We have developed a quantum memory that is completely compatible with current quantum information processing for continuous variables of light, where arbitrary frequency sidebands of a squeezed vacuum can be stored and retrieved using bichromatic electromagnetic induced transparency. The 2 MHz sidebands of squeezed vacuum pulses with temporal widths of 470 ns and a squeezing level of $-1.78\pm 0.02$ dB were stored for 3 $\mu$s in laser-cooled ${}^{87}\mathrm{Rb}$ atoms. Squeezing of $-0.44\pm 0.02$ dB, which is the highest squeezing reported for a retrieved pulse, was achieved.

We study the dissipative dynamics of a two-level system (TLS) exposed to strong ac driving. By combining Floquet theory with Van Vleck perturbation theory in the TLS tunneling matrix element, we diagonalize the time-dependent Hamiltonian and provide corrections to the renormalized Rabi frequency of the TLS, which are valid for both a biased and unbiased TLS and go beyond the known high-frequency and rotating-wave results. In order to mimic environmental influences on the TLS, we couple the system weakly to a thermal bath and solve analytically the corresponding Floquet-Bloch-Redfield master equation. We give a closed expression for the relaxation and dephasing rates of the TLS and discuss their behavior under variation of the driving amplitude. Further, we examine the robustness of coherent destruction of tunneling (CDT) and driving-induced tunneling oscillations (DITO). We show that also for a moderate driving frequency an almost complete suppression of tunneling can be achieved for short times and demonstrate the sensitiveness of DITO to a change of the external parameters.

The phenomenon of vacuum decay, that is, electron-positron pair production due to the instability of the quantum electrodynamics vacuum in an external field, is a remarkable prediction of Dirac theory whose experimental observation is still lacking. Here a classic wave optics analog of vacuum decay, based on light propagation in curved waveguide superlattices, is proposed. This photonic analog enables a simple and experimentally accessible visualization in space of the process of pair production as breakup of an initially negative-energy Gaussian wave packet, representing an electron in the Dirac sea, under the influence of an oscillating electric field.

We propose a strategy for perfect state transfer in spin chains based on the use of an unmodulated coupling Hamiltonian whose coefficients are explicitly time dependent. We show that, if specific and nondemanding conditions are satisfied by the temporal behavior of the coupling strengths, our model allows perfect state transfer. The paradigm put forward by our proposal holds the promises to set an alternative standard to the use of clever encoding and coupling-strength engineering for perfect state transfer.

The parametric deformations of quasienergies and eigenvectors of unitary operators are applied to the design of quantum adiabatic algorithms. The conventional, standard adiabatic quantum computation proceeds along eigenenergies of parameter-dependent Hamiltonians. By contrast, discrete adiabatic computation utilizes adiabatic passage along the quasienergies of parameter-dependent unitary operators. For example, such computation can be realized by a concatenation of parameterized quantum circuits, with an adiabatic though inevitably discrete change of the parameter. A design principle of adiabatic passage along quasienergy was recently proposed: Cheon’s quasienergy and eigenspace anholonomies on unitary operators is available to realize anholonomic adiabatic algorithms [A. Tanaka and M. Miyamoto, Phys. Rev. Lett. 98, 160407 (2007)], which compose a nontrivial family of discrete adiabatic algorithms. It is straightforward to port a standard adiabatic algorithm to an anholonomic adiabatic one, except an introduction of a parameter $|v\rangle$, which is available to adjust the gaps of the quasienergies to control the running time steps. In Grover’s database search problem, the costs to prepare $|v\rangle$ for the qualitatively different (i.e., power or exponential) running time steps are shown to be qualitatively different.

We consider the high-probability state transfers and entanglements between different nodes of the spin-$1/2$ chains governed by the $\mathit{XXZ}$ Hamiltonian using the inhomogeneous stationary external magnetic field. Examples of three-, four-, ten- and twenty-node chains are represented. A variant of realization of the proper inhomogeneous magnetic field is discussed.

The stabilizer code, one major family of quantum error-correcting codes (QECC), is specified by the joint eigenspace of a commuting set of Pauli observables. It turns out that noncommuting sets of Pauli observables can be used to construct more efficient QECCs, such as the entanglement-assisted QECCs, which are built directly from any linear classical codes whose detailed properties are needed to determine the parameters of the resulting quantum codes. Here we propose another family of QECCs, namely, the breeding QECCs, that also employ noncommuting sets of Pauli observables and can be built from any classical additive codes, either linear or nonlinear, with the advantage that their parameters can be read off directly from the corresponding classical codes. Besides, since nonlinear codes are generally more efficient than linear codes, our breeding codes have better parameters than those codes built from linear codes. The terminology is justified by the fact that our QECCs are related to the ordinary QECCs in exactly the same way that the breeding protocols are related to the hashing protocols in the entanglement purification.

