Based on a proposed coherence measure, we show that the local coherence of a bipartite quantum pure state (coherence of its reduced density matrix) is exactly the same as the minimal average coherence with all potential pure-state realizations under consideration. In particular, it is shown that bipartite concurrence of pure states just captures the maximal difference between local coherence and the average coherence of one subsystem induced by local operations on the other subsystem with the assistance of classical communications, which provides an alternative operational meaning for bipartite concurrence of pure states. The relation between concurrence and the proposed coherence measure can also be extended to bipartite mixed states.

This paper generalizes and expands upon the work [O. Oreshkov, T. A. Brun, and D. A. Lidar, Phys. Rev. Lett. 102, 070502 (2009)] where we introduced a scheme for fault-tolerant holonomic quantum computation (HQC) on stabilizer codes. HQC is an all-geometric strategy based on non-Abelian adiabatic holonomies, which is known to be robust against various types of errors in the control parameters. The scheme we present shows that HQC is a scalable method of computation and opens the possibility for combining the benefits of error correction with the inherent resilience of the holonomic approach. We show that with the Bacon-Shor code the scheme can be implemented using Hamiltonian operators of weights 2 and 3.

We consider controlled collisions between two ultracold atoms guided by external harmonic potentials. We derive analytical solutions of the Schrödinger equation for this system and investigate the properties of eigenergies and eigenstates for different trap geometries as a function of a trap separation and of the scattering length. When varying the trap separation the energy spectrum exhibits avoided crossings, corresponding to trap-induced shape resonances. Introducing an energy-dependent scattering length we investigate the behavior of the system in the vicinity of a magnetic Feshbach resonance. Finally, we illustrate our analytical results with two examples: the quantum phase gate controlled by the external magnetic field and a scheme for a coherent transport of atoms in optical lattices into higher Bloch bands.

High-energy cutoffs for individual electrons from nonsequential double ionization of helium by a 390 nm laser pulse at intensity $0.8\text{PW}/{\text{cm}}^2$ have been investigated using a classical 3D ensemble. Below the recollision-excitation threshold, both electrons are bound after recollision and get ionized one by one later. With the trajectory back analyzing of electrons with final energies above $2U_p$, we find an additional mechanism for the latter ionized electron after recollision. Both nuclear and laser forces contribute in this mechanism which leads to a broad transverse momentum distribution.

We develop a scheme to generate number squeezing in a Bose-Einstein condensate by utilizing interference between two hyperfine levels and nonlinear atomic interactions. We describe the scheme using a multimode quantum field model and find agreement with a simple analytic model in certain regimes. We demonstrate that the scheme gives strong squeezing for realistic choices of parameters and atomic species. The number squeezing can result in noise well below the quantum limit, even if the initial noise on the system is classical and much greater than that of a Poissonian (shot-noise limit) distribution.

We investigate the width of the Ginzburg critical region and experimental signatures of critical behavior in strongly interacting trapped Fermi gases close to unitarity, where the $s$-wave scattering length diverges. Despite the fact that the width of the critical region is of the order unity, evidence of critical behavior in the bulk thermodynamics of trapped gases is strongly suppressed by their inhomogeneity. The specific heat of a harmonically confined gas, for instance, is linear in the reduced temperature $t=\left(T-T_{\text{c}}\right)/T_{\text{c}}$ above $T_{\text{c}}$. We also discuss the prospects of observing critical behavior in the local compressibility from measurements of the density profile.

We theoretically demonstrate a deterministic conditional sign flip of a two-photon pulse that preserves the pulse profile. The nonlinear optical system used is a $V$-type three-level atom confined in a cavity, and the input photons are down-converted twin photons having temporal entanglement. The input and the output pulse profiles are nearly perfectly preserved when the reciprocal bandwidth of the input photons is tuned to the atomic linewidth.

