In this work, we study the coherent dynamics of an atomic Zeeman wave packet using a continuous pump-probe scheme. A polarized Zeeman wave packet is created in laser-cooled lithium atoms via few-photon excitation by a femtosecond laser pulse, producing a state with a magnetic moment tilted relative to an external magnetic field. The subsequent Larmor precession of the atomic magnetic moment is probed via continuous ionization by an optical dipole trap (ODT) laser. The resulting ionization fragments—photoelectrons and photoions—are detected in coincidence using a cold target recoil ion momentum spectrometer (COLTRIMS). While the ODT facilitates further cooling of the atoms, its continuous-wave laser field does not provide the pulsed timing reference typically required to extract photoelectron momentum distributions in COLTRIMS. Here, we present an extension of the standard COLTRIMS technique that exploits redundancies in the measured data to reconstruct the ionization time. The resulting time-dependent ionization signal reveals the coherent precession of the atomic magnetic moment, enabling real-time access to atomic dynamics on nanosecond timescales.

Ultracold atomic systems provide a versatile platform for exploring quantum phenomena, offering tunable interactions and diverse trapping geometries. In this study, we investigate a one-dimensional system of trapped fermionic atoms using the composite boson formalism, which describes pairs of opposite-spin fermions as cobosons (short for composite bosons). We solve the Schrödinger equation for a $2\uparrow +2\downarrow$ fermionic system across a broad range of attractive and repulsive interactions by constructing the system's wave function as a superposition of two-coboson states. We determine key observables such as particle density profiles and two-body correlations. The density profiles calculated agree with both the Tonks-Girardeau and Lieb-Liniger limits and provide a portrait of the transition between those regimes. In the strong repulsive regime, the ansatz captures the Fridel-Wigner transition characterized by a doubling of the peaks in the density profiles. Additionally, we compute the low-lying energy spectrum and estimate the pairing gap. Our results highlight the usefulness of the coboson superposition ansatz for exploring quantum phenomena in strongly correlated few-body systems.

Frequency-based synthetic dimensions are a promising avenue for expanding the dimensionality of photonic systems. In this work, we show how a tilted synthetic lattice is naturally realized by periodically modulating a single-mode resonator under a coherent monochromatic drive. We theoretically study the Bloch wave dynamics in the tilted synthetic lattice, which gives rise to peculiar features in the spectral distribution of the cavity field. Our predictions are experimentally confirmed using a tunable superconducting microwave resonator.

Recently, the intersection of quantum optics and attosecond physics has attracted growing interest. Here, we study the quantum optical signatures of high-harmonic generation (HHG) from solid-state material driven by squeezed coherent states. We find that squeezed coherent states can either enhance or suppress the HHG yields compared to that driven by coherent light, depending on the photon bunching or antibunching characteristics of driving light. The analysis on semiclassical trajectories indicates that the amplitude fluctuation alters the recombination energy, while the phase fluctuation affects the birth and recombination time of electron-hole pairs. By calculating the Husimi Q distributions of HHG, we demonstrate that the squeezing of the two conjugate quadratures of driving light asymmetrically influence the squeezing phase of harmonics. Surprisingly, we find that even under coherent driving, the harmonics still exhibit squeezing due to the highly nonlinear response of solid-state HHG, which was overlooked previously. Numerical calculations of the Mandel parameter are performed to characterize the photon statistics of emitted HHG arising from different microscopic mechanisms. The results indicate that intraband radiation can exhibit a super-Poissonian distribution, while interband emission exhibits sub-Poissonian distribution under coherent driving.

The efficient witnessing and certification of entanglement is necessitated by its ubiquitous use in various aspects of quantum technologies. In the case of continuous-variable bipartite systems, the Shchukin-Vogel hierarchy gives necessary conditions for separability in terms of moments of the mode operators. In this work we derive mode-operator-based witnesses for continuous-variable bipartite entanglement relying on the interference of two states. Specifically, we show how one can access higher moments of the mode operators, crucial for detecting entanglement of non-Gaussian states, using a single beamsplitter with variable phase and photon-number-resolving detectors. We demonstrate that the use of an entangled state paired with a suitable reference state is sufficient to detect entanglement in, e.g., two-mode squeezed vacuum, NOON states, and mixed entangled cat states. We also take into account experimental noise, including photon loss and detection inefficiency, as well as finite measurement statistics.

