Bell nonlocality is the resource that enables device-independent quantum information processing tasks. It is revealed through the violation of so-called Bell inequalities, indicating that the observed correlations cannot be reproduced by any local hidden-variable model. While well explored in few-body settings, the question of which Bell inequalities are best suited for a given task remains quite open in the many-body scenario. One natural approach is to assign Bell inequalities to physical Hamiltonians, mapping their interaction graph to two-body, nearest-neighbor terms. Here, we investigate the effect of boundary conditions in a two-dimensional square lattice, which can induce different topologies in lattice systems. We find a relation between the induced topology and the Bell inequality's effectiveness in revealing nonlocal correlations. By using a combination of tropical algebra and tensor networks, we quantify their detection capacity for nonlocality. Our work can act as a guide to certify Bell nonlocality in many-qubit devices by choosing a suitable Hamiltonian and measuring its ground-state energy, a task that many quantum experiments are purposely built for.

Vortex $\gamma$ photons, which carry large intrinsic orbital angular momenta (OAM), have significant applications in nuclear physics, atomic physics, hadron physics, high-energy particle physics, and astrophysics, but their production remains unclear. In this work, we investigate the generation of such photons from nonlinear Compton scattering of circularly polarized monochromatic lasers on vortex electrons. We develop a quantum radiation theory for ultrarelativistic vortex electrons in lasers by using the harmonics expansion and spin eigenfunctions, which allows us to explore the kinematical characteristics, angular momentum transfer mechanisms, and formation conditions of vortex $\gamma$ photons. The multiphoton absorption of electrons enables the vortex $\gamma$ photons, with fixed polarizations and energies, to exist in mixed states comprised of multiple harmonics. Each harmonic represents a vortex eigenmode and has transverse momentum broadening due to transverse momenta of the vortex electrons. The large topological charges associated with vortex electrons offer the possibility for $\gamma$ photons to carry adjustable OAM quantum numbers from tens to thousands of units, even at moderate laser intensities. $\gamma$ photons with large OAM and transverse coherence length can assist in influencing quantum selection rules and extracting phase of the scattering amplitude in scattering processes.

We perform an experimental study of quantum discord on spin-orbit modes of an intense laser beam, analogous to different X states. These modes are prepared through the incoherent superposition of different intense laser beams, encoding a two-qubit system in both polarization and the first-order Hermite-Gaussian modes [as proposed in Phys. Rev. A 103, 022411 (2021)]. We characterize these modes by using all-optical tomography and calculated entropic quantum discord. Remarkably, even for high-fidelity mode reconstruction, the computation of quantum discord showed significant deviations from theoretical expectations. Then, we proposed a simple model based on noisy measurements that showed a small degradation of the fidelity of the output modes, giving rise to the strong deviation of the discord. Our results, both experimental data and the theoretical predictions, demonstrate good agreement.

Clock interferometry refers to the coherent splitting of a clock into two different paths and recombining in a way that reveals the proper-time difference between them. Unlike the comparison of two separate clocks, this approach allows testing how nonflat space-time influences quantum coherence. Atomic clocks are currently the most accurate timekeeping devices. Here we propose using optical tweezers to implement clock interferometry. Our proposed clock interferometer employs an alkaline-earth-like atom held in an optical trap at the magic wavelength. Through a combination of adiabatic, tweezer-based splitting and recombining schemes and a modified Ramsey sequence of the clock states, we achieve linear sensitivity to the gravitational time dilation. Moreover, the measurement of the time dilation is insensitive to relative fluctuations in the intensity of the tweezer beams. We analyze the tweezer clock interferometer and show that it is feasible with current technological capabilities. The proposed interferometer could test the effect of gravitational redshift on quantum coherence and implement the quantum twin paradox.

