Weak measurement is an important technique for the detection of the tiny spin-dependent splitting in photonic spin Hall effect. The weak measurement is only valid when the probe wave function remains almost undisturbed during the procedure of measurements. However, it does not always satisfy such condition in some practical situations, such as in the strong-coupling regime or when the preselected and postselected states are nearly orthogonal. In this paper, we develop a modified weak measurement for the detection of the photonic spin Hall effect when the probe wave function is distorted. We find that the measuring procedure with preselected and postselected ensembles is still effective. This scheme is important for us to detect the photonic spin Hall effect in the case where neither weak nor strong measurements can detect the spin-dependent splitting. The modified theory is valid not only in the weak-coupling regime but also in the strong-coupling regime, and especially in the intermediate regime. The theoretical models of conventional weak measurements and modified weak measurements are established and compared. We show that the experimental results coincide well with the predictions of the modified theory.

Quantum walks can be defined in two quite distinct ways: discrete-time and continuous-time quantum walks (DTQWs and CTQWs). For classical random walks, there is a natural sense in which continuous-time walks are a limit of discrete-time walks. Quantum mechanically, in the discrete-time case, an additional “coin space” must be appended for the walk to have nontrivial time evolution. Continuous-time quantum walks, however, have no such constraints. This means that there is no completely straightforward way to treat a CTQW as a limit of a DTQW, as can be done in the classical case. Various approaches to this problem have been taken in the past. We give a construction for walks on $d$-regular, $d$-colorable graphs when the coin flip operator is Hermitian: from a standard DTQW we construct a family of discrete-time walks with a well-defined continuous-time limit on a related graph. One can think of this limit as a “coined” continuous-time walk. We show that these CTQWs share some properties with coined DTQWs. In particular, we look at a spatial search by a DTQW over the two-dimensional (2D) torus (a grid with periodic boundary conditions) of size $\sqrt{N}\times \sqrt{N}$, where it was shown that a coined DTQW can search in time $O(\sqrt{N}logN)$, but a standard CTQW takes $\mathrm{\Omega }\left(N\right)$ time to search for a marked element. The continuous limit of the DTQW search over the 2D torus exhibits the $O(\sqrt{N}logN)$ scaling, like the coined walk it is derived from. We also look at the effects of graph symmetry on the limiting walk, and show that the properties are similar to those of the DTQW as shown in Krovi and Brun, Phys. Rev. A 75, 062332 (2007).

The employ of a heralded noiseless linear amplifier has been proven as a useful tool for mitigating imperfections in quantum channels. Its analysis is usually conducted within specific frameworks, for which the set of input states for a given protocol is fixed. Here we obtain a more general description by showing that a noisy and lossy Gaussian channel followed by a heralded noiseless linear amplifier has a general description in terms of effective channels. This has the advantage of offering a simpler mathematical description, best suited for mixed states, both Gaussian and non-Gaussian. We investigate the main properties of this effective system, and illustrate its potential by applying it to loss compensation and reduction of phase uncertainty.

The electron-photon coincidence method in the coherence analysis version has been applied to characterize electron impact excitation of $4{}^1{P}_1$ state of zinc atoms for 80 eV. The experimental values of the Stokes parameters and the electron impact coherence parameters are presented together with convergent close coupling theoretical data. Our results are compared with recently published relativistic distorted-wave approximation calculations.

Single charge exchange in collisions between bare projectiles and heliumlike atomic systems at intermediate and high incident energies is examined by using the four-body formalism of the first- and second-order theories. The main purpose of the present study is to investigate the relative importance of the intermediate ionization continua of the captured electron compared to the usual direct path of the single electron transfer from a target to a projectile. In order to achieve this goal, comprehensive comparisons are made between the four-body boundary-corrected continuum-intermediate-states (BCIS-4B) method and the four-body boundary-corrected first Born (CB1-4B) method. The perturbation potential is the same in the CB1-4B and BCIS-4B methods. Both methods satisfy the correct boundary conditions in the entrance and exit channels. However, unlike the CB1-4B method, the second-order BCIS-4B method takes into account the electronic Coulomb continuum-intermediate states in either the entrance or the exit channel depending on whether the post or the prior version of the transition amplitude is used. Hence, by comparing the results from these two theories, the relative importance of the intermediate ionization electronic continua can be assessed within the four-body formalism of scattering theory. The BCIS-4B method predicts the usual second-order effect through double scattering of the captured electron on two nuclei as a quantum-mechanical counterpart of the Thomas classical two-step, billiard-type collision. The physical mechanism for this effect in the BCIS-4B method is also comprised of two steps such that ionization occurs first. This is followed by capture of the electron by the projectile with both processes taking place on the energy shell. Moreover, the role of the second, noncaptured electron in a heliumlike target is revisited. To this end, the BCIS-4B method describes the effect of capture of one electron by the interaction of the projectile nucleus with the other electron via the static electron-electron correlations in the target. This effect yields a novelty seen as the second Thomas peak. As an illustration, detailed computations were carried out involving both the differential and total cross sections for one-electron capture in the $p-\mathrm{He}$ collisions at intermediate and high impact energies. The results obtained in the BCIS-4B method are compared with those from the CB1-4B method and with the available experimental data. The overall usefulness of the BCIS-4B method is assessed in predicting experimental data for four-body single charge exchange both qualitatively (shapes of cross sections) and quantitatively (numerical values from measurements).

