A collective spin model is used to describe two species of mutually interacting ultracold bosonic atoms confined to a toroidal trap. The system is modeled by a Hamiltonian that can be split into two components, a linear part and a quadratic part, which may be controlled independently. We show the linear component is an analog of a Zeeman Hamiltonian, and the quadratic component presents a macroscopic simulator for spin-orbit and hyperfine interactions. We determine a complete set of commuting observables for both the linear and quadratic Hamiltonians, and derive analytical expressions for their respective spectra and density of states. We determine the conditions for generating maximal entanglement between the two species of atoms with a view to applications involving quantum correlations among spin degrees of freedom, such as in the area of quantum information.

We examine the low-energy excitations of a dilute supersolid state of matter with a one-dimensional crystal structure. A hydrodynamic description is developed based on a Lagrangian, incorporating generalized elastic parameters derived from ground state calculations. The predictions of the hydrodynamic theory are validated against solutions of the Bogoliubov–de Gennes equations, by comparing the speeds of sound, density fluctuations, and phase fluctuations of the two gapless bands. Our results are presented for two distinct supersolid models: a dipolar Bose-Einstein condensate in an infinite tube and a dilute Bose-Einstein condensate of atoms with soft-core interactions. Characteristic energy scales are identified, highlighting that these two models approximately realize the bulk incompressible and rigid lattice supersolid limits.

The present paper discusses the application of certain special Gaussian beams that, thanks to some polynomial prefactors, have uniquely designed holes in the irradiance. Such holes, or rather tubes, can constitute potential valleys for negatively polarizable particles, providing the possibility of guiding several objects of that kind, each along its own trajectory. The mechanism of creating these holes by interference of Gaussian beams which exhibit orbital angular momentum is discussed, and then the trajectories of particles moving in such a wave are numerically calculated. As it turns out, these particles, performing transverse oscillations, follow the designed tunnels of low irradiance. On the contrary, for particles with positive polarizability these areas are inaccessible.

Analysis of extreme-value statistics of stochastic laser pulses suggests a closed-form, quantitative criterion of self-focusing avoidance. We present an analytical solution for the excess kurtosis of the statistics of nonlinear-optical processes, which is shown to be a rapidly growing function of the nonlinearity order, thus indicating a physically significant redistribution of statistical weight within the probability distribution of the respective nonlinear readouts from its central part to its tails. Unlike deterministic self-focusing, whose criterion is expressed in terms of a well-defined self-focusing threshold $P_{\mathrm{cr}}$, its stochastic counterpart is a probabilistic process whose combined probability for a sample of $N$ laser pulses builds up as a function of $N$, leading to $N$-dependent self-focusing avoidance criteria. Specifically, for $N$ ≫ 1 laser pulses with a signal-to-noise ratio $a$, the criterion of self-focusing avoidance is shown to shift as $a^2{P}_{\mathrm{cr}}/\left(2lnN\right)$. Instead of dealing with a question as to how to completely avoid self-focusing, stochastic analysis has to deal with a question of how to effectively manage the self-focusing probability over a finite sample of laser shots. The occurrence of self-focusing in stochastic nonlinear optics is thus not a question of $if$, but a question of when.

We utilize click-counting theory for the reconstruction of photon statistics, employing an analytic pseudoinversion method. A reconfigurable time-bin multiplexing, click-counting detector is experimentally implemented, allowing for altering the photon-number resolution. A detector tomography is carried out, yielding vital measurement features, such as quantum efficiencies and cross-talk rates. We gauge the success of the pseudoinversion by applying the Mandel and binomial parameters, resulting in an additional interpretation of these parameters for the discrimination of distinct quantum statistics. For coherent states, which lie at the classical-nonclassical boundary, both parameters are highly sensitive measures, probing the kind of statistics and the reconstruction performance. In addition, we apply a loss deconvolution technique to account for detection losses.

Photon blockade in weak nonlinear regime is an exciting and promising subject that has been extensively studied in the steady state. However, how to achieve dynamic blockade in a single bosonic mode with weak nonlinearity using only pulsed driving field remains unexplored. Here, we propose to optimize the parameters of the pulsed driving field to achieve dynamic blockade in a single bosonic mode with weak nonlinearity via the particle swarm optimization (PSO) algorithm. We demonstrate that both Gaussian and rectangular pulses can be used to generate dynamic blockade in a single bosonic mode with weak nonlinearity. Based on the Fourier series expansions of the pulsed driving field, we identify that there are many paths for two-photon excitation in the bosonic mode, even only driven by pulsed field, and the dynamic blockade in a weak nonlinear regime is induced by the destructive interference between them. Our paper not only highlights the effectiveness of PSO algorithm in optimizing dynamical blockade but also opens a probable way to optimize the parameters for other quantum effects, such as quantum entanglement and quantum squeezing.

