We show here that the photonic states emitted by ensembles of multilevel atoms via a superradiance process exhibit entanglement in the modal (frequency) degree of freedom, making this collective emission process a favorable candidate for a fast, bright, and deterministic source of entangled photons. This entanglement is driven by two mechanisms: (i) selective excitation of the atomic ensemble to a superposition state, and (ii) degeneracies of the optical transitions owing to the internal structure of the emitting atoms. The latter induces intricate nonradiative virtual transitions in the ensemble, which create interatomic correlations that are imprinted onto the emitted photons. One of the important outcomes of this complexity is the generation of mode-independent entangled multiphoton states. In addition, we study the dynamics of the correlations of the superradiating multilevel atom ensembles, and demonstrate a case where they exhibit beating in steady state as a result of the aforementioned virtual transitions.

We study the response of a spin to two crossed magnetic fields: a strong and fast transverse field, and a weak and slow longitudinal field. We characterize the sideband response at the sum and the difference of driving frequencies over a broad range of parameters. In the strong transverse driving regime, the emission spectrum has a characteristic volcano lineshape with a narrow central transparency region surrounded by asymmetric peaks. Next, we couple the spin to a nonlinear cavity that both drives and measures it. In a sufficiently slow longitudinal field, the emission spectrum exhibits anomalous behavior, where the resonances in both the right and left sidebands lie on the same side of the central resonance. The theoretical results are compared to the experimental measurement of the emission of substitutional nitrogen P1 and nitrogen-vacancy NV$^-$ defects in diamond.

Scattering experiments with energetic particles, such as free electrons, have been historically used to reveal the quantum structure of matter. However, realizing coherent interactions between free-electron beams and solid-state quantum systems has remained out of reach, owing to their intrinsically weak coupling. Realizing such coherent control would open up opportunities for hybrid quantum platforms combining free electrons and solid-state qubits for coincident quantum information processing and nanoscale sensing. Here, we present a framework that employs negatively charged nitrogen-vacancy centers (NV-) in diamond as quantum sensors of a bunched electron beam. We develop a Lindblad master equation description of the magnetic free-electron--qubit interactions and identify spin relaxometry as a sensitive probe of the interaction. Experimentally, we integrate a confocal fluorescence microscopy setup into a microwave-bunched electron beam line. We monitor charge-state dynamics and assess their impact on key sensing performance metrics (such as spin readout contrast), defining safe operating parameters for quantum sensing experiments. By performing $T_1$ relaxometry under controlled electron beam exposure, we establish an upper bound on the free-electron--spin coupling strength. Our results establish NV- centers as quantitative probes of free electrons, providing a metrological benchmark for free-electron--qubit coupling under realistic conditions, and chart a route toward solid-state quantum control with electron beams.

Error mitigation is essential for unlocking the full potential of quantum algorithms and accelerating the timeline toward quantum advantage. As quantum hardware progresses to push the boundaries of classical simulation, efficient and robust error mitigation methods are becoming increasingly important for producing accurate and reliable outputs. We introduce QESEM, a reliable, high-accuracy, characterization-based software implementing efficient, unbiased quasi-probabilistic error mitigation. We explain the innovative components underlying the operation of QESEM and demonstrate its capabilities in the largest utility-scale error mitigation experiment based on an unbiased method. This experiment simulates the kicked transverse field Ising model with far-from-Clifford parameters on an IBM Heron device. We further validate QESEM's versatility across arbitrary quantum circuits and devices through high-accuracy error-mitigated molecular VQE circuits executed on IBM Heron and IonQ trapped-ion devices. Compared with multiple variants of the widely used zero-noise extrapolation method, QESEM consistently achieves higher accuracy. These results mark a significant step forward in accuracy and reliability for running quantum circuits on devices available today across diverse algorithmic applications. Finally, we provide projections of QESEM's performance on near-term devices toward quantum advantage.

Generation of squeezed light is usually implemented in nonlinear \c{hi}(2) or \c{hi}(3) materials. Semiconductor lasers and optical amplifiers (SOAs) also offer non-linearities but they differ from passive elements in that they add amplified spontaneous emission noise (ASE). In a semiconductor laser, squeezing to below the shot noise limit has been demonstrated. An SOA contains no cavity and it adds significant noise. Gain saturation can lead, in principle, to squeezing of the photon number quadrature to below the shot noise level but often the noise is reduced only to below the ASE level of a linear amplifier. At the same time, the noise in the phase quadrature increases according to the Heisenberg uncertainty principle. Short resonant pulses interacting with a quantum dot SOA induce coherent effects such as Rabi oscillations. Here, we demonstrate, for the first time, that Rabi oscillations cause cyclical noise squeezing which varies periodically with the excitation pulse area. The noise in the present experiments does not reach the quantum limit so we term this condition quasi squeezing. It occurs during the portions of the Rabi cycle when the quantum dots provide gain and repeats with every fourfold increase of the pulse excitation energy which amounts to a 2{\pi} increase in pulse area. In all other cases, the noise exhibits the properties of a coherent state.

