna designs, while providing theoretically optimal configurations for several additional types of nanostructures. Our results open a path to maximizing light-matter interactions in integrated platforms, for classical and quantum-optical applications.

We demonstrate enhanced Andreev reflection in a $\mathrm{Nb}/\mathrm{InGaAs}/\mathrm{InP}$-based superconductor-semiconductor hybrid device resulting in increased Cooper-pair injection efficiency, achieved by Cooper-pair tunneling into a semiconductor quantum well resonant state. We show this enhancement by investigating the differential conductance spectra of two kinds of samples: one exhibiting resonant states and one which does not. We observe resonant features alongside strong enhancement of Cooper pair injection in the resonant sample, and lack of Cooper pair injection in the nonresonant sample. The theoretical modeling for measured spectra by a numerical approach agrees well with the experimental data. Our findings open a wide range of directions in condensed matter physics and in quantum technologies such as superconducting light-emitting diodes and structures supporting exotic excitations.

Entanglement of photons is a fundamental feature of quantum mechanics, which stands at the core of quantum technologies such as photonic quantum computing, communication, and sensing. An ongoing challenge in all these is finding an efficient and controllable mechanism to entangle photons. Recent experimental developments in electron microscopy enable to control the quantum interaction between free electrons and light. Here, we show that free electrons can create entanglement and bunching of light. Free electrons can control the second-order coherence of initially independent photonic states, even in spatially separated cavities that cannot directly interact. Free electrons thus provide a type of optical nonlinearity that acts in a nonlocal manner, offering a way of heralding the creation of entanglement. Intriguingly, pre-shaping the electron’s wavefunction provides the knob for tuning the photonic quantum correlations. The concept can be generalized to entangle not only photons but also photonic quasiparticles such as plasmon-polaritons and phonons.

High-quality lattice resonances in arrays of infrared antennas operating in an open-cavity regime form polariton states by means of strong coupling to molecular vibrations. We studied polaritons formed by carbonyl stretching modes of (poly)methyl methacrylate on resonant antenna arrays using femtosecond 2DIR spectroscopy. At a normal incidence of excitation light, doubly-degenerate antenna-lattice resonances (ALR) form two polariton states: a lower polariton and an upper polariton. At an off-normal incidence geometry of the 2DIR experiments, the ALR degeneracy is lifted and, consequently, the polariton energies are split. We spectrally resolved and tracked the time-dependent evolution of a cross-peak signal associated with the excitation of reservoir states and the unidirectional transfer of the excess energy to lower polaritons. Bi-exponential decay of the cross-peak suggests that a reversible energy exchange between the bright and dark lower polaritons occurs with a characteristic transfer time of ~200 fs. The cross-peak signal further decays within ~800 fs, which is consistent with the relaxation time of the carbonyl stretching vibration and with the dephasing time of the ALR. An increase in the excitation pulse intensity leads to saturation of the cross-peak amplitude and to a modification of the relaxation dynamics. Using quantum-mechanical modelling, we found that kinetic scheme that captures all the experimental observations implies that only the bright lower polariton accepts the energy from the reservoir, suggesting that transfer occurs via a mechanism involving dipole-dipole interaction. Efficient reservoir-to-polariton transfer can play an important role in developing novel room-temperature quantum optical devices in the mid-infrared wavelength region.

In addition to extracting information, measurements of quantum systems are a resource for enhancing control and precision. They can be used to alter what we are detecting and allow access to new observables. Current hardware can perform near-ideal measurements of photon number or field amplitude, the ability to perform an ideal phase measurement is still lacking, even in principle. We implement a single-shot canonical phase measurement on a one-photon wave packet, which surpasses the current standard of heterodyne detection and is optimal for single-shot phase estimation. By applying quantum feedback to a Josephson parametric amplifier, our system adaptively changes its measurement basis during photon arrival to suppress amplitude information. We validate the detector’s performance by tracking the quantum state of the photon source. These results demonstrate that quantum feedback can enable access to new classes of physical observables.

Topological superconductors may harbor non-Abelian excitation on its boundaries such as Majorana zero mode – an essential ingredient for quantum information processing. Although, Majorana edge mode has been observed in several hybrid heterostructures and epitaxially grown materials, it is yet to be demonstrated in a layered stoichiometric material. This is essential to eliminate disorder induced artifacts and to design robust devices for quantum computing. Here, we used scanning tunneling microscopy to visualize the boundary states of intrinsic topological superconductivity in a transition metal dichalcogenide (TMD). We observed a zero-bias peak at the vortex core as well as an exponentially localized in-gap edge mode on the step edges along arbitrary direction and termination. The robust one-dimensional Majorana edge mode dispersing across the entire superconducting gap has slightly different localization length along the different directions of the step edges. We propose a simple model of nodal topological superconductivity pertinent to TMDs that captures the essential phenomenon of the one-dimensional edge mode along different directions and edge terminations. This is the first observation of Majorana edge mode in an intrinsically topologically superconducting material, marking a significant advance towards engineering two-dimensional devices for quantum computation.

Bosonic encoding is a nascent robust path to quantum computation, allowing error correction already at the hardware level. For an oscillator dispersively coupled to a two level ancilla, the universal control required for computation with any bosonic code was shown to be possible. However, the rate of control of such systems, as realized by Transmon coupled to a long-lived mode of a microwave cavity, is typically limited by the qubit-oscillator dispersive coupling strength [Heeres et al., 2017]. In our work, we propose and demonstrate experimentally a symmetry-inspired approach for fast displacement conditioned on the state of the qubit. Our novel method enables fast universal control while avoiding the undesired increase in higher-order terms, such as the Kerr nonlinearity. By focusing on the temporal form of the pulse, our method simplifies the implementation of the fast conditional displacement and greatly reduces the peak power required compared to previous schemes.

We describe how an entangling operation related to a CPHASE gate may be implemented by drawing on unique features of quantum measurement. The dynamics of a quantum system can be frozen by sufficiently strong monitoring, i.e., by the quantum Zeno effect. We show here that it is possible to combine local unitary operations and Zeno blocking of a single transition within a larger quantum system, in such a way that an effective non-local quantum operation is performed in an unmonitored subspace. We illustrate ideal realizations of such a process in detail, and then further describe the conditions under which it may be implemented using the dispersive continuous monitoring techniques commonly used in superconducting qubit systems. The scheme is experimentally feasible, and an implementation of it will be described in the talk by E. Blumenthal et al.

We study the fate of quantum criticality in a spin system coupled to gapless phonons. In one dimension, a recent study based on renormalization group (RG) analysis and density matrix renormalization group (DMRG) calculations reveals the possibility of the transition to remain second-order or driven to first-order, depending on the ratio of velocities of the spins and the phonons. Our goal is to understand how the coupling affects the transition in higher dimensions. We did a perturbative RG analysis of the Φ4 theory coupled to acoustic phonons, using an ε expansion near (3+1) dimensions. Our analysis is supplemented by a Quantum Monte Carlo simulation of the coupled system in two and three dimensions.

