We show that the chemical potential of a one-dimensional (1D) interacting Bose gas exhibits a nonmonotonic temperature dependence which is peculiar of superfluids. The effect is a direct consequence of the phononic nature of the excitation spectrum at large wavelengths exhibited by 1D Bose gases. For low temperatures $T$, we demonstrate that the coefficient in $T^2$ expansion of the chemical potential is entirely defined by the zero-temperature density dependence of the sound velocity. We calculate that coefficient along the crossover between the Bogoliubov weakly interacting gas and the Tonks-Girardeau gas of impenetrable bosons. Analytic expansions are provided in the asymptotic regimes. The theoretical predictions along the crossover are confirmed by comparison with the exactly solvable Yang-Yang model in which the finite-temperature equation of state is obtained numerically by solving Bethe-ansatz equations. A 1D ring geometry is equivalent to imposing periodic boundary conditions and arising finite-size effects are studied in detail. At $T=0$ we calculated various thermodynamic functions, including the inelastic structure factor, as a function of the number of atoms, pointing out the occurrence of important deviations from the thermodynamic limit.

Nonclassical light fields can be generated from two coupled cavities that are interacted with a two-level system. This scheme is drawing extensive attention because the mode coupling can reduce the requirement of coupling constants between the light fields and the two-level system. However, the effect of phase differences between different coupling constants on photon antibunching is always neglected. Considering a two-level system interacting with two coupled cavities, we analyze the statistical properties of the cavity field and show that the photon antibunching can be affected by the phase differences of coupling constants between the two-level system and the cavity modes, or between the cavity modes and the driving fields, and thus we can engineer the phases to optimally modulate single-photon sources or nonclassical light fields.

Molecules with $C_N$ or $C_{Nh}$ symmetry can absorb quanta of optical angular momentum to generate twisted excitons with well-defined quasiangular momenta of their own. Angular momentum is conserved in such interactions at the level of a paraxial approximation for the light beam. A sequence of absorption events can thus be used to create a range of excitonic angular momenta. Subsequent decay can produce radiation with a single angular momentum equal to that accumulated. Such molecules can thus be viewed as mediators for changing the angular momentum of light. This sidesteps the need to exploit nonlinear light-matter interactions based on higher-order susceptibilities. A tight-binding paradigm is used to verify angular momentum conservation and demonstrate how it can be exploited to change the angular momentum of light. The approach is then extended to a time-dependent density functional theory setting where the key results are shown to hold in a many-body, multilevel setting.

This work analyses the quantum properties of counterpropagating twin beams generated by a mirrorless optical parametric oscillator in the continuous-variable regime. Despite the lack of the filtering effect of a cavity, we show that in the vicinity of its threshold it may generate high levels of narrowband squeezing and Einstein-Podolsky-Rosen correlation, completely comparable to what can be obtained in standard optical parametric oscillators.

We analyze microscopic models of the particle source or sink which consist of a one- or two-site Bose-Hubbard model (the system) weakly coupled to a many-site Bose-Hubbard model (the reservoir). Assuming unequal filling factors for the system and reservoir, we numerically study equilibration dynamics and compare it with the solution of the master equation on the reduced density matrix of the system. Necessary conditions for the validity of the master equation approach are formulated.

For a bipartite state with equal local dimension $d$, we prove that one can obtain work gain under Landauer's erasure process on one party in the identically and independently distributed limit when the corresponding fully entangled fraction is larger than $\frac{1}{d}$. By processing a given state to the maximally mixed state, we give an approximation for the largest extractable work with an error in the energy scale, which becomes negligible in the large system limit. As a step to link quantum thermodynamics and quantum nonlocality, we also provide a simple picture to approximate the optimal work extraction and suggest a potential thermodynamic interpretation of the fully entangled fraction for isotropic states.

We consider the dynamics of an arbitrary quantum system coupled to a large arbitrary and fully quantum-mechanical environment through a random interaction. We establish analytically and check numerically the typicality of this dynamics, in other words, the fact that the reduced density matrix of the system has a self-averaging property. This phenomenon, which lies in a generalized central limit theorem, justifies rigorously averaging procedures over certain classes of random interactions and can explain the absence of sensitivity to microscopic details of irreversible processes, such as thermalization. It provides more generally an ergodic principle for embedded quantum systems.

