We discuss the kinetics of the disordered interacting Bose gas using the Boltzmann transport equation. The theory may serve as a unifying framework for studying questions of dynamics of the expanding Bose gas at different stages of the expansion. We show that the transport theory allows us to straightforwardly reproduce and generalize a number of results previously obtained from microscopic models in different formalisms. Based on estimates for the interparticle scattering rates, we discuss the relevance of interaction effects for the localization problem in the interacting disordered Bose gas. We argue that, if the number of particles is large enough, the size of the expanding cloud may exceed the localization length. We describe the spreading of the wave packet in this regime as collision-induced diffusion and compare the obtained rate of expansion to known results on subdiffusive spreading in nonlinear disordered lattices.

The resolution of optical lithography and the spacing of atom lithography are limited by about half of the wavelength due to the Rayleigh limit. Here we propose a coherent atom lithography experiment which can print nanometer structures. In this proposal, we show that subwavelength spatial distribution of the atoms in the excited state can be achieved by inducing spatially modulated Rabi oscillations in two-level atoms. We use the internal energy of the atoms to excite the photoresist and it can then print the subwavelength structure into the photoresist. For the peak Rabi frequency of about 2 GHz and the recoil limited collimation, we show that sub-$10\text{-nm}$ spacing in both one- and two-dimensional space is possible.

We propose a class of synthetic optical materials in which the refractive index satisfies $n(-\mathit{x})=-n^{*}\left(\mathit{x}\right)$. We term such systems antisymmetric parity-time ($\mathcal{APT}$) structures. Unlike $\mathcal{PT}$-symmetric systems, which require balanced gain and loss, i.e., $n(-\mathit{x})=n^{*}\left(\mathit{x}\right)$, $\mathcal{APT}$ systems consist of balanced positive- and negative-index materials. Despite the seemingly $\mathcal{PT}$-symmetric optical potential $V\left(\mathit{x}\right)\equiv n{\left(\mathit{x}\right)}^2{\omega }^2/c^2$, $\mathcal{APT}$ systems are not invariant under combined $\mathcal{PT}$ operations due to the discontinuity of the spatial derivative of the wave function. We show that $\mathcal{APT}$ systems can display intriguing properties, such as spontaneous phase transition of the scattering matrix, a flat total transmission band, and a continuous lasing spectrum.

Polarization-resolved second-harmonic microscopy has recently emerged as a valuable technique for in situ imaging of collagen structure in tissues. Nevertheless, collagen-rich tissues such as tendon, ligament, skin dermis, bone, cornea, or artery exhibit a heterogeneous and anisotropic architecture that results in complex optical properties. While experimental evidence of polarization distortions has been reported in various tissues, the physics of second-harmonic imaging within such tissues is not fully understood yet. In this work, we performed numerical simulations of polarization-resolved second-harmonic generation in a strongly focused regime within a birefringent tissue. We show that vectorial components due to strong focusing have a rather small effect on the measurement of the second-harmonic tensorial response, while birefringence and optical dispersion may affect these measurements dramatically. We show indeed that a difference in the focal field distribution for ordinary and extraordinary waves results in different phase-matching conditions, which strongly affects the relative efficacy of second-harmonic generation for different polarizations. These results are of great interest for extracting reliable quantitative parameters from second-harmonic images.

We employ a fiber-based optical microcavity with high finesse to study the enhancement of phonon sideband fluorescence of nitrogen-vacancy centers in nanodiamonds. Harnessing the full tunability and open access of the resonator, we explicitly demonstrate the scaling laws of the Purcell enhancement by varying both the mode volume and the quality factor over a large range. While changes in the emission lifetime remain small in the regime of a broadband emitter, we observe an increase of the emission spectral density by up to a factor of 300. This gives a direct measure of the Purcell factor that could be achieved with this resonator and an emitter whose linewidth is narrower than the cavity linewidth. Our results show a method for the realization of wavelength-tunable narrow-band single-photon sources and demonstrate a system that has the potential to reach the strong-coupling regime.

We utilize genetic algorithms aided by simulated annealing to find optimal dynamical decoupling (DD) sequences for a single-qubit system subjected to a general decoherence model under a variety of control pulse conditions. We focus on the case of sequences with equal pulse intervals and perform the optimization with respect to pulse type and order. In this manner, we obtain robust DD sequences, first in the limit of ideal pulses, then when including pulse imperfections such as finite-pulse duration and qubit rotation (flip-angle) errors. Although our optimization is numerical, we identify a deterministic structure that underlies the top-performing sequences. We use this structure to devise DD sequences which outperform previously designed concatenated DD (CDD) and quadratic DD (QDD) sequences in the presence of pulse errors. We explain our findings using time-dependent perturbation theory and provide a detailed scaling analysis of the optimal sequences.

