We investigate certain invariance properties of quantum fluids subject to a nonlinear gauge potential. In particular, we derive the covariant transformation laws for the nonlinear potentials under a space-time Galilean boost and consider $U\left(1\right)$ gauge transformations. We find that the hydrodynamic canonical field equations are form-invariant in the case of external gauge functions $\chi (\mathbf{r},t)$, but not for nonlinear gauge functionals $\chi \left[\rho \right]$. Hence, nonlinear gauge potentials are nontrivial potentials which may not be “gauged-away.” Notably, for a superfluid in dimension $d=1$, attempting to do so generates the gauge pressure of the fluid in the Hamiltonian density. Furthermore, we investigate how the field equations transform under arbitrary Galilean transformations. We find that the immediate lack of Galilean covariance is restored under a suitably chosen transformation rule set for the potentials, which is identical in form to that of a Schrödinger particle coupled to external scalar and vector potentials.

Square-root topological insulators are recently proposed intriguing topological insulators, where the topologically nontrivial nature of Bloch wave functions is inherited from the square of the Hamiltonian. In this paper, we propose that higher-order topological insulators can also have their square-root descendants, which we term square-root higher-order topological insulators. There, emergence of in-gap corner states is inherited from the squared Hamiltonian which hosts higher-order topology. As an example of such systems, we investigate the tight-binding model on a decorated honeycomb lattice, whose squared Hamiltonian includes a breathing kagome-lattice model, a well-known example of higher-order topological insulators. We show that the in-gap corner states appear at finite energies, which coincides with the nontrivial bulk polarization. We further show that the existence of in-gap corner states results in characteristic single-particle dynamics; namely, setting the initial state to be localized at the corner, the particle stays at the corner even after a long time. Such characteristic dynamics may experimentally be detectable in photonic crystals.

We propose a framework to study the topological properties of an optical bound state in the continuum (BIC). Here, the formation of a BIC has been shown using the interaction between proximity resonances undergoing avoided resonance crossing in a simple fabrication, feasible gain-loss assisted microcavity. Similar to a Friedrich-Wingten (FW)-type BIC, the formation of an ultrahigh-quality ($Q$) mode due to the precise destructive interference between resonances has been reported. The enhancement in the $Q$ factor was found to be more than four orders of magnitude greater than the other participating resonances. Furthermore, multiple such FW-type high-$Q$ operating points from the same set of proximity resonances have been identified. Upon further inspection, we report the formation of a special-BIC line in the system parameter space connecting the locations of these operating points. Further, we closely investigate the light dynamics of systems near single and multiple quasi-BICs. Aiming to develop a scheme to enhance the performance of optical sensing in a microcavity, we study the sensitivity of transmission coefficients and quality factor to sense even ultrasmall perturbations in the system configuration. Our proposed scheme would open up a vast potential for an enhanced desired optical response in enhanced nonlinear applications, low-threshold nano- and microlasers, and device-level sensors.

Optimizing scattering is the most sought-after goal in wave transport through photonic structures, which opens up many physical processes with prospective applications. Here, we uncover the appearance of a tunable frequency gap with negative anisotropic scattering ($g$) values in the visible wavelength range using photonic structures with short-range order. A scattering model is devised based on the structural morphology of samples to explain the wavelength-dependent frequency gap and the $g$ values. We show complete agreement between the experiment and theory, which is supported by the structure factor calculations. Specifically, we find a $g$ value of −0.7 ± 0.2 at the frequency gap in accordance with theoretical calculations. The study puts forward an amenable approach for tailoring the scattering anisotropy values using photonic structure with short-range order and thus mediates an inclination toward the localization.

We study analytically and numerically the properties of the Jaynes-Cummings model under monochromatic driving. The analytical results allow us to understand the regime of two branches of multiphoton excitation in the case of close resonance between resonator and driven frequencies. The rotating wave approximation allows us to reduce the description of the original driven model to an effective Jaynes-Cummings model with strong coupling between photons and qubit. The analytical results are in good agreement with the numerical ones even if there are certain deviations between the theory and numerics in the close vicinity of the resonance. We argue that the rich properties of the driven Jaynes-Cummings model represent a new area for experimental investigations with superconducting qubits and other systems.