Large-scale multiconfiguration Dirac-Fock calculations have been performed for the superheavy element eka-thorium, $Z=122$. The resulting atomic structure is compared with that obtained by various computational approaches involving different degrees of approximation in order to elucidate the role that correlation, relativistic, Breit, and quantum electrodynamics corrections play in determining the low-energy atomic spectrum. The accuracy of the calculations is assessed by comparing theoretical results obtained for thorium with available experimental data.

We present results for the corrections of order ${\alpha }^2(Z\alpha ){}^5$ to the Lamb shift. We compute all the contributing Feynman diagrams in dimensional regularization and a general covariant gauge using a mixture of analytical and numerical methods. We confirm results obtained by other groups and improve their precision. Values of the $32$ “master integrals” for this and similar problems are provided.

We present a detailed analysis of the interference effects observed for ionization in collisions of fast highly charged projectiles with molecular hydrogen. We propose a nonperturbative semiclassical approach to describe the process under consideration by solving the time-dependent Schrödinger equation fully numerically on a 3D spatial grid. We present results for ${\mathrm{Kr}}^{34+}$-${\mathrm{H}}_2$ collisions at $63$ MeV/u impact energy and discuss different structures observed experimentally in doubly differential cross sections. The presence of Young-type minima and the absence of high-frequency oscillations are especially addressed. We also report unexpected interference patterns which can be observed for fixed-in-space molecular targets.

The x-ray mass attenuation coefficients of zinc are measured in a high-accuracy experiment between 7.2 and 15.2 keV with an absolute accuracy of 0.044% and 0.197%. This is the most accurate determination of any attenuation coefficient on a bending-magnet beamline and reduces the absolute uncertainty by a factor of 3 compared to earlier work by advances in integrated column density determination and the full-foil mapping technique described herein. We define a relative accuracy of $0.006\%$, which is not the same as either the precision or the absolute accuracy. Relative accuracy is the appropriate parameter for standard implementation of analysis of near-edge spectra. Values of the imaginary components $f″$ of the x-ray form factor of zinc are derived. Observed differences between the measured mass attenuation coefficients and various theoretical calculations reach a maximum of about 5% at the absorption edge and up to 2% further than 1 keV away from the edge. The measurements invite improvements in the theoretical calculations of mass attenuation coefficients of zinc.

Dielectronic recombination (DR) of ions and electrons occurs via intermediate doubly excited Rydberg states converging to an excited state of the ion, and the total energy-integrated rate is the sum of the rates through the $n$,$\ell$ Rydberg states. To make stringent comparisons between calculated and observed rates, it is useful to resolve the contributions to DR of different $n$ and $\ell$ states. It is possible to separate experimentally the $n$ states due to their different energies, but it is not possible to separate the nearly degenerate high-$\ell$ states. We propose that it should be possible to measure the contribution to DR of energetically unresolved high-$\ell$ states by measuring the recombination rate as a function of electric field. As the field is raised, progressively lower $\ell$ states are converted to Stark states. Autoionization rates increase as $\ell$ is decreased, and when the field is raised to the point that an $\ell$ state with an autoionization rate in excess of the radiative decay rate is added to the manifold of Stark states, the DR rate will exhibit an observable increase. To test this proposal we have made measurements of autoionization yields of the autoionizing Ba $6p_{1/2}\mathit{nk}$ and $6p_{3/2}\mathit{nk}$ Stark states. The autoionization yield is complementary to DR, and the measurements indicate that the proposed approach should work well. Although it is not surprising that this approach works for excited ion states that are isotropic, such as the Ba${}^{+}$ $6p_{1/2}$ state, it is less obvious that it should work for an anisotropic ion state, such as the Ba${}^{+}$ $6p_{3/2}$ state. In the latter case there are four quantum defects for each $\ell$ state instead of one, and the Stark effect of the Ba $6p_{3/2}\mathit{nk}$ Stark states is far more complex than that of the Ba $6p_{1/2}\mathit{nk}$ Stark states. Calculations of the Stark effect reveal that, while there are four times as many $6p_{3/2}\mathit{nk}$ levels as $6p_{1/2}\mathit{nk}$ levels, many of the interactions of the $6p_{3/2}\mathit{nk}$ levels are negligible, and in practice the problem is no more complicated than the $6p_{1/2}\mathit{nk}$ Stark problem.

We apply the nonperturbative convergent close-coupling (CCC) and time-dependent close coupling (TDCC) formalisms to calculate fully differential energy and angular resolved cross sections of double photoionization (DPI) of lithium. The equal energy sharing case is considered in which dynamics of the DPI process can be adequately described by two symmetrized singlet and triplet amplitudes. The angular width of these amplitudes serves as a measure of the strength of the angular correlation between the two ejected electrons. This width is interpreted in terms of the spin of the photoelectron pair.