We present a frequency-filtering method for measuring and calculating frequency-filtered photon correlations. This method is a cavity-based system we call the multimode array filter, which consists of an array of tunable single-mode cavities that are equally spaced in frequency. By introducing a mode-dependent phase modulation, we produce a near rectangular frequency response, allowing us to increase the filter bandwidth—and thus the temporal response—without sacrificing frequency isolation. We model the frequency filtering using a cascaded quantum open systems approach which completely neglects any back-action of the filter onto the source system. This allows us to derive a closed set of operator moment equations for source and filter system operators, thus providing an extremely efficient method to calculate frequency-filtered first- and second-order correlation functions. We demonstrate this filtering method by applying it to a resonantly driven two-level atom. We present examples of frequency-filtered power spectra to demonstrate the improved frequency isolation of the multimode array filter over the single-mode filter. We then present results for the single-mode and multimode array filtered second-order auto- and cross-correlation functions. These are compared against expressions derived in the secular approximation. The improved frequency isolation of the multimode array filter allows us to investigate new regimes of frequency-filtered photon correlations, such as two-photon leapfrog processes, and the effect of vanishing bandwidth on filtered autocorrelation functions.

Entanglement potentials (EPs) enable the characterization and quantification of the nonclassicality of single-mode optical fields by measuring the entanglement generated through beam splitting. We experimentally generated single-photon states and tomographically reconstructed the corresponding two-qubit states to determine EPs defined via popular two-qubit measures of entanglement. These include the potentials for the relative entropy of entanglement (REEP), concurrence, and negativity. Among our experimental states, we found those that are very close (at least for some ranges of parameters) to the theoretical upper and lower bounds on relative EPs (or relative nonclassicality), i.e., when one EP is maximized or minimized for a given value of another EP. We experimentally confirmed the counterintuitive theoretical result of Miranowicz et al. [Phys. Rev. A 92, 062314 (2015)] that the relative nonclassicality (specifically, the negativity potential for given values of the REEP) of single-photon states can be increased by dissipation.

The quantum switch has seen multiple applications in quantum information and thermodynamical tasks. As it is constructed by placing two quantum channels in a coherent superposition of alternating causal orders, a composition known as indefinite causal order, these enhancements are often attributed to this indefinite causality and coherent superposition. However, as recent works have shown that the quantum switch also features non-Markovian effects that can contribute to the enhancement of communication capacities and work extraction, we attempt to show in this work that these non-Markovian effects can also enhance heat extraction tasks. In particular, we compare the quantum switch to the superposition of independent channels where two quantum channels are placed in a superposition, which have no non-Markovian effects, and show that the quantum switch can only outperform the superposition of independent channels in the prethermalization regimes, which also depends on the presence and amount of non-Markovianity. Our work reveals that positive heat extraction is still possible even when the working body is at a higher temperature than the interacting baths, allowing us to construct a refrigeration cycle utilizing this feature.

The no-masking theorem for quantum information proves that it is impossible to encode an arbitrary input state into a larger bipartite entangled state such that the full information is stored in the correlation but the individual subsystems have no information about the input state. Here, we ask the question: Is it possible to mask an observable such that the information about the observable is available in the joint system, but individual subsystems reveal nothing about the imprints of the observable? This generalizes the notion of masking to observables. We show that a universal unitary that can mask an arbitrary observable in any dimension does not exist. For a qubit system, we show that the masking operation for a given observable is locally unitarily connected to the swap operation. This suggests a conservation law for information content of observables that goes beyond the conservation laws under symmetry operations. Furthermore, we prove that the unconditional no-bit commitment result follows from the no-masking theorem for observables. Our results can have important applications in quantum information and quantum communication where we encode information not in states but in observables.

Two important classes of quantum channels, namely, the Werner-Holevo and the Landau-Streater channels, are known to be related only in three dimensions, i.e., when acting on qutrits. In this work, the definition of the Landau-Streater channel is extended in such a way that it retains its equivalence to the Werner-Holevo channel in all dimensions. This channel is then modified to be representable as a model of noise acting on qudits. We then investigate properties of the resulting noisy channel and determine the conditions under which it cannot be the result of a Markovian evolution. Furthermore, we investigate its different capacities for transmitting classical and quantum information with or without entanglement. In particular, while the pure (or high-noise) Landau-Streater or the Werner-Holevo channel is entanglement breaking and hence has zero capacity, by finding a lower bound for the quantum capacity, we show that when the level of noise is lower than a critical value the quantum capacity will be nonzero. Surprisingly, this value turns out to be approximately equal to 0.4 in all dimensions. Finally, we show that, in even dimensions, this channel has a decomposition in terms of unitary operations. This is in contrast with the three-dimensional, case where it has been proved that such a decomposition is impossible, even in terms of other quantum maps.