We present an error-correcting protocol that enhances the lifetime of stabilizer code–based qubits which are susceptible to the creation of pairs of localized defects (due to stringlike error operators) at finite temperature, such as the toric code. The primary tool employed is periodic application of a local, unitary operator, which exchanges defects and thereby translates localized excitations. Crucially, the protocol does not require any measurements of stabilizer operators and therefore can be used to enhance the lifetime of a qubit in the absence of such experimental resources.

Previous measurements of single photon $n{d}_{5/2}n{d}_{5/2}\to (n+1)d_j(n-2)f$ transitions by two of us were described as microwave transitions made possible by configuration interaction (CI) between the $n{d}_{5/2}n{d}_{5/2}$ and the nearly degenerate $(n+2)p_{3/2}(n-2)f$ states [Lee and Gallagher, Phys. Rev. A 93, 062509 (2016)]. Here we report the observation of the one photon $n{d}_{5/2}n{d}_{5/2}\to (n+3)s_{1/2}(n-2)f$, two photon $n{d}_{5/2}n{d}_{5/2}\to (n+3)p_j(n-2)f$, and three photon $n{d}_{5/2}n{d}_{5/2}\to (n+4)s_{1/2}(n-2)f$ microwave transitions. We show that both single and multiphoton microwave transitions are conveniently described as Forster resonant energy transfers between resonant Floquet states, and we show that the Floquet-Forster model reduces to the CI model used previously. Finally, to show that the transitions observed previously are by no means unique, we report pair transitions with different initial and final states as well as radio instead of microwave frequencies.

Interatomic Coulombic decay (ICD) is induced in helium nanodroplets by photoexciting the $n=2$ excited state of ${\mathrm{He}}^{+}$ using XUV synchrotron radiation. By recording multiple-coincidence electron and ion images we find that ICD occurs in various locations at the droplet surface, inside the surface region, or in the droplet interior. ICD at the surface gives rise to energetic ${\mathrm{He}}^{+}$ ions as previously observed for free He dimers. ICD deeper inside leads to the ejection of slow ${\mathrm{He}}^{+}$ ions due to Coulomb explosion delayed by elastic collisions with neighboring He atoms, and to the formation of ${\mathrm{He}}_k{}^{+}$ complexes.

We derive and study the hyperbolic reduced Maxwell-Bloch (HRMB) equations, a simplified model for the dynamics of strongly correlated Bose-Einstein condensates (BECs), and in particular for the interaction between the BEC atoms and its evaporated atoms under the strong interactions. This equation is one among four, which are proven to be integrable via the existence of a Lax pair, and thus the method of inverse scattering transform. Another equation is the reduced Maxwell-Bloch equation of quantum optics and the two others do not have physical applications yet. By studying the linear stability of the constant solutions of these four equations we observe various regimes, from stable, to modulational unstable, and unstable at all frequencies. The finite-dimensional reduction of the RMB equations is also used to give more insight into the constant solutions of these equations. From this study, we find that the HRMB equation arising from strongly correlated BECs is stable under the particular condition that the transition rate of evaporation is not too large compared to the number of evaporated atoms. We then derive explicit soliton solutions of the RMB equations and use numerical simulations to show collisions of solitons and kink solitons.

We study theoretically the black-hole lasing phenomenon in a flowing one-dimensional, coherently coupled two-component atomic Bose–Einstein condensate whose constituent atoms interact via a spin-dependent $s$-wave contact interaction. We show by a numerical analysis the onset of the dynamical instability in the spin branch of the excitations, once a finite supersonic region is created in this branch. We study both a spatially homogeneous geometry and a harmonically trapped condensate. Experimental advantages of the two-component configuration are pointed out, with an eye towards studies of backreaction phenomena.

Controlling quantum fluids at their fundamental length scale will yield superlative quantum simulators, precision sensors, and spintronic devices. This scale is typically below the optical diffraction limit, precluding precise wave-function engineering using optical potentials alone. We present a protocol to rapidly control the phase and density of a quantum fluid down to the healing length scale using strong time-dependent coupling between internal states of the fluid in a magnetic field gradient. We demonstrate this protocol by simulating the creation of a single stationary soliton and double soliton states in a Bose-Einstein condensate with control over the individual soliton positions and trajectories, using experimentally feasible parameters. Such states are yet to be realized experimentally, and are a path towards engineering soliton gases and exotic topological excitations.

