Non-Hermitian rotation-time reversal $\left(\mathcal{RT}\right)$-symmetric spin models possess two distinct phases, the unbroken phase in which the entire spectrum is real and the broken phase which contains complex eigenspectra, thereby indicating a transition point, referred to as an exceptional point. We report that the dynamical quantities, namely, the short- and long-time average of the Loschmidt echo, which is the overlap between the initial and the final states, and the corresponding rate function can faithfully predict the exceptional point. In particular, when the initial state is prepared as the ground state in the unbroken phase of the non-Hermitian Hamiltonian and the system is quenched to either the broken or unbroken phase, we analytically demonstrate that the rate function and the average Loschmidt echo can distinguish between the quench that occurred in the broken or the unbroken phase for the nearest-neighbor non-Hermitian $XY$ model with uniform and alternating magnetic fields, thereby indicating the exceptional point. Furthermore, we exhibit that such quantities are capable of identifying the exceptional point even in models like the non-Hermitian short- and long-range $XYZ$ model with magnetic field, which can only be solved numerically, thereby establishing it as detection criteria for recognizing exceptional points.

Quantum optical systems are typically affected by two types of noise: photon loss and dephasing. Despite extensive research on each noise process individually, a comprehensive understanding of their combined effect is still lacking. A crucial problem lies in determining the values of loss and dephasing for which the resulting loss-dephasing channel is antidegradable, implying the absence of codes capable of correcting its effect or, alternatively, capable of enabling quantum communication. A conjecture [Quantum 6, 821 (2022)] suggested that the bosonic loss-dephasing channel is not antidegradable if the loss is below $50\%$. In this paper we refute this conjecture, specifically proving that for any value of the loss, if the dephasing is above a critical value, then the bosonic loss-dephasing channel is antidegradable. While our result identifies a large parameter region where quantum communication is not possible, we also prove that if two-way classical communication is available, then quantum communication—and thus quantum key distribution—is always achievable, even for high values of loss and dephasing.

Dissipative quantum systems can under appropriate conditions exhibit bi- or multipartite entanglement at the steady state. The presence and properties of these quantum correlations depend on the relevant model parameters. Here, we characterize the steady-state entanglement in connection with the spatial structure of a small chain of three ions dispersively coupled with a pumped optical cavity. Within a semiclassical approximation, we describe the relation between entanglement, spatial organization, and vibrational modes of the ion chain. Upon increasing the pumping strength, our system undergoes a transition from a sliding to a pinned configuration, in which ions are expelled from the maxima of the optical potential. The features of the steady-state entanglement strongly depend on the kind of pinned configuration reached. We identify scenarios leading to entangled steady states, analyze the effect of defect formation upon entanglement between different system partitions, and observe the presence of multipartite quantum correlations.

From both intuitive and physical perspectives, it is generally recognized that within a resource theory framework, free operations cannot broadcast a resource state due to their inability to generate resource from free states. In the stabilizer formalism of fault-tolerant quantum computation, the basic ingredients of the corresponding resource theory consist of stabilizer states as free states and stabilizer operations as free operations. The celebrated Gottesman-Knill theorem shows that quantum advantages over classical computation come from the magic (nonstabilizer) resource, such as magic states or non-Clifford gates. In this work, we prove that broadcasting of any magic state via stabilizer operations is impossible, which is reminiscent of the no-broadcasting theorems for noncommuting states or quantum correlations. We further derive a trade-off relation between the magic resource consumed in the initial system and that gained in the target system. These results characterize magic states in the stabilizer formalism from the broadcasting angle, and may have implications for distributed quantum computation and quantum secret sharing.

In molecular photoionization, the two-center interference is widely described within the electric dipole approximation, neglecting the electric quadrupole effect. By numerically solving the two-dimensional time-dependent Schrödinger equation beyond the dipole approximation and using perturbation theory, we demonstrate that the two-center interference structure induced by the electric quadrupole transition in the one-photon ionization of ${\mathrm{H}}_2^{+}$ can be observed in the asymmetric photoelectron yield along the laser propagation direction. Compared with the dipole transition, the electron wave packets emitted from the two nuclei of the molecule due to the electric quadrupole transition exhibit an additional phase difference. This phase difference results in a distinct two-center interference structure compared to the dipole transition. Furthermore, an experimental scheme based on the reconstruction of attosecond beating by interference of two-photon transitions is proposed to reveal the two-center interference structure induced by the electric quadrupole transition. Our study provides a different perspective for investigating the nondipole effect in molecular photoionization.