We present experimental data on adiabatically driven frequency conversion with ultrashort (picosecond) laser pulses towards the vacuum-ultraviolet regime, enhanced by coherent population return and preparation of maximal atomic coherences. We generate the sum frequency of an intense pump pulse and a probe pulse via a two-photon resonance in xenon. When we slightly detune the pump laser from two-photon resonance, the atomic populations are adiabatically driven forth and back from the atomic ground state to an excited state. This coherent population return (CPR) prepares the medium in a transient state of maximal atomic coherence, which enhances frequency mixing with a probe laser. We thoroughly study the variation of CPR-enhanced frequency conversion with experimental parameters and compare CPR with conventional, resonantly enhanced frequency conversion. In particular, we investigate the temporal evolution of the atomic coherence and the effect of inevitable ac Stark shifts. We find that ac Stark shifts induce an asymmetry in the spectral characteristics of CPR, which nevertheless permits enhanced frequency conversion. For the case of resonantly enhanced frequency conversion, we show that the atomic coherences are maintained for several tens of picoseconds after the pump pulse, which permits time-delayed frequency conversion. Moreover, we analyze the pressure dependence of the atomic coherence' lifetime and observe a free induction decay at the two-photon coherence with the generation of a second harmonic field. These findings shall push applications of adiabatic light-matter interactions also to the regime of nonlinear optics, driven by intense laser pulses.

In this work, robustness of controlled density of optical states in doubly driven artificial atoms is studied under phonon dissipation. By using both perturbative and polaron approaches, we investigate the influence of carrier-phonon interactions on the emission properties of a two-level solid-state emitter, simultaneously coupled to two intense distinguishable lasers. Phonon decoherence effects on the main features of the emission spectra are found to be modest up to neon boiling temperatures ($\sim 30$ K), as compared with photon generation at the Fourier transform limit obtained in the absence of lattice vibrations (zero temperature). These results show that optical switching and photonic modulation by means of double dressing do not require ultralow temperatures for implementation, thus boosting their potential technological applications.

The development of a quantum network relies on the advances of hybrid systems which include ground-to-ground communication. However, the atmospheric turbulence of the environment poses a severe challenge to the optical quantum link. In this paper, we outline a theoretical and experimental investigation of the influence of atmospheric turbulence on the coincidence detection of the entangled photon pairs using a fully and partially spatially coherent pump beam. A spatial light modulator is encoded to mimic the atmospheric turbulence strength based on the Kolmogorov model of turbulence. The results show that the photon pairs generated using a partially spatially coherent pump are more robust towards varying atmospheric turbulence strengths than the photon pairs produced by a fully spatially coherent pump beam.

The paper “Physics without determinism: Alternative interpretations of classical physics” [Phys. Rev. A 100, 062107 (2019)] defines finite information quantities (FIQ). A FIQ expresses the available information about the value of a physical quantity. We show that a change in the measurement unit does not preserve the information carried by a FIQ and therefore that the definition provided in the paper is not complete.

In this short note we reply to a Comment by Callegaro et al. [Callegaro, Pennecchi, and Bich, Phys. Rev. A 102, 036201 (2020)] that points out some weakness of the model of indeterministic physics that we proposed in our work [Del Santo and Gisin, Phys. Rev. A 100, 062107 (2019)], based on what we named “finite information quantities” (FIQs). While we acknowledge the merit of their criticism, we maintain that it applies only to a concrete example that we discussed in our work, whereas the main concept of FIQ remains valid and suitable for describing indeterministic physical models. We hint at a more sophisticated way to define FIQs which, taking inspiration from intuitionistic mathematics, would allow the criticisms in the preceding Comment to be overcome.

We show that exciton-polariton condensates may exhibit a fundamental, self-localized nonlinear excitation in quantum hydrodynamical systems, which takes the form of a dark ring-shaped breather. We predict that these structures form spontaneously and remain stable under a combination of uniform resonant and nonresonant forcing. We study single-ring dynamics, ring interactions, and ring turbulence, and explain how direct experimental observations might be made. We discuss the statistics of ring formation and propose an experimental scheme by which these structures may be exploited to study the smooth crossover between equilibrium and nonequilibrium critical phase transitions. The observation of a breathing-ring soliton would represent a fundamental breathing soliton within the broad field of quantum hydrodynamics.

