We put forth a quantum simulation of a stroboscopic two-stroke thermal engine in the IBM quantum (IBMQ) processor. The system consists of a quantum spin chain connected to two baths at their boundaries, prepared at different temperatures using the variational quantum thermalizer algorithm. The dynamics alternates between heat and work strokes, which can be separately designed using independent quantum circuits. The results show good agreement with theoretical predictions, showcasing IBMQ as a powerful tool to study thermodynamics in the quantum regime, as well as the implementation of variational quantum algorithms in real-world quantum computers. It also opens the possibility of simulating quantum heat transport across a broad range of chain geometries and interactions.

Using the quantum-brachistochrone formalism, we address the problem of finding the fastest possible (time-optimal) deterministic conversion between $W$ and Greenberger-Horne-Zeilinger (GHZ) states in a system of three identical and equidistant neutral atoms that are acted upon by four external laser pulses. Assuming that all four pulses are close to being resonant with the same internal (atomic) transition—the one between the atomic ground state and a high-lying Rydberg state—each atom can be treated as an effective two-level system ($gr$-type qubit). Starting from an effective system Hamiltonian, which is valid in the Rydberg-blockade regime and defined on a four-state manifold, we derive the quantum-brachistochrone equations pertaining to the fastest possible $W$-to-GHZ state conversion. By numerically solving these equations, we determine the time-dependent Rabi frequencies of external laser pulses that correspond to the time-optimal state conversion. In particular, we show that the shortest possible $W$-to-GHZ state-conversion time is given by $T_{\text{QB}}=6.8\hslash /E$, where $E$ is the total laser-pulse energy used, this last time being significantly shorter than the state-conversion times previously found using a dynamical-symmetry-based approach [$T_{\text{DS}}=(1.33-1.66)T_{\text{QB}}$].

Nonreciprocal systems breaking time-reversal symmetry are essential tools in modern quantum technologies enabling the suppression of unwanted reflected signals or extraneous noise entering through detection ports. Here we propose a scheme enabling nonreciprocal conversion between optical and radio-frequency (rf) photons using exclusively optomechanical and electromechanical interactions. The nonreciprocal transmission is obtained by interference of two dissipative pathways of transmission between the two electromagnetic modes established through two distinct intermediate mechanical modes. In our protocol, we apply a bichromatic drive to the cavity mode and a single-tone drive to the rf resonator, and use the relative phase between the drive tones to obtain nonreciprocity. We show that perfect nonreciprocal transduction can be obtained in the limit of large cooperativity in both directions, from optical to rf and vice versa. We also study the transducer noise and show that mechanical thermal noise is always reflected back onto the isolated port. In the limit of large cooperativity, the input noise is instead transmitted in an unaltered way in the allowed direction; in particular one has only vacuum noise in the output rf port in the case of optical-to-rf conversion.

In this paper, we investigate the strong-coupling quantum thermodynamics of a hybrid quantum system far from equilibrium, based on the renormalization theory of quantum thermodynamics we developed recently [Phys. Rev. Research 4, 023141 (2022)]. The strong-coupling hybrid system consists of a superconducting microwave cavity and a spin ensemble of the NV centers in diamond under external driving. The non-Markovian dynamics of this strong-coupling hybrid system has been experimentally explored and theoretically investigated. We apply the renormalization theory of quantum thermodynamics to study the transient quantum heat and work in this strong-coupling hybrid system. We find that the dissipation and fluctuation dynamics of the system induce the transient quantum heat current which shows significant non-Markovian effects. On the other hand, the energy and driving field renormalization produces quantum work power. In particular, the driving-induced work power can be largely enhanced by non-Markovian dynamics through the cavity coupling strongly with the spin ensemble at the resonance. Our results show that non-Markovian dynamics makes faster energy conversion of the heat and work.

