We propose a mechanism for coherent densification in a tilted optical lattice. The starting point is a well-known property of Wannier-Stark ladders: a homogeneous resonant coupling between neighboring Wannier-Stark states restores coherent translation. When the coupling amplitude is position dependent and has a node, the local transport velocity vanishes at the node. A single wave packet is thereby transported toward this point, while a cloud distributed around the node undergoes a reduction of its spatial extent. The aim is to provide a reversible density-redistribution tool in a lattice of fixed period, complementary to matter-wave lenses, accordion lattices, box traps, and coherent-transport protocols. We introduce a nearest-neighbor effective model in which the coupling varies linearly with the site index. This model admits an exact analytic solution via a generating function and the method of characteristics; in the continuum limit it predicts exponential contraction of distances to the node. We discuss a primary implementation based on a spatially shaped Raman or Bragg coupling, and an alternative based on temporal modulation of the lattice depth. Estimates for cesium indicate that densification times ranging from a few tens to a few hundreds of milliseconds are achievable with realistic parameters. The mechanism is not dissipative cooling, but a coherent compression step that can be halted, reversed, or followed by a controlled expansion stage.

We report the observation of light storage in a dual memory associated with Zeeman coherence belonging to different hyperfine ground-state levels of cold cesium atoms. This double memory is based on the phenomenon of spontaneous Zeeman coherence transfer as predicted by the tensorial density-matrix formalism describing light interaction with a degenerate two-level system. We employ the two degenerate two-level systems $6S_{1/2},F=4\to 6P_{3/2},F^{\prime }=4$ and $6S_{1/2},F=3\to 6P_{3/2},F^{\prime }=3$, where a coupling beam and a probe beam orthogonally polarized interact resonantly with the first system, inducing Zeeman coherence in both the ground and excited hyperfine levels. The spontaneous Zeeman coherence transfer to the hyperfine ground state of the second system allows for storing the optical information of the probe beam simultaneously in the two hyperfine ground-state levels. Delayed four-wave mixing is employed to demonstrate the observation of the dual memory. Moreover, we also demonstrate the direct optical pumping of Zeeman coherence between the two hyperfine ground states induced by the incident coupling beam, which allows us to increase the storage time of the memory. A theoretical model using a simplified two degenerate two-level system is developed and accounts qualitatively well for the main observed results.

In quantum position verification (QPV), a prover certifies her location to a set of verifiers by performing a quantum computation. One of the first QPV protocols was proposed by Kent, Munro, and Spiller in 2011: The prover receives a qubit $Q$ from one direction, receives an orthogonal basis $\{v,v^{\perp }\}$ from the opposite direction, and then measures $Q$ in $\{v,v^{\perp }\}$ and broadcasts the result. Several variants of this protocol have been proposed and analyzed, but the question of whether the original protocol is secure has never been fully resolved. In this work we show that there is no perfect finite-dimensional cheating strategy for the single-qubit measurement protocol. Our proof uses tools from real algebraic geometry.

Building on hybrid quantum lattice gas algorithm, we revisit the possibilities of this quantum lattice model. By deriving a correction to the predicted viscosity, we provide analytical and numerical results that refine original formulation. We introduce a minimal two-dimensional generalization of the algorithm, which allows us to simulate anisotropic Burger-like equations while retaining only two lattice velocities. This approach opens a promising route toward embedding momentum conservation and advancing toward Navier-Stokestwo dynamics in two dimensions, going beyond Frisch, Hasslacher, and Pomeau and lattice Boltzmann method with a quantum native model. We highlight how the presented algorithm is more efficient for full state evolution than other quantum nonlinear solvers, nonetheless we clarify its advantages compared to classical lattice gas methods. Being that this model is between classical and quantum computation, it gives a unique perspective on simulating nonlinearities with quantum computers, confirming quantum lattice gas models as a crucial playground for quantum simulations of nonlinearities.

Superconducting circuits exhibit remarkable potential for constructing large-scale quantum simulation and computation systems, featuring numerous qubits, extended coherence time, and precise control. Nevertheless, the growing number of signal cables poses a challenge in dilution refrigerators due to space and heat-load constraints. To overcome this issue, we experimentally implement an optically assisted transmission line as an alternative to coaxial cables. By modulating microwave signals on laser intensities at room temperature and regenerating the signals at a cryogenic plate within the dilution refrigerator, we demonstrate full control of superconducting qubits using photocurrent. We demonstrate and benchmark both single-qubit and two-qubit gates on frequency-tunable transmon qubits, achieving fidelities of 99.915% $\pm$ 0.005% and 99.676% $\pm$ 0.041%, respectively, reaching the requirement of the surface code.

