Quantum batteries are quantum systems that store energy which can then be used for quantum tasks. One relevant question about such systems concerns the differences and eventual advantages over their classical counterparts, whether in the efficiency of the energy transference, input power, total stored energy, or other relevant physical quantities. Here, we show how a purely quantum effect related to the vacuum of the electromagnetic field can enhance the charging of a quantum battery. In particular, we demonstrate how an anti-Jaynes-Cummings interaction derived from an off-resonant Raman configuration can be used to increase the stored energy of an effective two-level atom when compared to its classically driven counterpart, eventually achieving full charging of the battery with zero entropic cost.

An investigation of the radiationless decay of core excited Xe atoms in the region of Xe $L_3{M}_{4,5}{M}_{4,5}$ Auger electron kinetic energies (using x-ray energies in the vicinity of the $L_3$ threshold) is presented for Xe adsorbed on Cu, Ag, and Au. The intensity distribution of the decay channels is different compared with Xe in the gas phase. Charge transfer of the core excited electron occurs within tens of attoseconds in all systems for excitation energies approaching the ionization threshold of the condensed system, whereas charge transfer times are substrate-dependent for lower excitation energies. The determination of partial yields into the decay channels allows for the observation of a decay channel present in the Xe/Cu and Xe/Ag systems but not in the case of Xe/Au. Theoretical calculations allow us to interpret this difference as emanating from varying amounts of the ground state hybridization between Xe and the substrates, which impacts the energy of the Auger final states enabling identification of these states giving rise to system specific features in the experimental data.

We study carrier-envelope-phase- (CEP-) dependent photoelectron momentum distributions (PEMDs) of Kr atoms and randomly oriented ${\mathrm{CO}}_2$ molecules exposed to sub-two-cycle intense infrared fields with linear polarization. The adiabatic theory predicts that, at fixed laser parameters (intensity, pulse duration, and CEP), the PEMDs for targets with equal ionization potentials considered across a backward rescattering caustic have identical shapes and only their absolute values depend on the target. We choose Kr and ${\mathrm{CO}}_2$ as targets with almost identical ionization potentials (approximately $14$ eV) to explore this behavior of PEMDs. We find that the measured PEMDs for Kr and ${\mathrm{CO}}_2$ have cutoff structures, which are associated with the caustics, with identical decaying profiles. We also reveal that CEP-dependent PEMDs around the cutoff exhibit target dependence, reflecting the differences in the differential cross sections (DCSs) of elastic rescattering and tunneling ionization rates. Furthermore, we extract the DCSs from the measured PEMDs around the cutoffs at different CEPs based on the adiabatic theory. The extracted DCSs agree with ones calculated by using the single-active-electron (SAE) scattering model, which includes all multiple-scattering processes, with orientation averaging for ${\mathrm{CO}}_2$. In contrast, the independent-atom model, which only considers single-scattering processes, cannot reproduce the extracted DCSs of ${\mathrm{CO}}_2$ at low scattering momenta. This finding indicates that multiple scattering is not negligible in the momentum range of 1.5–2.5 a.u. The extracted DCSs also show interference features assigned to multicenter scattering of O-O and C-O. These results confirm that the signature of electron interference is retained after orientation averaging, indicating that the internuclear distances of the constituent atoms can be retrieved from the CEP-dependent PEMDs for randomly oriented molecules using the SAE model.

We have calculated the polarization and Stark-shifted binding energy for ultraintense lasers interacting with highly charged ions across the periodic table from beryllium to uranium at intensities up to ${10}^{22}\mathrm{W}{\mathrm{cm}}^{-2}$. The induced dipole and Stark shifts for the bound states can be as large as $0.1e{a}_0$ and $50E_h$. Calculations of tunneling show the impacts of polarization and Stark shifts on the ionization rate are significant but counteracting. The work resolves a long-standing question of how field-free derivations of the tunneling response for highly charged ions have been quantitatively successful in relativistic, ultrahigh-intensity experiments. Using a scaling relationship, the results can be generalized to give the induced electric dipole for any species across an intensity range from ${10}^{15}$ to ${10}^{22}\mathrm{W}{\mathrm{cm}}^{-2}$.

