We compute physical properties across the phase diagram of the $t-J_{\perp }$ chain with long-range dipolar interactions, which describe ultracold polar molecules on optical lattices. Our results obtained by the density-matrix renormalization group indicate that superconductivity is enhanced when the Ising component $J_z$ of the spin-spin interaction and the charge component $V$ are tuned to zero and even further by the long-range dipolar interactions. At low densities, a substantially larger spin gap is obtained. We provide evidence that long-range interactions lead to algebraically decaying correlation functions despite the presence of a gap. Although this has recently been observed in other long-range interacting spin and fermion models, the correlations in our case have the peculiar property of having a small and continuously varying exponent. We construct simple analytic models and arguments to understand the most salient features.

Ultracold atomic systems have been of great research interest in the past, with more recent attention being paid to systems of mixed species. In this work, we carry out nonperturbative path-integral Monte Carlo (PIMC) simulations of $N$ distinguishable particles at finite temperature, which can be thought of as an ultracold atomic system containing $N$ distinct species. We use the PIMC approach to calculate thermodynamic properties of particles interacting via hard-sphere and hard-cavity potentials. The first problem we study is a two-particle system interacting via a hard-sphere and hard-cavity interaction in order to test the effectiveness of two approximations for the thermal density matrix corresponding to these potentials. We then apply the PIMC method to a system of many hard-sphere particles under periodic boundary conditions at varying temperature in order to calculate the energy per particle, pressure, and specific heat of the system. We examine how finite-size effects impact the results of PIMC simulations of hard-sphere particles and when the thermodynamic limit has been reached. Our results provide microscopic benchmarks for a system containing distinguishable particles, which can be thought of as a limiting case for ultracold atomic systems of mixed species.

Numerous inequalities involving moments of integrated intensities and revealing nonclassicality and entanglement in bipartite optical fields are derived using the majorization theory, nonnegative polynomials, the matrix approach, and the Cauchy-Schwarz inequality. Different approaches for deriving these inequalities are compared. Using the experimental photocount histogram generated by a weak noisy twin beam monitored by a photon-number-resolving intensified CCD camera, the performance of the derived inequalities is compared. A basic set of 10 inequalities suitable for monitoring entanglement of a twin beam is suggested. Inequalities involving moments of photocounts (photon numbers) as well as some containing directly the elements of photocount (photon-number) distributions are also discussed as a tool for revealing nonclassicality.

For passive electromagnetic scatterers, we explore a variety of extreme limits on directional scattering patterns in a phase diagram, regardless of the details on the geometric configurations and material properties. By demonstrating the extinction cross sections with the power conservation intrinsically embedded in the phase diagram, we give an alternative interpretation for Kerker's first and second conditions, associated with zero backward scattering (ZBS) and nearly zero forward scattering (NZFS). Physical boundaries and limitations for these directional radiations are illustrated along with a generalized Kerker's condition with implicit parameters. By taking the dispersion relations of gold silicon core-shell nanoparticles into account, we reveal the realistic parameters to experimentally implement ZBS and NZFS at optical frequencies by means of a phase diagram.

Narrow-band intensity-difference squeezing (IDS) beams have important applications in quantum metrology and gravitational wave detection. The best way to generate narrow-band IDS is to employ a parametrically amplified (PA) four-wave mixing (FWM) process in high-gain atomic media. Such IDS can be further enhanced by cascading multiple PA FWM processes in separate atomic media. The complicated experimental setup, added losses, and mechanical stability can limit the wide use of such a scheme in practical applications. Here we show that by modulating the internal energy level(s) with an additional laser (or lasers), the degree of original IDS can be substantially increased. With an initial IDS of $\left(-3.6\pm 0.4\right)$ dB using a PA nondegenerate FWM process in a three-level $\mathrm{\Lambda }$-type configuration, the degree of IDS can be enhanced to $\left(-7.0\pm 0.4\right)$ dB or $\left(-9.0\pm 0.4\right)$ dB when we use one (two) laser beam (beams) to modulate the involved ground (excited) state (states). Our results show a low-loss, robust, and efficient way to produce a high degree of IDS and facilitate its potential applications.