State preparation and measurement errors are commonly regarded as indistinguishable. The problem of distinguishing state preparation (SPAM) errors from measurement errors is important to the field of characterizing quantum processors. In this work, we propose a method to separately characterize SPAM errors using a novel type of algorithmic cooling protocol called measurement-based algorithmic cooling (MBAC). MBAC assumes the ability to perform (potentially imperfect) projective measurements on individual qubits, which is available on many modern quantum processors. We demonstrate that MBAC can significantly reduce state preparation error under realistic assumptions, with a small overhead that can be upper bounded by measurable quantities. Thus, MBAC can be a valuable tool not only for benchmarking near-term quantum processors, but also for improving the performance of quantum processors in an algorithmic manner.

Excitonic insulator is a quantum coherent phase resulting from formation of a macroscopic population of electron-hole pairs. For a semimetal with narrow overlap of the conduction and valence bands, a finite exciton binding energy could lead to excitonic instability. Candidate material Ta2NiSe5 shows a second-order structural phase transition at Tc=328K, with two mirror symmetries broken. Using polarization-resolved Raman spectroscopy, we demonstrate that Ta2NiSe5 is an excitonic insulator below Tc [npj Quantum Mater. 6, 52 (2021)]. The order parameter of the structural transition is of quadrupolar symmetry. In this symmetry channel, we observe strongly coupled excitonic and optical phonon modes at low energy. The resulting Fano line shape can be decomposed to reveal an overdamped exciton mode that exhibits critical softening on cooling towards Tc. In contrast, the energy of the bare optical phonons increases on cooling, which indicates that the phase transition does not result from lattice instability. We conclude that the transition is driven by the critical excitonic mode, and the transition temperature is enhanced by the coupling of this mode to the lattice modes of the same symmetry. We further show that for Ta2Ni(Se1−xSx)5 the excitonic instability, the transition temperature Tc, and the magnitude of the structural change across Tc are suppressed with increasing sulfur content x [PRB 104, 045102 (2021); arXiv 2104.07032 (2021)]. The Fano line shape at low energy persists up to x=0.67, up to which point Ta2Ni(Se1−xSx)5 has a semi metallic high-temperature phase. However, Ta2NiS5 is a semiconductor with more than 0.3 eV gap; the asymmetric line shape is absent because the exciton mode appears at high energy. As both the exciton and optical phonons of Ta2NiS5 do not show critical softening, we conclude that its phase transition is driven by ferroelastic instability.

Zigzag edges in graphitic systems exhibit localized electronic states that drastically affect their properties. Here, room-temperature charge transport experiments across a single graphitic interface are reported, in which the interlayer current is confined to the contact edges. It is shown that the current exhibits pronounced oscillations of up to ≈40 µA with a dominant period of ≈5 Å with respect to lateral displacement that do not directly correspond to typical graphene lattice spacing. The origin of these features is computationally rationalized as quantum mechanical interference of localized edge states showing significant amplitude and interlayer coupling variations as a function of the interface stacking configuration. Such interference effects may therefore dominate the transport properties of low-dimensional graphitic interfaces.

The normal modes of nonconservative systems coalesce at the so-called exceptional points (EPs) of their spectrum. These degeneracy points are the source of unusual phenomena, some of which are accessed by encircling the points in a suitable space. Here, we encircle the EPs of the transfer matrix of a periodic laminate, using a spatial perturbation in its stiffness. We investigate how, collectively, mode conversion in the laminate and the fields it scatters depend on the parameters of the loop. We find that the starting point of the loop has a significant effect on various counterintuitive phenomena: it determines if the laminate acts as a source or a sink of energy; how mode conversion takes place; if the reflectance is greater than one; and if there is spatial asymmetry in the energy flow with respect to the direction of the incident waves. Our findings are relevant for the development of devices for elastic wave manipulation.

An overview will be given on the progress of quantum dot laser materials addressing the telecom C band and their high potential for the application in optical communication systems, where temperature stability of the device performance as well as a narrow linewidth emission plays an important role. Device results of quantum dot lasers and optical amplifiers will be shown, and the physical background discussed.

This comment on the paper by Karlovets and Pupasov-Maksimov [Phys. Rev. A 103, 012214 (2021)] addresses their criticism of the combined experimental and theoretical study by Remez et al. [Phys. Rev. Lett. 123, 060401 (2019)]. We show, by means of simple optical arguments as well as numerical simulations, that the arguments raised by Karlovets and Pupasov-Maksimov do not hold in the experimental regime reported by Remez et al. Further, we discuss a clarification for the theoretical derivations presented by Karlovets and Pupasov-Maksimov, as they hold only when the final state of the emitting electron is observed in coincidence with the emitted photon. Although this scenario is feasible and may stimulate new experimental regimes that do correspond to the predictions reported by Karlovets and Pupasov-Maksimov, it is not the common scenario in cathodoluminescence, where only the light is measured. Upon lifting the concerns regarding the experimental regime reported by Remez et al., and explicitly clarifying the electron postselection, we believe that the paper by Karlovets and Pupasov-Maksimov may constitute a valuable contribution to the problem of spontaneous emission by shaped electron wave functions, as it presents new expressions for the emission rates beyond the ubiquitous paraxial approximation.

Ultracold atomic gas provides a useful tool to explore many-body physics. One of the recent additions to this experimental toolbox is Floquet engineering, where periodic modulation of the Hamiltonian allows the creation of effective potentials that do not exist otherwise. When subject to external modulations, however, generic interacting many-body systems absorb energy, thus posing a heating problem that may impair the usefulness of this method. For discrete systems with bounded local energy, an exponentially suppressed heating rate with driving frequency has been observed previously, leaving the system in a prethermal state for exceedingly long durations. However, for systems in continuous space, the situation remains unclear. Here, we show that Floquet engineering can be applied to a strongly interacting degenerate Fermi gas held in a flat boxlike potential without inducing excessive heating on experimentally relevant timescales. The driving eliminates the effect of a spin-dependent potential originating from the simultaneous magnetic levitation of two different spin states. We calculate the heating rate and obtain a power-law suppression with the drive frequency. To further test the many-body behavior of the driven gas, we measure both the pair-condensation fraction at unitarity and the contact parameter across the BEC-BCS crossover. At low driving frequencies, the condensate fraction is reduced by the time-dependent force, but at higher frequencies, it revives and attains an even higher value than without driving. Our results are promising for future exploration of exotic many-body phases of a bulk strongly interacting Fermi gas with dynamically engineered Hamiltonians.

For over 80 years of research, the conventional description of free-electron radiation phenomena, such as Cherenkov radiation, has remained unchanged: classical three-dimensional electromagnetic waves. Interestingly, in reduced dimensionality, the properties of free-electron radiation are predicted to fundamentally change. Here, we present the first observation of Cherenkov surface waves, wherein free electrons emit narrow-bandwidth photonic quasiparticles propagating in two-dimensions. The low dimensionality and narrow bandwidth of the effect enable to identify quantized emission events through electron energy loss spectroscopy. Our results support the recent theoretical prediction that free electrons do not always emit classical light and can instead become entangled with the photons they emit. The two-dimensional Cherenkov interaction achieves quantum coupling strengths over two orders of magnitude larger than ever reported, reaching the single-electron-single-photon interaction regime for the first time with free electrons. Our findings pave the way to previously unexplored phenomena in free-electron quantum optics, facilitating bright, free-electron-based quantum emitters of heralded Fock states.