Data representation is crucial for the success of machine-learning models. In the context of quantum machine learning with near-term quantum computers, equally important considerations of how to efficiently input (encode) data and effectively deal with noise arise. In this paper, we study data encodings for binary quantum classification and investigate their properties both with and without noise. For the common classifier we consider, we show that encodings determine the classes of learnable decision boundaries as well as the set of points which retain the same classification in the presence of noise. After defining the notion of a robust data encoding, we prove several results on robustness for different channels, discuss the existence of robust encodings, and prove a lower bound on the number of robust points in terms of fidelities between noisy and noiseless states. Numerical results for several example implementations are provided to reinforce our findings.

We study quantum entanglement in half-filled Hubbard dimers at finite temperatures under an external magnetic field. Due to the itinerant nature of the electrons and their fundamental indistinguishability, we employ a site-based evaluation of entanglement via the concurrence in three distinct sectors of the Hilbert space, namely the charge, zero-, and single-spin subspaces. At zero temperature these measures can be combined to produce an accurate estimate of the entanglement entropy. For finite temperatures we show that those concurrences display reentrant behavior upon varying the magnetic field. Furthermore, we unveil that charge and spin quantum correlations are quite distinctly degraded by thermal fluctuations.

Dynamical decoupling provides an effective method to protect nonadiabatic geometric quantum computation against environment-induced decoherence. Although some dynamical-decoupling-protected nonadiabatic geometric gates were proposed, they need to combine some other dynamical gates to perform universal quantum computation. In this paper we propose a scheme to realize a universal set of dynamical-decoupling-protected nonadiabatic geometric gates, which not only possesses the resilience to environment-induced decoherence but also allows us to perform universal quantum computation all by geometric operations.

The photoexcitation and photodissociation of diatomic molecules by intense pulse lasers has been the subject of extensive investigations over the past decades. However, the usually employed theoretical framework neglects the coupling between the molecular rotational angular momentum ($\mathbf{R}$) and the angular momentum of the electrons projected onto the molecular axis $\mathrm{\Omega }=\mathrm{\Lambda }+\mathrm{\Sigma }$, which results in the known $\mathrm{\Lambda }$-doubling phenomenon in high-resolution electronic spectra of diatomic molecules. While neglecting this coupling is an excellent approximation in the weak-field or perturbative regime owing to the large mass difference between the rotating atoms and the electrons, the approximation breaks down for intense laser pulses because of the repeated Rabi cycling of the electronic transitions, which can have a significant effect on the rotational degrees of freedom of the molecule. By correcting the transition dipole matrix elements and introducing angular basis sets based on Wigner D functions, the conventional theoretical treatment is generalized to a universal description valid for both the weak- and strong-field regimes. The theoretical treatment developed here is applied to the $|{}^1\mathrm{\Sigma }\rangle$ to $|{}^1\mathrm{\Pi }\rangle$ transitions in diatomic systems. Our results reveal that, for field intensities resulting in about one Rabi cycling for extreme ultraviolet or x-ray transitions, the theoretical predictions by the conventional theoretical frame need to be corrected when considering observables such as the molecular alignment and the angular distribution of the photofragments.

Weyl points, synthetic magnetic monopoles in the three-dimensional momentum space, are the key features of topological Weyl semimetals. The observation of Weyl points in ultracold atomic gases usually relies on the realization of high-dimensional spin-orbit coupling (SOC) for two pseudospin states (i.e., spin 1/2), which requires complex laser configurations and precise control of laser parameters, thus, has not been realized in experiment. Here, we propose that robust Weyl points can be realized using one-dimensional triple-well superlattices (spin-1/three-band systems) with two-dimensional transverse SOC achieved by Raman-assisted tunnelings. The presence of the third band is responsible to the robustness of the Weyl points against system parameters (e.g., Raman laser polarization, phase, incident angle, etc.). Different from a spin-1/2 system, the nontrivial topology of Weyl points in such spin-1 system is characterized by both spin vector and tensor textures, which can be probed using momentum-resolved Rabi spectroscopy. Our proposal provides a simple yet powerful platform for exploring Weyl physics and related high-dimensional topological phenomena using high pseudospin ultracold atoms.