We report on the observation of a conspicuous loss in an ultracold mixture of ${}^7\mathrm{Li}$ and ${}^{88}\mathrm{Sr}$ atoms confined in a far-off-resonance optical dipole trap. We attribute the trap loss to the three-body inelastic Li-Sr-Sr collision and extract the corresponding three-body-recombination coefficient $K_3$ at $T\sim 18.5,45,70,600\mu \mathrm{K}$. The measured three-body-recombination coefficient is about two to three orders of magnitude larger than the typical values convenient for realizing quantum degenerate gases, and essentially rules out the prospect of realizing ${}^7\mathrm{Li}$ and ${}^{88}\mathrm{Sr}$ mixtures of high phase space density. The measured three-body-recombination rates agree with the unitarity limit at high temperatures. Our results also confirm that the three-body-recombination loss is dominated by the light-heavy-heavy process, in agreement with the prediction for a system with a large mass-imbalanced and negligible intraspecies interaction.

The tunneling dynamics of a magnetic ${}^{164}\mathrm{Dy}$ quantum gas in an elongated or pancake skewed double-well trap is investigated with a time-dependent extended Gross-Pitaevskii approach. Upon lifting the energy offset, different tunneling regimes can be identified. In the elongated trap and for sufficiently large offset, the different configurations exhibit collective macroscopic tunneling. For smaller offset, partial reflection from and transmission through the barrier lead to density accumulation in both wells, and eventually to tunneling locking. One can also reach the macroscopic self-trapping regime for increasing relative dipolar interaction strength, while tunneling vanishes for large barrier heights. A richer dynamical behavior is observed for the pancake-like trap. For instance, the supersolid maintains its shape, while the superfluid density gets distorted, signifying the emergence of peculiar excitation patterns in the macroscopic tunneling regime. The findings reported here may offer alternative ways to probe distinctive dynamical features in the supersolid and droplet regimes.

We investigate the Fermi-polaron system with one mobile impurity immersed atop a two-component interacting fermion bath in one- and two-dimensional square lattices, where the impurity atom interacts with only one species (the spin-up component) of the background. The ground state with a given total momentum can be approximated by an extended Gaussian state, which is constructed from the combination of Gaussian states and a non-Gaussian polaronic transformation (Lee-Low-Pines transformation). In the few-body limit, the variational energies of two- and three-body systems show good agreement with exact results, which indicates the validity of the non-Gaussian variational method. We then move on to the many-body limit, i.e., the polaron problem with an interacting background. We choose two representative fillings of the background fermions, $\rho =1/4,1/2$, and obtain the corresponding ground states. Our results show that the system will undergo a smooth crossover from a Fermi-polaron to a Bose-polaron system as the interaction between background fermions increases. We further analyze the double occupancy and momentum distribution of the background fermions and find that the impurity will not significantly affect the background pairing.

The study of superfluid quantum vortices has long been an important area of research, with previous work naturally focusing on two-dimensional and three-dimensional systems, where rotation stabilizes point vortices and line vortices respectively. Interestingly, this physics generalizes for a hypothetical four-dimensional (4D) superfluid to include vortex planes, which can have a much richer phenomenology. In this paper we study the possibility of skewed and curved vortex planes, which have no direct analog in lower dimensions. By analytically and numerically studying the 4D Gross-Pitaevskii equation, we show that such vortex surfaces can be stabilized and favored by double rotation with unequal rotation frequencies. Our work raises open questions for further research into the physics of these vortex surfaces and suggests interesting future extensions to tilted vortex surfaces under equal-frequency double rotation and to more realistic 4D models.

