We study a quantum information engine (QIE) modeled by a multiqubit working medium (WM) collectively coupled to a single thermal bath. We show that one can harness the collective effects to significantly enhance the performance of the QIE, as compared to equivalent engines lacking collective effects. We use one bit of information about the WM magnetization to extract work from the thermal bath. We analyze the work output, noise-to-signal ratio, and thermodynamic uncertainty relation (TUR), and contrast these performance metrics of a collective QIE with that of an engine whose WM qubits are coupled independently to a thermal bath. We show that in the limit of high temperatures of the thermal bath, a collective QIE always outperforms its independent counterpart. In contrast to quantum heat engines, where collective enhancement in specific heat plays a direct role in improving the performance of the engines, here the collective advantage stems from higher occupation probabilities for the higher-energy levels of the positive magnetization states, as compared to the independent case. Furthermore, we show that in the limit of high temperatures, the TUR for both the collective as well as the independent QIEs violate the standard heat engine TUR bound of 2.

Variational quantum algorithms, especially quantum approximate optimization and the variational quantum eigensolver (VQE), have demonstrated their potential to provide computational advantage in the realm of combinatorial optimization. However, these algorithms suffer from classically intractable gradients limiting the scalability. This work addresses the scalability challenge for the VQE by proposing a classical gradient computation method which utilizes the parameter-shift rule but computes the expected values from the circuits using a tensor-ring approximation. The parametrized gates from the circuit transform the tensor ring by contracting the matrix along the free edges of the tensor ring. While the single-qubit gates do not alter the ring structure, the state transformations from the two-qubit rotations are evaluated by truncating the singular values, thereby preserving the structure of the tensor ring and reducing the computational complexity. This variation of the matrix-product-state approximation grows linearly in number of qubits and the number of two-qubit gates as opposed to the exponential growth in the classical simulations, allowing for a faster evaluation of the gradients on classical simulators.

Quantum state tomography is an essential component of modern quantum technology. In application to continuous-variable harmonic-oscillator systems, such as the electromagnetic field, existing tomography methods typically reconstruct the state in discrete bases, and are hence limited to states with relatively low amplitudes and energies. Here, we overcome this limitation by utilizing a feed-forward neural network to obtain the density matrix directly in the continuous position basis. An important benefit of our approach is the ability to choose specific regions in the phase space for detailed reconstruction. This results in a relatively slow scaling of the amount of resources required for the reconstruction with the state amplitude, and hence allows us to dramatically increase the range of amplitudes accessible with our method.

Taming a broader range of molecular species with large density has been a long-standing goal in molecular science. Heavy molecules with masses greater than 100 amu are of particular interest for precision measurements. However, decelerating a fast-moving beam of such heavy molecules to rest remains challenging for Zeeman deceleration. Moreover, the traditional approach of pulsed Zeeman decelerator suffers from serious molecular loss during deceleration, significantly limiting its potential applications. Herein, we present a proposal of ring-shaped traveling wave Zeeman decelerator (RTWZD) featured with true three-dimensional smoothly moving magnetic potential wells that effectively solve the above intractable problems. With the RTWZD approach, not only can the density of the molecule be greatly increased but also the range of molecular species for Zeeman deceleration can be extended from light to heavy. The performances of the RTWZD are characterized by theoretical analysis and numerical simulations, utilizing a group of atoms and molecules such as ${}^7\mathrm{Li}, {}^{16}\mathrm{O}_2, {}^{88}\mathrm{Sr}{}^{19}\mathrm{F}$, and ${}^{174}\mathrm{Yb}{}^{19}\mathrm{F}$ as testers. Notably, losses encountered in the traditional Zeeman decelerator can be avoided, yielding more than two orders of magnitude improvement in molecular density. These characteristics of the RTWZD make it an ideal toolbox to produce cold and dense atomic/molecular samples, with promising prospects for cold collision, sympathetic cooling, and precision measurement.