The realization of antiferromagnetic (AF) correlations in ultracold fermionic atoms on an optical lattice is a significant achievement. Experiments have been carried out in one, two, and three dimensions, and have also studied anisotropic configurations with stronger tunneling in some lattice directions. Such anisotropy is relevant to the physics of cuprate superconductors and other strongly correlated materials. Moreover, this anisotropy might be harnessed to enhance AF order. Here we numerically investigate, using the determinant quantum Monte Carlo method, a simple realization of anisotropy in the three-dimensional (3D) Hubbard model in which the tunneling between planes, $t_{\perp }$, is unequal to the intraplane tunneling $t$. This model interpolates between the three-dimensional isotropic ($t_{\perp }=t$) and two-dimensional (2D; $t_{\perp }=0$) systems. We show that at fixed interaction strength to tunneling ratio ($U/t$), anisotropy can enhance the magnetic structure factor relative to both 2D and 3D results. However, this enhancement occurs at interaction strengths below those for which the Néel temperature $T_{\mathrm{N}\acute{\mathrm{e}}\mathrm{el}}$ is largest, in such a way that the structure factor cannot be made to exceed its value in isotropic 3D systems at the optimal $U/t$. We characterize the 2D-3D crossover in terms of the magnetic structure factor, real space spin correlations, number of doubly occupied sites, and thermodynamic observables. An interesting implication of our results stems from the entropy's dependence on anisotropy. As the system evolves from 3D to 2D, the entropy at a fixed temperature increases. Correspondingly, at fixed entropy, the temperature will decrease going from 3D to 2D. This suggests a cooling protocol in which the dimensionality is adiabatically changed from 3D to 2D.

Fundamental entanglement related challenges have prevented quantum-interference-based control (i.e., coherent control) of collisional cross sections from being implemented in the laboratory. Here, differential cross sections for reactive scattering at low temperatures are shown to provide a unique opportunity to display such interference-based control by forming coherent superpositions of degenerate rotational states of reactant molecules $|jm\rangle$ with different $m$. In particular, we identify and quantify a unique signature of coherent control in reactive scattering with applications to $\mathrm{F}+ {\mathrm{H}}_2\to$ H $+\mathrm{HF}$ and HF $+$ D $\leftarrow$ F $+\mathrm{HD} \to \mathrm{HD} +$ F at $11\mathrm{K}$. Control is shown to be extensive.

Nonlocality plays a fundamental role in quantum information science. Recently, it has been theoretically predicted and experimentally demonstrated that the nonlocality of an entangled pair may be shared among multiple observers using weak measurements with moderate strength. Here we devise an optimal protocol of nonlocality sharing among three observers and show experimentally that nonlocality sharing may be also achieved using weak measurements with near-maximum strength. Our result sheds light on the interplay between nonlocality and quantum measurements and may find applications in quantum steering, unbounded randomness certification, and quantum communication network.

Wigner functions provide a way to do quantum physics using quasiprobabilities, that is, “probability” distributions that can go negative. Informationally complete positive operator-valued measures, a much younger subject than phase-space formulations of quantum mechanics, are less familiar but provide wholly probabilistic representations of quantum theory. In this paper, we show that the Born rule links these two classes of structure and discuss the art of interconverting between them. In particular, we demonstrate that the operator bases corresponding to minimal discrete Wigner functions (Wigner bases) are orthogonalizations of minimal informationally complete measurements (MICs). By not imposing a particular discrete phase-space structure at the outset, we push Wigner functions to their limits in a suitably quantified sense, revealing an extremality condition satisfied by the symmetric informationally complete measurements (SICs). This contributes another member to the set of optimality criteria that SICs are known to satisfy. Finally, we speculate that astute choices of MICs from the orthogonalization preimages of Wigner bases may in general give quantum measurements that are conceptually illuminating for the quasiprobability representations defined by those Wigner bases.