We report on a pulsewidth measurement of phase-coherent soft x-ray laser pulses generated by injection seeding a solid-target plasma amplifier with high harmonic pulses. A pulse duration of $1.13\pm 0.47\mathrm{ps}$ was measured for a seeded Ne-like Ti plasma amplifier operating at $32.6\mathrm{nm}$ using an ultrafast streak camera. This is the shortest pulse duration reported to date from a table-top soft x-ray laser amplifier. The result agrees with model simulations which suggest that intense femtosecond soft x-ray laser pulses could be obtained by injection seeding a tailored plasma amplifier in which the gain is confined to a high density region.

We explore how entanglement of a bipartite system evolves when one subsystem undergoes the action of an arbitrary noisy channel. It is found that the dynamics of entanglement of such system is determined by the channel’s action on the maximally entangled state, which includes as a special case the results for two-qubit systems [Konrad et al., Nat. Phys. 4, 99 (2008)]. In particular, for multiqubit or qubit-qudit systems, we get a general factorization law for the evolution equation of entanglement, with one qubit being subject to a noisy channel.

An entangling quantum gate based entirely on purely geometric operations is proposed in quantum computation for the Jaynes-Cummings model by the invariant theory, where the qubits include information about the states of photons. By controlling some arbitrary parameters in the invariant operators, the phase accumulated in the quantum gate is a pure geometric phase. This way may be extended to other physical systems.

Our recent work has demonstrated the superradiant writing and reading of collective coherence in a Bose-Einstein condensate [Phys. Rev. Lett. 99, 220407 (2007)]. This time we report the drastic improvement of the storage time realized by loading the atoms into an optical dipole trap, wherein an unfavorable spin-dependent phase shift and spatial diffusion of atoms can be suppressed. The measured storage time was $0.57\left(2\right)\mathrm{ms}$, which is limited by the temporal variation of atomic momentum due to harmonic oscillations in the trap.

In the original Bennett-Brassard 1984 protocol, the bit basis and the phase basis are used with equal probability. Lo et al. [J. Cryptology 18, 133 (2005)] proposed modifying the ratio between the two bases by increasing the final key generation rate. However, the optimum ratio has not yet been derived. In this Rapid Communication, in order to examine this problem, the ratio between the two bases is optimized for exponential constraints, given Eve’s information distinguishability and the final error probability.

We derive inequalities for the probabilities of projective measurements in mutually unbiased bases of a qudit system. These inequalities lead to wider ranges of validity and tighter bounds on entropic uncertainty inequalities previously derived in the literature.

We analyze the dynamics of a system qudit of dimension $\mu$ sequentially interacting with the $\nu$-dimensional qudits of a chain playing the role of an environment. Each pairwise collision has been modeled as a random unitary transformation. The relaxation to equilibrium of the purity of the system qudit, averaged over random collisions, is analytically computed by means of a Markov chain approach. In particular, we show that the steady state is the one corresponding to the steady state for random collisions with a single environment qudit of effective dimension ${\nu }_e=\nu \mu$. Finally, we numerically investigate aspects of the entanglement dynamics for qubits $(\mu =\nu =2)$ and show that random unitary collisions can create multipartite entanglement between the system qudit and the qudits of the chain.

A general expression for the energy of interparticle interaction induced by an arbitrary mode of light is determined using quantum electrodynamics, and it is shown that the Casimir-Polder potential is included within this quantum result. Equations are also derived for the corresponding coupling induced by multimode number states of light, and the dependence of the pair energy on the Poynting vector and polarization state is determined. Attention is then focused on the interactions between particles trapped in counterpropagating coherent beams, both with and without interference, and it is shown that the results afford insights into the multiparticle structures that can be optically fabricated with counterpropagating input. Brief consideration is also given to the effect of squeezing the optical coherent state. Extending previous studies of optical binding in Laguerre-Gaussian beams, results are given for the case of particles trapped at radially different locations within the beam structure. Finally, consideration is given to interparticle interactions induced by broadband light, and it is shown how the length of optically fabricated particle chains can be controlled by the use of wavelength filters.