Precise environmental-noise characterization in open quantum systems is a key step toward high-fidelity quantum control and targeted decoherence suppression in computing and sensing applications. Nonparametric quantum noise spectroscopy (QNS) provides a general-purpose, model-agnostic framework for estimating the spectral properties of an environment. The ability to perform such protocols under realistic constraints is key to their practical applicability. Notably, it is important to account for control constraints and understand how they limit the ability to learn about noise correlations as experiment-agnostic objects. We show how adopting a control-centric point of view allows one to recast the noise spectroscopy problem in such a way that (i) the central objects are now the time-ordered polyspectra and (ii) control filter functions are no longer encumbered by time ordering. In particular, we show that this approach enables the generalization of frequency-comb QNS protocols to arbitrary control scenarios without introducing additional control symmetries that remove time ordering from filter functions, improving estimation in typically pathological scenarios. We demonstrate the targeted reconstruction of the time-ordered polyspectra across classical Gaussian and quantum non-Gaussian environments via simulations.

Quantum effect enables enhanced estimation precision in metrology, with the Heisenberg limit (HL) representing the ultimate limit allowed by quantum mechanics. Although the HL is generally unattainable in the presence of noise, quantum error correction (QEC) can recover the HL in various scenarios. A notable example is estimating a Pauli $Z$ signal under bit-flip noise using the repetition code, which is both optimal for metrology and robust against noise. However, previous protocols often assume noise affects only the signal accumulation step, while the QEC operations, including state preparation and measurement, are noiseless. To overcome this limitation, we study fault-tolerant quantum metrology where all qubit operations are subject to noise. We focus on estimating a Pauli $Z$ signal under bit-flip noise, together with state preparation and measurement errors in all QEC operations. We propose a fault-tolerant metrological protocol where a repetition code is prepared via repeated syndrome measurements, followed by a fault-tolerant logical measurement. We demonstrate the existence of an error threshold, below which errors are effectively suppressed and the HL is attained.

The effective Hamiltonian of $m{\alpha }^6$- and $m{\alpha }^6(m/M)$-order corrections for hydrogen molecular ions has been derived by Zhong et al. [Z.-X. Zhong, W.-P. Zhou, and X.-S. Mei, Phys. Rev. A 98, 032502 (2018)]; in this work we express the energy correction in the form of finite-value effective operators. The cutoff regularization scheme is used to determine the finite part of divergent operators of the leading-order recoil corrections. Numerical calculations of first-order contributions are performed in the Hylleraas basis set. Combining the second-order terms calculated in recent work [V. I. Korobov, J.-P. Karr, and Z.-X. Zhong, Mol. Phys. e2563023 (2025)], the $m{\alpha }^6$-order corrections for the fundamental rovibrational transition are obtained with an uncertainty three times smaller than in previous calculations.

Superradiance is an extensive cooperative spontaneous emission phenomenon, exhibited by some atomic collective-spin states. However, distinct initial states differ in their decay dynamics. Dicke states $|j,m\rangle$ with distinct numbers of excitations $n=m+j$, driven by vacuum fluctuations, have their peak emission intensity shifted in time. Rotating Dicke states relative to the decay axis introduces an interesting parity structure that affects the pulse profile and photon correlations. Squeezed-bath prepared states undergo a squeezing-controlled crossover to the rotated Dicke states, making the emission character dependent on the amount of squeezing transferred from light to the atomic state. For semiclassical states with a macroscopic dipole moment, like the atomic coherent state, the emission intensity depends on their polarization. We present detailed results on the superradiant dynamics of a representative selection of states expanded in Dicke states to highlight the initial state as an independent dynamical control parameter. For large-$N$, we are able to predict fairly accurately the pulse profile in each case using the mean-field approximation, an approach based on the Fokker-Planck equation. We also present comparative results on the intensity correlation function, quantify the coherent and incoherent contributions of the emission, and contrast between small and large ensembles.

