The interplay between the non-Hermiticity and dimensionality gives rise to exotic characteristics in higher dimensions, with one representative phenomenon known as the geometry-dependent skin effect (GDSE), which refers to that the localization of extensive eigenstates depends on the system's geometry under open boundary conditions. In this paper, we demonstrate the emergence of GDSE in a two-dimensional $sp$ optical ladder lattice with on-site atom loss, which can be manifested by anisotropic dynamics of Bloch oscillations in the bulk of the system. By applying a static force in different directions, the wave-packet dynamics retrieves the complex energy spectra with either nonzero or zero spectral winding number, indicating the presence or absence of skin accumulation in the corresponding directions, respectively. We find that this bulk dynamical characterization can be extended to interacting multiparticle systems, offering a method for detecting GDSE and other types of skin effect in quantum many-body systems, even without a well-defined open boundary.

The optomechanical cavity system has been a paradigm in the manifestation of continuous variable quantum information over the past decade. This paper investigates how quantum phase synchronization is associated with bipartite Gaussian entanglement in coupled gain-loss mechanical oscillators, where the gain and loss rates can be controlled by driving the cavities with blue and red detuned lasers, respectively. We examine the role of the exceptional point (EP) in a deterministic way of producing self-sustained oscillations that generate robust quantum correlations among quadrature fluctuations of the mechanical oscillators. Particularly, steady phase synchronization dynamics along with the entanglement phenomena are observed in the effective weak-coupling regime above a critical driving power. These phenomena are further verified by observing the mechanical squeezing and phase-space rotations of the Wigner distributions. Moreover, the fidelity fluctuation shows how the quantum correlation dynamics are related to the EP of the system. We also discuss the impact of the mechanical oscillator's frequency mismatch and decoherence due to thermal phonons on system dynamics. These findings hold promise for applications in phonon-based quantum communication and information processing on the macroscopic scale.

Parity-time $\left(\mathcal{P}\mathcal{T}\right)$-symmetric dimers were introduced to highlight the unusual properties of non-Hermitian systems that are invariant after a combined parity and time reversal operation. They are also the building blocks of a variety of symmetry and topologically protected structures, especially on integrated photonic platforms. As the name suggests, it consists of two coupled oscillators, which can be optical, mechanical, electronic, and so on in nature. In this article, we show that its effective size, defined by the number of lattice sites inversely proportional to the lattice momentum, is surprisingly three instead of two from the perspective of energy quantization. More specifically, we show analytically that the complex energy levels of a one-dimensional concatenated chain with $N$ $\mathcal{P}\mathcal{T}$ dimers are determined by a system size of $1+2N$, which reduces to three in the case of a single $\mathcal{P}\mathcal{T}$ dimer. We note that while energy quantization conditions were established in various non-Hermitian systems, exact and explicitly quantized complex energies as reported here are still scarce. In connection, we also discuss the other symmetries of a $\mathcal{P}\mathcal{T}$ dimer and concatenated $\mathcal{P}\mathcal{T}$-dimer chain, including non-Hermitian particle-hole symmetry and chiral symmetry.

Gaussian boson sampling constitutes a prime candidate for an experimental demonstration of quantum advantage within reach with current technological capabilities. The original proposal employs photon-number-resolving detectors, however, these are not widely available. Nevertheless, inexpensive threshold detectors can be combined into a single click-counting detector to achieve approximate photon-number resolution. We investigate the problem of sampling from a general multimode Gaussian state using click-counting detectors and show that the probability of obtaining a given outcome is related to a matrix function which is dubbed as the Kensingtonian. We show how the Kensingtonian relates to the Torontonian and the Hafnian, thus bridging the gap between known Gaussian boson sampling variants. We then prove that, under standard complexity-theoretical conjectures, the model cannot be simulated efficiently.

The crucial limitation of improving the sensitivity of the detection of weak magnetic fields is the unavoidable measurement noise. In this paper, we propose a scheme to achieve precise sensing robust against additional noise by employing a dual frequency bias field modulation within a cavity-magnon system. We find that the antirotating wave term can amplify the signal of the detected magnetic field, but this amplification effect must coexist with the rotating wave term. In particular, by the bias field modulation, we find the robustness against cavity field thermal noise is substantially enhanced, quantum noise and cavity field thermal noise is greatly reduced, and the external magnetic field signal is amplified, thereby improving the weak-magnetic-field sensing system's sensitivity. Compared with the previous scheme, our scheme requires neither an ultrastrong- or deep-strong-coupling mechanism nor the suppression of the additional noise by increasing the electromagnetic cooperativity. Our scheme could provide a valuable candidate for weak-magnetic-field sensing.