In this work we investigate the nonequilibrium dynamics of one-dimensional spinless fermions loaded in a cavity with imbalanced pumping lasers. Our study is motivated by previous work on a similar setup using bosons, and we explore the unique properties of fermionic systems in this context. By considering the imbalance in the pumping, we find that the system exhibits multiple superradiant steady phases, a bistable regime, and an unstable phase. Furthermore, by making use of the hysteresis structure in the bistable regime, we propose a unidirectional topological pumping. Unlike the usual topological pumping in which the driving protocol breaks time-reversal symmetry, the driving protocol can be time-reversal invariant in our proposal.

In this paper we propose a scheme for achieving simultaneous nonreciprocal photon blockade for two coupled spinning resonators, one of which contains an optical parametric amplifier (OPA). Rotating two resonators gives rise to opposite Sagnac-Fizeau shifts for input from different ends. Depending on the opposite shifts, quantum destructive interference emerges and disappears in two-photon excitation. The emergence of destructive interference results in photon blockade in two resonators simultaneously for one end input, while the lack of complete destructive interference makes the blockade disappear for the other end input. The simultaneous nonreciprocal blockade is tunable by controlling the amplitude of the probe field and the driving field applied on the OPA. Our work paves the way to develop simultaneous nonreciprocal single-photon devices in multiple modes, which have potential applications in multichannel topological and chiral quantum technologies.

We investigate the behavior of relativistic electrons encountering a potential step through analogies with optical phenomena. By accounting for the conservation of the Dirac current, we elucidate that the Goos-Hänchen shift can be understood as a combination of two components: one arising from the current entering the transmission region and the other originating from the interference between the incident and reflected beams. This result has been proven to be consistent with findings obtained utilizing the stationary phase method. Moreover, we explore the transverse Imbert-Fedorov shift by applying both current conservation and total angular momentum conservation, revealing intriguing parallel to the spin Hall effect. Beyond enriching our comprehension of fundamental quantum phenomena, our findings have potential applications for designing and characterizing devices using Dirac and topological materials.

In a trapped spin-$1/2$ Bose-Einstein condensate with miscible interactions, a two-dimensional spin-orbit coupling can introduce an unconventional spatial separation between the two components. We reveal the physical mechanism of such a spin-orbit-coupling-induced phase separation. Detailed features of the phase separation are identified in a trapped Bose-Einstein condensate. We further analyze differences of phase separation in Rashba and anisotropic spin-orbit-coupled Bose gases. An adiabatic splitting dynamics is proposed as an application of the phase separation.

A generalized theory of Smith-Purcell radiation (SPR) from a two-dimensional (2D) array of subwavelength particles is presented. Unlike most theories of SPR, this one is valid in the prewave zone and the near zone. We have obtained a generalized dispersion relation that determines the points of concentration of radiation in the prewave zone rather than in the ordinary directions of propagation of the most intense radiation; in the wave zone, the dispersion relation transforms into the ordinary one. Analytically, we derived that it is the parabolic law that describes how the individual elements should be arranged for the most intense radiation to be achieved. Interestingly, for the ultrarelativistic limit, the parabolic law turns into the hyperbolic one. Therefore, to focus the radiation from ultrarelativistic electrons in the prewave zone, a hyperbolically arranged 2D grating should be used instead of a rectangular one. Our findings suggest that the radiation intensity at a point of focus in the prewave zone can be sensitive to numerically very small changes in the locations of the individual constituent elements of the array determining the topology—the way the array's elements are arranged. The theory is constructed for particles with arbitrary dielectric properties, including metal ones; numerical analysis has been performed for dielectric particles.

In this paper, we theoretically show the existence of a topological Berry's phase effect concerned with propagation of optical vortices (OVs) in multimode fibers. We study the transmission of OVs through a microcoil resonator (MCR) and show that for certain MCR parameters, by changing the degree of circular polarization of the incoming OV, one can continuously control the orbital angular momentum (OAM) of the outcoming optical field. We show that this effect is due to the combined action of the spin-orbit and orbit-orbit interactions that arise for light traveling along spatial trajectories. This property is potentially important for optical fiber OAM communications.

