We study the dynamics of a dipolar Bose-Einstein condensate, like, for example, a ${}^{52}\mathrm{Cr}$ or ${}^{164}\mathrm{Dy}$ condensate, interacting with a superconducting surface. The magnetic dipole moments of the atoms in the Bose-Einstein condensate induce eddy currents in the superconductor. The magnetic field generated by eddy currents modifies the trapping potential such that the center-of-mass oscillation frequency is shifted. We numerically solve the Gross-Pitaevskii equation for this system and compare the results with analytical approximations. We present an approximation that gives excellent agreement with the numerical results. The eddy currents give rise to anharmonic terms, which leads to the excitation of shape fluctuations of the condensate. We discuss how the strength of the excitation of such modes can be increased by exploiting resonances, and we examine the strength of the resonances as a function of the center-of-mass oscillation amplitude of the condensate. Finally, we study different orientations of the magnetic dipoles and discuss favorable conditions for the experimental observation of the eddy-current effect.

The many-electron weak-field asymptotic theory (ME-WFAT) of tunneling ionization [Tolstikhin et al., Phys. Rev. A 89, 013421 (2014)] is applied to atoms. The procedure to extract the asymptotic coefficient of a Dyson orbital needed to implement the ME-WFAT from many-electron wave functions given by a linear combination of Slater determinants composed of one-electron orbitals, as is typically the case in practical atomic structure calculations, is discussed. It is shown that in the one-configuration approximation such wave functions enable one to consistently implement the theory and calculate the ionization rate. The effect of relaxation of the ionic orbitals and the dependence of the rate on the total orbital momentum and spin states of the atom and ion are considered. However, wave functions constructed by mixing several electronic configurations, maybe more accurate in some sense, do not have the correct asymptotic behavior required for implementing the ME-WFAT. The theory is illustrated by calculations for atoms of the first three periods with the use of one-electron orbitals obtained by the Hartree-Fock method.

We use a finite-temperature effective field theory recently developed for superfluid Fermi gases to investigate the properties of dark solitons in these superfluids. Our approach provides an analytic solution for the dip in the order parameter and the phase profile across the soliton, which can be compared with results obtained in the framework of the Bogoliubov–de Gennes equations. We present results in the whole range of the BCS-BEC crossover, for arbitrary temperatures and taking into account Gaussian fluctuations about the saddle point. The obtained analytic solutions yield an exact energy-momentum relation for a dark soliton showing that the soliton in a Fermi gas behaves like a classical particle even at nonzero temperatures. The spatial profile of the pair field and for the parameters of state for the soliton are analytically studied.

We study single-photon transport in a single-mode waveguide, in which two-level atoms in spatial superpositions are embedded. We find that the transmission of the photon can be used as a nondestructive probe for the coherence of the superposition. Under certain trapping configurations, the protocol proposed is shown to be independent of which wells the atoms are initially trapped in. Furthermore, we show that by looking at the transmission in a suitable regime, a maximally entangled state can be postselected from an unknown superposition.

We propose a scheme for single-atom, quantum control of the direction of propagation of a coherent field incident, via a tapered fiber, upon a microtoroidal whispering-gallery-mode (WGM) resonator. The scheme involves overcoupling of the fiber taper to the resonator and strong coupling of an atom to the evanescent field of the WGM, i.e., an atom-field coupling that exceeds the total WGM linewidth. In contrast to previous, related schemes that operate in the bad-cavity regime, the proposed scheme can operate effectively with much stronger incident fields, while also preserving their coherent nature. It can also serve to prepare an entangled state of the atom and coherent optical pulses propagating in opposite directions along the fiber. We evaluate the fidelity of preparation of such a state taking into account absorption and atomic spontaneous emission and demonstrate that high fidelities should be possible with realistic parameters.

We study a modified Ramsey spectroscopy technique employing slowly decaying states for quantum metrology applications using dense ensembles. While closely positioned atoms exhibit super-radiant collective decay and dipole-dipole induced frequency shifts, recent results [L. Ostermann, H. Ritsch, and C. Genes, Phys. Rev. Lett. 111, 123601 (2013)] suggest the possibility to suppress such detrimental effects and achieve an even better scaling of the frequency sensitivity with interrogation time than for noninteracting particles. Here we present an in-depth analysis of this “protected subspace Ramsey technique” using improved analytical modeling and numerical simulations including larger three-dimensional (3D) samples. Surprisingly we find that using subradiant states of $N$ particles to encode the atomic coherence yields a scaling of the optimal sensitivity better than $1/\sqrt{N}$. Applied to ultracold atoms in 3D optical lattices we predict a precision beyond the single atom linewidth.

