The decoy-state method has been developed rapidly in quantum key distribution (QKD) since it is immune to photon-number splitting attacks. We examine two-basis-detector-efficiency asymmetry, which exists in realistic scenarios. By using the recent 4-intensity decoy-state optimization protocol, we report an implementation of high-rate QKD with asymmetric basis-detector efficiency, demonstrating a 1.9 to 33.2 times higher key rate than previous protocols in the situation of large basis-detector-efficiency asymmetry. The results ruled out an implicit assumption in QKD that the efficiency of $Z$ basis and $X$ basis are restricted to be the same. This work paves the way towards a morepractical QKD setting.

We investigate the Casimir forces between high-$T_c$ superconductors as functions of the distance and temperature, focusing on optimally doped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.95}$. We consider formerly studied configurations in normal metals lying in the short-distance (50 nm–600 nm), and long-distance (600 nm–10 microns) regimes. The dielectric properties of the material are described in terms of weakly interacting, short-correlated conducting pairs transported along quasi-two-dimensional layers. In the short-range regime, a continuous behavior of the Casimir forces arises, with no significant discontinuity at the transition temperature. This behavior also follows in the equivalent the normal conductor configuration. In the long-range regime, the forces show an abrupt increment at the critical temperature. Simultaneously, the Casimir entropy and the specific heat develop a strong discontinuous behavior, characteristic of the superconducting-normal phase transition. In every situation, the entropy vanishes at extremely low temperatures.

Optical frequency measurements of the intercombination line $\left(6s^2\right){}^{1}S_0-\left(6s6p\right){}^{3}P_1$ in the isotopes of ytterbium are carried out with the use of sub-Doppler fluorescence spectroscopy on an atomic beam. A dispersive signal is generated to which a master laser is locked, while frequency counting of an auxiliary beat signal is performed via a frequency comb referenced to a hydrogen maser. The relative separations between the lines are used to evaluate the ${}^{3}P_1$-level magnetic dipole and electric quadrupole constants for the fermionic isotopes. The center of gravity for the ${}^{3}P_1$ levels in ${}^{171}\mathrm{Yb}$ and ${}^{173}\mathrm{Yb}$ are also evaluated, where we find significant disagreement with previously reported values. These hyperfine constants provide a valuable testbed for atomic many-body computations in ytterbium.

We report the observation of a dimer-dimer inelastic collision resonance for ultracold Feshbach molecules made of bosonic sodium and rubidium atoms. This resonance, which we attribute to the crossing of the dimer-dimer threshold with a heteronuclear tetramer state, manifests itself as a pronounced inelastic loss peak of dimers when the interspecies scattering length between the constituent atoms is tuned. Near this resonance, a strong modification of the temperature dependence of the dimer-dimer scattering is observed. Our result provides insight into the heteronuclear four-body system consisting of heavy and light bosons and offers the possibility of investigating ultracold molecules with tunable interactions.

We calculate the effective three-body force for bosons interacting with each other by a two-body potential tuned to a narrow zero crossing in any dimension. We use the standard two-channel model parametrized by the background atom-atom interaction strength, the amplitude of the open-channel to closed-channel coupling, and the atom-dimer interaction strength. The three-body force originates from the atom-dimer interaction, but it can be dramatically enhanced for narrow crossings, i.e., for small atom-dimer conversion amplitudes. This effect can be used to stabilize quasi-two-dimensional dipolar atoms and molecules.

We study, both classically and quantum mechanically, enantioselective orientation of gas-phase chiral molecules excited by laser fields with twisted polarization. Counterintuitively, the induced orientation, whose direction is laser controllable, does not disappear after the excitation but stays long after the end of the laser pulses. We computationally demonstrate this long-lasting orientation, using propylene oxide molecules (${\mathrm{CH}}_3{\mathrm{CHCH}}_2\mathrm{O}$, or PPO) as an example, and consider two kinds of fields with twisted polarization: a pair of delayed cross-polarized pulses and an optical centrifuge. This chiral effect opens avenues for detecting molecular chirality, measuring enantiomeric excess, and separating enantiomers with the help of inhomogeneous external fields.