Quantum number-path entanglement is a resource for supersensitive quantum metrology and in particular provides for sub-shot-noise or even Heisenberg-limited sensitivity. However, such number-path entanglement is thought to have been resource intensive to create in the first place, typically requiring either very strong nonlinearities or nondeterministic preparation schemes with feedforward, which are difficult to implement. Recently [K. R. Motes et al., Phys. Rev. Lett. 114, 170802 (2015)], it was shown that number-path entanglement from a BosonSampling inspired interferometer can be used to beat the shot-noise limit. In this paper we compare and contrast different interferometric schemes, discuss resource counting, calculate exact quantum Cramér-Rao bounds, and study details of experimental errors.

We theoretically propose a scheme to produce Weyl points in two-dimensional (2D) optical microdisk-resonator arrays, which undergo a dynamical modulation with temporal periodicity. By mapping the set of modes supported by each resonator to a synthetic frequency dimension, the 2D lattice is equivalent to a three-dimensional (3D) time-independent lattice. We show that, by breaking the inversion symmetry ($I$ symmetry) or introducing artificial gauge fields, Weyl points can be formed, leading to anomalous topological edge states and Fermi arcs. This approach may provide a way to design robust topological photonic devices on chips and future applications for integrated photonics.

We describe the generation of photon pairs entangled in optical angular momentum through spontaneous four-wave mixing in circular arrays of nonlinear waveguides. We show that the incorporation of the array twist along the waveguides can enable robust generation even in the presence of defects and inhomogeneities. We present analytical results and numerical simulations based on coupled-mode equations for the classical pump field and a discrete Schrödinger equation for the biphoton wave function.

We theoretically investigate Klein tunneling processes in photonic artificial graphene. Klein tunneling is a phenomenon in which a particle with Dirac dispersion going through a potential step shows a characteristic angle- and energy-dependent transmission. We consider a generic photonic system consisting of a honeycomb-shaped array of sites with losses, illuminated by coherent monochromatic light. We show how the transmission and reflection coefficients can be obtained from the steady-state field profile of the driven-dissipative system. Despite the presence of photonic losses, we recover the main scattering features predicted by the general theory of Klein tunneling. Signatures of negative refraction and the orientation dependence of the intervalley scattering are also highlighted. Our results will stimulate the experimental study of intricate transport phenomena using driven-dissipative photonic simulators.

The classical problem of subwavelength particle detection on a flat surface is especially challenging when the refractive index of the particle is close to that of the substrate. We demonstrate a method to improve the detection ability several times for such a situation, by enhancing the “forbidden” evanescent waves in the substrate using the principle of super-resolution with evanescent waves amplification. The working mechanism of the system and experimental validation from a design with a thin single dielectric layer is presented. The resulting system is a simple but complete example of evanescent-wave generation, amplification, and the consequent modulation of the far field. This principle can have far reaching impact in the field of particle detection in several applications ranging from contamination control to interferometric scattering microscopy for biological samples.

One of the current challenges in second-harmonic generation (SHG) is to increase the efficiency of the second-harmonic conversion process while maintaining or even decreasing the fundamental-harmonic pump powers in a compact device. Here, we put forward an on-chip scheme to realize high-efficiency optical SHG in active-passive-coupled microring resonators with the aid of the intrinsic second-order nonlinearity. By careful analysis and extensive simulations, it is found that the introduction of an active microring resonator makes the strong SHG process feature an ultralow-power pump threshold, which is about four orders of magnitude lower than that in a single-microring resonator SHG system reported previously by X. Guo et al. [Optica 3, 1126 (2016)]. The observed SHG is enhanced by a factor of over 200 compared to the single-microring-resonator SHG system. The SHG conversion efficiency of over $72\%$ can be reached with optical pump power as low as a few microwatts for our proposed device. This investigation may open a new route towards development of easily fabricated radiation sources of coherent high-energy (shorter-wavelength) photons with an ultralow-power laser-triggered SHG process.

We experimentally and theoretically reinvestigate the significant third-harmonic enhancement via plasma generation using time-resolved methods with two-color, copropagating femtosecond laser pulses. By measuring plasma densities with digital in-line holography and using carrier-resolved unidirectional pulse propagation equations to model the experiment, we show that a fifth-order nonlinearity contributes to third-harmonic generation mainly via phase matching. In particular, our study confirms that self- and cross-phase modulation including higher-order nonlinear processes can play a crucial role in harmonic-generation phase matching, which has recently been proposed by Weerawarne $etal.$ [Phys. Rev. Lett. 114, 093901 (2015)] and should be potentially applicable to high-order harmonic generation.