Homomorphic encryption is a form of encryption which allows computation to be carried out on the encrypted data without the need for decryption. The success of quantum approaches to related tasks in a delegated computation setting has raised the question of whether quantum mechanics may be used to achieve information-theoretically-secure fully homomorphic encryption. Here we show, via an information localization argument, that deterministic fully homomorphic encryption necessarily incurs exponential overhead if perfect security is required.

Within exact electron density-functional theory, we investigate Kohn-Sham (KS) potentials, orbital energies, and noninteracting kinetic energies of the fractional ions of Li, C, and F. We use quantum Monte Carlo densities as input, which are then fitted, interpolated at noninteger electron numbers $N$, and inverted to produce accurate KS potentials $v_{\mathrm{s}}^N\left(\mathbf{r}\right)$. We study the dependence of the KS potential on $N$, and in particular we numerically reproduce the theoretically predicted spatially constant discontinuity of $v_{\mathrm{s}}^N\left(\mathbf{r}\right)$ as $N$ passes through an integer. We further show that, for all the cases considered, the inner orbital energies and the noninteracting kinetic energy are nearly piecewise linear functions of $N$. This leads us to propose a simple approximation of the KS potential $v_{\mathrm{s}}^N\left(\mathbf{r}\right)$ at any fractional electron number $N$ which uses only quantities of the systems with the adjacent integer electron numbers.

We derive exact analytic solutions of nonlinear Dirac equations with Kerr-like cubic nonlinearity. The equations model, among many physical systems, a Bose-Einstein condensate confined by a two-dimensional honeycomb optical lattice as well as relativistic solitons in optical waveguide arrays. The nonrelativistic limit of the solutions is derived, and the role of the nonlinearity on localization is discussed. The possibility of realizing some of these solutions in waveguide arrays and Bose-Einstein condensates is pointed out.

We investigate the disturbance of the state of a quantum system in a protective measurement for finite measurement times and different choices of the time-dependent system-apparatus coupling function. The ability to minimize this state disturbance is essential to protective measurement. We show that for a coupling strength that remains constant during the measurement interaction of duration $T$, the state disturbance scales as $T^{-2}$, while a simple smoothing of the coupling function significantly improves the scaling behavior to $T^{-6}$. We also prove that the shift of the apparatus pointer in the course of a protective measurement is independent of the particular time dependence of the coupling function, suggesting that the guiding principle for choosing the coupling function should be the minimization of the state disturbance. Our results illuminate the dynamics of protective measurement under realistic circumstances and may aid in the experimental realization of such measurements.

We investigate the construction of two universal three-qubit quantum gates in a hybrid system. The designed system consists of a flying photon and a stationary negatively charged nitrogen-vacancy (NV) center fixed on the periphery of a whispering-gallery-mode (WGM) microresonator, with the WGM cavity coupled to tapered fibers functioning as an add-drop structure. These gate operations are accomplished by encoding the information both on the spin degree of freedom of the electron confined in the NV center and on the polarization and spatial-mode states of the flying photon, respectively. Compared with previous schemes, our proposed scheme leads to a reduction of time and resource consumption in the processing. Moreover, these gates work in a deterministic way with no extra qubits required, which is feasible with current technologies. Both proposed gate operations could be physically implemented in one step, which provides a promising approach for quantum information processing ranging from distributed quantum computation to long-distance quantum communication.

The accessible information and the informational power quantify the amount of information extractable from a quantum ensemble and by a quantum measurement, respectively. So-called spherical quantum 2-designs constitute a class of ensembles and measurements relevant in testing entropic uncertainty relations, quantum cryptography, and quantum tomography. We provide lower bounds on the entropy of 2-design ensembles and measurements, from which upper bounds on their accessible information and informational power follow, as a function of the dimension only. We show that the statistics generated by 2-designs, although optimal for the above-mentioned protocols, never contains more than 1 bit of information. Finally, we specialize our results to the relevant cases of symmetric informationally complete sets and maximal sets of mutually unbiased bases, and we generalize them to the arbitrary-rank case.

