We probe the energy-loss enhancement in preequilibration and the charge-equilibration time for highly charged Xe ions $(q_{\mathrm{in}}=15–26)$ at near-Bohr velocity impinging on an Al target by measuring the dependence of the Xe $L$-shell x-ray yield on the initial charge state of the ions. The present results are found to be very consistent with those obtained in ion-transmission thin-foil experiments. The charge-equilibrium time is measured in the near-Bohr velocity region of highly charged ions in solids. Moreover, for the present collision system it is demonstrated by measuring the inner-shell x rays that the ionization of the inner-shell electrons of the projectile ions starts to contribute to ion energy loss at near-Bohr velocity.

Inspired by Analogue Gravity, superradiance has been previously investigated in Bose-Einstein condensates (BECs). In this work, we revisit this problem by introducing dissipation to the system. After establishing the possibility of quantum vortices in dissipative BECs, we analyze the propagation of elementary excitations, and we demonstrate the existence of superradiant modes, which can be interpreted in terms of the dissipation of “antiparticles.” Our findings support the possibility of superradiant scattering around dissipative quantum vortices, and they pave the way for future experimental realization of the phenomenon.

A method to realize controllable inversion of energy levels in a one-dimensional spin-orbit (SO)-coupled two-component Bose-Einstein condensate under the action of a gradient magnetic field and harmonic-oscillator (HO) trapping potential is proposed. The linear version of the system is solved exactly. By adjusting the SO coupling strength and magnetic-field gradient, the energy-level inversion makes it possible to transform any excited state into the ground state. The full nonlinear system is solved numerically, and it is found that the results are consistent with the linear prediction in the case of the repulsive intercomponent interaction. On the other hand, the intercomponent attraction gives rise to states of superposition and edge types. Similar results are also reported for the system with the HO trap replaced by the box potential. These results suggest a possibility to realize any excited state and observe it in the experiment.

We consider quantum statistical physics of many-body equilibrium fluctuations in an interacting Bose-Einstein-condensed (BEC) gas. We find a universal analytic formula for a characteristic function (Fourier transform) of a joint probability distribution for the particle occupation numbers in a BEC gas and discuss $♯\mathrm{P}$-hardness of computing this distribution. The latter is done by means of the Hafnian master theorem generalizing the classical permanent master theorem of MacMahon. We suggest an atomic boson sampling in the many-body interacting systems as an alternative to a widely studied Gaussian boson sampling of photons. We outline a multiqubit BEC trap, formed by a set of the single-qubit potential wells, as a convenient model for studying atomic boson sampling.

The extended Bose-Hubbard model with correlated tunneling exhibits staggered superfluid and supersolid quantum phases. We study finite-temperature phase transitions of quantum phases of dipolar bosons in a two-dimensional optical lattice using Gutzwiller mean-field and quantum Monte Carlo approaches. When nearest-neighbor repulsion is comparable to the on-site interaction, we find that the two topologically distinct superfluids are separated by a normal-fluid phase, while at stronger off-site interactions, density-modulated insulating quantum phases appear. We estimate the critical temperature of the staggered-superfluid to normal-fluid transition and show that this transition is of the Kosterlitz-Thouless type. Finally, we elucidate the coexistence of staggered quantum phases in the presence of an external trapping potential. Our study paves the way to observe novel staggered quantum phases in dipolar optical lattice experiments.

We study general collisions between chiral solitons in Bose-Einstein condensates subject to combined attractive and current-dependent interatomic interactions. A simple analysis based on the linear superposition of the solitons allows us to determine the relevant time and space scales of the dynamics, which is illustrated by extensive numerical simulations. By varying the differential amplitude, the relative phase, the average velocity, and the relative velocity of the solitons, we characterize the different dynamical regimes that give rise to oscillatory and interference phenomena. Apart from the known inelastic character of the collisions, we show that the chiral dynamics involves an amplitude reduction with respect to the case of regular solitons. To compare these results with feasible ultracold-gas experiments, the influence of harmonic confinement is analyzed in both the emergence and the interaction of chiral solitons.

Utilizing scattering theory, we quantify the consequences of physical constraints that limit the visibility of non-Hermitian effects in passive devices. The constraints arise from the fundamental requirement that the system obeys causality, and can be captured concisely in terms of an internal time-delay operator, which furthermore provides a direct quantitative measure of the visibility of specific non-Hermitian phenomena in the density of states. We illustrate the implications by contrasting two prominent non-Hermitian effects, exceptional points, and the non-Hermitian skin effect, whose underlying extreme mode nonorthogonality turns out to be undetectable in the density of states.

Super- and subradiance are usually described in the framework of Dicke collective states, which is an “atomic picture” in which the electromagnetic field only provides an effective interaction between the atoms. Here, we discuss a complementary picture, in which we describe the propagation and scattering of light in the atomic medium, which provides a complex susceptibility and scatterers. This “optical picture” is valid in the linear-optics regime for disordered samples and is mainly relevant at low density, when the susceptibility and scattering cross section can be computed from simple textbook formulas. In this picture, superradiance is a dispersion effect due to a single scattering event dressed by an effective refractive index, whereas subradiance is due to multiple scattering. We present numerical and experimental data supporting our interpretation.