The ionization of atoms with sequences of attosecond pulses gives rise to excited ionic states that are entangled with the emitted photoelectron. Still, the ionic ensemble preserves some coherence that can be controlled through the laser parameters. In helium, control of the $2s/2p{\mathrm{He}}^{+}$ coherence is mediated by the autoionizing states below the $N=2$ threshold [Phys. Rev. Res. 3, 023233 (2021)]. In the present work we study the role of the resonances both below and above the $N=3$ threshold on the coherence of the $N=3{\mathrm{He}}^{+}$ ion, in the attosecond pump-probe ionization of the helium atom, which we simulate using the newstock ab initio code. Due to the fine-structure splitting of the $N=3{\mathrm{He}}^{+}$ level, the ionic dipole beats on a picosecond timescale. We show how, from the dipole beating, it is possible to reconstruct the polarization of the ion at its inception.

We show that two particles interacting via spin exchange exhibit topological features found in one-dimensional single-particle lattice models. This is accomplished by absorbing all of the spatial degrees of freedom of the lattice into the spin degrees of freedom of the two particles. Comparing the spin system with the Su-Schrieffer-Heeger model, we show the existence of topologically protected edge spin states and establish the bulk-edge correspondence. Modifying the spin system with a chiral symmetry-breaking term results in it resembling the Rice-Mele model and can therefore act as a Thouless spin pump of one of the particles when periodically and adiabatically driven. By using the spin states as a synthetic spatial dimension, we show two particles are enough to simulate well-known topological properties in condensed matter physics.

High data acquisition rates and low-noise detection of ultracold neutral atoms present important challenges for the state tomography and interferometric application of entangled quantum states in Bose-Einstein condensates. In this paper, we present a high-flux source of ${}^{87}\mathrm{Rb}$ Bose-Einstein condensates combined with a number-resolving detection. We create Bose-Einstein condensates of $2\times {10}^5$ atoms with no discernible thermal fraction within 3.3 s using a hybrid evaporation approach in a magnetic and optical trap. For the high-fidelity tomography of many-body quantum states in the spin degree of freedom [M. Hetzel, et al., arXiv:2207.01270], it is desirable to select a single mode for a number-resolving detection. We demonstrate the low-noise selection of subsamples of up to 16 atoms and their subsequent detection with a counting noise below 0.2 atoms. The presented techniques offer an exciting path towards the creation and analysis of mesoscopic quantum states with improved fidelities and towards their exploitation for fundamental and metrological applications.

Recent experimental progress produced molecular superfluids (MSF) in thermal equilibrium; this opens the door to a new class of experiments investigating the associated thermodynamic and dynamical responses. We review the theoretical picture of the phase diagram and quasiparticle spectrum in the atomic superfluid (ASF) and MSF phases. We further compute the parametric dependence of the quasiparticle lifetimes at one-loop order. In the MSF phase, the $\text{U}\left(1\right)$ particle-number symmetry breaks to ${\mathbb{Z}}_2$ and the spectrum exhibits a gapless Goldstone mode in addition to a gapped ${\mathbb{Z}}_2$-protected atom-like mode. In the ASF phase, the $\text{U}\left(1\right)$ symmetry breaks completely, leaving behind a Goldstone mode and an unprotected gapped mode. In both phases, the Goldstone mode decays with a rate given by the celebrated Belyaev result, as in a single component condensate. In the MSF phase, the gapped mode is sharp up to a critical Cherenkov momentum beyond which it emits phonons. In the ASF phase, the gapped mode decays with a constant rate even at small momenta. These decay rates govern the spectral response in microtrap tunneling experiments and lead to sharp features in the transmission spectrum of atoms fired through molecular clouds.

We investigate the time-dependent Rényi entanglement entropy after a quantum quench starting from the Mott-insulating and charge-density-wave states in a one-dimensional free boson system. The second Rényi entanglement entropy is found to be the negative of the logarithm of the permanent of a matrix consisting of time-dependent single-particle correlation functions. From this relation and a permanent inequality, we obtain rigorous conditions for satisfying the volume-law entanglement growth. We also succeed in calculating the time evolution of the Rényi entanglement entropy in extremely large systems by brute-force computations of the permanent. We discuss possible applications of our findings to the real-time dynamics of noninteracting bosonic systems.

We propose and analyze a scheme for tunable coherent perfect absorption (CPA) and near-perfect reflection (near CPR) in a three-level $\mathrm{\Lambda }$-type atom-cavity system. With electromagnetically-induced-transparency-type interference induced by a coherent coupling laser, it provides a method of tunable near CPR at two-photon resonance. By virtue of different coupling strengths, CPA can be tuned from the linear regime to the nonlinear or bistable regime at a fixed input-field frequency. In addition, the CPA regime can be switched to the near-CPR regime with variable linear absorption or gain by an incoherent pump field. This work suggests a mechanism for manipulation on perfect absorption and reflection theoretically which can potentially be applied to logical gates and optical switches for coherent optical computing and communication.