The presence of (approximate) conservation laws can prohibit the fast relaxation of interacting many-particle quantum systems. We investigate this physics by studying the center-of-mass oscillations of two species of fermionic ultracold atoms in a harmonic trap. If their trap frequencies are equal, a dynamical symmetry (spectrum-generating algebra), closely related to Kohn's theorem, prohibits the relaxation of center-of-mass oscillations. A small detuning $\delta \omega$ of the trap frequencies for the two species breaks the dynamical symmetry and ultimately leads to a damping of dipole oscillations driven by interspecies interactions. Using memory-matrix methods, we calculate the relaxation as a function of frequency difference, particle number, temperature, and strength of interspecies interactions. When interactions dominate, there is almost perfect drag between the two species and the dynamical symmetry is approximately restored. The drag can either arise from Hartree potentials or from friction. In the latter case (hydrodynamic limit), the center-of-mass oscillations decay with a tiny rate, $1/\tau \propto {\left(\delta \omega \right)}^2/\mathrm{\Gamma }$, where $\mathrm{\Gamma }$ is a single-particle scattering rate.

We study fermions in a lattice, with on-site and nearest neighbor attractive interactions between two spin species. We consider two geometries: (1) both spins in a triangular lattice and (2) a mixed geometry with up spins in honeycomb and down spins in triangular lattices. We focus on the interplay between spin-population imbalance, on-site and valence bond pairing, and order parameter symmetry. The mixed geometry leads to a rich phase diagram of topologically nontrivial phases. In both geometries, we predict order parameters with simultaneous time-reversal and translational symmetry breaking.

In this article, we calculate the friction between two counterflowing bosonic and fermionic superfluids. In the limit where the boson-boson and boson-fermion interactions can be treated within the mean-field approximation, we show that the force can be related to the dynamical structure factor of the fermionic component. Finally, we provide asymptotic expressions for weakly and strongly attractive fermions and show that the damping rate obeys simple scaling laws close to the critical velocity.

In this paper, we show that photon blockade can be observed in a system with weak Kerr nonlinearities. The system consists of two spatially overlapping single-mode semiconductor cavities with tunable one- and two-photon tunneling. We find that both conventional and unconventional single-photon blockades are controllable by manipulating the phase in the hopping regardless of the weakness of nonlinearity. We refer to the photon blockade with weak nonlinearities as an unconventional photon blockade. The unconventional photon blockade effect originates from the destructive interference between different paths from the ground state to two-photon states. We analytically derive the exact optimal conditions for strong antibunching, which are in good agreement with those obtained by numerical simulations. The optimal parameters for photon blockade are derived. Based on these parameters, the semiconductor cavity systems can be used as a single-photon source in regimes of both weak and strong nonlinearities.

A route toward line-shape engineering of Fano resonances in photonic structures is theoretically proposed, which uses dynamic modulation of the refractive index of a microcavity. The method is exemplified by considering coupled-resonator optical waveguide systems. An exact Floquet analysis, based on coupled-mode theory, is presented. Two distinct kinds of resonances can be dynamically created, depending on whether or not the static structure sustains a localized mode. In the former case, a single Fano resonance arises, which can be tuned in both frequency and linewidth by varying the refractive index modulation amplitude and frequency. In the latter case, two resonances, in the form of narrow asymmetric dips in the transmittance, are found, which can overlap resulting in an electromagnetically induced transparency effect.

Transition frequencies between low-lying energy levels in a single trapped ${}^{138}\mathrm{Ba}^{+}$ ion have been measured with laser spectroscopy referenced to an optical frequency comb. By extracting the frequencies of one-photon and two-photon components of the line shape using an eight-level optical Bloch model, we achieved 0.1 MHz accuracy for the $5d{}^2{D}_{3/2}-6p{}^2{P}_{1/2}$ and $6s{}^2{S}_{\text{1/2}}-5d{}^2{D}_{3/2}$ transition frequencies, and 0.2 MHz for the $6s{}^2{S}_{\text{1/2}}-6p{}^2{P}_{1/2}$ transition frequency.

Macroscopic realism, the classical world view that macroscopic objects exist independently of and are not influenced by measurements, is usually tested using Leggett-Garg inequalities. Recently, another necessary condition called no-signaling in time (NSIT) has been proposed as a witness for nonclassical behavior. In this paper, we show that a combination of NSIT conditions is not only necessary but also sufficient for a macrorealistic description of a physical system. Any violation of macroscopic realism must therefore be witnessed by a suitable NSIT condition. Subsequently, we derive an operational formulation for NSIT in terms of positive operator-valued measures and the system Hamiltonian. We argue that this leads to a suitable definition of “classical” measurements and Hamiltonians, and we apply our formalism to some generic coarse-grained quantum measurements.