The Clauser-Horne-Shimony-Holt (CHSH) inequalities are the most famous examples of Bell inequalities. Cabello, Severini, and Winter came up with a graph approach to noncontextuality inequalities, which connects some graph-theoretic concepts to quantum and classical correlations. For example, the theta body of the exclusivity graph can be associated with the set of correlations achieved by quantum theory. Following the Cabello-Severini-Winter (CSW) approach, one may think that the theta body of the CHSH graph $\mathrm{TH}\left(G_{\text{CHSH}}\right)$ is equal to the quantum set of the CHSH Bell inequality ${\mathcal{Q}}_{\text{CHSH}}$, but is this really true? All assumptions about the CHSH inequalities come from Bell scenarios, while the CSW approach only demands the exclusivity structure of a noncontextuality (NC) scenario. To deal with the extra structure related to the presence of different players in a Bell scenario like the CHSH one, the colored-graph approach was introduced. Does it make any difference to think about the CHSH scenario as a Bell one or a more general NC scenario? The Bell CHSH inequality is represented by a bicolored graph $\mathcal{G}$ and the NC CHSH inequality by a simple graph $G$, which is the shadow of the colored graph $\mathcal{G}$. In general, we have that the theta body of the colored graph ${\theta }_c\left(\mathcal{G}\right)$ is a subset of the theta body of its shadow graph $\mathrm{TH}\left(G\right)$ in the same way that the Lovász number, which corresponds to the quantum bound, of the simple graph $\vartheta \left(G\right)$ is greater than or equal to the Lovász number of the colored graph ${\vartheta }_c\left(\mathcal{G}\right)$. In the case of the CHSH inequality, we have that $\vartheta \left(G\right)={\vartheta }_c\left(\mathcal{G}\right)$. Does this accident also hold for the corresponding quantum sets? Is it true that $\mathrm{TH}\left(G\right)={\theta }_c\left(\mathcal{G}\right)$, which would mean that every correlation reached by quantum theory applied to the CHSH NC scenario could also be obtained at the in principle more restrictive CHSH Bell scenario? In this paper our answer to such a question is negative. We show that ${\theta }_c\left(\mathcal{G}\right)⊈\mathrm{TH}\left(G\right)$ and therefore that there are quantum correlations which cannot be obtained under Bell restrictions.

The propagation of a fast particle in a low-density gas at thermal equilibrium is studied in the context of quantum mechanics. A quantum master equation in the Redfield form governing the reduced density matrix of the particle is derived explicitly from first principles. Under some approximations, this equation reduces to the linear Boltzmann equation. The issue of the positivity of the time evolution is also discussed by means of a Lindblad form. The Born and Markov assumptions underlying these equations, as well as other approximations regarding the bath correlation function, are discussed in detail. Furthermore, all these master equations are shown to be equivalent with each other if the density matrix of the particle is diagonal in the momentum basis, or if the collision rate is independent of the particle momentum.

The $n$-qubit stabilizer states are those left invariant by a $2^n$-element subset of the Pauli group. The Clifford group is the group of unitaries which take stabilizer states to stabilizer states; a physically motivated generating set, the Hadamard, phase, and controlled-not (cnot) gates which comprise the Clifford gates, impose a graph structure on the set of stabilizers. We explicitly construct these structures, the “reachability graphs,” at $n\le 5$. When we consider only a subset of the Clifford gates, the reachability graphs separate into multiple, often complicated, connected components. Seeking an understanding of the entropic structure of the stabilizer states, which is ultimately built up by cnot gate applications on two qubits, we are motivated to consider the restricted subgraphs built from the Hadamard and cnot gates acting on only two of the $n$ qubits. We show how the two subgraphs already present at two qubits are embedded into more complicated subgraphs at three and four qubits. We argue that no additional types of subgraph appear beyond four qubits, but that the entropic structures within the subgraphs can grow progressively more complicated as the qubit number increases. Starting at four qubits, some of the stabilizer states have entropy vectors which are not allowed by holographic entropy inequalities. We comment on the nature of the transition between holographic and nonholographic states within the stabilizer reachability graphs.

The problem of phase estimation (or amplitude estimation) admits a quadratic quantum speedup. Wang, Higgott, and Brierley [Wang, Higgott, and Brierley, Phys. Rev. Lett. 122, 140504 (2019)] have shown that there is a continuous tradeoff between quantum speedup and circuit depth [by defining a family of algorithms known as $\alpha$-quantum phase estimation $\left(\alpha \text{−}\mathrm{QPE}\right)$]. In this paper, we show that the scaling of $\alpha \text{−}\mathrm{QPE}$ can be naturally and succinctly derived within the framework of quantum singular value transformation (QSVT). From the QSVT perspective, a greater number of coherent oracle calls translates into a better polynomial approximation to the sign function, which is the key routine for solving phase estimation. The better the approximation to the sign function, the fewer samples one needs to determine the sign accurately. With this idea, we simplify the proof of $\alpha \text{−}\mathrm{QPE}$, while providing an interpretation of the interpolation parameters, and show that QSVT is a promising framework for reasoning about classical-quantum interpolations.