The ${}^{61}\mathrm{Ni}^{12+}$ ion is a promising candidate for a novel ultraprecise optical clock, owing to its two clock transitions with narrow natural linewidth between levels with low degeneracy. Using the multiconfiguration Dirac-Hartree-Fock (MCDHF) method, we calculated the hyperfine-induced Landé $g$ factors for the ${}^{3}P_{0}F=3/2$ and ${}^{3}P_{1,2}F=1/2$ states, as well as the hyperfine-induced electronic quadrupole moment of the ${}^{3}P_{0}F=3/2$ state in the ${}^{61}\mathrm{Ni}^{12+}$ ion. The effects of the electron correlations and relativistic effects on the concerned atomic parameters were evaluated in detail based on the active space approach. It is found that the valence-valence and core-valence electron correlations have the dominant influence on the atomic parameters concerned, while the core-core and the higher-order correlations are non-negligible for evaluating the uncertainties. Additionally, the relative quadratic Zeeman shifts, the relative electric quadrupole shift, and the corrections arising from the hyperfine-induced effect were evaluated. It is demonstrated that the hyperfine interaction strictly eliminates the electric quadrupole shift of the M1 transition $3s^{2}3p^4{}^{3}P_1-{{}^{3}P}_2$, and significantly reduces the electric quadrupole frequency shift for the E2 transition $3s^{2}3p^4{}^{3}P_0-{{}^{3}P}_2$ by a factor of $4\times {10}^{-6}$. The hyperfine-induced corrections to the relative quadratic Zeeman shifts are factors of $2\times {10}^{-4}$ and $6\times {10}^{-7}$ for the M1 and E2 transition, respectively. It means that the hyperfine-induced effects need to be identified precisely for an ultraprecise ${}^{61}\mathrm{Ni}^{12+}$ optical clock aiming for a precision better than ${10}^{-19}$ when the magnetic field exceeds $1\mu \mathrm{T}$ or the gradient of the environmental electric field exceeds ${10}^8\mathrm{V}/{\mathrm{m}}^2$.

The collision dynamics of ${\mathrm{H}}^{+}$ with ${\mathrm{O}}_2$ have been carried out at high collision energies $E_{\mathrm{c}.\mathrm{m}.}=0.096$–4.848 keV ($E_{\mathrm{lab}}=0.1$–5.0 keV) within the time-dependent semiclassical straight line trajectory approach. For high collision energy, ions (projectiles) are expected to follow a straight line trajectory, and the collision time scales allow us to consider sudden approximations for the rovibrational motions. Therefore, the dynamical calculations have been performed using rigid-rotor potential-energy surfaces. The ab initio adiabatic potential-energy surfaces exhibit nonadiabatic interactions between the entrance and charge-transfer (CT) channels. Dynamical attributes such as differential cross sections and integral cross sections for the electronically elastic and the CT processes have been calculated using the quasidiabatic surfaces and potential couplings and have been compared with the available data from the scattering experiments.

The well-known sub-Doppler polarization gradient cooling in a type-I transition ($F_{\mathrm{e}}=F_{\mathrm{g}}+1$) is caused by red-detuned lasers. On the other hand, in a type-II transition ($F_{\mathrm{e}}\le F_{\mathrm{g}}$), sub-Doppler cooling takes place through blue-detuned lasers. This opposite behavior for the two types of transitions is due to spontaneously transferred coherence (STC). In the absence of STC, both types of transitions show blue-detuned cooling. In this paper, we experimentally and theoretically demonstrate blue-detuned cooling for both types of transitions in ${}^{87}\mathrm{Rb}$. For completeness, we compare the temperatures in various configurations.