We propose a linear-optical implementation of a hyperentanglement-assisted quantum error-correcting code. The code is hyperentanglement assisted because the shared entanglement resource is a photonic state hyperentangled in polarization and orbital angular momentum. It is possible to encode, decode, and diagnose channel errors using linear-optical techniques. The code corrects for polarization “flip” errors and is thus suitable only for a proof-of-principle experiment. The encoding and decoding circuits use a Knill-Laflamme-Milburn-like scheme for transforming polarization and orbital angular momentum photonic qubits. A numerical optimization algorithm finds a unit-fidelity encoding circuit that requires only three ancilla modes and has success probability equal to 0.0097.

In this work, we study a system composed of four two-level particles, two source entangled qubits $AB$, and two ancilla entangled qubits $A^{\prime }{B}^{\prime }$, in the presence of two local common reservoirs, i.e., one reservoir for subsystem $A{A}^{\prime }$ and another for $B{B}^{\prime }$. For initial maximal entangled states, we study the entanglement of the source if after evolution the ancilla is measured at time $t$.

We investigate the process of entanglement transfer from a three-mode quantized field to a system of three spatially separated qubits, each one made of a two-level atom resonantly coupled to a cavity mode. The optimal conditions for entanglement transfer, evaluated by atomic tripartite negativity, are derived for radiation prepared in qubitlike and Gaussian entangled states in terms of field parameters, atom-cavity interaction time, cavity mirror losses, and atomic preparation. For qubitlike states we find that for negligible cavity losses some states may completely transfer their entanglement to the atoms and/or be exactly mapped to the atomic state, whereas for Gaussian states we find a range of field parameters needed to obtain a large entanglement transfer. The purity of the three-qubit states and the entanglement of two-qubit subsystems are also discussed in some detail.

We discuss the formation of molecular oxygen in ultracold collisions between hydroxyl radicals and atomic oxygen. A time-independent quantum formalism based on hyperspherical coordinates is employed for the calculations. Elastic, inelastic, and reactive cross sections as well as the vibrational and rotational populations of the product ${\mathrm{O}}_2$ molecules are reported. A $J$-shifting approximation is used to compute the rate coefficients. At temperatures $T=10–100\mathrm{mK}$ for which the OH molecules have been cooled and trapped experimentally, the elastic and reactive rate coefficients are of comparable magnitude, while at colder temperatures, $T<1\mathrm{mK}$, the formation of molecular oxygen becomes the dominant pathway. The validity of a classical capture model to describe cold collisions of OH and O is also discussed. While very good agreement is found between classical and quantum results at $T=0.3\mathrm{K}$, at higher temperatures, the quantum calculations predict a larger rate coefficient than the classical model, in agreement with experimental data for the $\mathrm{O}+\mathrm{O}\mathrm{H}$ reaction. The zero-temperature limiting value of the rate coefficient is predicted to be about $6\times {10}^{-12}{\mathrm{cm}}^3{\text{molecule}}^{-1}{\mathrm{s}}^{-1}$, a value comparable to that of barrierless alkali-metal atom-dimer systems and about a factor of five larger than that of the tunneling dominated $\mathrm{F}+{\mathrm{H}}_2$ reaction.

We investigate positron-impact ionization of hydrogen near threshold using the hyperspherical hidden crossing method (HHCM). Previously, Ihra et al. [Phys. Rev. Lett. 78, 4027 (1997)] used the HHCM to obtain the extended Wannier threshold law for zero angular momentum. We extend their analysis to higher angular momentum $L$ and show that the extended Wannier threshold law is $L$ independent. We also calculate the absolute partial-wave ionization cross sections for $L=0$, 1, 2, and 3 and compare our results with other calculations and with experimental measurements. The HHCM calculation provides an explanation for the very small $S$-wave and large $D$-wave contributions to the ionization cross section in terms of destructive and constructive interference, respectively.

