We consider a quantum oscillator coupled to a bath of $N$ other oscillators. The total system evolves with a quasi-Hermitian Hamiltonian. Associated to it is a family of Hermitian systems, parameterized by a unitary map $W$. Our main goal is to find the influence of $W$ on the entropy and the entanglement in the Hermitian systems. We calculate explicitly the reduced density matrix of the single oscillator for all Hermitian systems and show that, regardless of $W$, their von Neumann entropy oscillates with a common period which is twice that of the non-Hermitian system. We show that, generically, the oscillator and the bath are entangled for almost all times. While the amount of entanglement depends on the choice of $W$, the entanglement of the time-averaged density matrix is entirely independent of $W$. These results describe some universality in the physical properties of all Hermitian systems associated to a given non-Hermitian one.

The traditional Bayes' rule lays the foundation for causal reasoning and finding relations between cause (input) and effect (output). This causal reasoning is universally applied to all physical processes to establish causal relations. Here we show that it does not establish correct causal correspondence between quantum causes and effects in general. In fact, there are instances within the framework of quantum mechanics where the use of the traditional Bayes' rule leads to inconsistencies in quantum measurement inferences. We consider two such cases, inspired by Frauchiger-Renner's and Hardy's setups, where the traditional Bayes' rule results in paradoxical situations even after assuming quantum mechanics as a nonlocal theory. As a remedy, we introduce an input-output causal relation using the reasoning based on a quantum Bayes' rule. It applies to general quantum processes even when a cause (or effect) is in coherent superposition with other causes (or effects), involves nonlocal correlations as allowed by quantum mechanics, and applies in the cases where causes belonging to one system induce effects in some other system as happens in quantum measurement processes. This enables us to propose a resolution to the contradictions that appear in the context of Frauchiger-Renner's and Hardy's setups. Our results thereby affirm that quantum mechanics, equipped with a quantum Bayes' rule, can indeed consistently describe the use of itself.

When describing the effective dynamics of an observable in a many-body system, the repeated randomness assumption, which states that the system returns in a short time to a maximum entropy state, is a crucial hypothesis to guarantee that the effective dynamics is classical and Markovian and obeys local detailed balance. While the latter behavior is frequently observed in naturally occurring processes, the repeated randomness assumption is in blatant contradiction to the microscopic reversibility of the system. Here we show that the use of the repeated randomness assumption can be justified in the description of the effective dynamics of an observable that is both slow and coarse, two properties we will define rigorously. Then our derivation will invoke essentially only the eigenstate thermalization hypothesis and typicality arguments. While the assumption of a slow observable is subtle, as it provides only a necessary but not sufficient condition, it also offers a unifying perspective applicable to, e.g., open systems as well as collective observables of many-body systems. All our ideas are numerically verified by studying density waves in spin chains.

An essential action in quantum information processing is the manipulation (control) of a single qubit, ideally a closed two-level system. However, in realistic applications, quantum processes are often under the influence of the external environment, e.g., presenting some degree of dissipation and decoherence. In this paper we address the emerging difficulties in the (tracking) quantum control of a two-level system under the influence of both Markovian and non-Markovian noise. We employ a same framework to treat both situations, a Lindblad-type equation, but considering that for the former (latter) case, the decay rate $\mathrm{\Gamma }$ is time independent (dependent). We discuss the conditions leading to a breakdown of the quantum control and eventual ways to overcome the problem, like employing a fast control scheme or controlling the off-diagonal terms of the system density matrix. Surprisingly, for Markovian noise such breakdown time decreases with $\mathrm{\Gamma }$ not as an exponential but as a power law. This indicates that the quantum control should be possible for a coupling between the system and the environment stronger than previously expected. Moreover, we find that for non-Markovian noise, the breakdown time is longer when there is backflow, i.e., $\mathrm{\Gamma }\left(t\right)$ can be negative. The present theoretical results point to certain favorable scenarios to operate qubits even in a noisy medium.

In this paper, we theoretically propose a method to simulate the longitudinal relaxation of a single qubit by coupling it to an auxiliary qubit. In order to mimic the finite-temperature relaxation, we utilize the Hamiltonian-ensemble approach [C. M. Kropf, C. Gneiting, and A. Buchleitner, Phys. Rev. X 6, 031023 (2016)]. The longitudinal relaxation arises as a consequence of the ensemble average and the interaction between the working qubit and the auxiliary qubit. Furthermore, we apply this approach to investigate the influence of the longitudinal relaxation and the transverse relaxation on the entanglement dynamics of two qubits. It is discovered that the sudden death of the entanglement will occur as long as the longitudinal relaxation is present. The transverse relaxation assists the longitudinal relaxation and thus accelerates the finite-time disentanglement.