We study discrimination of $m$ quantum measurements in the scenario when the unknown measurement with $n$ outcomes can be used only once. We show that ancilla-assisted discrimination procedures provide a nontrivial advantage over simple (ancilla-free) schemes for perfect distinguishability and we prove that inevitably $m\le n$. We derive necessary and sufficient conditions of perfect distinguishability of general binary measurements. We show that the optimization of the discrimination of projective qubit measurements and their mixtures with white noise is equivalent to the discrimination of specific quantum states. In particular, the optimal protocol for discrimination of projective qubit measurements with fixed failure rate (exploiting maximally entangled test state) is described. While minimum-error discrimination of two projective qubit measurements can be realized without any need of entanglement, we show that discrimination of three projective qubit measurements requires a bipartite probe state. Moreover, when the measurements are not projective, the non-maximally entangled test states can outperform the maximally entangled ones. Finally, we rephrase the unambiguous discrimination of measurements as quantum key distribution protocol.

Chiral molecules exist in two configurations that are nonsuperposable mirror images of one another. The underlying molecular structure is referred to as the absolute configuration. In chiral environments, the handedness of molecules influences their chemical characteristics dramatically, and therefore the determination of absolute configurations is of fundamental interest in organic chemistry and biology. Commonly applied techniques to assign absolute configuration are anomalous single-crystal x-ray diffraction and vibrational circular dichroism. However, these techniques become increasingly more challenging when applied to molecules that are made out of light atoms exclusively. Furthermore, there is no established method to determine the absolute handedness of gas-phase molecules that are not optically active. In this work, we apply the foil-induced Coulomb explosion imaging technique to determine directly the absolute configuration of the chiral molecule trans-2,3-dideuterooxirane (${\mathrm{C}}_2{\mathrm{OD}}_2{\mathrm{H}}_2$) in the gas phase. The experiment leads to the definitive assignment of the ($R,R$) configuration to an enantio-selected dideuterooxirane sample with a statistical confidence of $5\sigma$. As the handedness of trans-2,3-dideuterooxirane is unambiguously linked by chemical synthesis to the stereochemical key reference glyceraldehyde, our results provide an independent verification of the absolute configuration of the stereochemical reference standard.

The effect of the Coriolis interaction upon the sharing of energy between rotational and vibrational excitation during an electronic transition is considered with particular emphasis on recoil-induced excitation during photoionization. If there is a large change in equilibrium bond length upon ionization, then Coriolis coupling leads to a significant transfer of energy between rotational and vibrational excitation. Experimental results for valence ionization of ${\mathrm{N}}_2$ and CO and for carbon $1s$ ionization of CO show evidence of this effect.

The hyperfine structure (hfs) splittings of the $4D_{5/2}$ state for two isotopes of ${}^{87}\mathrm{Rb}$ and ${}^{85}\mathrm{Rb}$ atoms are measured based on double-resonance optical pumping spectra in a $5S_{1/2}\text{−}5P_{3/2}\text{−}4D_{5/2}$ ladder-type atomic system. The frequency calibration is performed by employing a wideband fiber-pigtailed phase-type electro-optic modulator together with a Fabry-Pérot cavity to cancel the error arising from nonlinear frequency scanning. The hfs magnetic dipole constant $A$ of the $4D_{5/2}$ state is determined to be −16.801 ± 0.005 MHz for ${}^{87}\mathrm{Rb}$ and −4.978 ± 0.004 MHz for ${}^{85}\mathrm{Rb}$. The hfs electric quadrupole constant $B$ of the $4D_{5/2}$ state is determined to be 3.645 ± 0.030 MHz for ${}^{87}\mathrm{Rb}$ and 6.560 ± 0.052 MHz for ${}^{85}\mathrm{Rb}$. The values of $A$ and $B$ for the ${}^{87}\mathrm{Rb}4D_{5/2}$ state are twice as accurate as previous work with thermal atoms using a femtosecond laser comb and the values of $A$ and $B$ for the ${}^{85}\mathrm{Rb}4D_{5/2}$ state are 3 times and 25 times more accurate than previous work in laser-cooled atoms using Fabry–Pérot interferometer, respectively. According to this high precision of the hfs constants and the previously measured nuclear $g$ factors of the two isotopes, the value of the $d$-electron hyperfine anomaly ${}^{87}\mathrm{\Delta }{}^{85}\left(4D_{5/2}\right)$ is derived to be −0.0041 ± 0.0009.