In a recent work [Reible et al., Phys. Rev. Res. 5, 023156 (2023)], it was shown that the mean particle-particle interaction across an ideal surface that divides a system into two parts can be employed to estimate the size dependence for the thermodynamic accuracy of the system. In this work we propose its application to systems with finite-range interactions that model dense quantum gases and derive an approximate size-dependence scaling law. In addition, we show that the application of the criterion is equivalent to the determination of a free-energy response to a perturbation. The latter result confirms the complementarity of the criterion to other estimates of finite-size effects based on direct simulations and empirical structure or energy convergence criteria.

Quantum batteries, which are quantum systems to be used for the storage and transformation of energy, have been recently attracting research interest. A promising candidate for their investigation is the Dicke model, which describes an ensemble of two-level systems interacting with a single-mode electromagnetic wave in a resonator cavity. In order to charge the battery, a coupling between the ensemble of two-level systems and resonator cavity should be turned off at a certain moment of time. This moment of time is chosen in such a way that the energy gets fully stored in an ensemble of two-level systems. In our previous works we have investigated a bound-luminosity superradiant state of the extended Dicke model and found analytical expressions for the dynamics of coherent energy transfer between the superradiant condensate and the ensemble of two-level systems. Here, using our previous results, we have derived analytically the superlinear law for the quantum battery charging power $P\sim N^{3/2}$ as a function of the number $N$ of two-level systems in the battery, and also the $N$ dependence for the charging time $t_c\sim N^{-1/2}$. The $N$ exponent $3/2$ of the charging power is in quantitative correspondence with the recent result $1.541$ obtained numerically by other authors. The physics of Dicke quantum battery charging is considered in detail.

We investigate the properties of Chern insulator in a two-dimensional (2D) Su-Schrieffer-Heeger (SSH) model with next-nearest-neighbor (NNN) hopping. We find that the Weyl points, serving as phase-transition points, precisely coincide with high-symmetry points. In the phase diagram, there are two nontrivial phases with different nonzero Chern numbers, one trivial phase with zero Chern number, and two different types of phase-transition boundaries formed by the Weyl points. The system with a nonzero Chern number is topologically nontrivial, with localized edge states at the top and bottom. Interestingly, the eigenmodes on the phase-transition boundaries of two different nontrivial phases are localized at the bottom of the system, in contrast with the extended eigenmodes observed on the trivial and nontrivial phase-transition boundaries. Our work provides insights into exploring the correlation between edge state and phase transition boundary in Chern insulator based on the 2D SSH model with NNN hopping.

We generalize the recent work of Shibata and Katsura [Phys. Rev. B 99, 174303 (2019)], who considered a $S=\frac{1}{2}$ chain with alternating $XX$ and $YY$ couplings in the presence of dephasing, the dynamics of which are described by the GKLS master equation. Their model is equivalent to a non-Hermitian system described by the Kitaev formulation [Kitaev, Ann. Phys. 321, 2 (2006)] in terms of a single Majorana species hopping on a two-leg ladder in the presence of a nondynamical ${\mathbb{Z}}_2^{\phantom{*}}$ gauge field. Our generalization involves Dirac gamma matrix “spin” operators on the square lattice and maps onto a non-Hermitian square lattice bilayer which is also Kitaev solvable. We describe the exponentially many nonequilibrium steady states in this model. We identify how the spin degrees of freedom can be accounted for in the two-dimensional model in terms of the gauge-invariant quantities and then proceed to study the Liouvillian spectrum. We use simulated annealing to estimate the Liouvillian gap and the first decay modes for large system sizes. We observe a transition in the first decay modes, similar to that in the work of Shibata and Katsura. The results we obtain are compared to the results we obtained from a perturbative analysis for small and large values of the dissipation strength.

Einstein-Podolsky-Rosen (EPR) steering, a fundamental concept of quantum nonlocality, describes one observer's capability to remotely affect another distant observer's state by local measurements. Unlike quantum entanglement and Bell nonlocality, both associated with the symmetric quantum correlation, EPR steering depicts the unique asymmetric property of quantum nonlocality. With the local filter operation in which some system components are discarded, quantum nonlocality can be distilled to enhance the nonlocal correlation and even the hidden nonlocality can be activated. However, asymmetric quantum nonlocality in the filter operation still lacks a well-rounded investigation, especially considering the discarded parts where quantum nonlocal correlations may still exist with probabilities. Here, in both theory and experiment, we investigate the effect of reusing the discarded particles from local filter. We observe all configurations of EPR steering simultaneously and other intriguing evolution of asymmetric quantum nonlocality, such as reversing the direction of one-way EPR steering. This work provides a perspective to answer “what is the essential role of utilizing quantum steering as a resource?” and demonstrates a practical toolbox for manipulating asymmetric quantum systems with significant potential applications in quantum information tasks.