We consider a model of interacting Bose-Einstein condensates on small Sierpiński gaskets. We study eigenstates which are characterized by cyclic supercurrents for each triangular plaquette (loop states). For noninteracting systems we find at least three classes of loop eigenmodes: standard, chaotic, and periodic. Standard modes are those inherited from the basic three-site ring of condensates with phase differences locked to $2\pi /3$. Standard modes become unstable in the interacting system but only when the interaction exceeds a certain critical value $u_c$. Chaotic modes are characterized by very different circular currents per plaquette, so the usual symmetry of loop currents is broken. Circular supercurrents associated with chaotic modes become chaotic for any finite interaction, signaling the loss of coherence between the condensates. Periodic modes are described by alternating populations and two different phase differences. The modes are self-similar and are present in all generations of Sierpiński gasket. When the interaction is included, the circular current of such a mode becomes periodic in time with the amplitude growing linearly with the interaction. Above a critical interaction the amplitude saturates, signaling a transition to a macroscopic self-trapping state originally known from a usual Bose-Josephson junction. We perform a systematic analysis of this rich physics.

Confinement and deconfinement, captivating attributes of high-energy elementary particles, have recently garnered wide attention in quantum simulations based on cold atoms. However, partial confinement, an intermediate state between the confinement and deconfinement, remains underexplored. The partial confinement encapsulates the phenomenon that the confining behavior of charged particles is contingent upon their relative positions. In this paper we demonstrate that the spin-1 quantum-link model provides an excellent platform for exploring partial confinement. We conduct a comprehensive investigation of the physics emerging from partial confinement in the context of both equilibrium and nonequilibrium dynamics. Potential experimental setups using cold atoms are also discussed. Our work offers a simple and feasible routine for the study of confinement-related physics in the state-of-the-art artificial quantum systems subject to gauge symmetries.

We study the Josephson junction made of a one-dimensional ultradilute quantum liquid in a double-well potential. We analyze the dynamics as a function of the interaction strength and compare the results to the standard bosonic Josephson junction. It is found that the beyond-mean-field effects alter the dynamics, particularly in the regime, where the beyond-mean-field corrections dominate over the residual mean-field interaction. In that case, we observe nonlinear dynamics describing localization revivals instead of regular Josephson oscillations. In the regime where the ultradilute quantum liquids perform the regular Josephson oscillations, their frequency is also significantly changed in comparison with the regular Josephson junction. These results provide experimental characteristics of the ultradilute quantum liquids that contrast with the Josephson oscillations of a regular Bose-Einstein condensate.

We formulate the induced potential in a finite-temperature cold-atomic medium between two heavy impurities, or polarons, which is shown to be complex valued in general. The imaginary part of the complex-valued potential describes a decoherence effect, and thus the resulting Schrödinger equation for the two polarons acquires a non-Hermitian term. We apply the developed formulation to two representative cases of polarons interacting with medium particles through the $s$-wave contact interaction: (i) the normal phase of single-component (i.e., spin-polarized) fermions using the fermionic field theory and (ii) a superfluid phase using the superfluid effective field theory, which is valid either for a Bose-Einstein condensate (BEC) of a single-component Bose gas or for the BEC-BCS crossover in two-component fermions at a low-energy regime. Computing the leading-order term, the imaginary part of the potential in both cases is found to show a universal $r^{-2}$ behavior at long distance. We propose three experimental ways to observe the effects of the universal imaginary potential in cold atoms.

We study three-body systems in a mass-imbalanced, two-component, cold-atom mixture, and we investigate the three-body parameter of their Efimov states for both bosonic and fermionic systems, with a major focus on the Er-Er-Li Efimov states. For a system interacting solely via van der Waals interactions, the van der Waals universality of the three-body parameter is analytically derived using the quantum defect theory. With the addition of a perturbative dipole interaction between the heavy atoms, the three-body parameters of the bosonic and fermionic Efimov states are found to behave differently. When the dipole interaction is as strong as the van der Waals interaction, corresponding to realistic Er-Er-Li Efimov states, we show that the van der Waals universality persists once the effects of the nonperturbative dipole interaction are renormalized into the $s$-wave and $p$-wave scattering parameters between the heavy atoms. For a dipole interaction much stronger than the van der Waals interaction, we find that the universality of the Efimov states can be alternatively characterized by a quasi-one-dimensional scattering parameter due to a strong anisotropic deformation of the Efimov wave functions. Our work thus clarifies the interplay of isotropic and anisotropic forces in the universality of the Efimov states. Based on the renormalized van der Waals universality, the three-body parameter is estimated for specific isotopes of Er-Li cold-atom mixtures.

