We study thermal properties of a trapped Bose-Bose mixture in a dilute regime using quantum Monte Carlo methods. Our main aim is to investigate the dependence of the superfluid density and the condensate fraction on temperature for the mixed and separated phases. To this end we use the diffusion Monte Carlo method in the zero-temperature limit and the path-integral Monte Carlo method for finite temperatures. The results obtained are compared with solutions of the coupled Gross-Pitaevskii equations for the mixture at zero temperature. We notice the existence of an anisotropic superfluid density in some phase-separated mixtures. Our results also show that the temperature evolution of the superfluid density and condensate fraction is slightly different, showing noteworthy situations where the superfluid fraction is smaller than the condensate fraction.

We theoretically investigate the polariton-polariton interaction in microcavities beyond the commonly used Born approximation (i.e., mean field), by adopting a toy model with a contact interaction to approximately describe the attraction between electrons and holes in quantum well and by using a Gaussian pair fluctuation theory beyond mean field. We obtain a density or chemical potential independent polariton-polariton interaction strength even in two dimensions, which result from coupling to the photon field. We show that quantum fluctuations lead to about a factor of 2 reduction in the polariton-polariton interaction strength within our toy model. Together with corrections to the $1s$ exciton approximation at very strong light-matter coupling, we find the polariton-polariton interaction strength under typical experimental conditions is overestimated by a factor of 3 in the widely used theories, if our toy model can qualitatively simulate the polariton interaction in GaAs quantum wells. We compare our prediction with the most recent measurement and argue that the beyond-Born-approximation effect to the polariton-polariton interaction strength is crucial for a quantitative understanding of the experimental data by Estrecho et al. [Phys. Rev. B 100, 035306 (2019)].

It is well known that Tan's contact could be calculated by using any of the following three methods: by the asymptotic behavior of momentum distribution, by Tan's adiabatic sweep theorem, or by the operator product expansion as an expectation value of the interaction term. We argue that if a theory describing a Bose (or Fermi) system with the only contact interaction is self-consistent, then it should lead to the same result in all three cases. As an example, we considered mean-field theory (MFT)-based approaches and established that among existing approximations of MFT, the Hartree-Fock-Bogoliubov (HFB) approach is the most self-consistent. Actually, HFB is able to describe existing experimental data on Tan's contact for dilute Bose gas but fails to predict its expected behavior at large gas parameters $(\gamma >0.015)$. So, for appropriate description of properties of a Bose gas even at zero temperature, this approximation needs to be expanded by taking into account fluctuations in higher order then the second one.

We numerically investigate the vortex structures in a fast rotating two-dimensional Rydberg-dressed Bose-Einstein condensate (BEC) taking into account the Lee-Huang-Yang (LHY) quantum correction. In a rotating BEC of reduced dimensionality, it is shown that there is room to tune the LHY coupling against the short- and long-range couplings. In the absence of the LHY effect, hexagonal, square, striped, and honeycomb lattices can exhibit upon increasing the long-range interaction, which can be realized in the context of superfluid (SF)-supersolid (SS) transition. In the presence of the LHY effect, competition between LHY and long-range terms results in rich lattice structures. In particular, clustering of multiple vortices can occur due to the trapping effect of the SS triangles or grids. Estimates of number of vortices in the clustering are provided.

Coupled-mode theory (CMT) has been widely used in optics and photonics design. Despite its popularity, several different formulations of CMT exist in the literature, and their applicable range is not entirely clear, in particular when it comes to high-index-contrast photonics platforms. Here we propose an improved formulation of CMT and demonstrate its superior performance through numerical simulations that compare CMT-derived quantities with supermode calculations and full wave propagation simulations. In particular, application of the improved CMT to asymmetric waveguides reveals a necessary correction in the conventional phase matching condition for high-index-contrast systems, which could lead to more accurate photonic circuit designs involving asymmetric elements.

