The elements of reality coined by Einstein, Podolsky, and Rosen promoted a series of fundamental discussions involving the notion of quantum correlations and physical realism. The superposition principle applied in the double-slit experiment with matter waves highlights the need for a critical review of the adoption of physical realism in the quantum realm. In this work, we employ a measure of physical irrealism and consider an initial contractive state in the double-slit setup for which position and momentum variables of a single particle are initially correlated. We investigate how the behavior of the irrealism can help us to obtain information about the interference pattern, wavelike, and particlelike properties in the double-slit setup with matter waves. We find that there is a time of propagation that minimizes the irrealism, and around this point the state at the detection screen is squeezed in position and momentum in comparison with the standard Gaussian superposition. Interestingly, we show that the maximum visibility and the number of interference fringes are related to the minimum of the irrealism. Moreover, we demonstrate a monotonic relation between the irrealism and visibility around the time of minimum. Then, we argue how to use these results to indirectly measure the irrealism for position variable from the fringe visibility.

Efficient simulations of the dynamics of open systems is of wide importance for quantum science and technology. Here, we introduce a generalization of the transfer-tensor, or discrete-time memory kernel, formalism to multitime measurement scenarios. The transfer-tensor method sets out to compute the state of an open few-body quantum system at long times, given that only short-time system trajectories are available. Here, we show that the transfer-tensor method can be extended to processes which include multiple interrogations (e.g., measurements) of the open system dynamics as it evolves, allowing us to propagate high-order short-time correlation functions to later times, without further recourse to the underlying system-environment evolution. Our approach exploits the process-tensor description of open quantum processes to represent and propagate the dynamics in terms of an object from which any multitime correlation can be extracted. As an illustration of the utility of the method, we study the buildup of system-environment correlations in the paradigmatic spin-boson model and compute steady-state emission spectra, taking fully into account system-environment correlations present in the steady state.

Dimensionality reduction algorithms, which reduce the dimensionality of a given data set whereas preserving the information of the original data set as well as possible, play an important role in machine learning and data mining. Duan et al. proposed a quantum version of the A-optimal projection algorithm (AOP) for dimensionality reduction [Phys. Rev. A 99, 032311 (2019)] and claimed that the algorithm has exponential speedups on the dimensionality of the original feature space $n$ and the dimensionality of the reduced feature space $k$ over the classical algorithm. In this paper, we correct the time complexity of the algorithm of Duan et al. to $O\left[\frac{{\kappa }^{4s}\sqrt{k^s}}{{\epsilon }^s}{\mathrm{polylog}}^s\left(\frac{mn}{\epsilon }\right)\right]$, where $\kappa$ is the condition number of a matrix that related to the original data set, $s$ is the number of iterations, $m$ is the number of data points, and $\epsilon$ is the desired precision of the output state. Since the time complexity has an exponential dependence on $s$, the quantum algorithm can only be beneficial for high-dimensional problems with a small number of iterations $s$. To get a further speedup, we propose an improved quantum AOP algorithm with time complexity $O\left[\frac{s{\kappa }^6\sqrt{k}}{\epsilon }\mathrm{polylog}\left(\frac{nm}{\epsilon }\right)+\frac{s^2{\kappa }^4}{\epsilon }\mathrm{polylog}\left(\frac{\kappa k}{\epsilon }\right)\right]$ and space complexity $O[{log}_2(nk/\epsilon )+s]$. With space complexity slightly worse, our algorithm achieves, at least, a polynomial speedup compared to the algorithm of Duan et al.. Also, our algorithm shows exponential speedups in $n$ and $m$ compared with the classical algorithm when $\kappa ,k$, and $1/\epsilon$ are $O\left[\mathrm{polylog}\right(nm\left)\right]$.

We present bounds for distilling many copies of a pure state from an arbitrary initial state in a general quantum resource theory. Our bounds apply to operations that are able to generate no more than a $\delta$ amount of resource, where $\delta \ge 0$ is a given parameter. To maximize applicability of our upper bound, we assume little structure on the set of free states under consideration besides a weak form of superadditivity of the function $G_{\min }\left(\rho \right)$, which measures the overlap between $\rho$ and the set of free states. Our bounds are given in terms of this function and the robustness of resource. Known results in coherence and entanglement theory are reproduced in this more general framework.

Finite fermion systems are known to exhibit shell structure in the weakly interacting regime, as is well known from atoms, nuclei, metallic clusters, or even quantum dots in two dimensions. All these systems have in common that the particle interactions between electrons or nucleons are spatially isotropic. Dipolar quantum systems as they have been realized with ultracold gases, however, are governed by an intrinsic anisotropy of the two-body interaction that depends on the orientation of the dipoles relative to each other. Here we investigate how this interaction anisotropy modifies the shell structure in a weakly interacting two-dimensional anisotropic harmonic trap. Going beyond Hartree-Fock analysis by applying the so-called importance-truncated configuration-interaction (CI) method as well as quadratic CI with single and double substitutions, we show how the magnetostriction in the system may be counteracted upon by a deformation of the confinement.

