In this paper, we make a few corrections to a paper by Zha and Ren [Phys. Rev. A 77, 014306 (2008)]. The Zha-Ren protocol for teleportation in principle is equivalent to Rigolin’s protocol [G. RigolinPhys. Rev. A 71, 032303 (2005)] and the associated Comment [F. G. DengPhys. Rev. A 72, 036301 (2005)]. We feel that the transformation operator is not well suited as a criterion for the faithful teleportation, but can be used as a means to transform an arbitrary four-qubit entangled state into a tensor product state of two Bell states. We give all the transformation operators.

We agree with the correction of the index made by Chen et al. [previous Comment, Phys. Rev. A 79, 036301 (2009)]. We reconfirm that our protocol can be applied to multipartite quantum channels and nonsymmetric quantum channels and that Rogolin’s protocol is only a special case of ours.

We consider an electrostatic qubit located near a Bose-Einstein condensate (BEC) of noninteracting bosons in a double-well potential, which is used for qubit measurements. Tracing out the BEC variables we obtain a simple analytical expression for the qubit’s density matrix. The qubit’s evolution exhibits a slow $\left(\propto 1/\sqrt{t}\right)$ damping of the qubit’s coherence term, which however turns to be a Gaussian one in the case of static qubit. This is in contrast to the exponential damping produced by most classical detectors. The decoherence is, in general, incomplete and strongly depends on the initial state of the qubit.

A universal cloning machine is derived. The machine produces $c$ clones of an unknown qubit from $s$ identical replicas of qubit and $k$ identical replicas of its orthogonal qubit. For the standard cloning machine, i.e., $k=0$, the universal-not machine, $s=0$, and some other cases, the optimum machine is well known. The universal cloning machine derived in this paper gives clones whose fidelity is the same value as that for the optimum machine. In the case of sufficiently large $c$, we show that the clone fidelity generated by the cloning machine from $s\ne 0$ replicas and $k\ne 0$ orthogonal replicas is higher than that generated by the optimal cloning machine from $s+k$ replicas in case $s+k\le 300$.

We develop a numerical scheme to investigate the high-order harmonic generation (HHG) in intense laser-matter interactions. We represent the time-dependent wave function in the basis of the eigenstates of the field-free Hamiltonian and trace its temporal evolution. Our full quantum simulations reveal that continuum electrons with a broad energy distribution contribute equally to one harmonic and the excited state also plays an important role in the molecular HHG. These results imply a laser-intensity-dependent picture of intramolecular interference in the HHG.

Quantum accelerator modes were discovered in experiments with kicked cold atoms in the presence of gravity. They were shown to be tightly related to resonances of the quantum kicked rotor. In this paper a spinor formalism is developed for the analysis of modes associated with resonances of arbitrary order $q\ge 1$. Decoupling of spin variables from the orbital ones is achieved by means of an ansatz of the Born-Oppenheimer type that generates $q$ independent band dynamics. Each of these is described, in classical terms, by a map, and the stable periodic orbits of this map give rise to quantum accelerator modes, which are potentially observable in experiments. The arithmetic organization of such periodic orbits is briefly discussed.

We examine the spontaneous emission of a two-level emitter, quantum-dot exciton, into surface plasmons propagating on the surface of a cylindrical nanowire. The numerically obtained dispersion relations are found to influence the spontaneous emission rate strongly. At certain values of the exciton band gap, the emission rates can go to infinity due to the band-edge feature of the dispersion relations. Borrowing the idea from the photonic crystals, we model the quantum-dot exciton dynamics with a non-Markovian way and demonstrate that the decay can undergo an oscillatory behavior. In addition, we also show that the remote entangled states can be generated via super-radiance by using the one-dimensional propagating feature of the nanowire surface plasmons.

In the observation of multiple thresholds in the cavity-quantum electrodynamics microlaser there was deviation from the result predicted by uniform atom-cavity coupling theory, notably a frequency-pushing behavior. We analyze in the semiclassical limit the tilted injection of atoms, originally used to achieve a spatially uniform atom-cavity coupling, and show that the deviation originates from the interference between the induced dipole moments associated with two traveling-wave components of a standing-wave cavity mode. The criteria for sufficiently reduced interference and thus uniform atom-cavity interactions are derived in analytic considerations and verified by numerical calculation. Our analysis is well supported by experimental data.