Here, we show that superresolution optical fluctuation microscopy might not lead to ideally infinite superresolution enhancement with increasing of the order of measured cumulants. Using information analysis for estimating error bounds on the determination of point-source positions, we show that reachable precision per measurement might be saturated with increasing of the order of the measured cumulants in the superresolution regime. In fact, there is an optimal correlation order beyond which there is practically no improvement for objects of three and more point sources. However, for objects of just two sources, one still has an intuitively expected resolution increase with the cumulant order.

The spectral representation of an electric field in a two-constituent composite medium is revisited. A theory is developed for calculating the electromagnetic (EM) eigenstates of Maxwell's equations for such a composite where the magnetic permeability, as well as the electric permittivity, have different uniform values in the two constituents. The physical electric field $\mathbf{E}\left(\mathbf{r}\right)$ produced in the system either by a given incident field or by a given source current density is expanded in this set of biorthogonal eigenstates for any position r. If the microstructure consists of a cluster of separate inclusions in a uniform host medium, then the EM eigenstates of all the isolated inclusions can also be used to calculate $\mathbf{E}\left(\mathbf{r}\right)$. Once all these eigenstates are known for a given host and a given microstructure then the calculation of $\mathbf{E}\left(\mathbf{r}\right)$ only involves performing three-dimensional integrals of known functions and solving sets of linear algebraic equations.

We studied the effect of the spontaneous emission on the photoluminescence (PL) of photonic nanofibers doped with an ensemble of quantum dots (QDs) and quantum emitters. Quantum emitters can be molecular dyes or DNA molecules. Bound photonic states of the nanofiber hybrid are calculated using the transfer-matrix method based on the Maxwell equations. It is shown that the number of bound states in the nanofiber hybrid depends on the size and shape of the nanofiber along with the concentration of quantum dots and quantum emitters including their dielectric constants. The bound photon electric field induces dipoles in quantum dots and quantum emitters, and they interact with each other via the dipole-dipole interaction (DDI). We found that excited excitons decay spontaneously due to the interaction between excitons and the DDI field. It is found that the decay linewidth is enhanced when the bound photon energy is close to the exciton energy. On the other hand, we predicted that in the weak DDI coupling limit and when the bound photon energy is far away from the exciton energy the decay linewidths are suppressed (quenched). An analytical expression of the photoluminescence is found using the density-matrix method in the presence of the DDI coupling. We have predicted that in the strong DDI coupling limit the peak in the photoluminescence spectrum splits into two peaks when the bound photon energy is far away from the exciton energy. Furthermore, we have shown that when the bound photon energy is close to the exciton energy, two peaks merge into one peak. We have also compared our theory with PL experiments of a nanofiber doped with an ensemble of the CdSe QDs and sulforhodamine 101 dye molecules. A good agreement between theory and experiments is found. Our theory can be used by experimentalists to perform alternative types of experiments and for fabricating types of nanosensors and nanoswitches.

When a bound electron is photoionized from an atom or molecule, the remaining multielectron ion with excess internal energy will relax towards lower-energy states. The subsequent transverse angular momentum transfer between the parent ion and the ejected electron can lead to an electron parity-unfavored transition. Here we perform a self-calibrated timing experiment to measure the time delay of the angular momentum transfer of parity-unfavored transitions during the multiphoton ionization. We use a strong linearly polarized 400-nm light field to trigger a parity-unfavored transition in the channel of the spin-orbit excited ionic state of krypton atoms and then selectively probe the phases of the electron wave functions with different magnetic quantum numbers with a weak parallelly (orthogonally) polarized 800-nm light field. The parallel and orthogonal probes serve as the start and the stop of a stopwatch, respectively. Hence, this allows us to access the characteristic time of such an ultrafast intra-atom multielectron process.