We propose an application of quantum temporal imaging to restoring the indistinguishability of the signal and the idler photons produced in type-II spontaneous parametric down-conversion with a pulsed broadband pump. It is known that in this case, the signal and the idler photons have different spectral and temporal properties. This effect deteriorates their indistinguishability and the visibility of the Hong-Ou-Mandel interference, respectively. We demonstrate that inserting a time lens in one arm of the interferometer and choosing properly its magnification factor restores perfect indistinguishability of the signal and the idler photons and provides 100% visibility of the Hong-Ou-Mandel interference in the limit of high focal group delay dispersion of the time lens.

We describe a class of neighboring-block stabilizer quantum error correction codes and demonstrate that such a class of codes can be implemented in a resource-efficient manner using a single ancilla and circular near-neighbor qubit connectivity. We propose an implementation for syndrome-measurement circuits for codes from the class and illustrate its workings for cases of three-qubit repetition code, Laflamme's five-qubit code, and Shor's nine-qubit code. For three-qubit repetition code and Laflamme's five-qubit code, the suggested scheme has the property that it uses only native two-qubit controlled-not-swap gates, which potentially reduces the amount of noncorrectable errors due to the shorter gate time. Elements of the scheme can be used to implement surface code with near-neighbor connectivity using a single ancilla, as demonstrated in an example. We developed efficient decoding procedures for repetition codes and the Laflamme's five-qubit code using a minimum-weight perfect-matching approach to account for the specific order of measurements in our scheme. The analysis of noise levels for which the scheme could show improvements in the fidelity of a stored logical qubit in the three-qubit repetition code and Laflamme's five-qubit code cases is provided. We complement our results by realizing the developed scheme for a three-qubit code using an IBM quantum processor and the Laflamme's five-qubit code using the state-vector simulator.

Linear $n$-local networks are compatible with quantum-repeater-based entanglement distribution protocols. Different sources of imperfections such as error in entanglement generation, communication over noisy quantum channels, and imperfections in measurements result in decay of quantumness across such networks. From practical perspectives it becomes imperative to analyze the nonclassicality of quantum network correlations in the presence of different types of noise. The present discussion provides a formal characterization of non-$n$-local features of quantum correlations in a noisy network scenario. In this context, persistency of non-$n$-locality has been introduced. Such a notion helps in analyzing decay of non-$n$-local features of network correlations with increasing length of the linear network in the presence of one or more causes of imperfections.

We present a quantum algorithm for implementing ${\varphi }^4$ lattice scalar field theory on qubit computers. The field is represented in the discretized field amplitude basis. The number of qubits and elementary gates required by the implementation of the evolution operator is proportional to the lattice size. The algorithm allows efficient ${\varphi }^4$ state preparation for a large range of input parameters in both the normal and broken-symmetry phases. The states are prepared using a combination of variational and adiabatic evolution methods. First, the ground state of a local Hamiltonian, which includes the ${\varphi }^4$ self-interaction, is prepared using short variational circuits. Next, this state is evolved by switching on the coupling between the lattice sites adiabatically. The parameters defining the local Hamiltonian are adjustable and constitute the input of our algorithm. We present a method to optimize these parameters in order to reduce the adiabatic time required for state preparation. For preparing broken-symmetry states, the adiabatic evolution problems caused by crossing the phase transition critical line and by the degeneracy of the broken-symmetry ground state can be addressed using an auxiliary external field which gradually turns off during the adiabatic process. We show that the time dependence of the external field during the adiabatic evolution is important for addressing the broken-symmetry ground state degeneracy. The adiabatic time dependence on the inverse error tolerance can be reduced from quadratic to linear by using a field strength that decreases exponentially in time relative to one that decreases linearly.

We calculate the ground states of a dipolar Bose gas confined in an infinite tube potential. We use the extended Gross-Pitaevskii equation theory and present a numerical method to efficiently obtain solutions. A key feature of this method is an analytic result for a truncated dipole-dipole interaction potential that enables the long-ranged interactions to be accurately evaluated within a unit cell. Our focus is on the transition of the ground state to a crystal driven by dipole-dipole interactions as the short-ranged interaction strength is varied. We find that the transition is continuous or discontinuous depending upon the average system density. These results give deeper insight into the supersolid phase transition observed in recent experiments and validate the utility of the reduced three-dimensional theory developed in [Phys. Rev. Res. 2, 043318 (2020)] for making qualitatively accurate predictions.