We offer theoretical explanations for some recent observations in numerical simulations of quantum random walks (QRWs). Specifically, in the case of a QRW on the line with one particle (walker) and two entangled coins, we explain the phenomenon, called “localization,” whereby the probability distribution of the walker’s position is seen to exhibit a persistent major “spike” (or “peak”) at the initial position and two other minor spikes which drift to infinity in either direction. Another interesting finding in connection with QRWs of this sort pertains to the limiting behavior of the position probability distribution. It is seen that the probability of finding the walker at any given location eventually becomes stationary and nonvanishing. We explain these observations in terms of the degeneration of some eigenvalue of the time evolution operator $U\left(k\right)$. An explicit general formula is derived for the limiting probability, from which we deduce the limiting value of the height of the observed spike at the origin. We show that the limiting probability decreases quadratically for large values of the position $x$. We locate the two minor spikes and demonstrate that their positions are determined by the phases of nondegenerated eigenvalues of $U\left(k\right)$. Finally, for fixed time $t$ sufficiently large, we examine the dependence on $t$ of the probability of finding a particle at a given location $x$.

We show how extra entanglement shared between sender and receiver reduces the memory requirements for a general entanglement-assisted quantum convolutional code. We construct quantum convolutional codes with good error-correcting properties by exploiting the error-correcting properties of an arbitrary basic set of Pauli generators. The main benefit of this particular construction is that there is no need to increase the frame size of the code when extra shared entanglement is available. Then there is no need to increase the memory requirements or circuit complexity of the code because the frame size of the code is directly related to these two code properties. Another benefit, similar to results of previous work in entanglement-assisted convolutional coding, is that we can import an arbitrary classical quaternary code for use as an entanglement-assisted quantum convolutional code. The rate and error-correcting properties of the imported classical code translate to the quantum code. We provide an example that illustrates how to import a classical quaternary code for use as an entanglement-assisted quantum convolutional code. We finally show how to “piggyback” classical information to make use of the extra shared entanglement in the code.

We discuss the generation of states close to the boundary family of maximally entangled mixed states as defined by the use of concurrence and linear entropy. The coupling of two qubits to a dissipation-affected bosonic mode is able to produce a bipartite state having, for all practical purposes, the entanglement and mixedness properties of one of such boundary states. We thoroughly study the effects that thermal and squeezed characters of the bosonic mode have in such a process and we discuss tolerance to qubit phase-damping mechanisms. The nondemanding nature of the scheme makes it realizable in a matter-light-based physical setup, which we address in some details.

The low-lying energy eigenstates of a one-dimensional (1D) system of many impenetrable point bosons and one moving impurity particle with repulsive zero-range impurity-boson interaction are found for all values of the impurity-boson mass ratio and coupling constant. The moving entity is a polaronlike composite object consisting of the impurity clothed by a comoving gray soliton. The special case with impurity-boson interaction of point hard-core form and impurity-boson mass ratio $m_i/m$ unity is first solved exactly as a special case of a previous Fermi-Bose (FB) mapping treatment of soluble 1D Bose-Fermi mixture problems. Then a more general treatment is given using second quantization for the bosons and the second-quantized form of the FB mapping, eliminating the impurity degrees of freedom by a Lee-Low-Pines canonical transformation. This yields the exact ground state (total linear momentum $q=0$) and exact boson-impurity distribution function in the thermodynamic limit for arbitrary $m_i/m$ and arbitrary impurity-boson interaction strength. These results are then extended to states with $q>0$.

We develop a practical theoretical formalism for studying the critical properties of a trapped Bose-Einstein condensate using the projected Gross-Pitaevskii equation. We show that this approach allows us to investigate the behavior of the correlation length, condensate mode, and its number fluctuations about the critical point. Motivated by recent experiments [Donner et al., Science 315, 1556 (2007)], we calculate the critical exponent for the correlation length of the trapped system. We observe finite-size effects in our results when the correlation length becomes comparable to the Ginzburg length. We extend the Binder cumulant to the trapped system and discuss an experimental method for measuring number fluctuations.