This work investigates the projectile and temperature dependence of the energy loss of charged particles in matter. To this end, we analyze two dielectric approaches which consider the presence of bound and free electrons and the effects of the ionization process. With these approaches, we calculate the energy-loss moments of protons, positrons, and electrons traversing Si, Fe, and Al targets, both in the cold solid phase and in the plasma state. We compare the results from the unified-wave-packet model (UWPM) and the shellwise local plasma approximation (SLPA) on an extensive range of parameters, including low, intermediate, and high projectile energies and target temperatures going from cold solid-state densities to hot plasma with temperatures up to 1000 eV. We reformulate the SLPA to include light-particle restrictions. We give special consideration to the case of positrons and electrons, where the inner-shell effects have not been analyzed in our previous works. Comparisons with experimental results for cold solid targets are presented, and stopping enhancement effects for heated targets are described, showing the physical origin of these effects.

A version of the configuration interaction method, which has been recently developed to deal with a large number of valence electrons, has been used to calculate magnetic dipole and electric quadrupole hyperfine structure constants for a number of states of erbium and fermium. Calculations for fermium are done for extracting nuclear moments of Fm isotopes from recent and future measurements. Calculations for erbium, which has an electronic structure similar to that of fermium, are done to study the accuracy of the method.

We experimentally investigate the Rydberg state excitation (RSE) process of ${\mathrm{H}}_2$ molecules induced by a strong laser field in the tunneling ionization region. Both neutral parent molecules ${\mathrm{H}}_2$ and fragment atoms H are observed to survive the strong 800-nm femtosecond laser fields in high Rydberg states; however, their behaviors are quite different upon varying the laser intensity and laser ellipticity. The results are compared to single and double ionization as well as RSE from the companion atom Ar. Analysis indicates that the ${\mathrm{H}}_2$ RSE is produced by the recapture or frustrated tunneling ionization process, and the nonsequential double ionization of ${\mathrm{H}}_2$ induced by recollision of the tunneled electrons plays an important role in the fragment H RSE of ${\mathrm{H}}_2$ in strong laser fields.

We theoretically investigate a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type superfluid phase transition in a driven-dissipative two-component Fermi gas. The system is assumed to be in the nonequilibrium steady state, which is tuned by adjusting the chemical potential difference between two reservoirs that are coupled with the system. Including pairing fluctuations by extending the strong-coupling theory developed in the thermal-equilibrium state by Nozières and Schmitt-Rink to this nonequilibrium case, we show that a nonequilibrium FFLO (NFFLO) phase transition can be realized without spin imbalance, under the conditions that (1) the two reservoirs imprint a two-edge structure on the momentum distribution of Fermi atoms and (2) the system is loaded on a three-dimensional cubic optical lattice. While the two edges work like two Fermi surfaces with different sizes, the role of the optical lattice is to prevent the NFFLO long-range order from destruction by NFFLO pairing fluctuations. We also draw the nonequilibrium mean-field phase diagram in terms of the chemical potential difference between the two reservoirs, a fictitious magnetic field to tune the spin imbalance of the system, and the environmental temperature of the reservoirs to clarify the relation between the NFFLO state and the ordinary thermal-equilibrium FFLO state discussed in spin-imbalanced Fermi gases.

We consider collective motion and damping of dipolar Fermi gases in the hydrodynamic regime. We investigate the trajectories of collective oscillations—here dubbed “weltering” motions—in cross-dimensional rethermalization experiments via Monte Carlo simulations, where we find stark differences from the dilute regime. These observations are interpreted within a semiempirical theory of viscous hydrodynamics for gases confined to anisotropic harmonic potentials. The derived equations of motion provide a simple effective theory that show favorable agreement with full numerical solutions. To do so, the theory must carefully account for the size and shape of the effective volume within which the gas's behavior is hydrodynamic. Although formulated for close-to-threshold dipolar collisions, our theoretical framework can be repurposed for other elastic cross sections in future studies.

The established approach of perturbative continuous unitary transformations (pCUTs) constructs effective quantum many-body Hamiltonians as perturbative series that conserve the number of one quasiparticle type. We extend the pCUT method to similarity transformations—dubbed ${\text{pcst}}^{++}$—allowing for multiple quasiparticle types with complex-valued energies. This enlarges the field of application to closed and open quantum many-body systems with unperturbed operators corresponding to arbitrary superimposed ladder spectra. To this end, a generalized counting operator is combined with the quasiparticle generator for open quantum systems recently introduced by Schmiedinghoff and Uhrig [G. Schmiedinghoff and G. S. Uhrig, SciPost Phys. 13, 122 (2022)]. The ${\text{pcst}}^{++}$ then yields model-independent block-diagonal effective Hamiltonians and Lindbladians, allowing a linked-cluster expansion in the thermodynamic limit similar to the conventional pCUT method. We illustrate the application of the ${\text{pcst}}^{++}$ method by discussing representative closed, open, and non-Hermitian quantum systems.