We study synthesis of optimal Clifford circuits and apply the results to peephole optimization of quantum circuits. We report optimal circuits for all Clifford operations with up to four inputs. We perform peephole optimization of Clifford circuits with up to 40 inputs found in the literature, and demonstrate a reduction in the number of gates by about 50%. We extend our methods to the synthesis of optimal linear reversible circuits, partially specified Clifford unitaries, and optimal Clifford circuits with five inputs up to input-output permutation. The results find their application in randomized benchmarking protocols, quantum error correction, and quantum circuit optimization.

We present a full analysis of the mode spectrum in a cavity formed by two parallel plates, one of which is magnetodielectric (metamaterial) while the other one is metallic, and obtain dispersion relations in closed form. The optical properties of the cavity walls are described in terms of realistic models for effective permittivity and permeability. Surface polaritons, i.e., electromagnetic modes that have at least partly an evanescent character, are shown to dominate the Casimir interaction at small separations. We analyze in detail the $s$-polarized polaritons, which are a characteristic feature of a magnetodielectric configuration, and discuss their role in the repulsive Casimir force.

We report enhanced sub-Doppler cooling of the bosonic atoms of ${}^{39}$K facilitated by formation of dark states with the cooling and repumping lasers tuned to the Raman resonance in $\Lambda$ configuration near the $D_1$ transition. A temperature of about 12 $\mu$K and phase-space density $>$$2\times {10}^{-5}$ is achieved in the two-stage $D_2$-$D_1$ molasses and spans a very large parameter region where quantum interference persists robustly. We also present results on enhanced radiation heating with a subnatural linewidth (0.07$\Gamma$) and a signature Fano-like profile of a coherently driven three-level atomic system. The optical Bloch equations relevant for the three-level atom in a bichromatic light field are solved with the method of continued fractions to show that cooling occurs only for a small velocity class of atoms, emphasizing the need for precooling in the $D_2$ molasses stage.

The use of retroreflection in light-pulse atom interferometry under microgravity conditions naturally leads to a double-diffraction scheme. The two pairs of counterpropagating beams induce simultaneously transitions with opposite momentum transfer that, when acting on atoms initially at rest, give rise to symmetric interferometer configurations where the total momentum transfer is automatically doubled and where a number of noise sources and systematic effects cancel out. Here we extend earlier implementations for Raman transitions to the case of Bragg diffraction. In contrast with the single-diffraction case, the existence of additional off-resonant transitions between resonantly connected states precludes the use of the adiabatic elimination technique. Nevertheless, we have been able to obtain analytic results even beyond the deep Bragg regime by employing the so-called “method of averaging,” which can be applied to more general situations of this kind. Our results have been validated by comparison to numerical solutions of the basic equations describing the double-diffraction process.

We consider the effect of a momentum kick on the ground state of a noninteracting two-dimensional Fermi gas subject to Rashba spin-orbit coupling. Although the total momentum is a constant of motion, the gas does not obey the rules of Galilean relativity. Upon imprinting a small overall velocity to the noninteracting gas, we find that the Fermi sea is deformed in a nontrivial way. We also consider a weakly repulsive Fermi gas and find, from its Hartree shift, that the total ground state of the system may change into a deformed, finite-momentum ground state as the repulsion is increased beyond a critical value, without the need for any external Zeeman fields. We also discuss possible experimental signatures of these effects.

We show that repulsive neutral-atom impurities in a dilute gas Bose-Einstein condensate (BEC) can self-localize in bubble polaron states formally analogous to electron bubbles in helium. The BEC is then the first impurity host medium known to exhibit both Landau-Pekar polaron states akin to that of self-localized electrons in a dielectric lattice and self-localized bubble polaron states. We find that the neutral BEC-impurity system is fully characterized by only two dimensionless coupling constants and that a single BEC impurity can be steered adiabatically from the Landau-Pekar to the bubble region. The adiabatic change is that of a crossover, not a transition.