An intriguing regime of universal charge transport at high entropy density has been proposed for periodically driven interacting one-dimensional systems with Bloch bands separated by a large single-particle band gap. For weak interactions, a simple picture based on well-defined Floquet quasiparticles suggests that the system should host a quasisteady state current that depends only on the populations of the system's Floquet-Bloch bands and their associated quasienergy winding numbers. Here we show that such topological transport persists into the strongly interacting regime where the single-particle lifetime becomes shorter than the drive period. Analytically, we show that the value of the current is insensitive to interaction-induced band renormalizations and lifetime broadening when certain conditions are met by the system's non-equilibrium distribution function, and show that these conditions correspond to a quasisteady state. We support these predictions through numerical simulation of a system of strongly interacting fermions in a periodically-modulated chain of Sachdev-Ye-Kitaev dots. Our work establishes universal transport at high entropy density as a robust far from equilibrium topological phenomenon, which can be readily realized with cold atoms in optical lattices.

An excellent candidate molecule for the measurement of the electron's electric dipole moment (eEDM) is thorium monofluoride ( ${\mathrm{ThF}}^{+}$) because the eEDM–sensitive state, ${}^3{\Delta }_1$, is the electronic ground state, and thus is immune to decoherence from spontaneous decay. We perform spectroscopy on $X{}^3{\Delta }_1$ to extract three spectroscopic constants crucial to the eEDM experiment: the hyperfine coupling constant, the molecular frame electric dipole moment, and the magnetic $g$ factor. To understand the impact of thermal blackbody radiation on the vibrational ground state, we study the lifetime of the first excited vibrational manifold of $X{}^3{\Delta }_1$. We perform ab initio calculations, compare them to our results, and discuss prospects for using ${\mathrm{ThF}}^{+}$ in an eEDM experiment at JILA.

Systems harboring parafermion zero-modes hold promise as platforms for topological quantum computation. Recent experimental work (G\"{u}l et al., arXiv:2009.07836) provided evidence for proximity-induced superconductivity in fractional quantum Hall edges, a prerequisite in proposed realizations of parafermion zero-modes. The main evidence was the observation of a crossed Andreev reflection signal, in which electrons enter the superconductor from one chiral mode and are reflected as holes to another, counter-propagating chiral mode. Remarkably, while the probability for cross Andreev reflection was much smaller than one, it was stronger for $\nu=1/3$ fractional quantum Hall edges than for integer ones. We theoretically explain these findings, including the relative strengths of the signals in the two cases and their qualitatively different temperature dependencies. Beyond the coupling of the two counter-propagating modes through Andreev reflection and back-scattering, an essential part of our model is the coupling of the edge modes to normal states in the cores of Abrikosov vortices located close to the edges. These vortices are made dense by the magnetic field needed to form the quantum Hall states, and provide a metallic bath to which the edges are tunnel-coupled. The stronger crossed Andreev reflection in the fractional case originates from the suppression of electronic tunneling between the bath and the fractional quantum Hall edges. Our theory shows that the mere observation of crossed Andreev reflection signal does not necessarily imply the presence of localized parafermion zero-modes, and suggests ways to identify their presence from the behavior of this signal in the low temperature limit.

Photonic time-crystals (PTCs) are spatially homogeneous media whose electromagnetic susceptibility varies periodically in time, causing temporal reflections and refractions for any wave propagating within the medium. The time-reflected and time-refracted waves interfere, giving rise to Floquet modes with momentum bands separated by momentum gaps (rather than energy bands and energy gaps, as in photonic crystals). Here, we present a study on the emission of radiation by free electrons in PTCs. We show that a free electron moving in a PTC spontaneously emits radiation, and when associated with momentum-gap modes, the electron emission process is exponentially amplified by the modulation of the refractive index. Moreover, under strong electron–photon coupling, the quantum formulation reveals that the spontaneous emission into the PTC bandgap experiences destructive quantum interference with the emission of the electron into the PTC band modes, leading to suppression of the interdependent emission. Free-electron physics in PTCs offers a platform for studying a plethora of exciting phenomena, such as radiating dipoles moving at relativistic speeds and highly efficient quantum interactions with free electrons.

A Fano resonance, as often observed in scattering, absorption, or transmission experiments, arises from quantum interference between a discrete optical transition and a continuous background. Here, we present a temperature-dependent study on Fano resonances observed in photoluminescence from flakes of the layered semiconductor antiferromagnet chromium thiophosphate (CrPS₄). Two Fano resonances with a distinctly different temperature dependence were identified. The continuous background that is responsible for the Fano resonances is attributed to the d–d transition of the optically active Cr³⁺ center, predominantly the spin-forbidden ²E_g → ⁴A_2g transition with contributions of the broad-band ⁴T_2g → ⁴A_2g transition. The discrete states that interfere with this continuous background are suggested to arise from localized atomic phosphorus. A model idea for explaining the individual temperature dependence of the Fano resonances is presented.

Variational methods have proven to be excellent tools to approximate the ground states of complex many-body Hamiltonians. Generic tools such as neural networks are extremely powerful, but their parameters are not necessarily physically motivated. Thus, an efficient parametrization of the wave function can become challenging. In this Letter we introduce a neural-network-based variational ansatz that retains the flexibility of these generic methods while allowing for a tunability with respect to the relevant correlations governing the physics of the system. We illustrate the success of this approach on topological, long-range correlated, and frustrated models. Additionally, we introduce compatible variational optimization methods for the exploration of low-lying excited states without symmetries that preserve the interpretability of the ansatz.

Photonic Time Crystals (PTCs) - dielectric media with their refractive index modulated periodically in time, offer new opportunities in photonics arising from time reflections and momentum bandgaps. Here, we study the emission of light from a radiation source inside a PTC. We solve the general classical and quantum mechanical models of emission in a temporally-varying medium, and find that radiation is always exponentially amplified when associated with the momentum gap, whether initiated by a macroscopic source, an atom, or by vacuum fluctuations, drawing the amplification energy from the modulation. The radiation linewidth becomes narrower as time advances, and is centered in the middle of the momentum gap. We calculate the spontaneous decay rate of an atom embedded in a PTC and show that it vanishes at the band edge due to low density of photonic states. Finally, we propose the concept of nonresonant tunable PTC lasers.

Quantum candies (qandies) represent a type of pedagogical simple model that describes many concepts from quantum information processing (QIP) intuitively without the need to understand or make use of superpositions and without the need of using complex algebra. One of the topics in quantum cryptography that has gained research attention in recent years is quantum digital signatures (QDS), which involve protocols to securely sign classical bits using quantum methods. In this paper, we show how the “qandy model” can be used to describe three QDS protocols in order to provide an important and potentially practical example of the power of “superpositionless” quantum information processing for individuals without background knowledge in the field.