We numerically study free expansion of a few Lieb-Liniger bosons, which are initially in the ground state of an infinitely deep hard-wall trap. Numerical calculation is carried out by employing a standard Fourier transform, as follows from the Fermi-Bose transformation for a time-dependent Lieb-Liniger gas. We study the evolution of the momentum distribution, the real-space single-particle density, and the occupancies of natural orbitals. Our numerical calculation allows us to explore the behavior of these observables in the transient regime of the expansion, where they are nontrivially affected by the particle interactions. We derive analytically (by using the stationary phase approximation) the formula which connects the asymptotic shape of the momentum distribution and the initial state. For sufficiently large times, the momentum distribution coincides (up to a simple scaling transformation) with the shape of the real-space single-particle density (the expansion is asymptotically ballistic). Our analytical and numerical results are in good agreement.

Due to Pauli blocking of intermediate states, the scattering matrix (or $T$ matrix) of two fermionic atoms in a Fermi gas becomes different from that of two atoms in free space. This effect becomes particularly important near a Feshbach resonance, where the interaction in free space is very strong but becomes effectively suppressed in the medium. We calculate the in-medium $T$ matrix in ladder approximation and study its effects on the properties of collective modes of a trapped gas in the normal-fluid phase. We introduce the in-medium interaction on both sides of the Boltzmann equation, namely, in the calculation of the mean field and in the calculation of the collision rate. This allows us to explain the observed upward shift of the frequency of the quadrupole mode in the collisionless regime. By including the mean field, we also improve considerably the agreement with the measured temperature dependence of frequency and damping rate of the scissors mode, whereas the use of the in-medium cross section deteriorates the description, in agreement with previous work.

We present measurements of the resonance structure for nondegenerate four-wave mixing (4WM) in the diamond configuration in a warm atomic vapor of rubidium. We show that a narrow sub-Doppler feature exists and can be understood as a result of velocity-class selection. We measure the efficiency of the 4WM process and show it to be on the order of $1\times {10}^{-4}$. Our observations are consistent with the results of a theoretical treatment based on the semiclassical Maxwell-Bloch equations for the system with the appropriate coupling values derived from the experimental values in Rb.

We propose a strictly local protocol that is completely equivalent to global quantum state reconstruction for a bipartite system. We show that the joint density matrix of an arbitrary two-mode Gaussian state, entangled or not, is obtained via local operations and classical communication only. In contrast to previous proposals, simultaneous homodyne measurements on both modes are replaced with local homodyne detections and a set of local projective measurements.

We discuss a major theoretical generalization of existing techniques for handling the three-body problem that accurately describes the interactions among four fermionic atoms. Application to a two-component Fermi gas accurately determines dimer-dimer scattering parameters at finite energies and can give deeper insight into the corresponding many-body phenomena. To account for finite temperature effects, we calculate the energy-dependent complex dimer-dimer scattering length, which includes contributions from elastic and inelastic collisions. Our results indicate that strong finite-energy effects and dimer dissociation are crucial for understanding the physics in the strongly interacting regime for typical experimental conditions. While our results for dimer-dimer relaxation are consistent with experiment, they confirm only partially a previously published theoretical result.

A cesium fountain clock is operated utilizing a microwave oscillator that derives its frequency stability from a stable laser by means of a fiber-laser femtosecond frequency comb. This oscillator is based on the technology developed for optical clocks and replaces the quartz-based microwave oscillator commonly used in fountain clocks. As a result, a significant decrease in the frequency instability of the fountain clock is obtained, reaching $7.4\times {10}^{-14}{\left[\tau \left(\text{s}\right)\right]}^{-1/2}$. We could demonstrate that for a significant range of detected atom numbers the instability is limited by quantum projection noise only, and that for the current status of this fountain clock the microwave source poses no limit on the achievable frequency instability.