A double-time correlation function of arbitrary two quantum operations is studied for a nonstationary open quantum system which is in contact with a thermal reservoir. It includes a usual correlation function, a linear response function, and a weak value of an observable. Time evolution of the correlation function can be derived by means of the time-convolution and time-convolutionless projection operator techniques. For this purpose, a quasidensity operator accompanied by a fictitious field is introduced, which makes it possible to derive explicit formulas for calculating a double-time correlation function in the second-order approximation with respect to a system-reservoir interaction. The derived formula explicitly shows that the quantum regression theorem for calculating the double-time correlation function cannot be used if a thermal reservoir has a finite correlation time. Furthermore, the formula is applied for a pure dephasing process and a linear dissipative process. The quantum regression theorem and the the Leggett-Garg inequality are investigated for an open two-level system. The results are compared with those obtained by exact calculation to examine whether the formula is a good approximation.

Using a tripartite framework consisting of an open quantum system, its environment, and a passive reference system, in this study we discuss quantum mutual information (QMI) and data processing inequality (DPI). Without any restriction on the initial correlations, the necessary and sufficient conditions for the decrease or increase as well as for the conservation of QMI are obtained for any joint unitary evolution of the open quantum system and its environment. In the special case of the input Markov states, it has been shown that as long as the tripartite input state is a Markov state then DPI holds for every joint unitary evolution even in the presence of initial correlations encoded in the input. We also point out that the converse of the last statement, that is, sufficiency of the existence of a local quantum channel or of the fulfillment of DPI for the input Markov state does not hold in general and this fact is exhibited by counterexamples.

We have measured fully differential cross sections for dissociative single capture and Coulomb explosion through double capture in 75 keV $p+{\mathrm{H}}_2$ collisions. Data were analyzed for fixed kinetic energy releases and molecular orientations as a function of scattering angle. Two-center interference was identified for dissociative single capture. The interference pattern is not inconsistent with the symmetry of the dissociative electronic state affecting the phase angle of the interference term. No clear signatures of single-center interference were observed for either process. For double capture at most only a very weak two-center interference structure was found. This very small (or zero) visibility can probably be attributed to a convolution of two independent scatterings of the projectile with the two electrons yielding the measured scattering angle.

Apart from the electronic motion of the target in the molecular collision process, collision probabilities strongly depend on the dynamics of nuclear motions (rotation and vibration). Here we have studied this nuclear dynamical dependency of collision cross sections for a positron–carbon monoxide collision using rovibrational close-coupling, rotational laboratory frame close-coupling (rotational LFCC), and LFCC adiabatic nuclear vibration (LFCC-ANV) methods. Here we have computed the angle-integrated elastic and state-to-state rotational excitation, the elastic and state-to-state vibrational (summed over rotational) excitation, and the total (summed over rotational and vibrational) cross sections for the incident positron energy between 0.0 and 7.0 eV using the rovibrational close-coupling method. The rotational LFCC method is also employed to calculate elastic, (state-to-state) rotational excitation, and total cross sections. To estimate the effects of the nuclear dynamics we have calculated vibrational elastic and state-to-state vibrational excitation cross sections using the LFCC-ANV method. In these calculations the model correlation-polarization potential is used to include the correlation-polarization potential. The effect of nuclear dynamics on the collision probabilities is discussed comparing calculated total cross sections using the rovibrational close-coupling and the rotational LFCC method with other theoretical calculations and experimental results. The discussion includes comparison of the present vibrational $0\to 1$ excitation process with the theoretical and measured values. The other vibrational and rotational elastic and excitation cross sections are also compared with the existing theoretical results and we have discussed the relevance of the effect of nuclear rotational and vibrational dynamics. We have also presented a comparison among the energy transfer parameters using different coupling schemes and have discussed the implications of the results.

The single ionization triple differential cross sections (TDCS) of the Ar ($3p$) atoms are reported for the positron and electron impact at 1 keV. The calculated cross sections have been obtained using distorted wave Born approximation (DWBA) approach for the average ejected electron energies 13 and 26 eV at different momentum transfer conditions. The present attempt is helpful to probe the information on the TDCS trends for the particle-matter and antiparticle-matter interactions and to analyze the recent measurements [Phy. Rev. A 95, 062703 (2017)]. The binary electron emission is enhanced while the recoil emission is decreased for the positron impact relative to the electron impact in the DWBA calculation results. Systematic shift of peaks, shifting away from the momentum transfer direction for positron impact and shifting towards each other for electron impact, is observed with increasing momentum transfer.