The integration of semiconducting colloidal nanocrystals (NCs) with carbon nanotubes (CNTs) in a single device presents a unique platform that combines optical flexibility with high charge carrying capability. These qualities are desirable in many applications such as photovoltaic cells, photocatalysis, and light sensors. Here, we present hybrid devices that incorporate various CdSe/CdS core/shell NCs, such as seeded quantum dots (sQDs) and asymmetric seeded nanorods (a-sNRs), with single-wall carbon nanotube in a field-effect transistor geometry. We used electrical measurements to probe a light-induced charge transfer (LICT) between the CdSe/CdS NCs and the CNT. We investigate the effect of gate voltage on the LICT magnitude and temporal characteristics. Surprisingly, the measured photo-response depends on the gate voltage, and we observe both electrons and holes transfer from the a-sNRs to the …

We study a device composed of an optical interferometer integrated with a ferrimagnetic sphere resonator (FSR). Magneto-optic coupling can be employed in such a device to manipulate entanglement between optical pulses that are injected into the interferometer and the FSR. The device is designed to allow measuring the lifetime of such macroscopic entangled states in the region where environmental decoherence is negligibly small. This is achieved by recycling the photons interacting with the FSR in order to eliminate the entanglement before a pulse exits the interferometer. The proposed experiment may provide some insight into the quantum to classical transition associated with a measurement process.

We investigate experimentally and theoretically the temporal evolution of the spin of the conduction band electron and that of the valence band heavy hole, both confined in the same semiconductor quantum dot. We use all-optical pulse techniques to perform complete tomographic measurements of the spin as a function of time after its initialization and study the total spin purity (coherence), measured here. In the important limit of a weak externally applied magnetic field, comparable in strength to the Overhauser field due to fluctuations in the surrounding nuclei spins, the measured spin purity performs complex temporal oscillations. We use a central-spin model encompassing the spin' s Zeeman and the hyperfine interactions to reproduce the measured results quantitatively. Our studies are essential for designing and optimizing quantum-dot-spin-based entangled multiphoton sources that set stringent limitations on the magnitude of the externally applied field.

Noble-gas spins feature hours-long coherence times, owing to their great isolation from the environment, and find practical usage in various applications. However, this isolation leads to extremely slow preparation times, relying on weak spin transfer from an electron-spin ensemble. Here we propose a controllable mechanism to enhance this transfer rate. We analyze the spin dynamics of helium-3 atoms with hot, optically excited potassium atoms and reveal the formation of quasibound states in resonant binary collisions. We find a resonant enhancement of the spin-exchange cross section by up to 6 orders of magnitude and 2 orders of magnitude enhancement for the thermally averaged, polarization rate coefficient. We further examine the effect for various other noble gases and find that the enhancement is universal. We outline feasible conditions under which the enhancement may be experimentally observed and practically utilized.

The concept of tunneling injection was introduced in the 1990's to improve the dynamical properties of semiconductor lasers by avoiding the problem of hot carrier injection which increase the gain nonlinearity and hence limit the modulation capabilities. Indeed, tunneling injection led to record modulation speeds in quantum well lasers. Employing tunneling injection in quantum dot lasers is significantly more complicated. Tunneling injection is based on an energy band alignment between a carrier reservoir and the active region where laser oscillation takes place. However, the inherent inhomogeneity of self-assembled quantum dots prevents an unequivocal band alignment and can cause the tunneling injection process to actually deteriorate the laser performance compared to nominally identical quantum dot lasers that have no tunneling section. Understanding the complex process of tunneling injection in quantum dot lasers requires a comprehensive study where different aspects are analyzed theoretically and experimentally. In this paper we describe the technology of such lasers in the InP material system followed by a microscopic analysis of the detailed electrical characterization which is correlated to the electro-optic properties yields information about the exact carrier transport mechanism at bias levels of almost zero to well above threshold. A tunneling injection quantum dot optical amplifier was used for multi wavelength pump probe characterization from which it is clear why tunneling injection often deteriorates laser performance and determines how to design a structure which can take advantage of tunneling injection. Finally, we present a direct comparison between the modulation response of a tunneling injection quantum dot laser and a twin structure that has no tunneling injection section.

The broad study sheds light on the fundamental tunneling injection process that can guide the design of an optimum laser where tunneling injection will be taken full advantage of and will improve the dynamical properties.

Quantum entanglement and relativistic causality are key concepts in theoretical works seeking to unify quantum mechanics and gravity. In this article, a gedanken experiment that couples the spin to spacetime is proposed, and is then analyzed in the context of quantum information by using different approaches to quantum gravity. Both classical gravity theory and certain quantum theories predict that around a spin-half particle, the spherical symmetry of spacetime is broken by its magnetic field or merely by its intrinsic angular momentum. It is asserted that any spin-related deviation from spherical symmetry, upon appropriate measurement, can violate relativistic causality and quantum no-cloning. To avoid these violations, the measurable spacetime around the particle's rest frame shall typically remain spherically symmetric, potentially as a back-action by the act of a covariant measurement, or due to a quantized spin-dependence of the magnetic field. This way, this gedanken experiment suggests a censorship mechanism preventing the possibility of spacetime-based spin detection, which can shed light on the interface between quantum mechanics and gravity. Since this proposed gedanken experiment is independent of any specific theory, it is suitable for testing the coupling of quantum matter and spacetime in present and future candidate theories of quantum gravity.

Thermodynamic uncertainty relations unveil useful connections between fluctuations in thermal systems and entropy production. This work extends these ideas to the disparate field of zero temperature quantum mesoscopic physics where fluctuations are due to coherent effects and entropy production is replaced by a cost function. The cost function arises naturally as a bound on fluctuations, induced by coherent effects—a critical resource in quantum mesoscopic physics. Identifying the cost function as an important quantity demonstrates the potential of importing powerful methods from non-equilibrium statistical physics to quantum mesoscopics.

Quantum state tomography is an essential tool for the characterization and verification of quantum states. However, as it cannot be directly applied to systems with more than a few qubits, efficient tomography of larger states on mid-sized quantum devices remains an important challenge in quantum computing. We develop a tomography approach that requires moderate computational and quantum resources for the tomography of states that can be approximated by Gibbs states of local Hamiltonians. The proposed method, Hamiltonian Learning Tomography, uses a Hamiltonian learning algorithm to get a parametrized ansatz for the Gibbs Hamiltonian, and optimizes it with respect to the results of local measurements. We demonstrate the utility of this method with a high fidelity reconstruction of the density matrix of 4 to 10 qubits in a Gibbs state of the transverse-field Ising model, in numerical simulations as well as in experiments on IBM Quantum superconducting devices accessed via the cloud.

We show that muonium spectroscopy in the coming years can reach a precision high enough to determine the anomalous magnetic moment of the muon below one part per million (ppm). Such an independent determination of muon $g-2$ would certainly shed light on the $\sim 2\mathrm{ppm}$ difference currently observed between spin-precession measurements and ( $R$-ratio based) standard model predictions. The magnetic dipole interaction between electrons and (anti)muons bound in muonium gives rise to a hyperfine splitting (HFS) of the ground state which is sensitive to the muon anomalous magnetic moment. A direct comparison of the muonium frequency measurements of the HFS at J-PARC and the 1S-2S transition at PSI with theory predictions will allow us to extract muon $g-2$ with high precision. Improving the accuracy of QED calculations of these transitions by about 1 order of magnitude is also required. Moreover, the good agreement between theory and experiment for the electron $g-2$ indicates that new physics interactions are unlikely to affect muonium spectroscopy down to the envisaged precision.