As is well known, two-dimensional and three-dimensional superfluids under rotation can support topological excitations such as quantized point vortices and line vortices, respectively. Recently, we have studied how, in a hypothetical four-dimensional (4D) superfluid, such excitations can be generalized to vortex planes and surfaces. In this paper we continue our analysis of skewed and curved vortex surfaces based on the 4D Gross-Pitaevskii equation and show that certain types of such states can be stabilized by equal-frequency double rotations for suitable parameters. This work extends the rich phenomenology of vortex surfaces in four dimensions and raises interesting questions about vortex reconnections and the competition between various vortex structures which have no direct analog in lower dimensions.

The quantum fluctuations of a quantum Rabi triangle are investigated using an analytical method that goes beyond mean-field theory. An artificial magnetic field applied in three cavities leads to the breaking of time-reversal symmetry, which is evident in the directional transfer of photons. In contrast to previous studies, we focus on the scaling exponents of quantum fluctuations of the local photon number and the position variance near the critical point. Specifically, the fluctuations of photons do not diverge as the coupling strength approaches the critical value from below for the chiral superradiant phase transition. Attributing to geometric frustration, two distinct scaling laws arise for the frustrated cavity and the remaining cavities. Specifically, in the frustrated cavity, the scaling exponent in the chiral superradiant phase differs from that without an artificial magnetic field for the frustrated antiferromagnetic superradiant phase. The unusual scaling exponents indicate distinct universality classes in contrast to the single-cavity Rabi one. We suggest that accurate critical exponents in few-body systems are useful for identifying exotic quantum phase transitions in light-matter coupling systems.

We study entanglement degradation when one share of an entangled pair experiences noise. We consider the dissipative Landau-Zener model for describing local noise. The dissipative Landau-Zener model provides a convenient setting for modeling entanglement degradation with applications in quantum network developments and communications. Here, we study the problem in the fast- and slow-driving regimes. In the slow-driving regime, our results are analytical, while in the fast-driving regime, we use numerical techniques to obtain the results. Our study addresses the role of two main properties of the dynamics in entanglement degradation, namely, spin-coupling direction and adiabaticity of the dynamics. We derive an analytical expression for entanglement survival time versus bath temperature in the slow-driven regime. In both regimes, when the bath temperature is zero, we show that transversal spin-coupling has a less destructive effect on entanglement than longitudinal spin-coupling. We also show that entanglement degradation is weaker in the nonadiabatic regime compared to the adiabatic regime. Therefore, our results determine the proper choice of parameters for having less entanglement degradation in the presence of local noise described by the dissipative Landau-Zener model.

Cluster states are the essential resource used in the implementation of fusion-based quantum computation (FBQC). We introduce a method to generate high-fidelity optical cluster states by utilizing the concept of unitary averaging. This error-averaging technique is entirely passive and can be readily incorporated into the proposed PsiQuantum FBQC architecture. Using postselection and the redundant encoding of fusion gates, we observe an enhancement in the average fidelity of the output cluster state. As a generalization, we also show an improvement in the linear optical Bell-state measurement (BSM) success probability when the BSM is imperfect.

Quantum broadcasting is central to quantum information processing and characterizes the correlations within quantum states. Nonetheless, traditional quantum broadcasting encounters inherent limitations dictated by the principles of quantum mechanics. In a previous study, Parzygnat et al. [Phys. Rev. Lett. 132, 110203 (2024)] introduced a canonical broadcasting quantum map that goes beyond the quantum no-broadcasting theorem through a virtual process. In this work, we generalize the concept of virtual broadcasting to unilocal broadcasting by incorporating a reference system and introduce protocols that can be approximated using physical operations with minimal cost. First, we propose a universal unilocal protocol enabling multiple parties to share the correlations of a target bipartite state, which is encoded in the expectation value for any observable. Second, we formalize the simulation cost of a virtual quantum broadcasting protocol into a semidefinite programming problem. Notably, we propose a specific protocol with optimal simulation cost for the two-broadcasting scenario, revealing an explicit relationship between simulation cost and the quantum system's dimension. Moreover, we establish upper and lower bounds on the simulation cost of the virtual $n$-broadcasting protocol and demonstrate the convergence of the lower bound to the upper bound as the quantum system's dimension increases.

