We study a system of penetrable bosons embedded in a spherical surface. Under the assumption of weak interaction between the particles, the ground state of the system is, to a good approximation, a pure condensate. We employ thermodynamic arguments to investigate, within a variational ansatz for the single-particle state, the crossover between distinct finite-size “phases” in the parameter space spanned by the sphere radius and the chemical potential. In particular, for radii up to a few interaction ranges we examine the stability of the fluid phase with respect to a number of crystal-like arrangements having the symmetry of a regular or semiregular polyhedron. We find that, while quantum fluctuations keep the system fluid at low density, upon compression it eventually becomes inhomogeneous, i.e., particles gather together in clusters. As the radius increases, the nature of the high-density aggregate varies and we observe a sequence of transitions between different cluster phases (“solids”), whose underlying rationale is to maximize the coordination number of clusters, while ensuring at the same time the proper distance between each neighboring pair. We argue that, at least within our mean-field description, every cluster phase is supersolid.

In recent years, guided acoustic wave Brillouin scattering has become an important tool in photonics, serving as the basis for everything from new forms of information processing to silicon lasers. Due to low losses and long interaction lengths, fiber optic systems offer an intriguing platform to harness these guided-wave light-sound interactions. However, within typical fiber optic systems these interactions are exceedingly weak—requiring complex microstucturing to yield appreciable light-sound coupling. Here, we enhance this light-sound coupling by using a ${\mathrm{CS}}_2$-filled liquid core optical fiber. Owing to tight confinement of the optical and acoustic modes within the fiber core, as well as the large electrostrictive response of ${\mathrm{CS}}_2$, this system yields an unprecedented forward Brillouin gain for a fiber optic system. To demonstrate this physics, we measure multipeaked spontaneous forward Brillouin scattering power spectra, yielding information about the fiber geometry, material properties, and acousto-optic coupling strength. To interpret these data, we simulate the spontaneous Brillouin scattering power spectrum for this fiber system. These results reveal that hybridized acoustic excitations within the fiber core and cladding produce this characteristic multipeaked power spectrum. In the future, the large forward Brillouin coupling, long interaction lengths, and low losses of liquid-core fibers may enable new forms of distributed sensing, lasers with customizable emission, and physics including continuum optomechanical cooling.

Superposition of multiple indistinguishable two-photon paths is introduced by means of a digital wave front encoding technique to design a spatially phase-correlated wave front of a light field, which could result in a superbunching effect with a bunching peak much larger than 2, the theoretical bunching peak of thermal light. Experimentally, we demonstrated a bunching peak of $11.72\pm 0.05$ with the digitally designed phase-correlated wave front of classical light, which can be improved further on. A significant improvement of the visibility of the second-order interference fringes and ghost imaging was also experimentally demonstrated. Our method provides a convenient digitally controllable way to implement superposition of multiple different but indistinguishable two-photon paths, and therefore to modify the optical coherence properties of photons. Such a superbunching effect may have potential applications in improving the visibility of correlation interference and imaging, and enhancing the efficiency of two-photon nonlinear effects such as two-photon absorption effect.

We investigate the driven quantum phase transition between the oscillating motion and the classical nearly free rotations of the Josephson pendulum coupled to a harmonic oscillator in the presence of dissipation. We refer to this as the Josephson-Rabi model. This model describes the standard setup of circuit quantum electrodynamics, where typically a transmon device is embedded in a superconducting cavity. We find that by treating the system quantum mechanically this transition occurs at higher drive powers than expected from an all-classical treatment, which is a consequence of the quasiperiodicity originating in the discrete energy spectrum of the bound states. We calculate the photon number in the resonator and show that its dependence on the drive power is nonlinear. In addition, the resulting multiphoton blockade phenomenon is sensitive to the truncation of the number of states in the transmon, which limits the applicability of the standard Jaynes-Cummings model as an approximation for the pendulum-oscillator system. We calculate the $n\mathrm{th}$-order correlation functions of the blockaded microwave photons and observe the differences between the rotating-wave approximation and the full multilevel Josephson-Rabi Hamiltonian with the counter-rotating terms included. Finally, we compare two different approaches to dissipation, namely the Floquet-Born-Markov and the Lindblad formalisms.

We address a fundamental question of quantum optics: Can a beam of light mediate coherent Hamiltonian interactions between two distant quantum systems? This is an intriguing question whose answer is not a priori clear, since the light carries away information about the systems and might be subject to losses, giving rise to intrinsic decoherence channels associated with the coupling. Our answer is affirmative and we derive a particularly simple sufficient condition for the interactions to be Hamiltonian: The light field needs to interact twice with the systems and the second interaction has to be the time reversal of the first. We demonstrate that, even in the presence of significant optical loss, coherent interactions can be realized and generate substantial amounts of entanglement between the systems. Our method is directly applicable for building hybrid quantum systems, with relevant applications in the fields of optomechanics and atomic ensembles.