We study the electronic phase diagram of the excitonic insulator candidates ${\mathrm{Ta}}_2\mathrm{Ni}{\left({\mathrm{Se}}_{1-x}{S}_x\right)}_5$ $(x=0,\cdots ,1)$ using polarization resolved Raman spectroscopy. Critical excitonic fluctuations are observed that diminish with $x$ and ultimately shift to high energies, characteristic of a quantum phase transition. Nonetheless, a symmetry-breaking transition at finite temperatures is detected for all $x$, exposing a cooperating lattice instability that takes over for large $x$. Our study reveals a failed excitonic quantum phase transition, masked by a preemptive structural order.

Coupling both optical light and microwave with magnetic materials is a pursued area of research recently as it will lead to emerging quantum technologies. Although spin waves in magnetic materials can be strongly coupled to microwave radiation, effective coupling of optical light with spin wave in magnetic system is still a major challenge. We have set up a figure-8 laser and integrated it with a coupled ferrimagnetic sphere-microwave resonator. We demonstrate the modulation of optical spectrum of figure-8 laser pulse by interaction with the microwave coupled ferrimagnetic YIG sphere. We observe that spectral peaks of optical pulses are governed by cavity modes formed by the YIG element.

Dielectric laser accelerators (DLAs) hold great promise for producing economic and compact on-chip radiation sources. On-chip DLAs benefit from fabrication capabilities of the silicon industry and from breakthroughs in silicon-photonic nanostructures to enhance the interaction between particles and laser fields. Seemingly unrelated recent advances in the quantum interactions of electrons and light have raised interest in the underlying classical-quantum correspondence principle at the foundations of electron acceleration. Here, we present the observation of the underlying quantum nature of DLAs: observing quantized peaks in the electron-energy spectra. Our findings demonstrate quasi-phase-matching between an electron wave function and a light wave, which also demonstrates the role of the quantum wave function in the inverse Smith-Purcell effect. We harness the capabilities of an ultrafast transmission electron microscope (UTEM) to maintain a long electron-light interaction length extending over hundreds of periods of the laser pulse, mediated by a silicon-photonic nanograting DLA. The UTEM is shown as a new platform for characterization of future DLA concepts. The results raise fundamental questions regarding the role of quantum mechanics in DLA design, and more generally about the prospects of manipulating particles’ quantum wave functions in accelerator physics.

The field of quantum information is becoming more known to the general public. However, effectively demonstrating the concepts underneath quantum science and technology to the general public can be a challenging job. We investigate, extend, and greatly expand here "quantum candies" (invented by Jacobs), a pedagogical model for intuitively describing some basic concepts in quantum information, including quantum bits, complementarity, the no-cloning principle, and entanglement. Following Jacob's quantum candies description of the well-known quantum key distribution protocol BB84, we explicitly demonstrate additional quantum cryptography protocols and quantum communication protocols, using generalized quantum candies (including correlated pairs of qandies). These demonstrations are done in an approachable manner, that can be explained to high-school students, without using the hard-to-grasp concept of superpositions and its mathematics. The intuitive model we investigate has a fascinating overlap with some of the most basic features of quantum theory. Hence, it can be a valuable tool for science and engineering educators who would like to help the general public to gain more insights into quantum science and technology. For the experts, the model we present, due to not employing quantum superpositions, enables - in some sense - extending far beyond quantum theory. Most remarkably, "quantum" candies of some unique type can be defined, such that non-local boxes (of the Popescu-Rohrlich type) as well as regular (correlated) quantum candies can be generated by a single `"quantum" candies machine.

The low-temperature colloidal production of II–VI nanoplatelet heterostructures has stimulated the interest of researchers due to the possible uses of these materials in various optoelectronic devices. Here, we report a room-temperature coating by CdS or ZnS dots of preprepared CdSe nanoplatelets. The dot coating process made use of a synthesis developed for the formation of free-standing CdS and ZnS species, involving injection of a metal precursor into reactive sulfur–amine solutions at room temperature. CdSe structured nanoplatelets with 1.75 nm thickness were used as the core constituent. The structural properties were investigated using Fourier transform infrared spectroscopy and advanced electron microscopy, while the elemental mapping was verified using high-angle annular dark field transmission electron microscopy. The results showed a dots-on-plate structure, resembling an intermediate configuration between core/crown or core/shell heterostructures. A thorough study of optical properties demonstrated a dramatic spectral shift of the absorption edge to lower energies, regardless of the uniformity of the coating layer, maintaining spectral stability over two months while stored at ambient conditions. Other optical measurements included examination of temperature-dependent photoluminescence and transient photoluminescence decay, from room temperature down to ∼4 K. These measurements revealed excitons, trions, and trapped carrier recombination emission with variable relative intensities following the temperature change. The dots-on-plate structures studied displayed a short photoluminescence decay (≤nanosecond), compatible with that of core/shell (crown) structures prepared at high temperatures. As a result, the presented techniques are alternative pathways that eliminate the requirement for excessive temperatures and/or multistep processes, instead providing rapid, inexpensive, and scalable procedures with practical advantages.

Light and matter can now interact in a regime where their coupling is stronger than their bare energies. This deep-strong coupling (DSC) regime of quantum electrodynamics promises to challenge many conventional assumptions about the physics of light and matter. Here, we show how light and matter interactions in this regime give rise to electromagnetic nonlinearities dramatically different from those of naturally existing materials. Excitations in the DSC regime act as photons with a linear energy spectrum up to a critical excitation number, after which, the system suddenly becomes strongly anharmonic, thus acting as an effective intensity-dependent nonlinearity of an extremely high order. We show that this behavior allows for N-photon blockade (with $N \gg 1$), enabling qualitatively new kinds of quantum light sources. For example, this nonlinearity forms the basis for a new type of gain medium, which when integrated into a laser or maser, produces large Fock states (rather than coherent states). Such Fock states could in principle have photon numbers orders of magnitude larger than any realized previously, and would be protected from dissipation by a new type of equilibrium between nonlinear gain and linear loss. We discuss paths to experimental realization of the effects described here.

Distributed quantum computing can provide substantial noise reduction due to shallower circuits. An experiment illustrates the advantages in the case of a Grover search. This motivates study of the quantum advantage of the distributed version of the Simon and Deutsch-Jozsa algorithms. We show that the distributed Simon algorithm retains the exponential advantage, but the complexity deteriorates from $O\left(n\right)$ to $O\left(n^2\right)$, where $n={log}_2\left(N\right)$. The distributed Deutsch-Jozsa algorithm deteriorates to being probabilistic but retains a quantum advantage over classical random sampling.

Topological superconductors are an essential component for topologically protected quantum computation and information processing. Although signatures of topological superconductivity have been reported in heterostructures, material realizations of intrinsic topological superconductors are rather rare. Here we use scanning tunnelling spectroscopy to study the transition metal dichalcogenide 4Hb-TaS₂ that interleaves superconducting 1H-TaS₂ layers with strongly correlated 1T-TaS₂ layers, and find spectroscopic evidence for the existence of topological surface superconductivity. These include edge modes running along the 1H-layer terminations as well as under the 1T-layer terminations, where they separate between superconducting regions of distinct topological nature. We also observe signatures of zero-bias states in vortex cores. All the boundary modes exhibit crystallographic anisotropy, which—together with a finite in-gap density of states throughout the 1H layers—allude to the presence of a topological nodal-point superconducting state. Our theoretical modelling attributes this phenomenology to an inter-orbital pairing channel that necessitates the combination of surface mirror symmetry breaking and strong interactions. It, thus, suggests a topological superconducting state realized in a natural compound.