Most chirped-pulse experiments refer to a theoretical study from McGurk, Schmalz, and Flygare [J. Chem. Phys. 60, 4181 (1974)] which is well tailored to interpret the signals obtained with very fast chirped pulses, but is not sufficient to account for the signals in the case of slower chirped pulses used in spectroscopy to increase the signal-to-noise ratio. A theoretical study of the polarization of molecules subjected to a chirped pulse in a cell, uniform supersonic flow, or molecular beam is presented. Three degrees of approximation for the polarization are introduced and are compared with the numerical solution of the optical Bloch equations. These expressions enter the analytic expression of the free-induction-decay signal which is validated against experimental data on the rotational emission spectra of OCS molecules. A relation among the pulse duration, the line position in the chirped pulse, and the signal amplitude is proposed in the thermalized case. It assists in the optimization of the chirped-pulse parameters and in the estimation of the error associated with the line intensity.

Transport of coherent waves in multiple-scattering media may exhibit fundamental, nonintuitive phenomena such as halt of diffusion by disorder called Anderson localization. For electromagnetic waves, this phenomenon was observed only in one and two dimensions so far. However, none of these experiments studied the contribution of reciprocal paths nor their manipulation by external fields. In order to weaken the effect of reciprocity of coherent wave transport on Anderson localization in one dimension, we studied light propagation through stacks of parallel Faraday-active glass slides exposed to magnetic fields up to 18 T. Measurements of light transmission statistics are presented and compared to one-dimensional (1D) transfer-matrix simulations. The latter reveals a self-organization of the polarization states in this system leading to a saturation of the Faraday rotation-induced reciprocity breaking, an increase of the localization length, and a decrease of transmission fluctuations when reciprocity is broken. This is confirmed experimentally for samples containing small numbers of slides while for larger samples a crossover from a 1D to a quasi-1D transport regime is found.

Hermite‐Gaussian (HG), Laguerre‐Gaussian (LG), and Ince-Gaussian (IG) modes have been of considerable importance for photonics. They have revealed a unique way to optical manipulation and are particularly promising for optical communications. This paper combines HG modes, as a basis, inside the nonlinear crystal and the generated second harmonic field turns to LG and IG modes. Here we present a way to use second order nonlinear media to convert optical beam modes as we wish.

In self-induced harmonic potentials, the coaxial superposition of a fundamental Gaussian beam and a (0,1) mode Laguerre-Gaussian beam yields a two-dimensional (2D)–deflection-steerable breathing beam. The magnitude (orientation) of the deflection can be steered by tuning the ratio (initial phase difference) between the two input constituent beams. Besides, the beam rotates around the deflecting straight trajectory the mass center (not the constituent beams' center) undergoes. The results provide an approach for the 2D steering of beam trajectory, which might be of interest for the applications in all-optical information processing.

A theoretical model based on two-point scatterers is suggested to investigate scattering of partially coherent radiation by a non-Hermitian localized structure, invariant under the simultaneous symmetry operations of parity inversion and time reversal. Within the first-order Born approximation, and the formalism of classical coherence theory, the spectral density of the scattered field in the far zone is obtained analytically. We find that the unidirectional character of the scattered radiation, which is one of the principal effects present in non-Hermitian structures, may be suppressed due to the coherence properties of the radiation without changing the geometry of the structure. Asymmetric spectral changes of the scattered radiation due to the non-Hermitian character of the material are also observed.

We propose an approach to analytically solve the quantum dynamics of bosonic systems. The method is based on reconstructing the quantum state of the system from the moments of its annihilation operators, dynamics of which is solved in the Heisenberg picture. The proposed dynamical reconstruction method is general in the sense that it does not require assumptions on the initial conditions of the system such as separability, or the structure of the system such as linearity. It is an alternative to the standard master-equation approaches, which are analytically demanding especially for large multipartite quantum systems. To demonstrate the proposed technique, we apply it to a system consisting of two coupled damped quantum harmonic oscillators.

We investigate the gauge invariance of the semiconductor Bloch equations (SBEs) in solid high-order harmonic generation (HHG). It is found that the gauge dependence of the SBEs can be attributed to the absence of Berry connection terms in the SBEs in previous studies. When the Berry connection terms are considered, the gauge invariance of the SBEs can be naturally preserved under an arbitrary global phase transformation. To satisfy the demand of the continuity of transition dipole matrix elements in one-dimensional calculations, we propose a gauge that is easy to implement numerically. Finally, the saddle point analysis shows that Berry connections can influence many properties of HHG, such as ellipticity dependence and even harmonic generation. This work uncovers the importance of the Berry connection terms that have been neglected for a long time in simulations of solid HHG.