In this work, we propose a stable charging scheme mediated by a three-level system (qutrit), which renders a unidirectional energy flow from an external power source to an ($N+1$)-dimensional quantum battery. By virtue of the qutrit dissipation, the battery avoids the spontaneous discharging induced by the time-reversal symmetry of any unitary-charging scheme. Irrespective of the initial state, the battery can be eventually stabilized at the maximal-ergotropy state as long as the charger-battery interaction is present. We use a Dyson series of Lindbladian superoperator to obtain an effective master equation for the battery, which is found to be equivalent to the high-order Fermi's golden rule adapted to the non-Hermitian Hamiltonian and spontaneous decay. We extract the optimization condition for charging efficiency and justify it in the finite-size battery with uniform energy splitting, the large spin battery, and the truncated harmonic-oscillator battery.

Quantum noise spectroscopy is a well-known method for detecting environmental noise spectrum and has various applications in quantum sensing, quantum network design, and quantum computing. In this work, a protocol for quantum noise spectroscopy is presented by qudit spectators, that are arbitrary $d$-level quantum systems passively spectating noises, which possess stochastic and stationary Gaussian distributions. The protocol uses Weyl basis decomposition, consisting of generalized Pauli matrices, as the theoretical tool, and the Alvarez-Suter noise spectroscopy method, by which the qudit is being exposed to a series of pulse sequences, and the noise polyspectra is extracted by multiple measurements of an observable of the qudit. The formalism is further developed and simulated for three theoretical frameworks: The first two frameworks utilize a simplified Weyl basis and are capable of detecting $Z$-type dephasing noise. One is specifically modeled for a qutrit ($d$ = 3), and the second framework is designed for all qudits and has been tested through simulations of a ququad ($d$ = 4) and a quoct ($d$ = 8). The third framework employs the full Weyl basis, enabling it to identify all types of XYZ-dephasing noise. The simulations for this framework have been conducted for a quoct ($d$ = 8) implemented by the antimony atom. The analytical protocol for noise spectroscopy that has been introduced transforms qudit systems into quantum spectators, and the presented simulations demonstrate the accuracy and reliability of this noise spectroscopy protocol.

Quick detection of transmittance changes in an optical channel is crucial for secure communication. We demonstrate that preshared entanglement using two-mode squeezed vacuum states significantly reduces detection latency compared to classical and entanglement-augmented coherent-state probes. The change-detection latency is inversely proportional to the quantum relative entropy (QRE), which goes to infinity in the absence of thermal noise, suggesting idealized instantaneous detection. However, in realistic scenarios, we show that QRE scales logarithmically with the inverse of the thermal-noise mean photon number. We propose a receiver that achieves this scaling and quantify its performance gains over existing methods. Additionally, we explore the fundamental trade-off between communication capacity and change-detection latency, highlighting how preshared entanglement enhances both.

Conventional manipulations over quantum systems for, such as coherent population trapping and unidirectional transfer, focus on Hamiltonian engineering while regarding the system's manifold geometry and constraint equation as secondary causes. Here, we treat them on equal footing in controlling a finite-dimensional quantum system under a time-dependent non-Hermitian Hamiltonian, which is inspired by the D'Alembert principle of regarding active force, constraint force, and inertial force in an unbiased way. Under the biorthogonal condition, the non-Hermitian Hamiltonian could be triangularized in a constraint picture spanned by a set of completed and orthonormal basis states, which is found to be a sufficient condition to construct at least one universal nonadiabatic passage in both bra and ket spaces. The passage ends up with a desired target state that is automatically normalized without artificial normalization in the existing treatments for non-Hermitian quantum systems. Moreover, the passage is found to be robust against the parametric deviation when the real part of its global phase is rapidly varying with time. Our protocol is explicitly verified for the perfect population transfer in the two-level system and the chiral population transfer in the three-level system. It generalizes our framework of universal quantum control to the field of the biorthogonal quantum mechanics.