In one-dimensional (1D) quantum gases, the momentum distribution (MD) of the atoms is a standard experimental observable, routinely measured in various experimental setups. The MD is sensitive to correlations, and it is notoriously hard to compute theoretically for large numbers of atoms $N$, which often prevents direct comparison with experimental data. Here we report significant progress on this problem for the 1D Tonks-Girardeau (TG) gas in the asymptotic limit of large $N$, at zero temperature and driven out of equilibrium by a quench of the confining potential. We find an exact analytical formula for the one-particle density matrix $\langle {\widehat{\mathrm{\Psi }}}^{†}\left(x\right)\widehat{\mathrm{\Psi }}\left(x^{\prime }\right)\rangle$ of the out-of-equilibrium TG gas in the $N\to \infty$ limit, valid on distances $|x-x^{\prime }|$ much larger than the interparticle distance. By comparing with time-dependent Bose-Fermi mapping numerics, we demonstrate that our analytical formula can be used to compute the out-of-equilibrium MD with great accuracy for a wide range of momenta (except in the tails of the distribution at very large momenta). For a quench from a double-well potential to a single harmonic well, which mimics a “quantum Newton cradle” setup, our method predicts the periodic formation of peculiar, multiply peaked, momentum distributions.

Vapor-cell spectroscopy is an essential technique in many fields and is particularly relevant for quantum technologies. Most atom- and ion-trapping experiments rely on simultaneous spectroscopy of two atomic transitions, employing a separate apparatus for each transition. We demonstrate simultaneous spectroscopy on two atomic transitions, within a single apparatus, using spatially overlapped beams from two independent lasers. This method enables compact setups and offers superior spectroscopic performance, leading to sharper spectroscopy peaks, stronger absorption signals, and superior laser stability. Doppler-free locking features become accessible over a frequency range several hundred megahertz wider than for standard saturated absorption spectroscopy. Exploring the full parameter space associated with dual-frequency spectroscopy reveals a latticelike structure of sharp resonance features, which enhances experimental versatility by allowing laser frequency stabilization within a wide manifold of locations in two-dimensional frequency space. We present data for ${}^{133}\mathrm{Cs}$ and ${}^{85}\mathrm{Rb}$ and compare our results to a theoretical model. Employing the technique for frequency stabilization close to the $D_2$ line of ${}^{85}\mathrm{Rb}$ results in an improvement in frequency stability by a factor typically between 2 and 3 for averaging times of up to 1 s. The technique will advance portable quantum technologies and facilitate high-precision measurements.

The recently reported unidirectional guided resonances (UGRs) in asymmetric photonic crystal (PhC) slabs exhibit ultrahigh unidirectional radiation. Radiation on one side of the PhC slab at a particular direction is allowed while that on the other side is perfectly canceled. The UGR was previously explained as the single side far field polarization singularity. Here the formation mechanism of UGRs is explained from the viewpoint of multipole radiation. Two types of UGR are considered with one induced from the bound state in the continuum and the other arising from the avoided crossings of two guided resonances. The first type of UGR, similar to the Brewster effect, arises from the overlap of the null direction of the dipolelike radiation with the system radiation channel. The second type of UGR stems from the intermixing of multipoles in the two original bands which modifies the radiation singularities, resulting in the single side radiation in one of the newly formed bands. Our results provide an alternative perspective for the understanding of UGRs by connecting the near field information and the far field radiation.

In a quantum Stirling heat engine, the heat exchanged with two thermal baths is partly utilized for performing work by redistributing the energy levels of the working substance. We analyze the thermodynamics of a quantum Stirling engine operating in finite time. We develop a model in which a time-dependent potential barrier changes the energy-level structure of the working substance. The process takes place under a constant interaction with the thermal bath. We further show that in the limit of slow operation of the cycle and low temperature, the efficiency of such an engine approaches Carnot efficiency. We also show that the maximum output power, for the strokes that affect the energy levels, is obtained at an intermediate operating speed, demonstrating the importance of a finite-time analysis.