How fast an observable can evolve in time is answered by so-called observable speed limit. Here, we prove a stronger version of the observable speed limit and show that the previously obtained bound is a special case of the new bound. The stronger quantum speed limit for the state also follows from the stronger quantum speed limit for observables (SQSLO). We apply this to prove a stronger bound for the entanglement rate using the notion of capacity of entanglement (the quantum information theoretic counterpart of the heat capacity) and show that it outperforms previous bounds. Furthermore, we apply the SQSLO for the rate of modular Hamiltonian and in the context of interacting qubits in a quantum battery. These illustrative examples reveal that the speed limit for the modular energy and the time required to charge the battery can be exactly predicted using the new bound. This shows that for estimating the charging time of quantum battery SQSLO is actually tight, i.e., it saturates. Our findings can have important applications in quantum thermodynamics, the complexity of operator growth, predicting the time rate of quantum correlation growth, and quantum technology, in general.

In quantum metrology, information about unknown parameters $\mathit{\theta }=({\theta }_1,...,{\theta }_M)$ is accessed by measuring probe states ${\widehat{\rho }}_{\mathit{\theta }}$. In experimental settings where copies of ${\widehat{\rho }}_{\mathit{\theta }}$ can be produced rapidly (e.g., in optics), the information-extraction bottleneck can stem from high postprocessing costs or detector saturation. In these regimes, it is desirable to compress the information encoded in ${\widehat{\rho }}_{\mathit{\theta }}{}^{\otimes n}$ into $m<n$ copies of a postselected state: ${\widehat{\rho }}_{\mathit{\theta }}^{\text{ps}}{}^{\otimes m}$. Remarkably, recent works have shown that, in the absence of noise, compression can be lossless, for $m/n$ arbitrarily small. Here, we fully characterize the family of filters that enable lossless compression. Further, we study the effect of noise on quantum-metrological information amplification. Motivated by experiments, we consider a popular family of filters, which we show is optimal for qubit probes. Further, we show that, for the optimal filter in this family, compression is still lossless if noise acts after the filter. However, in the presence of depolarizing noise before filtering, compression is lossy. In both cases, information extraction can be implemented significantly better than simply discarding a constant fraction of the states, even in the presence of strong noise.

The incompatibility between two observables in quantum theory is described by Heisenberg's uncertainty principle. In this work, we study spatial confinement effects on Heisenberg's uncertainty principle for a hydrogen atom located at the center of an impenetrable spherical cavity with radius $r_o$. Both the radial and vector representation of the uncertainty principle are considered. For this, we solve the Schrödinger equation numerically within a finite-differences approach. We find that for small cavity sizes the values of $\mathrm{\Delta }\widehat{r}\mathrm{\Delta }{\widehat{p}}_r$ (radial) bunch according to the number of nodes and that for Rydberg states, i.e., large excitation, they become more coherent, satisfying exactly Heisenberg's uncertainty principle, in contrast to the vector description. However, for the vector case, we find that $\mathrm{\Delta }\widehat{\mathbf{r}}$ degenerates for small cavity sizes and bunches according to the principal quantum number $n$ for large cavities. We find that the behavior of $\mathrm{\Delta }\widehat{\mathbf{p}}$ is responsible for the breaking of the energy degeneracy for confined quantum systems. This occurs when the confinement radius is of the order of the orbital size, as determined by the electron average distance $\langle \widehat{r}\rangle$. In addition, we estimate the critical cavity size for which relativistic effects become relevant and verify that the relativistic corrections to the energy, obtained from first-order perturbation theory, become important when the total energy of the atom surpasses 93.845 Hartree, corresponding to 10% of the speed of light, which is fulfilled for cavity sizes $r_o<1$ a.u.

The nonadiabatic molecular alignment effect is commonly found helpful in enhancing nonlinear processes such as high-order harmonics and strong-field ionization. In this work, an opposite phenomenon is observed, in which the nonadiabatic molecular alignment effect prepared with a linear-polarized prepulse strongly suppresses the generation of ${\mathrm{N}}_2$ lasing induced by circularly polarized 800-nm femtosecond laser pulse in filament plasma. The presence of a weak prepulse periodically suppresses the lasing at each revival timing of the rotational wave packet of the ${\mathrm{N}}_2$ molecule. The fine structures of the lasing suppression at different revivals exhibits distinct features. The underlying mechanism of the lasing signal suppression is attributed to sudden change of the polarization ellipticity of the pump laser pulse as it experiences ultrafast birefringence induced by the linear prepulse. Theoretical simulations by numerically solving the time-dependent Schrödinger equation with the molecular alignment effect included confirm sensitive changes of the polarization ellipticity of the pump laser at every alignment revival, which reproduces most features of the fine structures of lasing suppression and hence largely supports our interpretation.

