In this paper, to investigate how atoms evaporate and cool in microgravity environments of the space station, we have developed the dynamic equation for evaporative cooling of trapped atoms and obtained the analytical expression of atomic temperature with respect to trapping laser parameters and gravitational acceleration. In our model, the evaporation of atoms is equivalent to applying a damping force to the trapped atoms, and the evaporative cooling process of trapped atoms is comprehended as the damped oscillation of trapped atoms in the optical dipole traps. By introducing the gravity in our model, we obtained the analytical model of temperature variation with gravity after cooling, and the theoretical results agree well with the evaporation experiment of rubidium-87 atoms on the ground. Our theoretical results show that, compared with the atomic evaporative cooling experiment on the ground, the microgravity environment of the space station can achieve cooler atomic gases when the losses of atoms, such as the one-body loss caused by background-gas collisions and the three-body recombination loss caused by interatomic inelastic collisions, can be ignored.

We investigate the breathing mode and the stability of a quantum droplet in a tightly trapped one-dimensional dipolar gas of bosonic atoms. When the droplet with a flat-top density profile is formed, the breathing-mode frequency scales as the inverse of the number of atoms in the cloud. This is straightforwardly derived within a phenomenological hydrodynamical approach and confirmed using both a variational method based on a generalized Gross-Pitaevskii action functional and the sum-rule approach. We extend our analysis also to the presence of axial confinement showing the effect of the trap on the density profile and therefore on the breathing-mode frequency scaling. Our analysis confirms the stability of the quantum droplet against the particle emission when the flat-top density profile is observed. Our results can be used as a guide to the experimental investigations of collective modes to detect the formation of quantum droplets in quasi-one-dimensional dipolar gases.

High-efficiency microwave-to-optical quantum transduction is crucial for quantum networks and distributed quantum computing. Cavity-based electro-optic transducers have been widely explored due to their cavity-enhanced conversion efficiency, albeit at the compromise of limited transduction bandwidth and strict frequency alignment requirements between microwave and optical modes. A recent advancement in meter-long superconducting electro-optic modulators (SEOM) has demonstrated conversion efficiency approaching that of the cavity-based transducers on a traveling-wave structure while maintaining a broad bandwidth of tens of gigahertz. This paper provides a further theoretical investigation into the dynamics of the microwave-to-optical conversion process in a traveling-wave geometry. Based on this analysis, we propose a traveling-wave electro-optic transducer design featuring near-unity conversion efficiency and tunable conversion frequency.

The present work reports the theoretical and experimental observation of Stokes scintillations (SS) for focused vector vortex beams (VVBs) with controllable spatial correlation. Initially, a Gaussian-Schell model (GSM) source is utilized to generate the partially coherent VVBs (PC-VVBs). The focusing of PC-VVBs results in the optimization of extremum values of SS parameters across the beam cross section. Additionally, the SS parameters of the focused PC-VVB show spatially structured behavior. It is found that the spatial correlation, the inhomogeneous polarization distribution, and the Poincaré-Hopf index (PHI) of VVBs are important factors for structuring the SS parameters. The SS parameters are observed to exhibit a complementary relationship. Furthermore, the degeneracy associated with the SS parameters is discussed to complete the study. The flower-patterned distribution of specific SS parameters is useful in determining the topological index of the PC-VVBs. Interestingly, the bound condition and complementary nature of SS parameters are useful for scintillation squeezing across the beam cross section in PC-VVBs, where one SS parameter is suppressed at the expense of another. Thus, the source can be finely tuned using the bound condition with correlation and topological features to harness this effect optimally.

Tunable Rydberg interactions in cold atoms can be used to generate strongly correlated photon pairs through a spontaneous four-wave-mixing process in which all driving fields are allowed to work on resonance. The scheme relies on electromagnetically induced transparency and an imbalanced dark state with most atoms trapped in a ground state and a weakly populated Rydberg state. We show in particular that the combination of these two effects results in negligible linear absorption and Raman gain at the Stokes and anti-Stokes frequencies of generated photon pairs, while the nonlocal (Rydberg) nonlinearities contribute significantly to the enhancement of photon-pair generation rates and cross correlations at very low pump strengths and optical depths.