Periodic driving is used to steer physical systems to unique stationary states or nonequilibrium steady states (NESS), producing enhanced properties inaccessible to non-driven systems. For open quantum systems, characterizing the NESS is challenging and existing results are generally limited to specific types of driving and the Born-Markov approximation. Here we go beyond these limits by studying a generic periodically driven $ N$-level quantum system interacting with a low-density thermal gas. Exploiting the framework of Floquet scattering theory, we establish general Floquet thermalization conditions constraining the nature of the NESS and the transition rates. Moreover, we examine theoretically the structure of the NESS at high temperatures, and find out that the NESS complies, rather surprisingly, with the Boltzmann law for any driving. Numerical calculations illustrate our theoretical elaborations for a simple toy model.

Wigner crystals formed by laser-cooled ions in traps are unconventional condensed-matter systems, characterized by interparticle distances of several micrometers and energy scales on the order of $\mu$eV. Their crystalline structure emerges from the interplay between Coulomb repulsion and the external confining potential, which can be readily tuned. Moreover, individual ions can be precisely manipulated with lasers and imaged via resonance fluorescence. These unusual and unique properties make ion crystals a powerful platform for studying phases of matter and their dynamics in the strongly correlated quantum regime. This review examines the theoretical framework and experimental characterization of ion Coulomb crystals from a condensed-matter perspective. We highlight their dynamical and thermodynamic properties in one, two, and three dimensions, along with recent investigations into their out-of-equilibrium behavior. We provide outlooks on future directions for exploring novel condensed matter phenomena with trapped ion crystals, as well as their many scientific and technical applications, which have driven advances in controlling and measuring ion crystals in the lab.

Artificial quantum systems with synthetic dimensions enable exploring novel quantum phenomena difficult to create in conventional materials. These synthetic degrees of freedom increase the system's dimensionality without altering its physical structure, accessing higher-dimensional physics in lower-dimensional setups. However, synthetic quantum systems often suffer from intrinsic disorder, causing rapid decoherence that limits scalability, a major obstacle in quantum information science. Here, we show that introducing just a few long-range interactions can mitigate decoherence, creating persistent collective coherence in highly symmetric collective excited states. We term this universal phenomenon "supercoherence" and show its exceptional robustness against disorder up to a dynamical phase transition at critical interaction strength and disorder. Supercoherence stabilizes not only coherence but also all other quantum properties of the states, challenging traditional views on the inevitability of decoherence in disordered interacting quantum systems and suggesting new opportunities for quantum memory and information processing.

This study investigates the influence of crystal orientation on nitrogen incorporation, bonding configurations, and thermal stability in (100)- and (111)-oriented single-crystal diamond (SCD) surfaces implanted with ultra-low-energy N₂⁺ ions (100 and 200 eV; dose: 1 × 10¹⁵ ions/cm²). Upon impact, N₂⁺ ions dissociate, and each interacting nitrogen atom receives half the kinetic energy (50 and 100 eV), making the process ideal for studying near-threshold defect formation. Low-energy electron diffraction revealed a reconstructed surface with coexisting (2 × 1) and (1 × 2) domains on (100) SCD, while the (111) surface maintained a well-ordered (1 × 1) structure. X-ray photoelectron spectroscopy showed that (100) SCD incorporates and retains more nitrogen than (111), with superior thermal stability up to 1000 °C. Despite the low implantation energy, both surfaces exhibit minimal structural defect (C_def), although (100) SCD is slightly more susceptible to defect formation. Implanted nitrogen forms primarily C–N/C $\text{=}$ N and N–(sp² C/C_def) bonds, with the former dominating. These bonding configurations evolve with temperature and crystal orientation. The N–(sp² C/C_def) component remains stable between 300 and 700 °C and decreases at higher temperatures. At lower temperatures, nitrogen remains in interstitial form (N_i), forming N_i–C_i complexes with mobile carbon interstitials (C_i), which stabilize N–(sp² C/C_def) bonding. At elevated temperatures (800–1000 °C), N_i converts to substitutional nitrogen (N_s), reducing the defect-associated bonding signal. Overall, (100) SCD shows a stronger tendency for defect-mediated nitrogen stabilization than (111). These insights into ultra-low-energy nitrogen–diamond interactions may guide future strategies for surface engineering and stabilization of shallow nitrogen-vacancy centers in diamond-based quantum technologies.

Practical implementations of Quantum Key Distribution (QKD) often deviate from the theoretical protocols, exposing the implementations to various attacks even when the underlying (ideal) protocol is proven secure. We present new analysis tools and methodologies for quantum cybersecurity, adapting the concepts of vulnerabilities, attack surfaces, and exploits from classical cybersecurity to QKD implementation attacks. We present three additional concepts, derived from the connection between classical and quantum cybersecurity: "Quantum Fuzzing", which is the first tool for black-box vulnerability research on QKD implementations; "Reversed-Space Attacks", which are a generic exploit method using the attack surface of imperfect receivers; and a concrete quantum-mechanical definition of "Quantum Side-Channel Attacks", meaningfully distinguishing them from other types of attacks. Using our tools, we analyze multiple existing QKD attacks and show that the "Bright Illumination" attack could have been fully constructed even with minimal knowledge of the device implementation. This work begins to bridge the gap between current analysis methods for experimental attacks on QKD implementations and the decades-long research in the field of classical cybersecurity, improving the practical security of QKD products and enhancing their usefulness in real-world systems.