The mean field approximation becomes applicable when entanglement is sufficiently weak. We explore a nonlinear term that can be added to the Schrödinger equation without violating unitarity of the time evolution. We find that the added term suppresses entanglement, without affecting the evolution of any product state. The dynamics generated by the modified Schrödinger equation is explored for the case of a two-spin 1/2 system. We find that for this example the added term strongly affects the dynamics when the Hartmann Hahn matching condition is nearly satisfied.

Measurement-based quantum communication relies on the availability of highly entangled multi-photon cluster states. The inbuilt redundancy in the cluster allows communication between remote nodes using repeated local measurements, compensating for photon losses and probabilistic Bell-measurements. For feasible applications, the cluster generation should be fast, deterministic, and its photons - indistinguishable. We present a novel source based on a semiconductor quantum-dot device. The dot confines a heavy-hole, precessing in a finely tuned external weak magnetic field while periodically excited by a sequence of optical pulses. Consequently, the dot emits indistinguishable polarization-entangled photons, where the field strength optimizes the entanglement. We demonstrate Gigahertz rate deterministic generation of >90% indistinguishable photons in a cluster state with more than 10 photons characteristic entanglement-length.

Optical drives at terahertz and mid-infrared frequencies in quantum materials are increasingly used to reveal the nonlinear dynamics of collective modes in correlated many-body systems and their interplay with electromagnetic waves. Recent experiments demonstrated several surprising optical properties of transient states induced by driving, including the appearance of photo-induced edges in the reflectivity in cuprate superconductors, observed both below and above the equilibrium transition temperature. Furthermore, in other driven materials, reflection coefficients larger than unity have been observed. In this paper we demonstrate that unusual optical properties of photoexcited systems can be understood from the perspective of a Floquet system; a system with periodically modulated system parameters originating from pump-induced oscillations of a collective mode. We present a general phenomenological model of reflectivity from Floquet materials, which takes into account parametric generation of excitation pairs. We find a universal phase diagram of drive induced features in reflectivity which evidence a competition between driving and dissipation. To illustrate our general analysis we apply our formalism to two concrete examples motivated by recent experiments: a single plasmon band, which describes Josephson plasmons in layered superconductors, and a phonon-polariton system, which describes upper and lower polaritons in materials such as insulating SiC. Finally we demonstrate that our model can be used to provide an accurate fit to results of phonon-pump - terahertz-probe experiments in the high temperature superconductor $\rm{YBa_2Cu_3O_{6.5}}$. Our model explains the appearance of a pump-induced edge, which is higher in energy than the equilibrium Josephson plasmon edge, even if the interlayer Josephson coupling is suppressed by the pump pulse.

Flicker noise causes decoherence in Josephson junction-based superconducting qubits, thus limiting their practical potential as building blocks for quantum computers. This is due to limited length and complexity of executable algorithms, and increased dependency on error-correcting measures. Therefore, identifying and subsiding the atomic sources of flicker noise are of great importance to the development of this technology. We developed a method that combines ab initio DFT calculations and quantum dynamics to model charge transport across a Josephson junction, by which it is possible to more accurately assess different defects as sources of flicker noise. We demonstrate the use of our method in an investigation of various atomic defects, including vacancies, trapping, and substitutions, in an Al|Al₂O₃|Al Josephson junction. This demonstration both reveals weaknesses in previous attempts to pinpoint the atomic sources of flicker noise and highlights new candidates.

Bi-stable mechanical resonators play a significant role in various applications, such as sensors, memory elements, and quantum computing. While carbon nanotube (CNT) based resonators have been widely investigated as promising nano electro-mechanical devices, a bi-stable CNT resonator has never been demonstrated. Here, we report a new class of CNT resonators in which the nanotube is buckled upward. We show that a small upward buckling yields record electrical frequency tunability, whereas larger buckling can achieve Euler-Bernoulli (EB) bi-stability, the smallest mechanical resonator with two stable configurations to date. We believe that these recently discovered CNT devices will open new avenues for realizing nano-sensors, mechanical memory elements and parametric amplifiers. Furthermore, we present a three-dimensional theoretical analysis revealing significant nonlinear coupling between the in-plane and out-of-plane static and dynamic modes of motion, and a unique three-dimensional EB snap-through transition. We utilize this coupling to provide a conclusive explanation for the low quality factor in CNT resonators at room temperature, key in understanding dissipation mechanisms at the nano scale.

The field of quantum information is becoming more known to the general public. However, effectively demonstrating the concepts underneath quantum science and technology to the general public can be a challenging job. We investigate, extend, and much expand here "quantum candies" (invented by Jacobs), a pedagogical model for intuitively describing some basic concepts in quantum information, including quantum bits, complementarity, the no-cloning principle, and entanglement. Following Jacob's quantum candies description of the well-known quantum key distribution protocol BB84, we explicitly demonstrate various additional quantum cryptography protocols using quantum candies in an approachable manner. The model we investigate can be a valuable tool for science and engineering educators who would like to help the general public to gain more insights into quantum science and technology: most parts of this paper, including many protocols for quantum cryptography, are expected to be easily understandable by a layperson without any previous knowledge of mathematics, physics, or cryptography.

The science of x-rays is by now over 125 years old, starting with Wilhelm Röntgen's discovery of x-rays in 1895, for which Röntgen was awarded the first Nobel Prize in Physics. X-rays have fundamentally changed the world in areas, including medical imaging, security scanners, industrial inspection, materials development, and drugs spectroscopy. X-ray science has been so far responsible for over 25 Nobel Prizes in Physics, Chemistry, and Medicine/Physiology. With x-ray generation being a highly commercialized, widely adopted technology, it may appear that there is little left to discover regarding the fundamentals of x-ray science. Contrary to this notion, recent years have shown renewed interest in the research and development of innovative x-ray concepts. We highlight, in this Perspective, promising directions for future research in x-ray science that result from advances in quantum science and in nanomaterials. Specifically, we describe three key opportunities for advancing x-ray science in the near future: (1) emerging material platforms for x-ray generation, especially 2D materials and their heterostructures; (2) free-electron-driven emission of entangled photon–photon and electron–photon pairs for x-ray quantum optics; and (3) shaping free-electron wavepackets for controllable x-ray emission. These research directions could lead to improvements in x-ray resonance fluoroscopy, high-contrast x-ray imaging, stimulated coherent x rays, x-ray superradiance, and other prospects for x-ray quantum optics.