We derive a Hamiltonian for the external and internal dynamics of an electromagnetically bound, charged two-particle system in external electromagnetic and gravitational fields, including leading-order relativistic corrections. We apply this Hamiltonian to describe the relativistic coupling of the external and internal dynamics of cold ions in Paul traps, including the effects of micromotion, excess micromotion, and trap imperfections. This provides a systematic and fully quantum-mechanical treatment of relativistic frequency shifts in atomic clocks based on single trapped ions. Our approach reproduces well-known formulas for the second-order Doppler shift for thermal states, which were previously derived on the basis of semiclassical arguments. We complement and clarify recent discussions in the literature of the role of time dilation and mass defect in ion clocks.

Hybrid photon-atom integrated circuits, which include photonic microcavities and trapped single neutral atoms in their evanescent field, have great potential for quantum information processing. In this platform, the atoms provide single-photon nonlinearity and long-lived memory, which are complementary to the excellent passive photonic devices in conventional quantum photonic circuits. In this work, we propose a stable platform for realizing hybrid photon-atom circuits based on an unsuspended photonic chip. By introducing high-order modes in the microring, a feasible evanescent-field trap potential well $\sim 0.26\mathrm{mK}$ could be obtained by only 10-$\mathrm{mW}$-level power in the cavity, compared with the 100-mW-level power required in the scheme based on fundamental modes. Based on our scheme, stable single-atom trapping with relatively low laser power is feasible for future studies on high-fidelity quantum gates, single-photon sources, and many-body quantum physics based on a controllable atom array in a microcavity.

An impurity in a Bose gas is commonly referred to as a Bose polaron. For a dilute Bose gas, its properties are expected to be universal, i.e., dependent only on a few parameters characterizing the boson-impurity interactions. It has been known for some time that when boson-impurity interactions are weak, the properties of the polaron depend only on the scattering length of these interactions. In this paper, which accompanies Phys. Rev. Lett. 126, 123403 (2021) (where some of these results have already been reported), we examine stronger boson-impurity interactions, keeping their range finite. We demonstrate that for attractive interactions between impurity and the bosons up to and including the unitary point, all static properties of a Bose polaron in a dilute Bose gas can be calculated in terms of the scattering length and an additional parameter that characterizes the range of the impurity-boson interactions. Our approach is valid if this parameter does not deviate too much from the scattering length of intraboson interactions, with the precise criterion given in the text. We produce explicit expressions for the energy and other properties of a polaron for the case when the impurity-boson scattering length is tuned to unitarity, and we also provide the first correction away from it.

We propose a numerically exact method for a mixture with a single impurity immersed in several majority fermions, confined in a harmonic potential. We separate one of the degrees of freedom through an appropriately tailored canonical transformation and perform exact diagonalization on the simplified Hamiltonian. This method is especially effective for a heavy impurity, where it outmatches the typical exact diagonalization approach. We used our method to calculate energy and density profiles of the first few eigenstates for a mixture with up to ten majority fermions.

Over the last decade there have been many advances in studies of quantum walks (QWs) including a momentum-space QW recently realized in our spinor Bose-Einstein condensate system. This QW possessed behaviors that generally agreed with theoretical predictions; however, it also showed momentum distributions that were not adequately explained by the theory. We present a theoretical model which proves that the coherent dynamics of the spinor condensate is sufficient to explain the experimental data without invoking the presence of a thermal cloud of atoms as in the original theory. Our numerical findings are supported by an analytical prediction for the momentum distributions in the limit of zero-temperature condensates. This current model provides more complete explanations to the momentum-space QWs that can be applied to study quantum search algorithms and topological phases in Floquet-driven systems.

We theoretically investigate the isothermal compressibility ${\kappa }_T$ in the normal state of an ultracold Fermi gas. Including pairing fluctuations, as well as preformed-pair formations, within the framework of the self-consistent $T$-matrix approximation, we evaluate the temperature dependence of this thermodynamic quantity over the entire BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region. While ${\kappa }_T$ in the weak-coupling BCS regime is dominated by Fermi atoms near the Fermi surface, correlations between tightly bound Cooper-pair molecules are found to play crucial roles in the strong-coupling BEC regime. In the latter region, besides a two-body molecular interaction, a three-body one is shown to sizably affect ${\kappa }_T$ near the superfluid phase-transition temperature. Our results indicate that the strong-coupling BEC regime of an ultracold Fermi gas would provide a unique opportunity to study multibody correlations between Cooper-pair molecules.