We report calculated elastic integral and differential cross sections for electron collisions with the hydrocarbons 1,3-butadiene (${\mathrm{C}}_4{\mathrm{H}}_6$), 2-methyl-1,3-butadiene (${\mathrm{C}}_5{\mathrm{H}}_8$), and 2,3-dimethyl-1,3-butadiene (${\mathrm{C}}_6{\mathrm{H}}_{10}$) for impact energies up to 15 eV. Our calculations were performed with the Schwinger Multichannel Method with pseudopotentials, in the static-exchange and static-exchange plus polarization approximations. These molecules differ for the presence of one methyl group, in the case of ${\mathrm{C}}_5{\mathrm{H}}_8$, and two methyl groups, in the case of ${\mathrm{C}}_6{\mathrm{H}}_{10}$ in substitution of one and two hydrogen atoms in ${\mathrm{C}}_4{\mathrm{H}}_6$, respectively (methylation effect). For the polar molecule 2-methyl-1,3-butadiene, we included the Born closure procedure in order to account for the long-range potential. We found two ${\pi }^{*}$ shape resonances in the integral cross section of each one of the molecules studied. The present results are also compared with the experimental values for the resonances positions and with total cross sections available in the literature. In particular, we show that the minimum in the total cross section of ${\mathrm{C}}_5{\mathrm{H}}_8$ located at around 1.6 eV and assigned by the authors as a Ramsauer-Townsend minimum is, actually, a valley between the two ${\pi }^{*}$ shape resonances. Also for the ${\mathrm{C}}_5{\mathrm{H}}_8$ molecule, the enhancement in the total cross section below 1.6 eV is the tail of the low-lying shape resonance and not an effect due to its permanent dipole moment, as suggested by the authors. We discuss the influence of the methylation effect in the shape and magnitude of the elastic cross sections and also in the location of the ${\pi }^{*}$ shape resonances of these hydrocarbons.

The transfer of the linear photon momentum to the electron in the few-photon ionization of an atom is accurately computed through a numerical solution to the three-dimensional time-dependent Schrödinger equation (TDSE) beyond the dipole approximation. The characteristics of the transfer is studied in detail by comparing the TDSE results with those calculated by the perturbation theory (PT) using the exact scattering states. Our fully quantum-mechanical calculations show a discernible photoelectron momentum shift along the laser's propagation direction, which is caused by the nondipole effects. The momentum transfer rule for the single-photon ionization is shown to agree with results obtained from the perturbation theory. We identify some extraordinary “dips” in the photon-momentum transfer in the case of two-photon ionization, which depend on the laser ellipticity and are caused by the suppression of the nondipole transitions. In addition, the Coulomb screening effect on the photon-momentum transfer has also been investigated and we find that the Coulomb tail is negligible in the single-photon ionization case. However, the potential in the short range near the Coulomb center may influence the initial state, which will change the amount of the momentum that can be transferred.

The dynamics following laser-induced molecular photodissociation of gas-phase ${\mathrm{CH}}_2\mathrm{BrI}$ at 271.6 nm were investigated by time-resolved Coulomb-explosion imaging using intense near-IR femtosecond laser pulses. The observed delay-dependent photofragment momenta reveal that ${\mathrm{CH}}_2\mathrm{BrI}$ undergoes C-I cleavage, depositing 65.6% of the available energy into internal product states, and that absorption of a second UV photon breaks the C-Br bond of ${\mathrm{CH}}_2\mathrm{Br}$. Simulations confirm that this mechanism is consistent with previous data recorded at 248 nm, demonstrating the sensitivity of Coulomb-explosion imaging as a real-time probe of chemical dynamics.

A system with externally polarized dipole molecules at half-filling moving along a one-dimensional zigzag chain is studied, including ground-state phase diagrams. The dipoles are oriented in-plane. Together with the geometry of the chain, this gives rise to a bond-alternating nearest-neighbor interaction due to simultaneous attractive and repulsive interactions. By tuning the ratio between the nearest-neighbor interaction and hopping, various phases can be accessed by controlling the polarization angle. In the ultrastrong coupling limit, the system simplifies to a frustrated extended axial Ising model. For the small coupling limit, qualitative discussion of the ordering behavior using effective field theory arguments is provided. We show that when the chain angle is small, the system mostly exhibits BKT-type phase transitions, whereasa large chain angle would drive the system into a gapped (Ising) dimerized phase, where the hopping strength is closely related to the orientation of dimerized pairs.