Understanding the elementary mechanism for the dissipation of vortex energy in quantum liquids is one central issue in quantum hydrodynamics, such as quantum turbulence in systems ranging from neutron stars to atomic condensates. In a two-dimensional (2D) Bose-Einstein condensate (BEC) at zero temperature, in addition to the vortex drift-out process from the boundary, a vortex-antivortex pair can annihilate in bulk, but controversy remains on the number of vortices involved in the annihilation process. We find that a dynamic crossover exists from four-body to three-body vortex annihilation processes with time evolution in decaying 2D quantum turbulence of a boundary-less uniform quasi-2D BEC. Such a dynamical crossover depends on the initial vortex pair density and occurs when the sound waves generated in the vortex annihilation process surpass a threshold energy. As the confinement along the third direction is relaxed in a quasi-2D BEC, the threshold sound wave energy decreases due to the three-dimensional (3D) vortex line curve and reconnection, shifting the dynamical crossover to the early time. Our work reveals an elementary mechanism for the dissipation of vortex energy that may help understand exotic matter and dynamics in quantum liquids.

We study the propagation of dark-bright solitons in two-component nonlinear systems with general nonlinear parameters, and explore how nonlinear interactions enrich the soliton dynamics giving rise to nonsinusoidal oscillations under constant forces. Treating the bright soliton as an effective barrier, we reveal that such oscillations are characterized by the Josephson equations with self-adapted critical current and bias voltage, whose explicit analytic expressions are derived using the Lagrangian variational method. The dynamical phase diagram in nonlinear parameter space is presented, identifying oscillation regions with different skewed sinusoidal dependence, and diffusion regions with irreversible soliton spreading due to instability of the barrier. Furthermore, we obtain periodic dispersion relations of the solitons, indicating a switch between positive and negative inertial masses, consistent with the oscillation behaviors. Our results provide a general and comprehensive theoretical framework for soliton oscillation dynamics and pave the way for investigating various nonlinear transports and their potential applications.

Single-mode semiconductor lasers support fast nonlinear dynamics for investigating extreme events. Although one laser subject to perturbation is sufficient for yielding extreme events, multiple lasers with mutual interactions have yet to be considered. Here, extreme events are found experimentally using two detuned lasers under delayed mutual injection. The mutually injected lasers each emit an intensity that exhibits locking to a constant, except for interruptions by intermittent pulses, including very strong ones as extreme events. Such extreme eruptions from a constant background under mutual injection are different from the continual chaotic oscillations usually obtained under unidirectional injections. By varying the detuning frequency between the lasers, the intermittent pulsations with extreme events are found in a route that involves regular switching between constant and periodic oscillatory intensities. By adopting space-time representation, the extreme events are observed to emerge stochastically, together with some partial repetitions of pulses corresponding to a slightly lengthened round trip. By optimizing the detuning, the maximal normalized occurrence of extreme events reaches 0.8%, which is relatively high among approaches on lasers without external modulations for excitations. On average, about six extreme events are yielded within one round trip of 130 ns, while their short pulse widths are of the order of 0.1 ns. Applicable to generating strong random electrical pulses, the experiments point to the importance of mutual interactions in single-mode lasers for extreme events.

We investigate the single-photon scattering spectra in a giant-molecule waveguide quantum electrodynamics system, where a giant molecule consisting of two coupled giant atoms is connected to two waveguides via two separate sites, respectively. The phases of coupling strengths between the giant molecule and waveguides are considered. Using a real-space method, we obtain the exact analytical expressions for the single-photon scattering coefficients, which are valid both in the Markovian and non-Markovian regimes. It is shown that the transmission spectra strongly depend on phases, including the phases accumulated by photons traveling between coupling points and the phase difference between two coupled channels of the giant-molecule upper (lower) waveguide. Under the Markovian limit, we find that by selecting appropriate accumulated phases and local phase difference, targeted routing and asymmetrical scattering of photons can be achieved without breaking the symmetry in the coupling configuration between the giant molecule and waveguides. Moreover, all the optimal phase combinations for implementing single-photon routing have been summarized. In the non-Markovian regime, the scattering spectra show more complicated line shapes with multiple staggered peaks and dips. More interestingly, the scattering spectrum exhibits periodic changes during the transition from the Markovian to the non-Markovian regime, which makes one obtain a means to control photon routing in addition to phases. Our results may be useful in controlling and manipulating light-matter interactions involving giant atoms at the single-photon level.