Doppler broadening presents a major limitation for high-resolution spectroscopy and nonlinear optics in room-temperature atomic vapors. Here, we demonstrate the suppression of Doppler broadening accompanied by pronounced absorption on the upper transition of a three-level ladder system, achieved by dressing the intermediate state with a strong control field. As a concrete realization, we study a hot vapor of ${}^{87}\mathrm{Rb}$ where the lower transition is driven by a strong control field resonant with the $D_2$ line at a wavelength of 780 nm, while a weak counterpropagating probe field at the telecom C-band wavelength of 1529 nm $(5P_{3/2}\leftrightarrow 4D_{5/2})$ interrogates the dressed states. We observe absorption features with a resonant optical depth of approximately 4 and a full width at half maximum of about 17 MHz. Remarkably, this corresponds to an order-of-magnitude reduction relative to the Doppler width, while the optical depth on the upper transition of the ladder scheme exceeds that of the Doppler-broadened lower transition. The measured spectra are in good agreement with theoretical modeling. Combining high optical density with sub-Doppler-width absorption lines typically requires laser-cooled atoms, while our approach profits from the experimental simplicity of a hot-vapor platform.

Interference provides a fundamental mechanism for generating and manipulating entanglement in many-body quantum systems. Here, we develop an interference framework in which the nonlinear dynamics of collective spin-$\frac{1}{2}$ ensembles are mapped onto phase accumulation and self-interference in phase space, providing a direct and physically transparent description of entanglement formation. Within this framework, one-axis twisting produces Greenberger-Horne-Zeilinger (GHZ) states, while two-axis twisting generates multicomponent GHZ superpositions relevant for multiparameter quantum metrology. Building on this interference-based description, we analyze metrological performance under realistic Markovian noise, including local and collective emission, pumping, and dephasing, and examine the role of Hamiltonian control based on linear and nonlinear interactions. We show that the optimal control enhances sensitivity in both single- and multiparameter estimation across noise-dependent regimes. These results establish interference as a unifying principle linking nonlinear dynamics, entanglement generation, and metrological performance. This framework offers a broadly applicable route to robust quantum-enhanced sensing in noisy many-body systems.

Bell measurements (BMs) are ubiquitous in quantum information and technology. They are basic elements for quantum commmunication, computation, and error correction. In particular, when performed on logical qubits encoded in physical photonic qubits, they allow for a read-out of stabilizer syndrome information to enhance loss tolerance in qubit-state transmission and fusion. However, even in an ideal setting without photon loss, BMs cannot be done perfectly based on the simplest experimental toolbox of linear optics. Here we leverage that any logical BM on stabilizer codes can always be mapped onto a single physical BM perfomed on any qubit pair from the two codes. As a necessary condition for the success of a logical BM, this provides a general upper bound on its success probability, especially ruling out the possibility that the stabilizer information obtainable from only partially succeeding, physical linear-optics BMs could be combined into the full logical stabilizer information. We formulate sufficient criteria to find schemes for which a single successful BM on the physical level will always allow to obtain the full logical information by suitably adapting the subsequent physical measurements. Our approach based on stabilizer group theory is generally applicable to any stabilizer code, which we demonstrate for quantum parity, five-qubit, standard and rotated planar surface, tree, and seven-qubit Steane codes. Our schemes attain the general upper bound for all these codes, while this bound had previously only been reached for the quantum parity code.

Discrete time crystals are nonequilibrium states of periodically driven quantum matter characterized by robust subharmonic responses. We investigate how such temporal order behaves in the presence of stochastic measurements. Using trajectory-resolved simulations of a monitored Floquet-Ising model, we identify an intermediate regime in which the local subharmonic response survives only intermittently in time. The dynamics develops finite-lived temporal plateaus of locally coherent subharmonic response punctuated by stochastic disruptions correlated with projective measurements. While local temporal staggering remains robust over broad intervals, the spectral coherence of the subharmonic response exhibits substantially stronger fluctuations, revealing a temporally fragmented dynamics not fully captured by global long-time observables alone. The resulting plateau statistics are further consistent with an effective two-state kinetic description based on finite-rate stochastic disruption events. These results show that monitored dynamics can support fluctuating local synchronization in which finite temporal domains repeatedly emerge and disappear along individual quantum trajectories, without establishing globally persistent discrete time-crystalline order under stochastic monitoring.

Quantum information masking (QIM) encodes single-qubit information into bipartite entanglement, but the absence of a universal masker restricts standard bipartite implementations to specific Bloch sphere disks. Here, we experimentally demonstrate an automated quantum masking machine that can be reconfigured for target disks of different orientations. Driven by a machine-learning-assisted feedback algorithm, a self-calibrating adapter determines the required rotation directly from photon-count measurements for a fixed base masker and maps a selected target disk to a maskable latitudinal configuration. Leveraging this framework, we experimentally demonstrate a proof-of-principle quantum secret sharing scheme. For the tested states, quantum state tomography shows that the encoded information is retained in the global entanglement, while the corresponding marginal states exhibit no resolvable input dependence within experimental uncertainty. Rather than altering the no-masking principle, this closed-loop architecture broadens the input compatibility of a fixed masker and provides a reconfigurable platform for masking-based quantum communication.