Modulational instability of a uniform two-dimensional binary Bose-Einstein condensate (BEC) in the presence of quantum fluctuations is studied. The analysis is based on the coupled Gross-Pitaevskii equations. It is shown that quantum fluctuations can induce instability when the BEC density is below a threshold. The dependence of the growth rate of modulations on the BEC parameters is found. It is observed that an asymmetry of the interaction parameters and/or initial densities of the components typically decreases the growth rate. Further development of the instability results in the BEC breaking up into a set of quantum droplets. These droplets merge dynamically with each other, so the total number of droplets decreases rapidly. The rate of this decrease is evaluated numerically for different initial parameters.

Causal modeling is a tool for generating causal explanations of observed correlations and has led to a deeper understanding of correlations in quantum networks. Existing frameworks for quantum causality tend to focus on acyclic causal structures that are not fine-tuned i.e., where causal connections between variables necessarily create correlations between them. However, fine-tuned causal models (which permit causation without correlation) play a crucial role in cryptography, and cyclic causal models can be used to model physical processes involving feedback and may also be relevant in exotic solutions of general relativity. Here we develop a causal modeling framework capable of dealing with these general scenarios. The key feature of our framework is that it allows operational and relativistic notions of causality to be independently defined and for connections between them to be established in a theory-independent manner. The framework first gives an operational way to study causation that allows for cyclic, fine-tuned, and nonclassical causal influences. We then consider how a causal model can be embedded in a space-time structure (modeled as a partial order) and propose a compatibility condition for ensuring that the embedded causal model does not allow signaling outside the space-time future. We identify several distinct classes of causal loops that can arise in our framework, showing that compatibility with a space-time can rule out only some of them. We discuss conditions for preventing superluminal signaling within arbitrary (and possibly cyclic) causal structures and consider models of causation in postquantum theories admitting so-called jamming correlations. Finally, this work introduces the concept of a higher-order affects relation, which is useful for causal discovery in fined-tuned causal models.

The reduced dynamics of an open quantum system $S$, interacting with its environment $E$, is not completely positive, in general. In this paper, we demonstrate that if the two following conditions are satisfied, simultaneously, then the reduced dynamics is completely positive: (1) the reduced dynamics of the system is linear, for arbitrary system-environment unitary evolution $U$; and (2) the reduced dynamics of the system is linear, for arbitrary initial state of the system ${\rho }_S$.

A three-level system can be used in a $\mathrm{\Lambda }$-type configuration in order to construct a universal set of quantum gates through the use of non-Abelian nonadiabatic geometrical phases. Such construction allows for high-speed operation times which diminish the effects of decoherence. This might be, however, accompanied by a breakdown of the validity of the rotating-wave approximation (RWA) due to the comparable timescale between counter-rotating terms and the pulse length, which greatly affects the dynamics. Here, we investigate the trade-off between dissipative effects and the RWA validity, obtaining the optimal regime for the operation of the holonomic quantum gates.

We examine entanglement of thermal states for spin-1/2 dimers in external magnetic fields. Entanglement transition in the temperature-magnetic-field plane demonstrates a duality in spin-spin interactions. This identifies a pair of dual categories of symmetric and antisymmetric dimers with each category classified into toric entanglement classes. The entanglement transition line is preserved from each toric entanglement class to its dual toric class. The toric classification is an indication of the topological signature of the entanglement, which bring about topological stability that could be relevant for quantum information processing.