The standard method for approaching quantum electrodynamic (QED) field theory uses a perturbative $S$-matrix approach. This approach is explicitly nondynamical and provides only a one-time, static map between an initial state to be evolved by the “full propagator” of a bona-fide interacting field theory and an asymptotically equivalent effective initial state to be evolved by the “free propagator” of the corresponding noninteracting field theory. We provide a detailed derivation of a nonperturbative and dynamical approach to QED that allows one to study the space-time dynamics of electron-photon interactions directly. As an example of this method, we compute the time-resolved dynamics of Compton scattering for a system with a nontrivial spatial structure in only one dimension while restricting to the case of a single electron and at most one photon. This approach retains the massless photon of quantum electrodynamics in contrast to previous approaches that resorted to using massive bosons [T. Cheng, E. R. Gospodarczyk, Q. Su, and R. Grobe, Ann. Phys. 325, 265 (2010)] to represent the photon. The dynamics of Compton scattering are illustrated using joint probability distributions that evolve in time. This information is compared to that provided by the $S$ matrix.

We show that the quantum approximate optimization algorithm (QAOA) can construct, with polynomially scaling resources, the ground state of the fully connected $p$-spin Ising ferromagnet, a problem that notoriously poses severe difficulties to a vanilla quantum annealing (QA) approach due to the exponentially small gaps encountered at first-order phase transition for $p\ge 3$. For a target ground state at arbitrary transverse field, we find that an appropriate QAOA parameter initialization is necessary to achieve good performance of the algorithm when the number of variational parameters $2P$ is much smaller than the system size $N$ because of the large number of suboptimal local minima. Instead, when $P$ exceeds a critical value $P_N^{*}\propto N$, the structure of the parameter space simplifies, as all minima become degenerate. This allows achieving the ground state with perfect fidelity with a number of parameters scaling extensively with $N$ and with resources scaling polynomially with $N$.

The Solovay-Kitaev theorem allows us to approximate any single-qubit gate to arbitrary accuracy with a finite sequence of fundamental operations from a universal set of gates. Inspired by this decomposition, we present a quantum channel simulator capable of implementing any completely positive trace-preserving map. Our realization consists of one ancillary qubit, encoded in the transverse mode of a laser beam (orbital degree of freedom), one qubit system, encoded in its polarization (spin), one spin-orbit CNOT gate, and four single-qubit operations performed with prisms and polarization components. Our results describe the implementation of arbitrary single-qubit channels on the photon polarization using the transverse mode as the ancillary qubit.

Reference-frame-independent quantum key distribution (RFI-QKD) can dispense with the requirements of active alignment on reference frames between legitimate users, which is beneficial for free-space implementation. However, the fluctuating transmittance due to atmospheric turbulence still remains a great challenge for improving the performance and has been seldom addressed. In this paper we extend the recently proposed prefixed-threshold real-time selection method to RFI-QKD while combining practical consideration of the transmittance probability distribution model based on the finite aperture theory. Through numerical simulations, we present an estimation for the variance of the log-normal model with respect to distance and receiving aperture radius, and demonstrate the effectiveness of using this method in the RFI protocol. Considering the finite-key effects, simulation results show that the gap of the key rate with different reference frame deviations can be alleviated by increasing the data size. By adopting this method one can tolerate more serious transmission loss, especially in strong turbulence cases, which is helpful for future free-space experimental designs.

High-order harmonic generation by a femtosecond laser pulse in the presence of a moderately strong terahertz (THz) field is studied under the strong field approximation. We demonstrate a simple proportionality of near-cutoff even-order harmonic (NCEH) amplitude to the THz electric field. The formation of the THz-induced NCEHs is analytically shown for both continuous-wave and Gaussian pulses. The perturbation analysis with regard to the frequency ratio of the THz field to the femtosecond pulse shows the THz-induced NCEHs originate from its first-order correction of the action in the presence of the THz vector potential, and the available parametric conditions for the phenomenon are also clarified. As the complete characterization of the time-domain waveform of the broadband THz field is essential for a wide variety of applications, the paper provides an alternative time-resolved field-detection technique, allowing for a robust broadband characterization of pulses in THz spectral range.