We study the quasiparticle properties of a dipolar impurity immersed in a two-dimensional dipolar bath. We use the ab initio diffusion Monte Carlo technique to determine the polaron energy, effective mass, and quasiparticle residue. We find that both the polaron energy and quasiparticle residue follow a universal behavior with respect to the polarization angle when properly scaled in terms of the scattering length. This trend is maintained over a wide range of values of the gas parameter, even in the highly correlated regime. Instead, the effective mass shows growing anisotropy as the tilting angle is increased, which is induced, mainly, by the anisotropy of the impurity-boson dipole-dipole interaction. Surprisingly, the effective mass is larger in the direction of minimum interparticle repulsion. Finally, we use our Monte Carlo results to check the accuracy of perturbative approaches and determine their range of validity in terms of the gas parameter.

We experimentally investigate the Su-Schrieffer-Heeger (SSH) Hamiltonian using Rydberg-atom synthetic dimensions constructed, in a single atom, from a ladder of six neighboring $n{}^{3}S_1$ Rydberg states in which adjacent states are coupled with two-photon transitions using microwave fields. Alternating strong or weak tunneling rates, controlled by adjusting the microwave amplitudes, are varied to explore the topological phase transition inherent in the SSH Hamiltonian as a function of the ratio of the tunneling rates. For each ratio, quench dynamics experiments, in which the system is initially prepared in one of the bulk Rydberg states and then subjected to the microwave fields, are performed to measure the population evolution of the system. From the dynamics measurements, we extract the mean chiral displacement and verify that its long-time average value converges towards the system winding number. The loss of nontrivial topology is also examined by probing the energy spectrum of the system in steady state and observing the disappearance of the zero-energy edge states. The results show that even a system with as few as six levels can demonstrate the essential characteristics of the SSH Hamiltonian.

Parity-time ($\mathcal{PT}$) symmetry provides a new technique for the manipulation of oscillation modes in the field of microwave photonics. In this paper, we conceive and experimentally demonstrate a simple $\mathcal{PT}$-symmetric all-optical microwave oscillator (AOMO) based on a single polarimetric loop, overcoming the challenges of manipulating gain and loss and mutual coupling ratio and perfect control of the geometry in the dual-loop configurations. We also uncover the analytics of the $\mathcal{PT}$-symmetry condition in an AOMO with a detailed nonquantum description. By controlling the polarization rotation angle of the $\mathcal{PT}$-symmetric structure, two mutually coupled loops with balanced gain and loss are easily conceived, enabling the proposed AOMO to operate in the single mode. The optical-optical feedback modulation and oscillatory frequency selection are implemented using a semiconductor optical amplifier (SOA) and the stimulated Brillouin scattering (SBS) effect, respectively. Benefiting from the SOA's cross-gain modulation, the SBS-based frequency selection, and the $\mathcal{PT}$-symmetric breaking, dual-wavelength optical signals can be achieved with wideband tunability, very low beating phase noise, and high stability. In the experiment, we successfully generated an oscillating signal tuning from 3 to 30 GHz, and the measured phase noise reached −103.2 dBc/Hz at 10 GHz and −90.69 dBc/Hz at 30 GHz at 10-kHz offset frequency.

The computational cost of quantum algorithms for physics and chemistry is closely linked to the spectrum of the Hamiltonian, a property that manifests in the necessary rescaling of its eigenvalues. The typical approach of using the 1-norm as an upper bound to the spectral norm to rescale the Hamiltonian suits the most general case of bounded Hermitian operators but neglects the influence of symmetries commonly found in chemical systems. In this work, we introduce a hierarchy of symmetry-aware spectral bounds that provide a unified understanding of the performance of quantum phase estimation algorithms using block-encoded electronic structure Hamiltonians. We present a variational and numerically tractable method for computing these bounds, based on orbital optimization, to demonstrate that the computed bounds are smaller than conventional spectral bounds for a variety of molecular benchmark systems. We also highlight the unique analytical and numerical scaling behavior of these bounds in the thermodynamic and complete basis set limits. Our work shows that there is room for improvement in reducing the 1-norm, not yet achieved through methods like double factorization and tensor hypercontraction, but highlights the potential challenges in improving the performance of current quantum algorithms beyond small constant factors through 1-norm reduction techniques alone.