Metamaterials based on mechanical elements have been developed over the past decade as a powerful platform for exploring analogs of electron transport in exotic regimes that are hard to produce in real materials. In addition to enabling new physics explorations, such developments promise to advance the control over acoustic and mechanical metamaterials, and consequently to enable new capabilities for controlling the transport of sound and energy. Here, we demonstrate the building blocks of highly tunable mechanical metamaterials based on real-time measurement and feedback of modular mechanical elements. We experimentally engineer synthetic lattice Hamiltonians describing the transport of mechanical energy (phonons) in our mechanical system, with control over local site energies and loss and gain as well as over the complex hopping between oscillators, including a natural extension to nonreciprocal hopping. Beyond linear terms, we experimentally demonstrate how this measurement-based feedback approach makes it possible to independently introduce nonlinear interaction terms. Looking forward, synthetic mechanical lattices open the door to exploring phenomena related to topology, non-Hermiticity, and nonlinear dynamics in nonstandard geometries, higher dimensions, and with novel multibody interactions.

Quantum gate teleportation (QGT) is an excellent candidate for implementing remote quantum gate operations in large-scale quantum computer networks as it has the minimum resource demand to realize such computing models. The Bell nonlocality of quantum entanglement channel in QGT can be transformed into the corresponding posterior statistics correlation obtained by local measurements after QGT operation, and this Bell-type-like correlation can be used to check device and measurement loopholes or information leaking in the teleportation systems. In this paper, we propose a verification scheme based on Bell nonlocality for quantum cnot gate teleportation in a black-box scenario where the vendors may not be trusted. The double criteria of high fidelity ($\overline{F}>97.53\%$) and high CHSH inequality violation ($\mathrm{CHSH}>2$) are used for assessing a vendor's reliability. The clients can use our scheme to check whether vendors employ classical simulation techniques to forge nonlocal quantum computing processes and it could provide a secure manner for the construction of quantum computer networks in the future.

We present an experimentally practical method to reveal Einstein-Podolsky-Rosen (EPR) steering in non-Gaussian spin states by exploiting a connection to quantum metrology. Our criterion is based on the quantum Fisher information, and uses bounds derived from generalized spin-squeezing parameters that involve measurements of higher-order moments. This leads us to introduce the concept of conditional spin-squeezing parameters, which quantify the metrological advantage provided by conditional states, as well as detect the presence of an EPR paradox.

Multipartite Einstein-Podolsky-Rosen (EPR) steering and multimode quantum squeezing are essential resources for various quantum applications. The paper focuses on studying a coupled three-mode squeezed vacuum (C3MSV), which is a typical multimode squeezed Gaussian state and will exhibit peculiar steering property. Using the technique of integration within ordered products, we give the normal-ordering form for the coupled three-mode squeezing operator and derive the general analytical expressions of the statistical quantities for the C3MSV. Under Gaussian measurements, we analyze all bipartite Gaussian steerings (including no steering, one-way steering, and two-way steering) in detail and study the monogamy relations for the C3MSV. Then, we study the decoherence of all these steerings in noisy channels and find that sudden death will happen in a certain threshold time. Through the steerings shared in the C3MSV, we propose conceptual (and ideal) schemes of remotely generating Wigner negativity by performing appropriate photon subtraction(s) in the local position. Our obtained results may lay a solid theoretical foundation for a future practical study. We also believe that the C3MSV will be one of good candidate resources in future quantum protocols.

In this paper, we theoretically calculate the natural lifetime of the $5s5p{}^3\mathrm{P}_0^o$ state of the ${}^{87}\mathrm{Sr}$ atom and evaluate the effect of an external magnetic field on the lifetimes of Zeeman sublevels. Using the multiconfiguration Dirac-Hartree-Fock method, the hyperfine-induced $5s5p{}^3\mathrm{P}_0^o–5s^2{}^1\mathrm{S}_0$ clock transition rate of ${}^{87}\mathrm{Sr}$ is estimated as $6.39\left(37\right)\times {10}^{-3}{\mathrm{s}}^{-1}$, corresponding to a lifetime of 156(9) s. Our result closely agrees with previous measurements after correcting the decay rate due to the black-body radiation effect. Additionally, the transition rates of Zeeman sublevels are calculated as a function of the magnetic-field strength. Our theoretical results reveal that transition rates between different excited Zeeman states are discrepant, and the lifetime of magnetic sublevel $+M_F^e(-M_F^e$; the superscript $e$ refers to the excited state, i.e., the $5s5p{}^3\mathrm{P}_0^o$ state) will increase (decrease) with the magnetic-field strength. Moreover, we measure the coupling strength between the clock laser and the ${}^{87}\mathrm{Sr}$ atoms, and our findings align well with theoretical estimations regarding changes in normalized coupling strength with external magnetic-field strength.