Despite the promise that fault-tolerant quantum computers can efficiently solve classically intractable problems, it remains a major challenge to find quantum algorithms that may reach computational advantage in the present era of noisy, small-scale quantum hardware. Thus, there is a substantial ongoing effort to create new quantum algorithms (or adapt existing ones) to accommodate depth and space restrictions. By adopting a hybrid query perspective, we identify and characterize two methods of breaking down quantum algorithms into rounds of lower (query) depth, designating these approaches as “parallelization” and “interpolation.” To the best of our knowledge, these had not been explicitly identified and compared side by side, although one can find instances of them in the literature. We apply them to two problems with known quantum speedup: calculating the $k$-threshold function and computing a NAND tree. We show that for the first problem parallelization offers the best performance, while for the second interpolation is the better choice. This illustrates that no approach is strictly better than the other, and that there is more than one good way to break down a quantum algorithm into a hybrid quantum-classical algorithm.

The measurement of quantum states is one of the most important problems in quantum mechanics. We introduce a quantum state tomography technique in which the state of a qubit is reconstructed, while the qubit remains undetected. The key ingredients are (i) employing an additional qubit, (ii) aligning the undetected qubit with a known reference state by using path identity, and (iii) measuring the additional qubit to reconstruct the undetected qubit state. We establish theoretically and demonstrate experimentally the method with photonic polarization states. The principle underlying our method could also be applied to quantum entities other than photons.

Using the nascent concept of quantum spin-transfer torque [A. Zholud et al., Phys. Rev. Lett. 119, 257201 (2017); M. D. Petrović et al., Phys. Rev. X 11, 021062 (2021)],  we demonstrate that a current pulse can be harnessed to entangle quantum localized spins of two spatially separated ferromagnets (FMs) which are initially unentangled. The envisaged setup is composed of a spin-polarizer (${\mathrm{FM}}_p$) and a spin-analyzer (${\mathrm{FM}}_a$) FM layers separated by a normal metal (NM) spacer. The injection of a current pulse into the device leads to a time-dependent superposition of many-body states characterized by a high degree of entanglement between the spin degrees of freedom of the two distant FM layers. The nonequilibrium dynamics are due to the transfer of spin angular momentum from itinerant electrons to the localized spins via a quantum spin-torque mechanism that remains active even for collinear but antiparallel arrangements of the ${\mathrm{FM}}_p$ and ${\mathrm{FM}}_a$ magnetizations (a situation in which the conventional spin torque is absent). We quantify the mixed-state entanglement generated between the FM layers by tracking the time evolution of the full density matrix and analyzing the build-up of the mutual logarithmic negativity over time. The effect of decoherence and dissipation in the FM layers due to coupling to bosonic baths at finite temperature, the use of multielectron current pulses, and the dependence on the number of spins are also considered in an effort to ascertain the robustness of our predictions under realistic conditions. Finally, we propose a “current-pump–x-ray-probe” scheme, utilizing ultrafast x-ray spectroscopy, that can witness nonequilibrium and transient entanglement of the FM layers by extracting its time-dependent quantum Fisher information.

In quantum multiparameter estimation, multiple to-be-estimated parameters are encoded in a quantum dynamics system by a unitary evolution. As the parameters vary, the system may undergo a topological phase transition (TPT). In this paper, we investigate two SU(2) TPT models and propose the singular behavior of the quantum metric tensor around the TPT point as a tool for the simultaneous optimal estimation of multiple parameters. We find that the proposed TPT sensing protocol can achieve the same metrology performance as the quantum-control-enhanced one. Moreover, the probe state of the TPT sensing protocol is only the ground state of the Hamiltonian rather than the entangled state required in the control-enhanced one. In addition, an adaptive multiparameter estimation strategy is developed for updating the estimated values until the desired quantum Cramér-Rao bound is approached. Our work reinforces the connection between quantum multiparameter estimation and topology physics, with potential inspiration for quantum critical metrology.

We describe the design and operation of a surface-electrode Paul trap for parallel entangling gate implementation. In particular, we demonstrate the possibility of separating or coupling ion motion by adjusting the DC-voltages on a set of electrodes and show the possibility of parallel MS-gate operations for specific voltage configurations. We verify the scalability of this approach and characterize the performance of these gates in the presence of the finite phonon mode occupation and of the finite drift of the phonon frequencies. Additionally, we investigate how the number of ions per individual trapping site and anharmonic potential terms affect the coupling between the wells.