We couple the full three-dimensional ab initio quantum evolution of the light pulse polarization in interaction with an atom with a propagation model to simulate the propagation of ultrashort laser pulses over macroscopic dimensions, in the presence of self-generated harmonics up to order 11. We evidence a clear feedback of the generated harmonics on propagation, with an influence on the ionization probability as well as the yield of the harmonic generation itself.

We investigate a uniformly coupled non-Hermitian system with asymmetric coupling amplitude. The asymmetric coupling equals to a symmetric coupling threaded by an imaginary gauge field. In a closed configuration, the imaginary gauge field leads to an imaginary magnetic flux, which induces a non-Hermitian phase transition. For an open boundary, the imaginary gauge field results in an eigenstate localization. The eigenstates under Dirac and biorthogonal norms and the scaling laws are quantitatively investigated to show the affect of asymmetric coupling induced one-way amplification. However, the imaginary magnetic flux does not inevitably induce the non-Hermitian phase transition for systems without translation invariance, which is elucidated from the non-Hermitian phase transition in the non-Hermitian ring with a single coupling defect. Our findings provide insights into the non-Hermitian phase transition and one-way localization.

In this paper, we extend the results of an earlier paper [G. Mouloudakis and P. Lambropoulos, Phys. Rev. A 97, 053413 (2018)] in which we had demonstrated the limitations of the notion of nonresonant multiphoton ionization, in the exploration of photon statistics effects in nonlinear processes. Through the quantitative analysis of specific realistic processes, we provide the connection to conditions of intensity and pulse duration necessary in relevant experiments, including a recent seminal experiment demonstrating the effect of superbunching found in squeezed radiation. The outlook for a future era of studies on the interplay between stochasticity and nonlinearity in transitions driven by electromagnetic radiation is also discussed.

We study interacting ultracold atoms in a three-dimensional (3D) harmonic trap with spin-selective dissipations, which can be effectively described by non-Hermitian parity-time ($\mathcal{PT}$) symmetric Hamiltonians. By exactly solving the non-Hermitian two-body problem of spin-1/2 (spin-1) bosons in a 3D harmonic trap, we find that the system can exhibit third-order (fifth-order) exceptional points (EPs) with ultrasensitive cube-root (fifth-root) spectral response due to interaction anisotropies in spin channels. We also present the general principle for the creation of high-order EPs and their spectral sensitivities with arbitrary particle number $N$ and arbitrary spin $s$. Generally, with spin-independent interactions, the EP order of bosons can be as high as $2Ns+1$, and the spectral response around EP can be as sensitive as $\sim {\epsilon }^{1/(2ks+1)}$ under a $k$-body interaction anisotropy. Moreover, we propose to detect the ultrasensitive spectral response through the probability dynamics of certain state. These results suggest a convenient route towards more powerful sensor devices in spinor cold atomic systems.

In this paper we study the thermal equilibration of small bipartite Bose-Hubbard systems, both quantum mechanically and in mean-field approximation. In particular we consider small systems composed of a single-mode “thermometer” coupled to a three-mode “bath,” with no additional environment acting on the four-mode system, and test the hypothesis that the thermometer will thermalize if and only if the bath is chaotic. We find that chaos in the bath alone is neither necessary nor sufficient for equilibration in these isolated four-mode systems. The two subsystems can thermalize if the combined system is chaotic even when neither subsystem is chaotic in isolation, and under full quantum dynamics there is a minimum coupling strength between the thermometer and the bath below which the system does not thermalize even if the bath itself is chaotic. We show that the quantum coupling threshold scales like $1/N$ (where $N$ is the total particle number), so that the classical results are obtained in the limit $N\to \infty$.

Recent developments in real-time ultrafast measurement techniques have enabled us to prove experimentally that soliton molecules execute internal motions with some aspects similar to those of a matter molecule. Such an analogy between the dynamics of soliton molecules and the dynamics of matter molecules is based on the assumption that the dissipative solitons constituting a molecule are rigid entities sharing a common profile. Whereas this assumption drastically reduces the number of degrees of freedom, it does not hold true in general and we demonstrate that it overlooks some of the essential dynamical features of the soliton molecule. We present a theoretical study based on the principle that the different pulse constituents of a soliton molecule are deformable entities. Specifically, by using of a collective coordinate approach to investigate bi- and trisoliton molecules, we reveal features such as symmetric or asymmetric distortions of their profiles, and energy exchange processes between them. This implies, in subsequent experiments, the use of characterization techniques that can be used to retrieve a larger number of degrees of freedom.

The perturbation theory by Ge et al. [Phys. Rev. A 87, 023833 (2013)] for transverse-electric polarized modes in weakly deformed microdisks omits terms related to the variation of the normal derivative of the magnetic field along the boundary. Here, we show that these terms are necessary to accurately describe microdisks with a strongly winding boundary. In particular, it is demonstrated that the corrected perturbation theory allows one to describe the counterintuitive phenomenon of $Q$-factor enhancement due to weak boundary deformation. We discuss in detail the microflower cavity and the limaçon cavity. Good agreement of the corrected perturbation theory with full numerical results is observed.