The zero-temperature phase diagrams of imbalanced two-species Fermi gases are investigated in asymmetric optical lattices with arbitrary potential depths, based on the exact spectrum instead of the Fermi-Hubbard model. We study the effect of lattice potentials and atomic densities to the fully paired Bardeen-Cooper-Schrieffer (BCS) state and particularly the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. It is found that the increasing lattice potential favors BCS at low densities because of the enhanced effective coupling, whereas FFLO is favored at intermediate densities when the system undergoes a dimensional crossover. Finally, using the local density approximation, we study the evolution of the phase profile in the presence of external harmonic traps by merely tuning the lattice potentials.

The behavior of the Goos-Hänchen (GH) shifts in the reflected and transmitted light beam which is incident on a cavity containing an intracavity medium of three-level or four-level atoms with electromagnetically induced transparency (EIT) is discussed. We report a coherent control of the GH shift in a fixed configuration or device via superluminal and subluminal wave propagation. For superluminal wave propagation, we observe negative GH shifts in the reflected part of the incident light whereas the shifts are positive in the transmitted light beam. This corresponds to the negative group index of the cavity in the former case and positive group index of the cavity in the latter. For subluminal wave propagation, the behavior of the GH shifts in the reflected light changes and positive shifts appear; however, the GH shifts in the transmitted light remains positive. The corresponding group index of the cavity is positive in both cases.

We derive semiclassical laser equations valid in all orders of nonlinearity. With the help of a diagrammatic representation, the perturbation series in powers of electric field can be resummed in terms of a certain class of diagrams. The resummation makes it possible to take into account a weak effect of population pulsations in a controlled way while treating the nonlinearity exactly. The proposed laser theory reproduces the all-order nonlinear equations in the approximation of constant population inversion and the third-order equations with population-pulsation terms as special cases. The theory can be applied to arbitrarily open and irregular lasers, such as random lasers.

We address security of the quantum key distribution scheme based on the noisy modulation of coherent states and investigate how it is robust against noise in the modulation regardless of the particular technical implementation. As the trusted preparation noise is shown to be security breaking even for purely lossy channels, we reveal the essential difference between two types of trusted noise, namely sender-side preparation noise and receiver-side detection noise, the latter being security preserving. We consider the method of sender-side state purification to compensate the preparation noise and show its applicability in the realistic conditions of channel loss, untrusted channel excess noise, and trusted detection noise. We show that purification makes the scheme robust to the preparation noise (i.e., even the arbitrary noisy coherent states can in principle be used for the purpose of quantum key distribution). We also take into account the effect of realistic reconciliation and show that the purification method is still efficient in this case up to a limited value of preparation noise.

A fully quantum-mechanical approach is utilized to study the collision process of He${}^{+}$ with neutral neon, and the radiative charge transfer (RCT) and radiative association (RA) cross sections are presented in the energy range from 0.08 meV to 1 eV, while the optical potential and semiclassical methods are adopted to calculate the total radiative decay cross sections for energies from 0.08 meV to 5 keV. The potential energy curves and dipole transition matrix elements are obtained by an ab initio multireference configuration interaction package. For the related three lowest $X$ ${}^2{\Sigma }^{+}$, $A$ ${}^2\Pi$, and $B$ ${}^2{\Sigma }^{+}$ states, the spectroscopic data are in good agreement with other theoretical calculations and experimental measurements. Our results indicate that the RCT cross section is much larger than the nonradiative charge transfer cross section for collision energy $E$ $<$ 20 eV, and when $E$ $>$ 40 eV, the nonradiative process becomes dominant. Especially, we found that in the present collision system the RA process is more important than the RCT process when $E$ $<$ 1 meV. The RCT and RA rate coefficients are also given for temperatures from 1 to 4 $\times {10}^3$ K.

The theory of a laser system with a ladder-type configuration is studied in detail based on the quantum Langevin approach. By using an external field to link the lower lasing level with another atomic level, whose decay rate is much larger, laser intensity significantly increases and the quantum-limited linewidth can be quenched. We also discuss the spectrum of fluctuations of the output field, and the result shows that the fluctuations at low frequencies can be much suppressed too. On the other hand, this quenching approach can realize a laser output between two atomic levels, whose decay rates do not satisfy the usual lasing condition that the decay rate of the lower lasing level should be larger than that of the upper lasing level. It will be very useful to realize a laser output with the wavelength we want. This quenching approach has been widely used in the absorption spectrum of the ytterbium optical lattice clock and in the laser cooling approach for calcium atoms. Here we apply it in the stimulated emission of lasers.

For highly divergent emission of broad-area vertical-cavity surface-emitting lasers, a rotation of the polarization direction by up to ${90}^{°}$ occurs when the pump rate approaches the lasing threshold. Well below threshold the polarization is parallel to the direction of the transverse wave vector and is determined by the transmissive properties of the Bragg reflectors that form the cavity mirrors. In contrast, near-threshold and above-threshold emission is more affected by the reflective properties of the reflectors and is predominantly perpendicular to the direction of transverse wave vectors. Two qualitatively different types of polarization transition are demonstrated: an abrupt transition, where the light polarization vanishes at the point of the transition, and a smooth transition, where it is significantly nonzero during the transition.