A theory of quantum damped oscillator with arbitrary time dependence of the frequency and damping coefficient, based on the Heisenberg-Langevin equations with delta-correlated stochastic force operators, is applied to the case of the dynamical Casimir effect in a cavity with a periodically photo-excited semiconductor boundary. Accompanying results for the mean number of created photons, its variance, and the photon distribution function are given. In the asymptotical regime, the field mode goes to the so-called superchaotic quantum state.

We examine the quantum correlations at steady state in a three-mode quantum-beat laser that operates well above threshold. The analysis is presented by employing the dressed-atom combination-mode picture, in which only one combination mode of three cavity fields is coupled to the active medium, while the other two combination modes are decoupled. For suitable operating conditions, the coupled mode has the intensity fluctuations below the shot noise, while the two decoupled modes retain in their vacuum states. This determines that all of the original modes exhibit sub-Poissonian statistics. When the system operates well above threshold, the van Loock-Furusawa inequality is satisfied, concluding that the genuine tripartite entanglement among three sub-Poissonian fields is established. Physically, the responsible mechanism is the combined effect of atomic coherence controlled correlated-spontaneous emission and intrinsic feedback.

The general problem of performance advantage obtainable by the use of nonclassical transmitted states over classical ones is considered. Attention is focused on the situation where system loss is significant and additive Gaussian noise may be present at the receiver. Under the assumption that the total received state is classical, rigorous output density operator representations and their trace distance bounds are developed for classical and nonclassical transmitted states. For applications with high loss in all modes, a practical no-go theorem is enunciated that rules out the possibility of significant advantage of nonclassical over classical states. The recent work on quantum illumination is discussed as an example of our no-go approach.

We present a detailed theoretical treatment of a three-level $\Lambda$ system in a hot atomic vapor interacting with a coupling and a probe field of arbitrary strengths, leading to electromagnetically induced transparency and slow light under the two-photon resonance condition. We take into account all the relevant decoherence processes including collisions. Velocity-changing collisions (VCCs) are modeled in the strong-collision limit effectively, which helps in achieving optical pumping by the coupling beam across the entire Doppler profile. The steady-state expressions for the atomic density-matrix elements are numerically evaluated to yield the experimentally measured response characteristics. The predictions, taking into account a dynamic rate of influx of atoms in the two lower levels of the $\Lambda$, are in excellent agreement with the reported experimental results for ${{}^4\text{H}\text{e}}^{\ast }$. The role played by the VCC parameter is seen to be distinct from that by the transit time or Raman coherence decay rate.

We present a time-dependent density-functional-theory (TDDFT) Ehrenfest dynamics approach to study the lifetime and the charge neutralization rate of a lithium ion near an aluminum cluster surface. The lifetime of the excited state as a function of the surface-atom distance can be determined, including the effects of level crossings, without prior quantitative information about the coupling between atomic levels and surface states. This method can be used to compute lifetimes of excited atomic states near a surface in both the weak- and the strong-coupling regions and in the avoided crossing region. Because TDDFT Ehrenfest dynamics is a mean-field theory, the wave function consists of contributions from several different excited states during the time propagation. The shortest lifetime is predicted near the region of the avoided crossing between the ${\text{Li}}^{+}\text{-Al}$ and the ${\text{Li-Al}}^{+}$ states.

This Brief Report shows the behavior of photoelectrons produced by the irradiation of x-ray free-electron lasers (XFELs) onto a cluster or a biomolecule and the measurement of the x-ray fluxes by using the photoelectrons. It is found that plasmas, which are produced through the interaction of XFELs with the target, give little contribution to photoelectrons. The minimum energies of the photoelectrons inform us of the x-ray flux for each target size. Further, these energies approximated by a simple equation agree well with the calculation results for relatively small x-ray fluxes.

We demonstrate both efficient control of polarizations and high tunability of high-order sum and difference frequencies generated by adding an intense parallel- and perpendicular-polarized infrared laser field. When 805 nm pulses from a Ti:sapphire laser system and 1300 nm pulses from an optical parametric amplifier (OPA) are combined with perpendicular polarizations, the sum frequencies with two or four OPA photons are generated stronger than those with one or three OPA photons. This observation directly reflects the difference in their polarizations of the generated sum frequencies. Sum frequencies absorbing up to eight OPA photons are also observed for the parallel polarizations. Our observations are successfully reproduced by the theoretical calculations with the Lewenstein model including a weighting factor.