Logical operations are essential for quantum computation within quantum error-correcting codes. However, discovering their physical realizations is challenging, especially for nonadditive codes that lack a stabilizer description. We present a general learning-based framework that, given only an encoding circuit, constructs physical implementations of logical operations while enforcing structural properties such as transversality or shallow depth. Our approach is validated by rediscovering known logical operations of standard stabilizer codes. We then extend it to a codesign procedure, dubbed variational early fault-tolerant quantum computing (VarEFTQC), which tailors nonadditive encodings to a given noise model and enforces desired logical gate sets, such as transversal IQP-type families or low-depth universal sets. A software library implements the complete learning pipeline, including loss-function variants, ansatz families, and optimization routines. Together, these results position VarEFTQC as a proof-of-concept framework for discovering hardware-adapted logical gadgets for early fault-tolerant quantum computing.

Quantum simulation is a promising application for quantum computing. Quantum simulation algorithms may require the ability to control the time evolution unitary. Naive techniques to control a unitary can substantially increase the required computational resources. A standard approach to controlling Trotterized time evolution doubles the number of single-qubit arbitrary rotations. Here, we describe a compilation scheme to control a symmetric Trotterization that does not increase the number of arbitrary rotations. This halves the number of arbitrary rotations required to implement controlled, Trotterized time evolution compared to the standard approach. Arbitrary rotations contribute significantly to $T$ cost in a fault-tolerant architecture due to the number of required magic states. Therefore, arbitrary rotations often dominate resource estimates for fault-tolerant implementations of quantum simulation algorithms. This construction reduces the number of arbitrary rotations for controlled Trotter evolution to that of uncontrolled Trotter evolution, thereby reducing the cost of fault-tolerant quantum simulation.

The standard stimulated Raman adiabatic passage (STIRAP) protocol enables high-fidelity quantum state transfer in an ideal three-level system via adiabatic following of a dark state. However, in practical systems with more energy levels, control pulses with finite spectral selectivity often couple the three-level subspace to the remaining subspace, introducing leakage that fundamentally limits the transfer performance. Here, we adopt a multilevel chain model for STIRAP that explicitly incorporates this leakage subspace. Using Pontryagin's maximum principle, we formulate a leakage-penalized quantum optimal control problem with the control pulses constrained to experimentally feasible Gaussian pulse families. We derive explicit gradients of the objective functional with respect to the pulse parameters, enabling efficient low-dimensional optimization that suppresses leakage while preserving the counterintuitive STIRAP pulse ordering. Numerical simulations for a superconducting transmon platform demonstrate that the optimized control pulses significantly enhance the target-state transfer fidelity and improve robustness against amplitude miscalibration and detuning drifts.

In this article we report on an experimental study of the polarization of atoms in magnetized plasma (PAMP) in ${}^3\mathrm{He}$ gas under various conditions of radio-frequency (rf) excitation of plasma. Among parameters influencing the plasma dynamics are frequency, average and peak rf powers. Our data strongly indicate a significant influence of electron density in the plasma on the polarization value and buildup rate. In particular, it was found that polarization scales as $U^{\beta }$, where $U$ is the voltage across the coil in the tank circuit generating the plasma and $\beta$ is in the range of 1.3–1.5, similar to what is expected for the electron density. Experiments with the modulation of rf power show that the peak rf power governs the polarization rather than the average power. An unexpected increase in polarization related to the appearance of a small amount of impurities in the ${}^3\mathrm{He}$ gas at room temperature is found. Strong correlations between polarization and amount of oxygen impurity, which is desorbed as the temperature rises, support the hypothesis that polarization is proportional to electron density because it is expected that electron density linearly increases with impurity concentration. It was also found that the polarization and buildup rate are proportional to each other, a behavior that is common for all rf plasma frequencies. We propose a scenario that can explain the experimental observations: The polarization is limited by nuclear magnetic relaxation, while the growth rate associated with the intrinsic polarization process depends quadratically on the electron density. Our study identifies the ways to increase the efficiency of the PAMP method by maximizing electron density using different means.