The feasibility of laser cooling $M\mathrm{H}$ ($M=\mathrm{Be},\mathrm{Mg},\mathrm{Ca},\mathrm{Sr},\mathrm{and}\mathrm{Ba}$) is investigated using ab initio quantum chemistry. The ground state $X{}^2{\Sigma }^{+}$ and first excited state $A{}^2\Pi$ of $M\mathrm{H}$ species are calculated using the multireference configuration interaction (MRCI) level of theory. For $\mathrm{CaH}$, $\mathrm{SrH}$, and $\mathrm{BaH}$, the spin-orbit coupling effects are also taken into account in electronic structure calculations at the MRCI level. Calculated spectroscopic constants for ${}^9\mathrm{BeH}$, ${}^{24}\mathrm{MgH}$, ${}^{40}\mathrm{CaH}$, ${}^{86}\mathrm{SrH}$, and ${}^{138}\mathrm{BaH}$ show good agreement with available theoretical and experimental results. The permanent dipole moments and transition dipole moments (TDMs) of the $X{}^2{\Sigma }^{+}$ and $A{}^2\Pi$ states of $M\mathrm{H}$ species are also calculated at the MRCI level of theory, and they are in good agreement with previous theoretical results. With the potential energy curves and TDMs obtained, the highly diagonally distributed Franck-Condon factors $f_{00}\left({}^9\mathrm{BeH}:0.998{,}^{24}\mathrm{MgH}:0.954{,}^{40}\mathrm{CaH}:0.961{,}^{86}\mathrm{SrH}:0.978,\mathrm{and}{}^{138}\mathrm{BaH}:0.971\right)$ for the $A{}^2\Pi \left(v^{\prime }=0\right)\to X{}^2{\Sigma }^{+}\left(v=0\right)$ transition are determined. Radiative lifetime calculations of the $A{}^2\Pi \left(v^{\prime }=0\right)$ state (${}^9\mathrm{BeH}:82.0\mathrm{ns}$, ${}^{24}\mathrm{MgH}:48.6\mathrm{ns}$, ${}^{40}\mathrm{CaH}:33.3\mathrm{ns}$, ${}^{86}\mathrm{SrH}:33.2\mathrm{ns}$, and ${}^{138}\mathrm{BaH}$: 68.6 ns) are found to be short enough for rapid laser cooling. The proposed main cycling laser drives the $X{}^2{\Sigma }^{+}\left(v=0\right)\to A{}^2\Pi \left(v^{\prime }=0\right)$ transition at wavelength ${\lambda }_{00}\left({}^9\mathrm{BeH}:497.2\mathrm{nm}{,}^{24}\mathrm{MgH}:525.5\mathrm{nm}{,}^{40}\mathrm{CaH}:675.4\mathrm{nm}{,}^{86}\mathrm{SrH}:740.3\mathrm{nm},\mathrm{and}{}^{138}\mathrm{BaH}:952.6\mathrm{nm}\right)$. The results demonstrate the possibility of laser cooling $M\mathrm{H}$ species, and provide a promising theoretical reference for further theoretical and experimental research on alkaline-earth-metal monohydrides.

Cross sections for the dissociative recombination of ${\mathrm{N}}_2^{+}$ for $v_i^{+}=0–3$ are computed using multichannel quantum defect theory with molecular data generated using the $R$-matrix method. The calculation is completely ab initio and includes three electronic cores of the ion. Extensive comparisons are made with previous experimental and theoretical studies. Our cross section is in excellent agreement with experimental results and other theoretical results. Cross sections and isotropic rate coefficients are provided for all computed vibrational levels.

We investigate theoretically the single and double ionization of the He atom by antiproton impact for projectile energies ranging from 3 keV up to 1000 keV. We obtain accurate total cross sections by directly solving the fully correlated two-electron time-dependent Schrödinger equation. The cross sections are in excellent agreement with the available experimental data. We also present fully ab initio doubly differential data for single ionization at 10 and 100 keV impact energies and compare to classical-trajectory Monte Carlo calculations. In these differential cross sections we identify the binary-encounter peak along with the anticusp minimum. Furthermore, we also point out the importance of the postcollisional electron-projectile interaction at low antiproton energies, which significantly suppresses electron emission in the forward direction.

We report integral and differential cross sections for elastic and electronically inelastic ($X{}^1{A}_g\to \tilde{a}{}^3{B}_{1u}$) electron scattering by ethylene. The Schwinger multichannel method with pseudopotentials in the $N_{\mathrm{open}}$-channel-coupling scheme at the static-exchange-plus-polarization approximation is employed to calculate the scattering amplitudes at impact energies ranging from 5.7 to 50 eV. We discuss the multichannel-coupling effects in the calculated cross sections, in particular, how the number of excited states included in the open-channel space impacts the convergence of the elastic and the ($X{}^1{A}_g\to \tilde{a}{}^3{B}_{1u}$) excitation cross sections at higher collision energies. We found good agreement between the present calculated total cross section (which includes elastic, inelastic, and ionization contributions, the latter estimated with the binary-encounter-Bethe model) and the experimental data.