Generalizing Amari’s work titled “Information geometry on hierarchy of probability distributions” [IEEE Trans. Inf. Theory 47, 1701 (2001)], we define the degrees of irreducible multiparty correlations in a multiparty quantum state based on quantum relative entropy. We prove that these definitions are equivalent to those derived from the maximal von Neumann entropy principle [Phys. Rev. Lett. 89, 207901 (2002) and Phys. Rev. Lett. 101, 180505 (2008)]. Based on these definitions, we find a counterintuitive result on irreducible multiparty correlations: although the degree of the total correlation in a three-party quantum state does not increase under local operations, irreducible three-party correlations can be created by local operations from a three-party state with only irreducible two-party correlations. In other words, even if a three-partite state is initially completely determined by measuring two-party Hermitian operators, the determination of the state after local operations has to resort to the measurements of three-party Hermitian operators.

We demonstrate a state reconstruction technique which provides either the Wigner function or the density matrix of a field mode and requires only avalanche photodetectors, without any phase or amplitude discrimination power. It represents an alternative to quantum homodyne tomography of simpler implementation.

In this paper we investigate the use of quantum information to share classical secrets. While every quantum secret sharing scheme is a quantum error-correcting code, the converse is not true. Motivated by this we sought to find quantum codes which can be converted to secret sharing schemes. If we are interested in sharing classical secrets using quantum information, then we show that a class of pure ${\left[\left[n,1,d\right]\right]}_q$ Calderbank-Shor-Steane (CSS) codes can be converted to perfect secret sharing schemes. These secret sharing schemes are perfect in the sense that the unauthorized parties do not learn anything about the secret. Gottesman had given conditions to test whether a given subset is an authorized or unauthorized set; they enable us to determine the access structure of quantum secret sharing schemes. For the secret sharing schemes proposed in this paper the access structure can be characterized in terms of minimal codewords of the classical code underlying the CSS code. This characterization of the access structure for quantum secret sharing schemes is thought to be notable.

We propose a potentially practical scheme for quantum-information processing (QIP) with spatially distributed fullerene-caged electron spins using optical and microwave manipulations. Each doped fullerene located in a semiconducting single-walled carbon nanotube is embedded in a two-mode optical cavity. The caged spins have long decoherence time and the optical manipulation makes sure short operational time. Compared to the conventional QIP proposals involving an array of fullerene-caged electron spins based on nearest-neighbor coupling, our scheme corresponds to a network which could much reduce overhead in implementing distant qubits. We discuss the experimental feasibility and challenge based on currently available techniques and we show the possibility of high-fidelity operations.

We model the population and coherence dynamics of a singly charged quantum dot driven by a constant optical field. Using a separation of time scales we predict the occurrence of quantum jumps in the spin state of the excess electron or hole. Our analysis extends the description of intermittent fluorescence from a three-level to a four-level description; the former is common in atomic systems, the later occurs in many solid-state systems. The statistics of the quantum jumps as function of the intensity of the driving field provide detailed information on physical processes that limit the ground-state coherence of these systems and their use as a solid-state quantum bit. In particular it enables a discrimination of coherent spin coupling, induced by the nuclear magnetic field, from incoherent spin flips; the coherent coupling can gradually be suppressed by increasing the pump intensity due to the quantum Zeno effect.

The thermal entanglement is generated by weakly interacting atoms with an isotropic spin-1 chain. The decoherence of the entanglement is mainly investigated. The effective Hamiltonian is analytically obtained by the approximation method of perturbation. The scaling behavior of the effective coupling is numerically illustrated by the exact diagonalization. It is found out that the decay of the entanglement is slow in the case of noninteracting spins. The long-distance thermal entangled states can be used as the noisy channel for the achievement of the quantum teleportation.