Spectroscopic measurements of transition frequencies in various atomic systems are a significant part of modern physics. They enable the testing of fundamental interactions, the determination of physical constants, and the study of fundamental symmetries that occur in nature with unprecedented accuracy. Studies based on two-photon spectroscopy of simple atoms represent some of the most accurate experiments to date. The verification of precision experimental results is largely supported by theoretical analysis, which is most rigorous for light, nonrelativistic atoms and ions. In the last decade, much attention in the literature has been paid to the study of the quantum interference effect (QIE) in hydrogen and hydrogenlike atomic systems. This has made it possible to significantly reduce the experimental error in determining the appropriate transition frequency. The theoretical description of the QIE corresponds to the consideration of similar pathways arising for close-lying resonant states into which the transition frequency is measured. In the present work, the influence of the emission process on the absorption profile formation is investigated. The results of the studies carried out in this work show the need to take into account the effect of quantum interference in cascade radiation for the precise determination of the absorption transition frequency to highly excited states in two-photon spectroscopic experiments.

Chainwise stimulated Raman adiabatic passage (C-STIRAP) in a M-type molecular system is a good alternative in creating ultracold deeply bound molecules when the typical STIRAP in a $\mathrm{\Lambda }$-type system does not work due to weak Frank-Condon factors between states. However, its creation efficiency under smooth evolution is generally low. During the process, the population in the intermediate states may decay out quickly and the strong laser pulses may induce multiphoton processes. In this paper, we find that the shortcut to adiabaticity (STA) fits very well in improving the performance of the C-STIRAP. Currently, related discussions on the so-called chainwise stimulated Raman shortcut-to-adiabatic passage (C-STIRSAP) are rare. Here, we investigate this topic under adiabatic elimination. Given a relation among the four incident pulses, it is quite interesting that the M-type system can be generalized into an effective $\mathrm{\Lambda }$-type structure with the simplest resonant coupling. Consequently, all possible methods of STA for the three-state system can be borrowed. We take the counterdiabatic driving and “chosen path” methods as instances to demonstrate our treatment of the molecular system. Although the chosen path method does not work well in a real three-state system if there is strong decay in the excited state, our C-STIRSAP protocol under both methods can create ultracold deeply bound molecules with high efficiency in the M-type system. The evolution time is shortened without strong laser pulses and the robustness of STA is well preserved. Finally, the detection of ultracold deeply bound molecules is discussed.

We investigate a single particle in one-dimensional Wannier-Stark ladders with either a linear potential or a mosaic potential with spacing $\kappa =2$. In both cases, we exactly determine the critical energies separating the weakly ergodic states from ergodic states for a finite potential height. Especially in the latter case, we demonstrate a rich phase diagram with ergodic states, weakly ergodic states, and strongly Wannier-Stark-localized states. Our results also exhibit that critical energies are highly dependent on the height of the ladder and ergodic states only survive at $E\approx 0$ for the high ladder. Importantly, we find that the number of ergodic states can be adjusted by changing the interval of the nonzero potential. These interesting features will shed light on the study of disorder-free systems.

We study the spectrum and stationary states in a ring-shaped lattice potential in the context of ultracold atoms with attractive interatomic interactions. We determine analytical solutions in the absence of a lattice by mapping them to those for repulsive interactions, and then we numerically follow the transformation of those solutions as the lattice is introduced and strengthened. Several features emerge that are specific to negative nonlinearity, which include soliton branches detaching to create new ground states, gaps opening up at the bottom of the primary spectral branch, and multiple splitting and rejoining of some branches. We correlate the spectral features with the behavior of the density and phase of the corresponding eigenstates, and track them along branches and as various system parameters change. We find that the phase is sensitive to how a specific point in the spectrum is approached, particularly relevant at certain persistent gaps in the spectrum. The symmetry and stability properties are generally found to be opposite of that found for repulsive interactions.

We investigate the properties of natural orbitals and their occupation numbers of the ground state of two noninteracting anyons characterized by the fractional statistics parameter $\alpha$ and confined in a harmonic trap. We work in the boson magnetic gauge where the anyons are modeled as composite bosons with magnetic flux quanta attached to their positions. We derive an asymptotic form of the weakly occupied natural orbitals, and show that their corresponding (ordered descendingly) occupation numbers decay according to the power law $n^{-(4+2\alpha )}$, where $n$ is the index of the natural orbital. We find remarkable numerical agreement of the theory with the natural orbitals and their occupation numbers computed from the spectral decomposition of the system's wave function. We explain that the same results apply to the fermion magnetic gauge.