We consider how the reduced dynamics of an open quantum system coupled to an environment is realized in the Poincaré symmetry. The reduced dynamics is described by a dynamical map, which is a quantum channel (a completely positive and trace-preserving linear map) given by tracing out the environment from the total unitary evolution without initial correlations. We investigate the dynamical map invariant under the Poincaré transformations and discuss how the invariance constrains the form of the map. Based on the unitary representation theory of the Poincaré group, we develop a systematic way to construct the dynamical map with the Poincaré invariance. Using this method, we derive such a dynamical map for a spinless massive particle, and the conservation of the Poincaré generators is discussed. We then find the map with the Poincaré invariance and the four-momentum conservation. Further, we show that the conservation of the angular momentum and the boost operator makes the map of a spinless massive particle unitary.

Straightforward logical operations contrasting with complex state preparation are the hallmarks of the bosonic encoding proposed by Gottesman, Kitaev, and Preskill (GKP). The recently reported generation and error correction of GKP qubits in trapped ions and superconducting circuits thus holds great promise for the future of quantum computing architectures based on such encoded qubits. However, these experiments rely on error-syndrome detection via an auxiliary physical qubit, whose noise may propagate and corrupt the encoded GKP qubit. We propose a simple module composed of two oscillators and a physical qubit, operated with two experimentally accessible quantum gates and elementary feedback controls to implement an error-corrected GKP qubit protected from such propagating errors. In the idealized setting of periodic GKP states, we develop efficient numerical methods to optimize our protocol parameters and show that errors of the encoded qubit stemming from flips of the physical qubit and diffusion of the oscillators state in phase space may be exponentially suppressed as the noise strength over individual operations is decreased. Our approach circumvents the main roadblock towards fault-tolerant quantum computation with GKP qubits.

We demonstrate that, among all quantum teleportation protocols giving rise to the same average fidelity, those with aligned Bloch vectors between the input and output states exhibit the minimum average trace distance. This defines optimal protocols. Furthermore, we show that optimal protocols can be interpreted as the perfect quantum teleportation protocol under the action of correlated one-qubit channels. In particular, we focus on the deterministic case for which the final Bloch vector length is equal for all measurement outcomes. Within these protocols, there exists one type that corresponds to the action of uncorrelated channels: these are depolarizing channels. Thus, we established the optimal quantum teleportation protocol under a very common experimental noise.

We investigate whether it is possible to teleport the coherence of an unknown quantum state from Alice to Bob by communicating a smaller number of classical bits in comparison to what is required for teleporting an unknown quantum state. We find that we cannot achieve perfect teleportation of coherence with one bit of classical communication for an arbitrary qubit. However, we find that if the qubit is partially known, i.e., chosen from the equatorial and polar circles of the Bloch sphere, then teleportation of coherence is possible with the transfer of one cbit of information when we have maximally entangled states as a shared resource. In the case of the resource being a nonmaximally entangled state, we can teleport the coherence with a certain probability of success. In a general teleportation protocol for coherence, we derive a compact formula for the final state at Bob's laboratory in terms of the composition of the completely positive maps corresponding to the shared resource state and a joint positive operator-valued measure (POVM) performed by Alice on her qubit and the unknown state. Using this formula, we show that teleportation of the coherence of a partially known state with real matrix elements is perfectly possible with the help of a maximally entangled state as a resource. Furthermore, we explore the teleportation of coherence with Werner states and show that even when Werner states become separable, the amount of teleported coherence is nonzero, implying the possibility of teleportation of coherence without entanglement.

The field of space communications is the realm of communication technologies where diffraction and atmospheric effects, both of which contribute to loss and noise, become overriding. The pertinent questions here are how and at which rate information (secret keys) can be securely transferred (shared) among users under such supposedly severe circumstances. In the present work we study continuous-variable quantum key distribution in a measurement-device-independent configuration over free-space optical links. We assess the turbulence regime and provide a composable finite-size key rate analysis of the protocol for such links. We study both short-range, horizontal communication links as well as slant paths to, e.g., high-altitude platform station systems.