Programmable photonic integrated circuits are an emerging technology that amalgamates photonics and electronics, paving the way for light-based information processing at high speeds and low power consumption. Considering its wide range of applications as one of the most fundamental mathematical operations, there has been considerable interest in developing reliably implementable programmable circuit architectures that perform matrix-vector multiplication. Recently, it was shown that discrete unitary operations can be parameterized by interlacing fixed operators with diagonal phase parameters realized with phase shifter arrays. We show that these decompositions are a special case of a broader class of factorizations that enable parametrization of arbitrary complex matrices. The proposed representation of an $N\times N$ matrix is given by $N+1$ amplitude-and-phase-modulation layers interlaced with a fixed unitary layer that can be implemented, for example, via a coupled waveguide array. Thus, we introduce an architecture for physically implementing discrete linear operations, enabling the development of novel families of programmable photonic circuits for on-chip analog information processing.

We show how the area theorem is applicable to the analytical description of the nonlinear interaction of surface plasmon modes with resonant two-level atoms. A closed analytical solution is obtained and analyzed for a dielectric-negative index metamaterial interface, which shows that surface plasmons can form long-propagating $2\pi$ pulses when interacting with an optically dense two-level atomic ensemble. The possible applications of the surface pulse area theorem and the conditions for the detection of $2\pi$ surface plasmon pulses are discussed.

Several years ago, Guo et al. [Phys. Rev. Lett. 103, 093902 (2009)] demonstrated a counterintuitive phenomenon wherein the transmission of classical light through a coupled pair of waveguides is enhanced as the level of loss in one waveguide surpasses a critical threshold. In this paper, we employ the Heisenberg-Langevin formalism to explore a quantum perspective of this phenomenon. In the case where light is incident in both waveguides, a generally nonzero interference term in the presence of loss appears. Our analysis reveals that loss-induced transparency can manifest at the single-photon level for both separable and entangled photonic states.

The coexistence of coherent perfect absorber and laser or amplifier (CPAL) point is a peculiar spectral singularity associated with the scattering matrices of non-Hermitian systems. While the potential of CPAL systems for sensing application has been highlighted recently, the extreme sensitivity of parity-time ($\mathrm{P}\mathrm{T}$)-symmetric CPAL devices to the input signal deviations has so far impeded their practical utilization. Here we explore a strategy for implementing CPAL circuits by exploiting another type of non-Hermitian symmetry, namely anti-$\mathrm{P}\mathrm{T}$ ($\text{A}\mathrm{P}\mathrm{T}$) symmetry. We demonstrate that the condition for building CPAL in our proposed $\text{A}\mathrm{P}\mathrm{T}$-symmetric electronic circuits additionally requires parity symmetry, which simplifies the circuit design and implementation. Additionally, we show that this newly proposed structure is 1.85 times more robust compared to previous CPAL devices studied in the literature.

Quantum-enhanced cavity optomechanical (COM) sensors, utilizing quantum resources, have become powerful tools for precisely measuring a wide range of physical quantities, from gravitational waves to accelerations. However, quantum-enhanced COM torque sensing remains an area of exploration. In this work, we show that utilizing quantum squeezing can suppress the quantum noise of a COM torque sensor below the standard quantum limit. We theoretically predict that our COM torque sensor can achieve an approximately 20-dB quantum advantage by optimizing the homodyne detection. Our approach is compatible with available engineering techniques of advanced COM sensors, with potential applications ranging from nanoscale magnetism to the quantum geometric phase.