The rise of atomically thin two-dimensional (2D) materials brings a revolution in material science and engineering, and encouraged worldwide scientists to integrate desired 2D materials into electrical circuitry by non-covalent interactions. Regardless of some unique properties including super-flexibility, broadband absorption and high carrier mobility, the weak optical absorption due to the thinness of 2D layers restricts their commercial use for photodetection. On contrary, quantum dot (QD) of 2D materials can overcome the weak absorption problem due to the strong quantum confinement, high absorption coefficient combined with the superlative features and properties of 2D layers. Therefore, 2D QDs have become a powerful contender for next-generation photodetection technology. Moreover, bandgap engineering by controlling the QD size due to strong quantum confinement and further integration with other materials are the most adaptable strategy to design high-performance, wavelength-tunable multicolour photodetector devices. It should be noted that the optical sensing for the infrared spectral region is of paramount importance in recent years, for many applications, especially environmental monitoring, security camera, military application, etc., but such applications in 2D-based QDs are limited. Alternatively, QD-based photodetector fabrication strategies such as epitaxial growth, heterostructure integration, functionalization and colloidal synthesis became mature from a material science perspective to achieve a broad spectral tunability. This chapter provides a comprehensive summary over the latest scientific and technological progress of QD-based photodetectors fabricated for the ultraviolet to infrared regime using van der Waals (vdW) materials, focusing mainly on carbon-based QDs, transition metal dichalcogenides (TMDCs) and black phosphorus (BP) QDs. We first start with the structural and optical properties of various 2D QDs. Secondly, the several strategies to synthesize 2D QDs and the different state-of-the-art device fabrication technology to improve the device performance have been described by addressing the inherent critical challenges. Thereafter, a brief but essential survey of emerging 2D QDs and their on-chip integration with other semiconductors in heterostructure form for photodetection application have been discussed. Finally, the chapter summarizes the comparison of performance of several 2D QD-based photodetectors highlighting the underlying challenges and opportunities for the future development of vdW QD-based research to realize their industrial scale-up.

Floquet systems often exhibit dynamical symmetries (DS) that govern the time-dependent dynamics and result in selection rules. When a DS is broken, selection rule deviations are expected. Typically, information about the symmetry-breaking perturbation/phase and the time-dependent dynamics can be extracted from these deviations, hence they are regarded as a background free gauge of symmetry breaking. However, to date, DS breaking & selection rule deviations are not described by a general approach, thus there is no universal insight about the interplay between selection rule deviations, the symmetry breaking perturbation, and the broken DS. Here we consider DS breaking in Floquet systems from a general standpoint, formulating a general theory that analytically connects the symmetry-broken and fully symmetric systems. Using an external laser (of arbitrary frequency and polarization), as a model DS breaking perturbation, we discover that the broken symmetry systematically imposes selection rules on the symmetry-broken system, which physically manifest as scaling laws of selection rule deviations. We term these rules "selection rules for breaking selection rules" – a new concept in physics. We numerically validate the analytical theory in the context of high harmonic generation (HHG). Our discovery is a general feature of nonlinear wave-mixing phenomena, and we expect it to apply to any Floquet system (classical & quantum) and to any DS breaking mechanism (either by intrinsic or extrinsic elements of the system).

Superconductivity serves as a unique solid-state platform for electron interference at a device-relevant lengthscale, which is essential for quantum information and sensing technologies. As opposed to semiconducting transistors that are operated by voltage biasing at the nanometer scale, superconductive quantum devices cannot sustain voltage and are operated with magnetic fields, which impose a large device footprint, hindering miniaturization and scalability. Here, we introduce a system of superconducting materials and devices that have a common interface with a ferroelectric layer. An amorphous superconductor was chosen for reducing substrate-induced misfit strain and for allowing low-temperature growth. The common quantum pseudowavefunction of the superconducting electrons was controlled by the nonvolatile switchable polarization of the ferroelectric by means of voltage biasing. A controllable change of 21% in the critical temperature was demonstrated for a continuous film geometry. Moreover, a controllable change of 54% in the switching current of a superconducting quantum interference device was demonstrated. The ability to voltage bias superconducting devices together with the nonvolatile nature of this system paves the way to quantum-based memory devices.

Floquet engineering uses coherent time-periodic drives to realize designer band structures on-demand, thus yielding a versatile approach for inducing a wide range of exotic quantum many-body phenomena. Here we show how this approach can be used to induce non-equilibrium correlated states with spontaneously broken symmetry in lightly doped semiconductors. In the presence of a resonant driving field, the system spontaneously develops quantum liquid crystalline order featuring strong anisotropy whose directionality rotates as a function of time. The phase transition occurs in the steady state of the system achieved due to the interplay between the coherent external drive, electron-electron interactions, and dissipative processes arising from the coupling to phonons and the electromagnetic environment. We obtain the phase diagram of the system using numerical calculations that match predictions obtained from a phenomenological treatment and discuss the conditions on the system and the external drive under which spontaneous symmetry breaking occurs. Our results demonstrate that coherent driving can be used to induce non-equilibrium quantum phases of matter with dynamical broken symmetry.

We demonstrate that the finite difference grid method (FDM) can be simply modified to satisfy the variational principle and enable calculations of both real and complex poles of the scattering matrix. These complex poles are known as resonances and provide the energies and inverse lifetimes of the system under study (e.g., molecules) in metastable states. This approach allows incorporating finite grid methods in the study of resonance phenomena in chemistry. Possible applications include the calculation of electronic autoionization resonances which occur when ionization takes place as the bond lengths of the molecule are varied. Alternatively, the method can be applied to calculate nuclear predissociation resonances which are associated with activated complexes with finite lifetimes.

The interaction between free electrons and light stands at the base of both classical and quantum physics, with applications in free-electron acceleration, radiation sources, and electron microscopy. Yet to this day, all experiments involving free-electron–light interactions are fully explained by describing the light as a classical wave. We observed quantum statistics effects of photons on free-electron–light interactions. We demonstrate interactions that pass continuously from Poissonian to super-Poissonian and up to thermal statistics, revealing a transition from quantum walk to classical random walk on the free-electron energy ladder. The electron walker serves as the probe in nondestructive quantum detection, measuring the second-order photon-correlation g(2)(0) and higher-orders g(n)(0). Unlike conventional quantum-optical detectors, the electron can perform both quantum weak measurements and projective measurements by evolving into an entangled joint state with the photons. These findings inspire hitherto inaccessible concepts in quantum optics, including free-electron–based ultrafast quantum tomography of light.

Synthetic-space topological insulators are topological systems with at least one spatial dimension replaced by a periodic arrangement of modes, in the form of a ladder of energy levels, cavity modes, or some other sequence of modes. Such systems can significantly enrich the physics of topological insulators, in facilitating higher dimensions, nonlocal coupling, and more. Thus far, all synthetic-space topological insulators relied on active modulation to facilitate transport in the synthetic dimensions. Here, we propose dynamically invariant synthetic-space photonic topological insulators: a two-dimensional evolution-invariant photonic structure exhibiting topological properties in synthetic dimensions. This nonmagnetic structure is static, lacking any kind of modulation in the evolution coordinate, yet it displays an effective magnetic field in synthetic space, characterized by a Chern number of one. We study the evolution of topological states along the edge, and on the interface between two such structures with opposite synthetic-space chirality, and demonstrate their robust unidirectional propagation in the presence of defects and disorder. Such topological structures can be realized in photonics and cold atoms and provide a fundamentally new mechanism for topological insulators.