We present an interesting dynamical temporal localization of a strongly driven two-level system (TLS), a plateau with quantized oscillation, by an analytical and transparent method, the counter-rotating-hybridized rotating-wave (CHRW) method. This approach, which is based on unitary transformations with a single parameter, treats the rotating and counter-rotating terms on equal footing. In the unitarily transformed representation, we find that it is the multiple-harmonic terms shown in the transformed Hamiltonian that make a crucial contribution to the generation of the exotic plateau phenomenon. By comparing the results of the numerically exact calculation and several other methods, we show that the CHRW results obtained by analytical formalism involving the collective effects of multiple harmonics are in good agreement with the numerical results, which illustrates not only the general tendency of the dynamical evolution of strongly driven TLS, but also the interesting phenomena of plateaus. The developed CHRW method reveals two kinds of plateau patterns: zigzag plateau and armchair plateau, and quantitative analysis is given to comprehensively describe the features of the plateau phenomenon. The plateau phenomenon has a periodical pattern whose frequency is double the driving frequency, and possesses quantized oscillations, the number of which has a certain, precise value. Also, fast oscillation is produced on every plateau that is determined by the relevant driving parameters of the TLS. Our main results are as follows: (i) in the large-amplitude oscillatory case, it turns out that the collective effects of all even harmonics contribute to the generation of zigzag plateau with quantized oscillation, and the general tendency of evolution coincides with the result of the original CHRW method because of a linear trend of the phase function; (ii) in the small-amplitude oscillatory case, the dynamics of the coherent destruction of tunneling under strong driving is exactly exhibited by including the odd-harmonic effect, which offers an attractive dynamical pattern, namely, armchair plateau possessing a two-stair structure rather than the complete destruction. Besides, the characteristics of the plateau (position, frequency, the envelope, and number of quantized oscillations) are revealed by our analytical formalism. Both of the dynamical patterns are of great interest to strongly driven physics in the ongoing research on driven TLS systems.

We investigate the photoionization of ${\mathrm{H}}_2$ by an attosecond pulse train using reduced density matrices and reveal that the phase of the photoelectron introduces a time delay in the coherent vibrational motion of ${\mathrm{H}}_2^{+}$. We show that, even when the ionization laser pulse is Fourier limited, the reduced density matrix of ${\mathrm{H}}_2^{+}$ contains an intrinsic phase ascribed to the phase of the photoelectron wave function and that the intrinsic phase can be extracted by pump-probe measurements as long as the pump-probe time delay is measured with precision of the order of tens of attoseconds.

We study the generation of spin squeezing in arrays of long-lived dipoles subject to collective emission, coherent drive, elastic interactions, and single-particle relaxation. It is found that not only does single-particle relaxation not necessarily degrade the squeezing generated in the collective dynamics, but the interplay of single-particle and collective effects can in fact facilitate the generation of squeezing in a specific parameter regime. This latter behavior is connected to the dynamical self-tuning of the system through a dissipative phase transition that is present in the collective system alone. Our findings will be applicable to next-generation quantum sensors with an eye towards atomic clocks in cavity-QED setups and trapped ion systems.

The electrodynamic theory of continuous media is probably the most convenient platform when trying to construct analog gravity theories. Quite naturally, this topic has gained considerable interest. One peculiar but not so well known feature in this context is the unconventional behavior of radiation energy and momentum in cases where superluminal fluid velocities are encountered, which, as is known, is a major ingredient in analog gravity theories. These peculiar features are intimately connected with the spacelike character of Minkowski's four-momentum in electrodynamics. Here, we first consider an artificial model in which a Kerr-induced superluminal region is created in the right-hand region ($z>0$) in a left-moving, originally subluminal fluid. We analyze the behavior of the energy density, Poynting vector, and momentum density and calculate the force on the artificial black-hole horizon. Also, we delve into quantal aspects, looking for eventual production of particles associated with the sudden creation of the horizon, finding, however, that no particles are predicted to occur. The present paper continues a previous investigation by me on the same topic [Phys. Rev. A 100, 032109 (2019)]. The subject as such is closely related to the famous Abraham-Minkowski problem.

Driven-dissipative phase transitions are currently a topic of intense research due to the prospect of experimental realizations in quantum optical setups. The most paradigmatic model presenting such a transition is the Kerr model, which predicts the phenomenon of optical bistability, where the system may relax to two different steady states for the same driving condition. These states, however, are inherently out-of-equilibrium and are thus characterized by the continuous production of irreversible entropy, a key quantifier in thermodynamics. In this paper, we study the dynamics of the entropy production rate in a quench scenario of the Kerr model, where the external pump is abruptly changed. This is accomplished using a recently developed formalism, based on the Husimi $Q$-function, which is particularly tailored for driven-dissipative and non-Gaussian bosonic systems, Goes et al. [Phys. Rev. Res. 2, 013136 (2020)]. Within this framework, the entropy production can be split into two contributions, one being extensive with the drive and describing classical irreversibility, and the other being intensive and directly related to quantum fluctuations. The latter, in particular, is found to reveal the high degree of nonadiabaticity for quenches between different metastable states.