We present a theoretical study of elastic photon scattering by atomic targets. This process is of special interest since various channels from atomic and nuclear physics as well as quantum elctrodynamics contribute to it. In this work, we focus on Delbrück scattering which proceeds via production of virtual $e^{+}{e}^{-}$ pairs. In particular, we explore whether and how the Delbrück channel can be “seen” in present synchrotron experiments which employ strongly linearly polarized light in the energy range of a few hundred keV. In order to answer this question, detailed calculations have been performed for the scattering of 300- and 889.2-keV photons off helium-like tin ions. Based on these calculations, we argue that the Delbrück scattering for the energies below the threshold for $e^{+}{e}^{-}$ pair creation leads to a shift in the angular distribution and the polarization of the scattered photons which can be observed by state-of-the-art solid-state detectors.

This paper presents both experimental and theoretical investigations into the spectroscopy of dilute cesium (Cs) atoms within a solid argon (Ar) matrix at cryogenic temperatures. This system is relevant for matrix isolation spectroscopy and in particular for recently proposed methods for investigating phenomena that extend beyond the standard model of particle physics. We record absorption spectra at various deposition temperatures and examine the evolution of these spectra after deposition with respect to temperature changes. Taking advantage of Cs-Ar and Ar-Ar pairwise interaction potentials, we conduct a stability study of trapping sites, which indicates a preference for $T_d$ (tetrahedral, four vacancies) and $O_{\mathrm{h}}$ (cubic, six vacancies) symmetries. By implementing a mean-field analysis of the long-range Cs($6s,6p$)-Ar-Ar triple dipole interaction, combined with a temperature-dependent shift in zero-point energy, we propose effective Cs($6s,6p$)-Ar pairwise potentials. Upon integrating these pairwise potentials with spin-orbit coupling, we achieve a satisfactory agreement between the observed and simulated absorption line positions. The observed line broadening is reasonably well reproduced by a semiclassical thermal Monte Carlo approach based on Mulliken-type differences between excited and ground potential curves. Additionally, we develop a simple, first-order crystal-field theory featuring only six interaction mode coordinates. It uses the reflection approximation and incorporates quantized (phonon) normal modes. This produces a narrow triplet structure but not the observed amount of splitting.

We propose a scheme of deep laser cooling of ${}^{171}\mathrm{Yb}^{+}$, which is based on the effect of electromagnetically induced transparency (EIT) in a polychromatic field with three frequency components resonant with optical transitions of the ${}^{2}S_{1/2}\to {}^{2}P_{1/2}$ line. The deep cooling down to the ground motional state in a trap allows for a significant suppression of the second-order Doppler shift in the frequency standard. Moreover, in our scheme, there is no need to use a magnetic field, which is required for Doppler cooling of ${}^{171}\mathrm{Yb}^{+}$ in a field with a two frequency component. Cooling without the use of a magnetic field is important for the deep suppression of quadratic Zeeman shifts of clock transitions due to an uncontrolled residual magnetic field.

Air-water interface deformation has been used as a testbed to resolve the momentum of light debate (Abraham-Minkowski) inside the dielectric medium, but to date, experiments or theory which have resolved this debate precisely are lacking. Here, we present the experimental observation of both momenta in a single setup. The model employed in interpreting our experimental results suggests that the total momentum associated with light inside a dielectric is represented by the mean value of the Abraham and Minkowski momenta, which is $\frac{1}{2}(n+1/n)\hslash \omega /c$. We used a clean and relatively simple pump-probe-type setup to investigate the light momentum inside the dielectric medium. Using a pump beam, we generate a standing wave, formed due to an incident and reflected pump beam in a water drop, placed on a partially metallic coated prism. A weak red probe laser beam was used to detect the nanoscale height variation of the air-water interface, where natural evaporation enabled us to measure the radiation pressure variations (spacing between the node and antinodes, $\lambda /2n$) in the standing wave. Our results clearly depict the dependence of radiation pressure upon the phase of the incident and reflected light. We also performed numerical simulations in realistic experimental settings and found good agreement with the experimental results, and offer potential applications in microfluidics and optofluidics.