We apply the recently proposed quantum probing protocols with an unknown system-probe coupling to probe the convex coefficients in mixtures of commuting states. By using two reference states instead of one as originally suggested, we are able to probe both the lower and upper bounds for the convex coefficient. We perform extensive analysis for the roles of the parameters characterizing the double peaked Gaussian frequency spectrum in the Markovian-to-non-Markovian transition of the polarization dynamics of a single photon. We apply the probing of the convex coefficient to the transition-inducing frequency parameter and show that the non-Markovianity of the polarization dynamics can be confirmed with a single snapshot measurement of the polarization qubit performed at unknown time. Furthermore, we change the “known” measurement coupling that corresponds to our dynamics of interest to unknown measurement coupling and show that even for unknown measurement couplings the probing method is precise enough to confirm that using the “known coupling” would lead to non-Markovian polarization dynamics. We also show how the protocol can identify Markovian and non-Markovian time intervals in the dynamics. The results are validated with single-photon experiments.

We propose a deterministic scheme of generating genuine multiparty entangled states in quantum networks of arbitrary size having various geometric structures—we refer to it as entanglement circulation. The procedure involves optimization over a set of two-qubit arbitrary unitary operators and the entanglement of the initial resource state. We report that the set of unitary operators that maximize the genuine multipartite entanglement quantified via generalized geometric measure (GGM) is not unique. We prove that the GGM of the resulting state of arbitrary qubits coincides with the minimum GGM of the initial resource states. By fixing the output state as the six-qubit one, we find the optimal way to create such states according to the available resource. Moreover, we show that the method proposed here can be implemented by using logic gates, or by using the time dynamics of realizable spin Hamiltonians. In case of an ordered system, GGM varies periodically with time while the evolution via disordered models lead to a low but constant multipartite entanglement in outputs at a critical time, which decreases exponentially with the increase of the strength of the disorder.

We have developed a Fock-space relativistic coupled-cluster theory-based method for the calculation of electric dipole polarizability of one-valence atoms and ions. We employ this method to compute the ground-state and spin-orbit coupled excited state electric dipole polarizability of Al and In. To check the quality of many-electron wave functions, we also compute the excitation energies of some low-lying states of Al and In. The effects of the Breit interaction and QED corrections from the Uehling potential and the self-energy are included to improve the accuracy of $\alpha$ further. Our recommended values of the ground-state $\alpha$ for both atoms are in good agreement with the previous theoretical results. From our computations, we find that more than 65% of contributions come from the dipolar mixing of $3p(5p$) with $3d(5d$) and $4s(6s$) electrons for Al(In). The largest Breit and QED contributions are found to be 1.3% and 0.6%, respectively.

Ground and excited-rovibrational states of weakly bound triatomic molecules containing two helium and one silver atoms are investigated. We consider three systems, ${}^4\mathrm{He}\text{−}{}^4\mathrm{He}\text{−}\mathrm{Ag}$, ${}^3\mathrm{He}\text{−}{}^4\mathrm{He}\text{−}\mathrm{Ag}$, and ${}^3\mathrm{He}\text{−}{}^3\mathrm{He}\text{−}\mathrm{Ag}$. The three-atom Schrödinger equation is solved in hyperspherical coordinates using the potential-energy surface represented as the addition of He-He and He-Ag pair interaction potentials. We compute the energy levels and analyze the structural properties by considering averaged pair distances and bond angles, one-dimensional pair and angle distribution functions, and two-dimensional pair-pair and angle-angle distribution functions. All the He-He-Ag bound states are found to possess similar characteristics, displaying obtuse isosceles (${}^4\mathrm{He}_2\mathrm{Ag}$ and ${}^3\mathrm{He}_2\mathrm{Ag}$) or scalene (${}^3\mathrm{He}{}^4\mathrm{He}\mathrm{Ag}$) triangle configurations where the He-Ag distances are nearly fixed and the Ag bond angle is variable.

We discuss procedures for error-tolerant spin control in environments that permit transient, large-angle reorientation of a magnetic bias field. Short sequences of nonresonant magnetic-field pulses in a laboratory-frame meridional plane are derived. These are shown to have band-pass excitation properties comparable to established amplitude-modulated, resonant pulses used in static, high-field magnetic resonance. Using these meridional pulses, we demonstrate robust $z$ inversion in proton (${}^1\mathrm{H}$) nuclear magnetic resonance near Earth's field.