Electron elastic-scattering phase shifts and cross sections as well as total cross sections of low-frequency bremsstrahlung and its angular-asymmetry and polarization parameters upon low-energy electron collision with endohedral fullerenes $A@{\mathrm{C}}_{60}$ are theoretically scrutinized versus the nature, size, and spin of the encapsulated atom $A$. This is achieved by choosing Ar, Xe, Ba, Cr, Mn, and Eu as the case-study atoms $A$. The aim is to uncover the variety of effects which might occur in the above processes rather than to make thorough predictions for one particular spectrum. To that end, the study makes use of a simple model static-exchange approximation. There, both the encapsulated atom $A$ and the ${\mathrm{C}}_{60}$ cage are regarded as nonpolarizable targets and the ${\mathrm{C}}_{60}$ cage itself is modeled by an attractive spherical annular potential well. Calculated results identify the most interesting and/or useful future measurements or more rigorous calculations to perform.

Recent experiments have shown that the enhanced charge transfer by Rydberg excited clusters (ECTREC) reduces the highly charged ions very efficiently to neutral atoms and negative ions with little loss of momentum. Neutral-atom emission is anisotropic with respect to the laser polarization and the anisotropy is larger than that of the ion emission from Coulomb explosion of isolated single clusters. In such a scenario, it is expected that the negative-ion emission (like neutrals) should be anisotropic and have larger propensity along the laser polarization than in the perpendicular direction. Further, it may be anticipated that negative-ion emission is more anisotropic than neutral-atom emission if ECTREC is taken in to account. We demonstrate that the negative-ion emission is anisotropic. Contrary to expectations, the negative-ion emission anisotropy is not more than that of the neutral-atom emission. We show that this can be rationalized if low-energy (about 10 eV) electron collisional detachment of the negative ions is taken into account. Electron collisional detachment depletes the negative-ion yield preferentially along the laser polarization direction and reduces the negative-ion emission anisotropy.

We investigate the nonadiabatic dynamics of a driven spin-orbit-coupled Bose-Einstein condensate in both weak and strong driven force. It is shown that the standard Landau-Zener (LZ) tunneling fails in the regime of weak driven force and/or strong spin-orbital coupling, where the full nonadiabatic dynamics requires a new mechanism through multichannel effects. Beyond the semiclassical approach, our numerical and analytical results show an oscillating power-law decay in the quantum limit, different from the exponential decay in the semiclassical limit of the LZ effect. Furthermore, the condensate density profile is found to be dynamically fragmented by the multichannel effects and enhanced by interaction effects. Our work therefore provides a complete picture to understand the nonadiabatic dynamics of a spin-orbit coupled condensate, including various ranges of driven force and interaction effects through multichannel interference. The experimental indication of these nonadiabatic dynamics is also discussed.

We investigate the fragmented many-body ground states of a spin-2 Bose gas in zero magnetic field. We point out that the exact ground state is not simply an average over rotationally-invariant mean-field states, in contrast to the spin-1 case with even number of particles $N$. While for some certain parameters the exact ground state is an averaged mean-field state like in the spin-1 case, for other parameters this is not so. We construct the exact ground states and compare them with the angular-averaged polar and cyclic states. The angular-averaged polar states in general fail to retrieve the exact eigenstate at $N\ge 6$ while angular-averaged cyclic states sustain only for $N$ with a multiple of 3. We calculate the density matrices and two-particle density matrices to show how deviant the angular-averaged state is from the exact one.

In this paper we show that the coherence properties of the radiation emanating from an aperture in the single wall of a half-space blackbody cavity can be determined by considering how free-space blackbody radiation is scattered by those apertures. That is, although the presence of the cavity wall modifies the blackbody radiation field, it has no effect on the field components that are relevant in the scattering computations. This conclusion is essential since it justifies the methodology employed in earlier papers. Furthermore, our analysis shows that this computationally simple procedure is rigorous only for the half-space geometry and it also gives indications of when the results obtained by the procedure provide good approximations of the radiated field properties. This work thereby defines the domain of validity of earlier results connected with the coherence properties of blackbody radiation outside the cavity.

We propose a scheme for the ultrafast control of the emitter-field coupling rate in cavity quantum electrodynamics. This is achieved by the control of the vacuum field seen by the emitter through a modulation of the optical modes in a coupled-cavity structure. The scheme allows the on-off switching of the coupling rate without perturbing the emitter and without introducing frequency chirps on the emitted photons. It can be used to control the shape of single-photon pulses for high-fidelity quantum state transfer, to control Rabi oscillations, and as a gain-modulation method in lasers. We discuss two possible experimental implementations based on photonic crystal cavities and on microwave circuits.