A tetragonal photonic crystal composed of high-index pillars can exhibit a frequency-isolated accidental degeneracy at a high-symmetry point in the first Brillouin zone. A photonic band gap can be formed there by introducing a geometrical anisotropy in the pillars. In this gap, gapless surface and domain-wall states emerge under a certain condition. We analyze their physical properties in terms of an effective Hamiltonian, and a good agreement between the effective theory and numerical calculation is obtained.

Using a nondegenerate four-wave mixing process based on a double-$\mathrm{\Lambda }$ scheme in hot cesium vapor, we generate quantum correlated twin beams with a maximum intensity-difference squeezing of 6.5 dB. The substantially improved squeezing can be mainly attributed to very good frequency and phase-difference stability between the pump and probe beams in our experiment. Intensity-difference squeezing can be observed within a wide experimental parameter range, which guarantees its robust generation. Since this scheme produces multi-spatial-mode twin beams at the Cs $D_1$ line, it is of interest for experiments involving quantum imaging and coherent interfaces between atomic and solid-state systems.

It has previously been shown that a dye-filled microcavity can produce a Bose-Einstein condensate of photons. Thermalization of photons is possible via repeated absorption and reemission by the dye molecules. In this paper, we theoretically explore the behavior of the polarization of light in this system. We find that, in contrast to the near complete thermalization between different spatial modes of light, thermalization of polarization states is expected to generally be incomplete. We show that the polarization degree changes significantly from below to above threshold, and explain the dependence of polarization on all relevant material parameters.

Lidar is a well-known optical technology for measuring a target's range and radial velocity. We describe two lidar systems that use entanglement between transmitted signals and retained idlers to obtain significant quantum enhancements in simultaneous measurements of these parameters. The first entanglement-enhanced lidar circumvents the Arthurs-Kelly uncertainty relation for simultaneous measurements of range and radial velocity from the detection of a single photon returned from the target. This performance presumes there is no extraneous (background) light, but is robust to the round-trip loss incurred by the signal photons. The second entanglement-enhanced lidar—which requires a lossless, noiseless environment—realizes Heisenberg-limited accuracies for both its range and radial-velocity measurements, i.e., their root-mean-square estimation errors are both proportional to $1/M$ when $M$ signal photons are transmitted. These two lidars derive their entanglement-based enhancements from the use of a unitary transformation that takes a signal-idler photon pair with frequencies ${\omega }_S$ and ${\omega }_I$ and converts it to a signal-idler photon pair whose frequencies are $({\omega }_S+{\omega }_I)/2$ and $({\omega }_S-{\omega }_I)/2$. Insight into how this transformation provides its benefits is provided through an analogy to continuous-variable superdense coding.

The supersensitivity attained in quantum phase estimation is known to be compromised in the presence of decoherence. This is particularly patent at blind spots—phase values at which sensitivity is totally lost. One remedy is to use a precisely known reference phase to shift the operation point to a less vulnerable phase value. Since this is not always feasible, we present here an alternative approach based on combining the probe with an ancillary degree of freedom containing adjustable parameters to create an entangled quantum state of higher dimension. We validate this concept by simulating a configuration of a Mach-Zehnder interferometer with a two-photon probe and a polarization ancilla of adjustable parameters, entangled at a polarizing beam splitter. At the interferometer output, the photons are measured after an adjustable unitary transformation in the polarization subspace. Through calculation of the Fisher information and simulation of an estimation procedure, we show that optimizing the adjustable polarization parameters using an adaptive measurement process provides globally supersensitive unbiased phase estimates for a range of decoherence levels, without prior information or a reference phase.

We propose a scheme for entangling a movable mirror and two atomic ensembles in a coupled hybrid optomechanical system. Using the logarithmic negativity for the entanglement measure, we analyze the effect of the coupling strength between the two cavities on the entanglement of the movable mirror and the atomic ensembles, and we analyze the effect of the atom-cavity coupling constant on the entanglement of the movable mirror and the atomic ensembles. It is found that the entanglements of the movable mirror with one atomic ensemble transfer to the movable mirror with another atomic ensemble, and the robustness of the entanglement is enhanced by the coupling between the two cavities. The entanglements between the movable mirror and atomic ensembles are enhanced together with atom-cavity coupling increasing, and the entanglements have maxima in the accessibly parameters regime of the atom-cavity coupling constant.