Graph states are a unique resource for quantum information processing, such as measurement-based quantum computation. Here, we theoretically investigate the use of continuous-variable graph states for single-parameter quantum metrology, including both phase and displacement sensing. We identified the optimal graph states for the two sensing modalities and showed that Heisenberg scaling of the accuracy for both phase and displacement sensing can be achieved with local homodyne measurements.

Dissociative electron attachment (DEA) to difluoromethane (${\mathrm{CH}}_2{\mathrm{F}}_2$) has been studied in the electron energy range of 0 to 16 eV. Two resonant states around 2 and 11.4 eV leading to three different fragment anions (${\mathrm{F}}^{-}, \mathrm{CH}{\mathrm{F}}^{-}$, and ${\mathrm{F}}_2{}^{-}$) are observed. Ion-yield curves of the negative ions help us to locate the position of the resonant states. In the ion-yield curve of ${\mathrm{F}}^{-}$ ions, one small hump near 9.8 eV followed by a peak around 15.2 eV is also observed. Absolute DEA cross sections of the ${\mathrm{F}}^{-}$ ion is measured by using the well-known relative flow technique. Dissociation channels associated with each resonant states are identified by density-functional theory (DFT) calculations of the threshold energies. The theory matches quite well within the experimental uncertainty.

The ground-state properties of the Hubbard chain with on-site repulsion and anisotropic nearest-neighbor attraction are investigated by means of density matrix renormalization group calculations. The nonlocal attraction acts between fermions of one spin component only, mimicking the effect of $p$-wave Feshbach resonances in cold-atom systems. We analyze the onset of itinerant ferromagnetism, pinpointing the critical attraction strength where partially and fully ferromagnetic states occur. In the cold-atom setup, where the two (pseudo)spin populations are separately conserved, ferromagnetism occurs with the nucleation of a fully imbalanced band-insulating domain hosting the attractive component only. The size of this domain grows with the attraction strength, therefore increasing the (opposite) imbalance of the other domain, until the two spin components are fully separated. In the presence of a harmonic trap, the ferromagnetic state hosts a partially imbalanced domain in the center with an excess of the attractive component and filling lower than one. This central region is surrounded by fully imbalanced domains, located in the trap tails, hosting only fermions belonging to the other component.

We propose and analyze a microscopic Stirling heat engine based on an optomechanical system. The working fluid is a single vibrational mode of a mechanical resonator, which interacts by radiation pressure with a feedback-controlled optical cavity. The cavity light is used to engineer the thermal reservoirs and to steer the resonator through a thermodynamic cycle. In particular, the feedback is used to properly modulate the light fluctuations inside the cavity and hence to realize efficient thermodynamic transformations with realistic optomechanical devices.

A comparative study on the chromatic dispersions and the interactions with ultrashort pulses in epsilon-near-zero (ENZ) transparent conducting oxides (TCOs) is theoretically presented. The ENZ TCOs exhibit rapidly varying and unprecedentedly large $N\mathrm{th}$-order dispersions in the ENZ region, which is exclusive to ENZ plasmonic materials. However, it is found that in both physical and mathematical senses, the concepts of high $N\mathrm{th}$-order chromatic dispersions are inapplicable for ENZ TCOs with Drude-like permittivity. Subsequently, the impacts of the complex permittivity dispersion profiles on ultrashort pulse interactions are discussed. Comparisons are made between propagation patterns in an ENZ AZO waveguide obtained from the nonlinear Schrödinger equation (NLSE) and Maxwell's equations, as well as between interactions of femtosecond pulses with different temporal widths in a subwavelength ENZ AZO slab. Results show that the well-received NLSE is not suitable for modeling ENZ TCOs in either case, and subwavelength interaction patterns with different pulse widths are quite different, where the shorter pulse can excite stronger localized resonance.

Heralded single-photon sources are the most commonly used sources for optical quantum technology applications. There is strong demand for accurate prediction of their spectral features and temporal correlations with ever-increasing precision. This is particularly important in connection with the intrinsically stochastic photon-pair generation process in heralded sources. Here we present a complete theoretical description of the temporal correlation of a signal-idler, signal-signal, and signal-signal-idler coincidences of photons generated by continuous-wave pumped cavity-enhanced spontaneous parametric down-conversion. The theory excellently predicts the measurements, which has been experimentally confirmed in our setup utilizing single-photon detectors with high temporal resolution. This enables us to resolve and analyze the multiphoton correlation functions in great detail.

We derive a generalized master equation for multiphoton pulses interacting with multiple emitters in a waveguide-quantum electrodynamics system where the emitter frequency can be modulated and the effects of nonguided modes can also be considered. Based on this theory, we can calculate the real-time dynamics of an array of interacting emitters driven by an incident photon pulse which can be vacuum, a coherent state, a Fock state, or their superpositions. Moreover, we also derive generalized input-output relations to calculate the reflectivity and transmissivity of this system. We can also calculate the output photon pulse shapes. Our theory can find important applications in the study of waveguide-based quantum systems.