The strongest quantum nonlocality is a stronger form of nonlocality than that manifested by locally indistinguishable quantum states. It has efficient applications in quantum information hiding and quantum secret sharing. Constructing a smaller-size set with the strongest nonlocality contributes to reducing resource consumption. In this paper we establish a sufficient and necessary condition for the strongest nonlocality in general $N$-partite systems. Then we derive the minimum size of the strongest nonlocal set with symmetry in the system ${\left({\mathbb{C}}^3\right)}^{\otimes N}$. Notably, the genuinely entangled sets constructed in our previous work [Phys. Rev. A 109, 022220 (2024)] achieved this minimum size. In addition, we construct the strongest nonlocal genuinely entangled sets in the system ${\left({\mathbb{C}}^d\right)}^{\otimes N}$ for $d\ge 4$, which have a smaller size than the existing sets as $N$ increases. Our results provide a meaningful complement to the current understanding of the strongest nonlocality.

With rapid advances in quantum hardware, a central question is whether quantum devices with or without full error correction can outperform classical computers on practically relevant problems. Variational quantum algorithms (VQAs) have gained significant attention as promising candidates in this pursuit, particularly for combinatorial optimization problems. While reports of their challenges and limitations continue to accumulate, many studies remain optimistic based on small-scale, idealized testing setups, leaving doubt about the scalability of VQAs for large-scale problems. We systematically investigate this scaling behavior by analyzing how classical optimizers minimize variational quantum loss functions for random Quadratic Unconstrained Binary Optimization instances in the presence of uncertainty, modeled as effective Gaussian noise. We find that the critical noise threshold for successful optimization decreases rapidly as system size grows. This decline exceeds what can be explained solely by shrinking loss variance, confirming deeper, fundamental limitations in the loss landscapes of VQAs beyond barren plateaus. Translating these thresholds into required measurement shots reveals that achieving sufficient precision in the evaluated loss values quickly becomes impractical, even for moderatelysized problems. Our findings demonstrate serious scalability challenges for VQAs in optimization stemming from mere uncertainty, indicating potential barriers to achieving practical quantum advantage with current hybrid approaches.

Leakage out from the qubit subspace compromises standard quantum error correction protocols and poses a challenge for practical quantum computing. We propose here a passive leakage removal unit based on an array of coupled disordered transmons and last-site reset by feedback-measurement or dissipation. We find that the unit effectively removes leakage with minimal effect on the qubit subspace by taking advantage of energy level disorder for the qubit subspace and resonant leakage energy levels. There are two optimal measurement rates for removing leakage, which we show analytically to correspond to the characteristic timescales of the propagation and disintegration of the leakage excitation. The performance of the leakage removal unit depends on the strength of disorder, coupling between the transmons, and the length of the array. We simulate the system under experimentally feasible parameters and present an optimal configuration. Our approach is readily compatible with existing superconducting quantum processors, considering realistic conditions.

Mitigation of quantum errors is critical for current NISQ devices. In the present work, we address this task by treating the execution of quantum algorithms as the time evolution of an idealized physical system. We use knowledge of its physics to assist the mitigation of the quantum noise produced on the real device. In particular, the time evolution of the idealized system obeys a corresponding Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) hierarchy of equations. This is the basis for the novel error mitigation scheme that we propose. Specifically, we employ a subset of the BBGKY hierarchy as supplementary constraints in the zero-noise extrapolation (ZNE) method for error mitigation. We ensure that the computational cost of the scheme scales polynomially with the system size. We test our method on digital quantum simulations of the lattice Schwinger model under noise levels mimicking realistic quantum hardware. We demonstrate that our scheme systematically improves the error mitigation for the measurements of the particle number and the charge within this system. Relative to ZNE, we obtain an average reduction of the error by $(18.2\pm 0.5)\%$ and $(52.8\pm 6.3)\%$ for the respective above observables. We propose further applications of the BBGKY hierarchy for quantum error mitigation.