Isolating past information relevant for future prediction is central to quantitative science. Quantum models offer a promising approach, enabling statistically faithful modeling while using less past information than any classical counterpart. Here we introduce a class of phase-enhanced quantum models, representing the most general means of simulating a stochastic process unitarily in causal order. The resulting constructions surpass previous state-of-art methods—both in reducing the information they need to store about the past and in the minimal memory dimension they require to store this information. Moreover, these two features are generally competing factors in optimization—leading to an ambiguity in optimal modeling that is unique to the quantum regime. Our results simultaneously offer quantum advantages for stochastic simulation and illustrate further qualitative differences between classical and quantum notions of complexity.

Performances of work-to-work conversion are studied for a dissipative nonlinear quantum system with two isochromatic phase-shifted drives. It is shown that for weak Ohmic damping simultaneous maximization of efficiency with finite power yield and low power fluctuations can be achieved. Optimal performances of these three quantities are accompanied by a shortfall of the tradeoff bound recently introduced for classical thermal machines. This bound can be undercut down to zero for sufficiently low temperature and weak dissipation, where the non-Markovian quantum nature dominates. Analytic results are given for linear thermodynamics. These general features can persist in the nonlinear driving regime near a maximum of the power yield and a minimum of the power fluctuations. This broadens the scope to an operation field beyond linear response.

We propose a scheme for covert active sensing using floodlight illumination from a terahertz-bandwidth amplified spontaneous emission (ASE) source and heterodyne detection. We evaluate the quantum-estimation-theoretic performance limit of covert sensing, wherein a transmitter's attempt to sense a target phase is kept undetectable to a quantum-equipped passive adversary, by hiding the signal photons under the thermal noise floor. Despite the quantum state of each mode of the ASE source being mixed (thermal), and hence inferior compared to the pure coherent state of a laser mode, the thousand-times-higher optical bandwidth of the ASE source results in achieving a substantially superior performance compared to a narrow-band laser source by allowing the probe light to be spread over many more orthogonal temporal modes within a given integration time. Even though our analysis is restricted to single-mode phase sensing, this system could be applicable or extendible for various practical optical sensing applications.

Quantum information and communication processing within quantum networks usually employ identical particles. Despite this, the physical role of the quantum statistical nature of particles in large-scale networks remains elusive. Here, we show that just the indistinguishability of fermions allows a new mechanism of entanglement transfer in many-node quantum networks. This process activates remote entanglement among distant sites, which do not share a common past, by only locally counting identical particles and classical communication. These results constitute the key achievement of the present technique and open the way to a more stable multistage transfer of nonlocal quantum correlations based on fermions.

In this work we demonstrate experimentally how generative model training can be used as a benchmark for small (fewer than five qubits) quantum devices. Performance is quantified using three data analytic metrics: the Kullback-Leibler divergence and two adaptations of the $F_1$ score. Using the $2\times 2$ bars and stripes data set, we train several different circuit constructions for generative modeling with superconducting qubits. By taking hardware connectivity constraints into consideration, we show that sparsely connected shallow circuits outperform denser counterparts on noisy hardware.

We report measurements of the electric dipole matrix elements of the ${}^{133}\mathrm{Cs}6s{}^{2}S_{1/2}\to 7p{}^{2}P_{1/2}$ and $6s{}^{2}S_{1/2}\to 7p{}^{2}P_{3/2}$ transitions. Each of these determinations is based on direct, precise comparisons of the absorption coefficients between two absorption lines. For the $\langle 6s{}^{2}S_{1/2}\left|\right|r\left|\right|7p{}^{2}P_{3/2}\rangle$ matrix element, we measure the ratio of the absorption coefficient on this line to that of the $D_1$ transition $6s{}^{2}S_{1/2}\to 6p{}^{2}P_{1/2}$. The matrix element of the $D_1$ line has been determined previously with high precision by many groups. For the $\langle 6s{}^{2}S_{1/2}\left|\right|r\left|\right|7p{}^{2}P_{1/2}\rangle$ matrix element, we measure the ratio of the absorption coefficient on this line to that of the $6s{}^{2}S_{1/2}\to 7p{}^{2}P_{3/2}$ transition. Our results for these matrix elements are $\langle 6s{}^{2}S_{1/2}\left|\right|r\left|\right|7p{}^{2}P_{3/2}\rangle =0.57417\left(57\right)a_0$ and $\langle 6s{}^{2}S_{1/2}\left|\right|r\left|\right|7p{}^{2}P_{1/2}\rangle =0.27810\left(45\right)a_0$. These measurements have implications for the interpretation of parity nonconservation in atoms.

We report the demonstration of a quasi-one-dimensional Stern-Gerlach effect on a supersonic beam of lithium-7 atoms. This method combines a pulsed magnetic field gradient plus a strong bias field to create a quasi-one-dimensional force. We discuss the application of this technique to a cooling method and earth-based microgravity experiments.