We study the emergence of anticoncentration and approximate unitary design behavior in local Brownian circuits. The dynamics of circuit-averaged moments of the probability distribution and entropies of the output state can be represented as imaginary-time evolution with an effective local Hamiltonian in the replica space. This facilitates large-scale numerical simulation of the dynamics in $1+1$ dimensions of such circuit-averaged quantities using tensor network tools as well as identifying the various regimes of the Brownian circuit as distinct thermodynamic phases. In particular, we identify the emergence of anticoncentration as a sharp transition in the collision probability at $lnN$ timescale, where $N$ is the number of qubits. We also show evidence for a specific classical approximation algorithm undergoing a computational hardness transition at the same timescale. In the presence of noise, we show there is a noise-induced first-order phase transition in the linear cross entropy benchmark when the noise rate is scaled down as $1/N$. At longer times, the Brownian circuits approximate a unitary 2-design in $O\left(N\right)$ time. We directly probe the feasibility of quantum error correction by such circuits and identify a first-order transition at $O\left(N\right)$ timescales. The scaling behaviors for all these phase transitions are obtained from the large-scale numerics and corroborated by analyzing the spectrum of the effective replica Hamiltonian.

Determining the properties of molecules, such as bond angles and bond lengths, is an important part of materials science. Quantum simulators are expected to efficiently determine them by calculating their energy spectrum. Compared with the previous single-resonant quantum eigensolver adopted in simulating water molecule [Phys. Rev. Lett. 122, 090504 (2019)], we propose a multiresonant quantum eigensolver that can search for the energy spectrum of molecules in a parallel manner. Our approach can exponentially save the cost of finding the energy spectrum by a factor of $2^r$, where $r$ is the number of ancillary qubits. As an interesting demonstration, we have designed and implemented an experiment to determine the bond angle of water molecules on a four-qubit nuclear spin quantum processor. We experimentally estimate the ground and first excitation energies and their corresponding states for effective water Hamiltonians with different H-O bond angles. The experimental results clearly show that the molecule structure with $\angle \text{H-O}\sim {100}^{\circ }$ bond angle is most stable. We believe that our approach sheds light on further applications in solving quantum chemical problems.

Coupling of a quantum battery (QB) to two thermal reservoirs brings it to a nonequilibrium steady-state (NESS). In this situation, although, the QB can be charged through the NESS heat current, no work may be extracted from it through unitary cyclic processes. We exemplify this statement by studying the two- and three-qubit $XXZ$ spin-chain QBs coupled weakly to two different thermal reservoirs in a symmetric configuration wherein each end of chain interacts with an individual reservoir. We show that the work can be extracted at positive temperature bias when two ends of a $XXZ$ chain are collectively coupled to the hot reservoir.

We examine the properties of the atomic current in a superfluid oscillating circuit consisting of a mesoscopic channel that connects two reservoirs of a Bose-Einstein condensate. We investigate the presence of a critical current in the channel and examine how the amplitude of the oscillations in the number imbalance between the two reservoirs varies with the system parameters. In addition to highlighting that the dissipative resistance stems from the formation of vortex pairs, we also illustrate the role of these vortex pairs as a quantum current regulator. The dissipation strength is discrete based on the number imbalance, which corresponds to the emergence of vortex pairs in the system. Our findings indicate that the circuit demonstrates characteristics of both voltage-limiting and current-limiting mechanisms. To model the damping behavior of the atomic superfluid circuit, we develop an equivalent LC oscillator circuit with a quantum current regulator.

The Talbot interferometer, as a periodic reproduction of momentum distribution in the time domain, finds significant applications in multiple research. The interparticle interactions during the diffraction and interference process introduce numerous many-body physics problems, leading to unconventional interference characteristics. This work investigates both experimentally and theoretically the influence of interaction in a Talbot interferometer with a ${}^6\mathrm{Li}_2$ molecular Bose-Einstein condensate in a one-dimensional optical lattice, with interaction strength directly tunable via magnetic Feshbach resonance. A clear dependence of the period and amplitude of signal revivals on the interaction strength can be observed. While interactions increase the decay rate of the signal and advance the revivals, we find that over a wide range of interactions, the Talbot interferometer remains highly effective over a certain evolutionary timescale, including the case of fractional Talbot interference. This work provides insight into the interplay between interaction and the coherence properties of a temporal Talbot interference in optical lattices, paving the way for research into quantum interference in strongly interacting systems.