Quantum repeaters are critical to the development of quantum networks, enabling rates of entanglement distribution beyond those attainable by direct transmission. We consider multiple continuous-variable, squeezed light-based entanglement flows through a repeater involving noiseless linear amplification and dual homodyne detection. By analyzing a single-repeater-enhanced channel model with asymmetric half-channel losses across the repeater, we determine placements of the central repeater hub in a four-user hub-and-spoke network that enhance the rate of each entanglement flow.

Precise and efficient control of quantum systems is essential to perform quantum information-processing tasks. In terms of adiabatic speedup via leakage elimination operator approach, for a closed system, the ideal pulse control conditions have been theoretically derived by P-Q partitioning technique. However, it is a challenge to design the corresponding control pulses for an open system, which requires noisy environments to be addressed. In this paper, we apply the stochastic search procedures to an open qutrit system and successfully obtain the optimal control pulses for significant adiabatic speedup. The calculation results show that these optimal pulses allow us to acquire higher fidelities than the ideal pulses. The improvement of fidelity is large for relatively strong system-bath coupling strength and high bath temperature. For certain coupling strength and bath temperature, the maximal improvement can be achieved for a critical characteristic frequency which represents the memory time of the environment. Our investigation indicates that the stochastic search procedures are powerful tools to design control pulses for combating the detrimental effects of the environment.

In the standard Landau-Zener-Stückelberg-Majorana (LZSM) problem, the bias sweep rate and gap are both time independent and fully characterize the LZSM problem. We consider the nonlinear LZSM problem, in which at least one of the two characteristic parameters varies as the system traverses the avoided-crossing region. This situation results in what could be thought of as a more accurate description of any realistic situation as compared with the idealized linear LZSM problem. We consider both the case of perturbative nonlinearities, where the nonlinearity adds small corrections to the linear problem, and the case of essential nonlinearities, where the sweep and/or minimum-gap functions are qualitatively different from those of the linear LZSM problem. In the case of perturbative nonlinearities, we derive analytic expressions for the LZSM transition probability based on the Dykhne-Davis-Pechukas (DDP) formula, taking into account the leading corrections to the standard LZSM formula. We compare the derived approximate expressions with numerical simulation results and comment on the validity of the approximations. In particular, if the nonlinear term is small in comparison to the linear term throughout the finite duration of the avoided crossing traversal, the perturbative approximation is valid. Our results also provide information about the validity of the DDP formula. In addition to reviewing cases of essential nonlinearity treated previously in the literature, we analyze the case of an essentially nonlinear sweep function that describes an almost square pulse.

Interaction of a strong laser pulse with matter transfers not only energy but also linear momentum of the photons. Recent experimental advances have made it possible to detect the small amount of linear momentum delivered to the photoelectrons in strong-field ionization of atoms. Linear momentum transfer is a unique signature of the laser-atom interaction beyond its dipolar limit. Here, we present a decomposition of the subcycle time-resolved linear momentum transfer in terms of its multipolar components. We show that the magnetic dipole contribution dominates the linear momentum transfer during the dynamical tunneling process while the postionization longitudinal momentum transfer in the field-driven motion of the electron in the continuum is primarily governed by the electric quadrupole interaction. Alternatively, exploiting the radiation gauge, we identify nondipole momentum transfer effects that scale either linearly or quadratically with the coupling to the laser field. The present results provide detailed insights into the physical mechanisms underlying the subcycle linear momentum transfer induced by nondipole effects.

The spin Hall shift (SHS) refers to a transverse shift in the scattering plane during the spin-orbit interaction (SOI). SHS is considerably small when light is scattered by a sphere due to the absence of unidirectional scattering. This work shows that the chiral property of a sphere can offer a platform to optimize the SOI by achieving nearly zero forward scattering with significant backward scattering. As a result, a polarization transformation takes place and the scattered field transfers its maximum spin angular momentum to orbital angular momentum. Consequently, an enhanced transversal shift is observed due to the momentum conservation of the scattered field. The results reveal that the shift strongly depends on the handedness of the chiral sphere and the helicity of the incident beam. Thus, offering an alternative paradigm for the quantitative measurement of chirality parameters of a single chiral nanoparticle. We hope that these results will potentially shed new light on the SOI of various forms that can find potential applications such as optical sensing, chiral resolution of single nanoparticles, precision measurements, and the manipulation of subwavelength nanoparticles.

We characterize all Gaussian dynamical semigroups in continuous-variables quantum systems of $n$-bosonic modes which have a thermal Gibbs state as a stationary solution. This is performed through an explicit relation between the diffusion and dissipation matrices, which characterize the semigroup dynamics, and the covariance matrix of the thermal equilibrium state. We also show that Alicki's quantum detailed-balance condition, based on a Gelfand-Naimark-Segal inner product, allows the determination of the temperature dependence of the diffusion and dissipation matrices and the identification of different Gaussian dynamical semigroups which share the same thermal equilibrium state.