We investigate the attosecond streaking spectrum of ${\mathrm{H}}_2{}^{+}$ by numerically solving the time-dependent Schrödinger equation. The results show that the streaking spectrum of ${\mathrm{H}}_2{}^{+}$ depends sensitively on the internuclear distance $R$. In addition to the regular streaking structure, some streaking spectra of ${\mathrm{H}}_2{}^{+}$ with the splitting and distorted structures are also observed as $R$ changes. The splitting and distorted streaking spectra of ${\mathrm{H}}_2{}^{+}$ are demonstrated to be related to the destructive two-center interference and resonant transition triggered by the IR field, respectively. Meanwhile, the mixture structure of split and distortion on the streaking spectrum of ${\mathrm{H}}_2{}^{+}$ is attributed to the coupling of the destructive two-center interference and resonant transition. The $R$-dependent streaking spectra might pave an accessible route toward monitoring the ultrafast nuclear dynamics of molecules.

Most interferometers operate with photons or dilute, noncondensed cold atom clouds in which collisions are strongly suppressed. Spinor Bose-Einstein condensates (BECs) provide an alternative route toward realizing three-mode interferometers; in this realization, spin-changing collisions provide a resource that generates mode entanglement. Working in the regime where the pump mode, i.e., the $m=0$ hyperfine state, has a much larger population than the side or probe modes ($m=\pm 1$ hyperfine states), $f=1$ spinor BECs approximate SU(1,1) interferometers. We derive analytical expressions within the undepleted pump approximation for the phase sensitivity of such an SU(1,1) interferometer for two classes of initial states: pure Fock states and coherent spin states. The interferometer performance is analyzed for initial states without seeding, with single-sided seeding, and with double-sided seeding. The validity regime of the undepleted pump approximation is assessed by performing quantum calculations for the full spin Hamiltonian. Our analytical results and the associated dynamics are expected to guide experiments as well as numerical studies that explore regimes where the undepleted pump approximation makes quantitatively or qualitatively incorrect predictions.

We present a proposal to realize and detect topological phase transition in optical lattices with alkaline-earth-metal atoms. Using state-dependent lattices and laser-assisted tunneling, we realize a dimerized spin-$1/2$ antiferromagnetic chain with next-nearest-neighbor coupling. We show that the spin chain hosts two symmetric topological phases and undergoes the topological phase transition when we change the alternating strength of nearest-neighbor couplings. We propose to measure the topological phase transition by detecting the entanglement spectrum and nonlocal string order with a quantum gas microscope. Furthermore, from measurement results, we demonstrate the difference of statistical properties between two topological phases. Our proposal can be a promising platform for simulation of topological phase transition.

We investigate optical nonlinear interactions in a dynamic environment by studying the generation of photons in spontaneous parametric down conversion inside a nonlinear cavity where the optical path length is periodically modulated in time. We show that the temporal dynamics of the cavity modify the nonlinear interaction and the generated continuous-variable time-frequency entangled biphoton state evolves into a tunable discrete higher-dimensional state in the nonadiabatic modulation regime where the modulation timescales are much faster than the photon lifetime. In this regime, the system mimics effects of a quantum random walk in a photonic lattice with many associated effects, including localized and delocalized wave functions of the generated photons. We also propose generation of time-frequency hyperentangled states in the adiabatic limit. Our analysis shows that the proposed system is promising for applications in quantum simulation and information processing in the time-frequency domain.

We introduce a method for the conditional generation of nonclassical states of light in a cavity. We consider two-level atoms traveling along the transverse direction to the cavity axis and show that by conditioning on one of the output measurements nonclassical field states are generated. The two-level atoms are prepared in the ground state and we conditioned them on the events in which they are also detected in the ground state. Nonclassical properties of the cavity mode are identified and characterized. This includes quadrature squeezing, sub-Poissonian photon-number distributions, and negative Wigner functions. We determine the optimal parameter regions where the corresponding nonclassical features are most distinct.

We consider a realistic model for calculating the cross-spectral density of partially coherent beams from an x-ray undulator in a modern storage ring. This two-point coherence function is seen to have a speckled structure associated with the presence of x-ray coherence vortices and domain walls. Such cross-spectral density speckle is associated with a network of spatial pairs of points for which there is zero correlation. X-ray coherence vortices and domain walls are seen to emerge naturally as the number of coherent modes required increases. An understanding of the existence and nature of such correlation singularities enhances our ability to exploit partially coherent x-ray radiation from new or upgraded synchrotron sources, for both imaging and diffraction applications.