The Uhlmann-Jozsa fidelity (or, equivalently, the Bures distance) is a basic concept of quantum communication and quantum information, which however is very difficult to measure efficiently without recourse to quantum tomography. Here we propose a direct experimental method to estimate the fidelity between two unknown two-qubit mixed states via the measurement of the upper and lower bounds of the fidelity, which are referred to as the superfidelity and subfidelity, respectively. Our method enables a direct measurement of the first- and second-order overlaps between two arbitrary two-qubit states. In particular, the method can be applied to measure the purity (or linear entropy) of a single two-qubit mixed state in a direct experiment. We also propose and critically compare several experimental strategies for measuring the sub- and superfidelities of polarization states of photons in various linear-optical setups.

It is well known that the violation of Bell's inequality in the form given by Clauser, Horne, Shimony, and Holt (CHSH) in two-qubit systems requires entanglement, but not vice versa, i.e., there are entangled states which do not violate the CHSH inequality. Here we compare some standard entanglement measures with violations of the CHSH inequality (as given by the Horodecki measure) for two-qubit states generated by Monte Carlo simulations. We describe states that have extremal entanglement according to the negativity, concurrence, and relative entropy of entanglement for a given value of the CHSH violation. We explicitly find these extremal states by applying the generalized method of Lagrange multipliers based on the Karush-Kuhn-Tucker conditions. The found minimal and maximal states define the range of entanglement accessible for any two-qubit states that violate the CHSH inequality by the same amount. We also find extremal states for the concurrence versus negativity by considering only such states which do not violate the CHSH inequality. Furthermore, we describe an experimentally efficient linear-optical method to determine the highest Horodecki degree of the CHSH violation for arbitrary mixed states of two polarization qubits. By assuming to have access simultaneously to two copies of the states, our method requires only six discrete measurement settings instead of nine settings, which are usually considered.

We perform a proof-of-principle demonstration of the measurement-device-independent quantum key distribution protocol using weak coherent states and polarization-encoded qubits over two optical fiber links of 8.5 km each. Each link was independently stabilized against polarization drifts using a full-polarization control system employing two wavelength-multiplexed control channels. A linear-optics-based polarization Bell-state analyzer was built into the intermediate station, Charlie, which is connected to both Alice and Bob via the optical fiber links. Using decoy states, a lower bound for the secret-key generation rate of 1.04$\times$10${}^{-6}$ bits/pulse is computed.

The problem of optimally discriminating between two completely unknown qubit states is generalized by allowing an error margin. It is visualized as a device—the programmable discriminator—with one data and two program ports, each fed with a number of identically prepared qubits—the data and the programs. The device aims at correctly identifying the data state with one of the two program states. This scheme has the unambiguous and the minimum-error schemes as extremal cases, when the error margin is set to zero or it is sufficiently large, respectively. Analytical results are given in the two situations where the margin is imposed on the average error probability—weak condition—or it is imposed separately on the two probabilities of assigning the state of the data to the wrong program—strong condition. It is a general feature of our scheme that the success probability rises sharply as soon as a small error margin is allowed, thus providing a significant gain over the unambiguous scheme while still having high confidence results.

We analyze the $XY$ model characterized by an anisotropy $\gamma$ in an external magnetic field, $h$, with respect to its genuine multipartite entanglement content (in the thermodynamic and finite-size case). Despite its simplicity we show that the quantity—detecting genuine multipartite entanglement through permutation operators and being a lower bound on measures—witnesses the presence of genuine multipartite entanglement for nearly all values of $\gamma$ and $h$. We further show that the phase transition and scaling properties are fully characterized by this multipartite quantity. Consequently, we provide a useful toolbox for other condensed-matter systems, where bipartite entanglement measures are known to fail.

We report an experimental study of the charge-transfer process in collisions of Xe${}^{q+}$ ions ($16\le q\le 20$) with magnesium atoms at an energy of 5.5$q$ keV. With charge-selective and time-coincidence techniques, we separated the pure capture and capture accompanied by transfer-ionization processes. The experimental data indicate that the magnesium target is around two times more likely to lose two electrons than one in the collision. This finding is very different compared to the calculation based on the extended classic over-the-barrier model. The Xe${}^{q+}$-Mg collision also behaves very differently from “traditional” collisions between highly charged ions and noble gases. We suggest a one-step dielectronic mechanism for the capture process. The data also show that autoionization dominates the relaxation process after the capture, and fluctuation of the autoionization fraction versus the projectile charge state indicates that for the relaxation processes, the projectile core structure plays a more important role than the detailed characteristics of the projectile states where the target electrons are initially captured.