Engines are open systems that can generate work cyclically at the expense of an external disequilibrium. They are ubiquitous in nature and technology, but the course of mathematical physics over the last 300 years has tended to make their dynamics in time a theoretical blind spot. This has hampered the usefulness of statistical mechanics applied to active systems, including living matter. We argue that recent advances in the theory of open quantum systems, coupled with renewed interest in understanding how active forces result from positive feedback between different macroscopic degrees of freedom in the presence of dissipation, point to a more realistic description of autonomous engines. We propose a general conceptualization of an engine that helps clarify the distinction between its heat and work outputs. Based on this, we show how the external loading force and the thermal noise may be incorporated into the relevant equations of motion. This modifies the usual Fokker–Planck and Langevin equations, offering a thermodynamically complete formulation of the irreversible dynamics of simple oscillating and rotating engines.

The Zeno effect occurs in quantum systems when a very strong measurement is applied, which can alter the dynamics in non-trivial ways. Despite being dissipative, the dynamics stay coherent within any degenerate subspaces of the measurement. Here we show that such a measurement can turn a single-qubit operation into a two- or multi-qubit entangling gate, even in a non-interacting system. We demonstrate this gate between two effectively non-interacting transmon qubits. Our Zeno gate works by imparting a geometric phase on the system, conditioned on it lying within a particular non-local subspace.

Characterizing noisy quantum devices requires methods for learning the underlying quantum Hamiltonian which governs their dynamics. Often, such methods compare measurements to simulations of candidate Hamiltonians, a task which requires exponential computational complexity. Here, we analyze and optimize a method which circumvents this difficulty using measurements of short time dynamics. We provide estimates for the optimal measurement schedule and reconstruction error. We demonstrate that the reconstruction requires a system-size independent number of experimental shots, and characterize an informationally-complete set of state preparations and measurements for learning local Hamiltonians. Finally, we show how grouping of commuting observables and use of Hamiltonian symmetries can improve the Hamiltonian reconstruction.

We report temperature-dependent current-voltage (I - V - T) and output light power-voltage or current (P - V - T) or (P - I - T) characteristics of 1550 nm tunneling injection quantum dot (TI QD) laser diodes. Experimental data is accompanied by physical models that distinguish between different current flow and light emission mechanisms for different applied voltages and temperature ranges. Three exponential regimes in the I - V characteristics were identified for low bias levels where no optical radiation takes place. At the lowest bias levels, the diffusion-recombination mechanism based on the classical Shockley-Reid-Hall theory dominates. This is followed, at low and near room temperature, by a combination of weak tunneling and generation-recombination, respectively. In the third exponential region, for all temperatures carrier transport is dictated by strong tunneling, which is characterized by a temperature-independent slope of the I - V curves and a variable ideality factor. The I - V results were compared to a conventional QD laser in which the current flow mechanisms of the first and third types are absent, which clearly demonstrates the key role played by the TI layer. In the post-exponential voltage range, when the diodes are in the high injection regime, the characteristics of the two types of diodes are identical. The typical behavior at the threshold current, where the output power increases fast has a clear signature in the I - V characteristics. Finally, an analytical quantitative relationship is established between the light output power and the applied voltage and current as well as the carrier density participating in radiative recombination.

We present a novel method for quantum tomography of multi-qubit states. We apply the method to spin-multi-photon states, which we produce by periodic excitation of a semiconductor quantum-dot- confined spin every 1/4 of its coherent precession period. These timed excitations lead to the deterministic generation of strings of entangled photons in a cluster state. We show that our method can be used for characterizing the periodic process map, which produces the photonic cluster. From the measured process map, we quantify the robustness of the entanglement in the cluster. The 3-fold enhanced generation rate over previous demonstrations reduces the spin decoherence between the pulses and thereby increases the entanglement.

Orbital angular momentum of light is a core feature in photonics. Its confinement to surfaces using plasmonics has unlocked many phenomena and potential applications. Here, we introduce the reflection from structural boundaries as a new degree of freedom to generate and control plasmonic orbital angular momentum. We experimentally demonstrate plasmonic vortex cavities, generating a succession of vortex pulses with increasing topological charge as a function of time. We track the spatiotemporal dynamics of these angularly decelerating plasmon pulse train within the cavities for over 300 femtoseconds using time-resolved photoemission electron microscopy, showing that the angular momentum grows by multiples of the chiral order of the cavity. The introduction of this degree of freedom to tame orbital angular momentum delivered by plasmonic vortices could miniaturize pump probe–like quantum initialization schemes, increase the torque exerted by plasmonic tweezers, and potentially achieve vortex lattice cavities with dynamically evolving topology.

When multiple quantum emitters radiate, their emission rate may be enhanced or suppressed due to collective interference in a process known as super- or subradiance. Such processes are well known to occur also in light emission from free electrons, known as coherent cathodoluminescence. Unlike atomic systems, free electrons have an unbounded energy spectrum, and, thus, all their emission mechanisms rely on electron recoil, in addition to the classical properties of the dielectric medium. To date, all experimental and theoretical studies of super- and subradiance from free electrons assumed only classical correlations between particles. However, dependence on quantum correlations, such as entanglement between free electrons, has not been studied. Recent advances in coherent shaping of free-electron wave functions motivate the investigation of such quantum regimes of super- and subradiance. In this Letter, we show how a pair of coincident path-entangled electrons can demonstrate either super- or subradiant light emission, depending on the two-particle wave function. By choosing different free-electron Bell states, the spectrum and emission pattern of the light can be reshaped, in a manner that cannot be accounted for by a classical mixed state. We show these results for light emission in any optical medium and discuss their generalization to many-body quantum states. Our findings suggest that light emission can be sensitive to the explicit quantum state of the emitting matter wave and possibly serve as a nondestructive measurement scheme for measuring the quantum state of many-body systems.

Over the past few years, quantized interactions between coherent free electrons and femtosecond laser pulses have shown intriguing new prospects for light-matter interactions. The talk will present theory and experiments of free electrons in laser-driven (ultrafast) transmission electron microscopy. Our experiment achieved what is, in many respects, the most powerful nearfield optical microscope in the world today. We resolve photonic bandstructures as a function of energy, momentum, and polarization, simultaneously with capturing the temporal dynamics and spatial distribution of the photonic modes at deepsubwavelength resolution. Recently, we used these new capabilities to observe coherent free-electron interactions with light trapped in photonic crystals and with polariton wavepackets in 2D materials.

We examine the radiation emitted by high-energy positrons channeled into silicon crystal samples. The positrons are modeled as semiclassical vector currents coupled to an Unruh-DeWitt detector to incorporate any local change in the energy of the positron. In the subsequent accelerated QED analysis, we discover a Larmor formula and power spectrum that are both thermalized by the acceleration. Thus, these systems explicitly exhibit thermalization of the detector energy gap at the celebrated Fulling-Davies-Unruh (FDU) temperature. Our derived power spectrum, with a nonzero energy gap, is then shown to have an excellent statistical agreement with high-energy channeling experiments and also provides a method to directly measure the FDU temperature. We also investigate the Rindler horizon dynamics and confirm that the Bekenstein-Hawking area-entropy law is satisfied in these experiments. As such, we present the evidence for the first observation of acceleration-induced thermality in a nonanalog system.