We propose a different type of nonlinear interferometer, namely, four-beam nonlinear interferometer, which can realize phase-sensitivity enhancement. We compare the phase sensitivity of the four-beam nonlinear interferometer and the corresponding shot-noise limit. Our results demonstrate that the four-beam nonlinear interferometer can achieve a phase-sensitivity enhancement compared with the corresponding shot-noise limit. Moreover, taking the losses into consideration, the phase sensitivity of the four-beam nonlinear interferometer can also beat the corresponding shot-noise limit. Our scheme may find potential applications in multiparameter quantum metrology.

Considerable attention has been devoted to microscopic heat engines in both theoretical and experimental aspects. Notably, the fundamental limits pertaining to power and efficiency, as well as the trade-off relations between these two quantities, have been intensively studied. This study aims to shed further light on the ultimate limits of heat engines by exploring the relationship between the geometric length along the path of cyclic heat engines operating at arbitrary speeds and their power and efficiency. We establish a trade-off relation between power and efficiency using the geometric length and the timescale of the heat engine. Remarkably, because the geometric quantity comprises experimentally accessible terms in classical cases, this relation is useful for the inference of thermodynamic efficiency. Moreover, we reveal that the power of a heat engine is always upper bounded by the product of its geometric length and the statistics of energy. Our results provide a geometric characterization of the performance of cyclic heat engines, which is universally applicable to both classical and quantum heat engines operating far from equilibrium.

Quantum and random walks have been shown to be equivalent in the following sense: a time-dependent random walk can be constructed such that its vertex distribution at all time instants is identical to the vertex distribution of any discrete-time coined quantum walk on a finite graph. This equivalence establishes a deep connection between the two processes, far stronger than simply considering quantum walks as quantum analogs of classical random walks. The present paper strengthens this connection by providing a construction that establishes this equivalence in the reverse direction: a unitary time-dependent quantum walk can be constructed such that its vertex distribution is identical to the vertex distribution of any random walk on a finite graph at all time instants. The construction shown here describes a quantum walk that matches a random walk without measurements at all time steps (an otherwise trivial statement): measurement is performed in a quantum walk that evolved unitarily until a given time $t$ such that its vertex distribution is identical to the random walk at time $t$. The construction procedure is general, covering both homogeneous and nonhomogeneous random walks. For homogeneous random walks, unitary evolution implies time dependency for the quantum walk, since homogeneous quantum walks do not converge under arbitrary initial conditions, while a broad class of random walks does. Thus, the absence of convergence demonstrated for a quantum walk in its debut comes from both time homogeneity and unitarity, rather than unitarity alone, and our results shed light on the power of quantum walks to generate samples for arbitrary probability distributions. Finally, the construction here proposed is used to simulate quantum walks that match uniform random walks on the cycle and the torus.

Adaptive variational quantum eigensolvers (ADAPT-VQEs) are promising candidates for simulations of strongly correlated systems on near-term quantum hardware. To further improve the noise resilience of these algorithms, recent efforts have been directed towards compactifying, or layering, their Ansatz circuits. Here, we broaden the understanding of the algorithmic layering process in three ways. First, we investigate the noncommutation relations between the different elements that are used to build ADAPT-VQE Ansätze. In doing so, we develop a framework for studying and developing layering algorithms, which produce shallower circuits. Second, based on this framework, we develop a new subroutine that can reduce the number of quantum-processor calls by optimizing the selection procedure with which a variational quantum algorithm appends Ansatz elements. Third, we provide a thorough numerical investigation of the noise-resilience improvement available via layering the circuits of ADAPT-VQE algorithms. We find that layering leads to an improved noise resilience with respect to amplitude-damping and dephasing noise, which, in general, affect idling and nonidling qubits alike. With respect to depolarizing noise, which tends to affect only actively manipulated qubits, we observe no advantage of layering.