We have measured the linear and circular polarization of $391.4\mathrm{nm}$ $\mathrm{N}_2{}^{+}$ $B{}^2\Sigma _u^{+}(v^{\prime }=0)\to X{}^2\Sigma _g^{+}(v^{″}=0)$ $P$-branch fluorescence produced by spin-polarized electron impact excitation of ${\mathrm{N}}_2$ $X{}^1\Sigma _g^{+}(v=0)$ ground states. The resulting nonzero values of the circular polarization indicate that spin-rotation coupling, in conjunction with electron exchange excitation, acts to produce oriented rotational angular momenta in the excited molecular state prior to decay.

In this paper we describe an experiment on laser cooling of ${}^{87}\mathrm{Rb}$ atoms directly from a vapor background in diffuse light. Diffuse light is produced in a ceramic integrating sphere by multiple scattering of two laser beams injected through multimode fibers. A probe beam, whose propagation direction is either horizontal or vertical, is used to detect cold atoms. We measured the absorption spectra of the cold atoms by scanning the frequency of the probe beam, and observed both the absorption signal and the time of flight signal after we switched off the cooling light, from which we estimated the temperature and the number of cold atoms. This method is clearly attractive for building a compact cold atom clock.

We study the photodetachment cross sections of ${\mathrm{H}}^{-}$ near a metal surface using closed-orbit theory. Formulas for cross sections are presented. We find the metal surface mainly modifies the photodetachment cross sections near threshold. It induces a strong oscillation in the photodetachment cross section between threshold and an energy $\frac{1}{4d}$ above threshold, where $d$ is the distance between the negative ion and the metal surface. Below threshold the photodetachment cross section near a metal becomes finite because of quantum tunneling effect. The oscillation in the cross section is identified with a closed orbit and explained as an interference of the detached electron.

We evaluate photoelectron angular anisotropy $\beta$ parameters for the process of sequential two-photon double electron ionization of helium within the lowest-order time-independent perturbation theory. Our results indicate that for photoelectron energies outside the interval $(E_{\text{slow}},E_{\text{fast}})$, where $E_{\text{slow}}=\omega -V_{{\mathrm{He}}^{+}}$ and $E_{\text{fast}}=\omega -V_{\mathrm{He}}$, there is a considerable deviation from the dipole angular distribution, thus indicating the effect of electron correlation.

We study two-band effects on ultracold fermionic atoms in optical lattices by means of dynamical mean-field theory. We find that at half filling an atomic-density-wave (ADW) state emerges owing to the two-band effects in the attractive interaction region, while an antiferromagnetic state appears in the repulsive interaction region. As the orbital splitting is increased, quantum phase transitions from the ADW state to the superfluid state and from the antiferromagnetic state to the metallic state occur in the corresponding regions. By systematically changing the orbital splitting and the interaction, we obtain the phase diagram at half filling. The results are discussed using the effective boson model derived for strong attractive interaction.

We present a method to reduce the scattering from arbitrary objects by surrounding them with shells composed of several layers of homogeneous anisotropic materials. An optimization procedure is used to find the material parameters for each layer, the starting point of which is a discretized approximation of a coordinate transformation cloaking shell. We show that an optimized, three-layer shell can reduce the maximum scattering of an object by as much as $15\mathrm{dB}$ more than a 100-layer realization of a coordinate transformation cloaking shell. Moreover, using an optimization procedure can yield high-performance cloaking shell solutions that also meet external constraints, such as the maximum value of permittivity or permeability. This design approach can substantially simplify the fabrication of moderate-size cloaking shells.

An experimental study has been carried out on high-order harmonic generation from the interaction of $45\mathrm{fs}$ Ti:sapphire laser pulses with low-excited silver plasma plumes. The dependence of high-order harmonic intensity $\left(I_H\right)$ on the plasma plume length $\left(L_{\mathrm{med}}\right)$ shows slower than quadratic scaling $I_H\sim L_{\mathrm{med}}^p$ with “$p$”$\approx 0.9$, 0.8, and 0.7 for 21st, 33rd, and 41st harmonics, respectively. The harmonic intensity is observed to decrease with increasing harmonic order, and this decrease is faster for longer plasma plumes. The results are explained from physical considerations of phase mismatch between the driving laser pulse and the harmonic radiation during propagation, and reabsorption of the harmonic radiation in the plasma plume.