A stable and high-quality single-magnon state is desired by the single-magnon source for quantum-information application with a macroscopic spin system. We consider a hybrid system where a magnon mode is directly coupled to a nonresonant superconducting qubit via the exchange interaction. The magnon and qubit are under the driving and probing fields with the same frequency, respectively. We find that the single-magnon probability $P_1$ can be maximized when the product of the magnon-driving field detuning and the qubit-probing field detuning is equivalent to the square of the magnon-qubit coupling strength, ${\mathrm{\Delta }}_q{\mathrm{\Delta }}_m\approx J^2$. Then, the double-magnon probability $P_2$ can be minimized by tuning the ratio of the probing intensity to the driving intensity and the relative phase between the two fields. Under these optimized conditions with accessible strong driving intensity and low decay rate, strong magnon blockade gives rise to a stable single-magnon state with a high quality. It features a large brightness (the single-magnon probability) $P_1\approx 0.40$ and a high purity (the equal-time second-order correlation function) $g^{\left(2\right)}\left(0\right)\sim {10}^{-5}$. The two indicators as a whole prevail over the existing results for photon, phonon, and magnon modes with respect to a stable single-quantum state. The optimized conditions with a scalable modification ${\mathrm{\Delta }}_q{\mathrm{\Delta }}_m\approx N{J}^2$ apply to the situation when one focuses on any one of the $N$ magnon modes that are simultaneously coupled to a common qubit.

We perform an analysis supplementing the metrology toolbox in the time-frequency domain. While the relevant time-frequency-based metrological protocols can be borrowed from the spatial domain, where they have recently been well developed, their ultimate practical usefulness is shown to be restricted by limits put on the bandwidth of both the signal and measurements, as well as by the finite measurement time. As we demonstrate for the well-known problem of multiparameter estimation for two incoherent, pointlike sources, the impact of these experimental limitations on the optimal protocol's efficiency can be detrimental. Nonetheless, we propose necessary operational criteria for attainability of the quantum Cramér-Rao bound under the discussed restrictions.

Sideband cooling enables preparation of trapped ion motion near the ground state and is essential for many scientific and technological applications of trapped ion devices. Here, we study the efficiency of continuous and pulsed sideband cooling using both first- and second-order sidebands applied to an ion where the motion starts outside the Lamb-Dicke regime. We find that after optimizing these distinct cooling methods, pulsed and continuous cooling achieve similar results based on simulations and experiments with a ${}^{40}\mathrm{Ca}^{+}$ ion. We consider optimization of both average phonon number $\overline{n}$ and population in the ground state. We also demonstrate the disparity between $\overline{n}$ as measured by the sideband ratio method of trapped ion thermometry and the $\overline{n}$ found by averaging over the ion's motional state distribution.

The minimization of the magnetic field plays a crucial role in ultracold gas research. For instance, the contact interaction dominates all the other energy scales in the zero-magnetic-field limit, giving rise to novel quantum phases of matter. However, lowering magnetic fields well below the mG level is often challenging in ultracold gas experiments. In this paper, we apply Landau-Zener spectroscopy to characterize and reduce the magnetic field on an ultracold gas of sodium atoms to a few tens of $\mu \mathrm{G}$. The lowest magnetic field achieved here opens the way to observing novel phases of matter with ultracold spinor Bose gases.

The second law of thermodynamics posits that in closed macroscopic systems the rate of entropy production must be positive. However, small systems can exhibit negative entropy production over short timescales, seemingly in contradiction with this law. The fluctuation theorem quantitatively connects these two limits, predicting that entropy-producing trajectories become exponentially dominant as the system size and measurement time are increased. Here, we explore the predictions of the fluctuation theorem for a fluid of point vortices, in which the long-range Lorentz-like interactions and existence of negative-absolute-temperature states provide an intriguing test bed for the theorem. Our results suggest that while the theorem broadly holds even at negative absolute temperatures, the form of the interactions inherent in the vortex matter lead to anomalously large entropy production over short time intervals. The predictions of the fluctuation theorem are only fully recovered when sufficient noise is introduced to the dynamics to overwhelm the vortex-vortex interactions.