Assessing the presence of chemical, biological, radiological, and nuclear threats is a crucial task which is usually dealt with in spectroscopic measurements by analyzing the presence of spectral features in a measured absorption profile. The use of quantum ghost spectroscopy opens up the enticing perspective to perform these measurements remotely without compromising the measurement accuracy. However, in order to have the necessary signal-to-noise ratio, long acquisition times are typically required, hence subtracting from the benefits provided by remote sensing. In many instances, though, reconstructing the full spectral lineshape of an object is not needed and the interest lies in ascertaining the presence of a spectrally absorbing object. Here, we present an experimental investigation on the employ of the hypothesis testing framework to obtain a fast and accurate discrimination, carried out by ghost spectrometry. We discuss the experimental results obtained with different samples and complement them with simulations to explore the most common scenarios.

Motivated by recent experiments [Zhang et al., Nat. Phys. 19, 1466 (2023)], we study the interconversion between ultracold atomic and molecular condensates, quantifying the resulting oscillations and their slow decay. We find that near equilibrium the dominant damping source is the decay of condensed molecules into noncondensed pairs, with a pair kinetic energy that is resonant with the frequency of the oscillating atom-molecule interconversions. The decay, however, is nonexponential, as strong population of the resonant pairs leads to Bose enhancement. Introducing an oscillating magnetic field, which periodically modulates the molecular binding energy, enhances the oscillations at short times. However, the resulting enhancement of the pair-production process results in an accelerated decay, which rapidly cuts off the initial oscillation growth.

We construct a central-spin battery model affected by noise or thermal bath and consider the impact of environment on battery performance. In a noisy environment, we show that the steady-state energy and ergotropy are related to the number of spins exposed to the noisy channel. In addition, we reveal the relationship between the steady-state energy and ergotropy with the quantum information lost in the battery. In a thermal bath, we find that the higher the mean occupation number of the thermal bath, the greater the steady-state energy. In particular, we find that, in two different environments, the steady-state energy and ergotropy are related to the effective magnetic-field strength of battery $B$, but not charger $h$. It is worth noting that, in the central-spin model, when $B$ is a constant and $|h/B-1|$ reaches a certain value, the steady-state energy and ergotropy with noise or thermal bath are always greater than or equal to the maximum energy and ergotropy of the corresponding closed battery. At the same time, we generalize the conclusions to the two-qubit, two-harmonic-oscillator, and harmonic-oscillator-qubit models.

Quantum-phase-estimation algorithms are critical subroutines in many applications for quantum computers and in quantum-metrology protocols. These algorithms estimate the unknown strength of a unitary evolution. By using coherence or entanglement to sample the unitary $N_{\mathrm{tot}}$ times, the variance of the estimates can scale as $O(1/N_{\mathrm{tot}}^2)$, compared to the best “classical” strategy with $O(1/N_{\mathrm{tot}})$. The original algorithm for quantum phase estimation cannot be implemented on near-term hardware as it requires large-scale entangled probes and fault-tolerant quantum computing. Therefore, alternative algorithms have been introduced that rely on coherence and statistical inference. These algorithms produce quantum-boosted phase estimates without interprobe entanglement. This family of phase-estimation algorithms have, until now, never exhibited the possibility of achieving optimal scaling $O(1/N_{\mathrm{tot}}^2)$. Moreover, previous works have not considered the effect of noise on these algorithms. Here, we present a coherence-based phase-estimation algorithm which can achieve the optimal quadratic scaling in the mean absolute error and the mean squared error. In the presence of noise, our algorithm produces errors that approach the theoretical lower bound. The optimality of our algorithm stems from its adaptive nature: Each step is determined, iteratively, using a Bayesian protocol that analizes the results of previous steps.

The nonadiabatic geometric quantum gate (NGQG) is promising for the realization of high-fidelity operation for large-scale quantum processing. Normally, conventional NGQGs can be especially robust to either the Rabi error or the detuning error, which are two typical errors in many quantum computing platforms. However, it is difficult to suppress these two types of errors at the same time. This remains a big challenge for NGQGs. Here we present a general framework to implement the optimized geometric gate, where the evolution path is performed by using a family of optimized composite pulses. These composite pulses can reduce the sensitivity to the detuning error without introducing an extra Rabi error for this path. Thus, they help fulfill the cyclic evolution condition for the geometric gate. In addition, the inserted composite pulses would not introduce unwanted dynamical phase accumulation. As a result, the designed optimized geometric gate can simultaneously mitigate both the Rabi and detuning errors even in the presence of decoherence related to the real experiments. Our work paves a way to achieve geometric quantum computation robust against multiple errors.