We consider attractive and repulsive condensates in a ring trap stirred by a weak link, and analyze the spectrum of solitonic trains dragged by the link, by means of analytical expressions for the wave functions, energies, and currents. The precise evolution of current production and destruction in terms of defect formation in the ring and in terms of stirring is studied. We find that any excited state can be coupled to the ground state through two proposed methods: either by adiabatically tuning the link's strength and velocity through precise cycles which avoid the critical velocities and thus unstable regions or by keeping the link still while setting an auxiliary potential and imprinting a nonlinear phase as the potential is turned off. We also analyze hysteresis cycles through the spectrum of energies and currents.

We study the thermodynamic properties of binary Bose mixtures by developing a beyond mean-field Popov theory which properly includes the effects of quantum and thermal fluctuations in both the density and spin channels. Results for key thermodynamic quantities, such as the isothermal compressibility and the magnetic susceptibility, are derived from a perturbative calculation of the grand-canonical potential. We find that thermal fluctuations can play a crucial role on the miscibility condition of a binary mixture, favoring phase separation at finite temperature even if the mixture is soluble at zero temperature, as already anticipated in a previous work [M. Ota et al., Phys. Rev. Lett. 123, 075301 (2019)]. We further investigate the miscibility condition for binary mixtures in the presence of asymmetry in the intraspecies interactions, as well as in the masses of the two components. Furthermore, we discuss the superfluid behavior of the mixture and the temperature dependence of the Andreev-Bashkin effect.

Undiscovered properties of surface creeping wave states given by complex angular momentum poles, so-called Regge poles (RPs) in optical microcavity scattering, are transparently elucidated on the frameworks of non-Hermitian physics. In particular, we analytically show that the nature of external resonances (ERs) given by complex wave numbers is equivalent to surface creeping wave RPs. Moreover, by obtaining the full structural characteristics of RPs for different polarizations, it is found that notable nontrivial RPs in transverse-magnetic polarization arise owing to the classical Brewster angle. Finally, we explicitly demonstrate the emergence of non-Hermitian degeneracies incorporating different classes of RPs.

Radiation from a charged particle moving in a medium with a Maxwell's fish-eye refraction index profile is considered. It is shown that the radiation spectrum has a discrete character. The main emitted wavelength is proportional to the refractive profile's radius and has a dipole character in a regular medium. A Cherenkov-like threshold velocity is established. A cardinal rearrangement of angular distribution in a lossless medium is predicted. This behavior is caused by the total internal reflection in a lossless medium as opposed to photons' attenuated total reflection in the regular medium. A lossless medium ensures that both directed and monochromatic emission can serve as a light source in the corresponding regions.

The three-dimensional holography technique offers an efficient and convenient tool for storing depth information of the object and provides more realistic images compared to the two-dimensional case, which has been widely used in the fields of optical tweezers, image encryption, and optical microscopy. However, such a three-dimensional nonlinear holography technique is still lacking in the field of nonlinear optics. Here, we demonstrate the realization of three-dimensional nonlinear optical holograms. Without loss of generality, a second-order nonlinearity is used in our experiment and the generated second harmonic with multiplane patterns at different depths is dynamically realized. This study opens up the field of three-dimensional spatial nonlinear harmonic manipulation and may have applications in all-optical wave-band three-dimensional optical imaging, parallel micromachining, holographic displays, and so on.

We study the dynamics of a single excitation coherently shared among an ensemble of atoms and coupled to a one-dimensional wave guide. The coupling between the matter and the light field gives rise to collective phenomena such as superradiant states with an enhanced initial decay rate, but also to the coherent exchange of the excitation between the atoms. We find that the competition between the two phenomena provides a characteristic dynamics for the decay of the excitations, and remarkably exhibits an algebraic behavior, instead of the expected standard exponential one, for a large number of atoms. The analysis is first performed for a chiral waveguide, where the problem can be solved analytically. Remarkably, we demonstrate that a bidirectional waveguide exhibits the same behavior for large number of atoms and, therefore, it is possible to experimentally access characteristic properties of a chiral waveguide also within a bidirectional waveguide.