We theoretically study the superradiant phase transition (SPT) in an indirect Rabi model, where both a two-level system and a single-mode bosonic field couple to an auxiliary bosonic field. We find that the indirect spin-field coupling induced by the virtual excitation of the auxiliary field can allow the occurrence of a SPT at a critical point, and the influence of the so-called $A^2$ term in the normal Rabi model is naturally avoided. In the large detuning regime, we present the analytical expression of quantum critical point in terms of the original system parameters. The critical atom-field coupling strength is tunable, which will loosen the conditions on realizing the SPT. Considering a hybrid magnon-cavity-qubit system, we predict the squeezed cat state of magnon generated with feasible experimental parameters, which has potential applications in quantum metrology and quantum information processing.

The storage and retrieval of photonic quantum states, quantum memory, is a key resource for a wide range of quantum applications. Here we investigate the sensitivity of $\mathrm{\Lambda }$-type quantum memory to experimental fluctuations and drift. We use a variance-based approach, focusing on the effects of fluctuations and drift on memory efficiency. We consider shot-to-shot fluctuations of the memory parameters, and separately we consider longer timescale drift of the control field parameters. We find the parameters that a quantum memory is most sensitive to depend on the quantum memory protocol being employed, where the observed sensitivity agrees with physical interpretation of the protocols. We also present a general framework that is applicable to other figures of merit beyond memory efficiency. These results have practical ramifications for quantum memory experiments.

Comprehending the connections between the geometric, topological, and dynamical structures of integrable quantum systems with quantum phenomena exploitable in quantum information tasks, such as quantum entanglement, is a major problem in geometric information science. In this work we investigate these issues in a physical system of $\mathcal{N}$ interacting spin-$\mathtt{s}$ under the long-range Ising model. We discover the relevant dynamics, identify the corresponding quantum phase space, and we derive the associated Fubini-Study metric. Through the application of the Gauss-Bonnet theorem and the derivation of the Gaussian curvature, we proved that the dynamics occurs on a spherical topology manifold. Afterwards, we analyze the gained geometrical phase under the arbitrary and cyclic evolution processes and solve the quantum brachistochrone problem by establishing the time-optimal evolution. Moreover, by narrowing the system to a two spin-$\mathtt{s}$ system, we explore the relevant entanglement from two different perspectives. The first is geometrical in nature and involves the investigation of the interplay between the entanglement degree and the geometrical structures, such as the Fubini-Study metric, the Gaussian curvature, and the geometrical phase. The second is dynamical in nature and tackles the entanglement effect on the evolution speed and geodesic distance. Additionally, we resolve the quantum brachistochrone problem based on the entanglement degree.

Quantum algorithmics with single spins poses serious technological challenges such as precision fabrication, rapid decoherence, atomic-scale addressing, and readout. To circumvent atomic-scale challenges, we examine the case of fully polarized mesoscopic spin ensembles (spin-coherent states) whose total angular momenta states map to qudit submanifolds. We show that in the limit where the size of the ensembles is small compared to their separation, it is possible to treat them as qubits with an effective coupling strength that scales with the number of spins. If the spins within each ensemble are decoupled (e.g., via control fields, spinning or diffusional averaging, or materials engineering), one- and two-qubit gate operations can be implemented with high fidelities.

We design quantum refrigerators based on spin-$j$ quantum $XYZ$ and bilinear-biquadratic models with individual spins attached to bosonic thermal baths. By considering both local and global master equations, we illustrate an enhancement in the performance of the refrigerators with an increase in the spin dimension irrespective of the choice of the spin models. To assess the performance of the refrigerators, we introduce a distance-based measure to quantify the local temperature of a particle with arbitrary spin quantum number $j$. Interestingly, we find that the local temperature quantifier, defined via minimizing the distance between a spin-$j$ thermal state and the evolved state of the spin-$j$ particle in the steady state, coincides with the population-based definition of local temperature known in the literature for spin-$\frac{1}{2}$ particles. Moreover, we demonstrate that the qualitative behavior of the distance-based local temperature is independent of the choice of the distance measure by comparing the trace distance, Uhlmann's fidelity, and relative entropy distance. We further observe by computing a local master equation that the quantum refrigerator consisting of a spin-$1/2$ and a spin-$j$ particle can lead to a lower local temperature compared to a refrigerator with two identical spin-$j$ particles following the $XYZ$ interactions.