We have measured the coincidence time spectra of two Lyman-$\alpha$ photons emitted by a pair of H($2p$) atoms in the photodissociation of ${\text{H}}_2$ at the incident photon energy of 33.66 eV and at the hydrogen gas pressures of 0.40 and 0.02 Pa. The decay time constant at 0.02 Pa is approximately half the lifetime of a single H($2p$) atom, 1.60 ns, while the decay time constant at 0.40 Pa is in agreement with the lifetime of a single H($2p$) atom. It turns out that the decay faster than the lifetime of a single H($2p$) atom originates from the entanglement in the pair of H($2p$) atoms. We have demonstrated an effect of entanglement on atomic decay.

Triple-differential cross sections for two-photon double ionization of molecular hydrogen are presented for a central photon energy of $30$ eV. The calculations are based on a fully ab initio, nonperturbative approach to the time-dependent Schrödinger equation in prolate spheroidal coordinates, discretized by a finite-element discrete-variable representation. The wave function is propagated in time for a few femtoseconds using the short, iterative Lanczos method to study the correlated response of the two photoelectrons to short, intense laser radiation. The current results often lie in between those of Colgan et al. [J. Phys. B 41, 121002 (2008)] and Morales et al. [J. Phys. B 42, 134013 (2009)]. However, we argue that these individual predictions should not be compared directly with each other, but preferably with experimental data generated under well-defined conditions.

We demonstrate that repeated measurements in disordered systems can induce a quantum anti-Zeno effect under certain conditions to enhance quantum transport. The enhancement of energy transfer is really exhibited in multisite models under repeated measurements. The optimal measurement interval for the anti-Zeno effect and the maximal efficiency of energy transfer are specified in terms of the relevant physical parameters. Since the environment acts as frequent measurements on the system, the decoherence-induced energy transfer, which has been discussed recently for photosynthetic complexes, may be interpreted in terms of the anti-Zeno effect. We further find an interesting phenomenon in a specific three-site case, where local decoherence or repeated measurements may even promote entanglement generation between the nonlocal sites.

We study the implementation of quantum phase measurement in a superconducting circuit, where two Josephson phase qubits are coupled to the photon field inside a resonator. We show that the relative phase of the superposition of two Fock states can be imprinted in one of the qubits. The qubit can thus be used to probe and store the quantum coherence of two distinguishable Fock states of the single-mode photon field inside the resonator. The effects of dissipation of the photon field on the phase detection are investigated. We find that the visibilities can be greatly enhanced if the Kerr nonlinearity is exploited. We also show that the phase measurement method can be used to perform the Gauss sum factorization of numbers ($⩾{10}^4$) into a product of prime integers, as well as to precisely measure both the resonator’s frequency and the nonlinear interaction strength. The largest factorizable number is mainly limited by the coherence time. If the relaxation time of the resonator were to be $~10$ $\mu$s ($~1$ ms), then the largest factorizable number can be $⩾{10}^{4}N$ ($⩾{10}^{7}N$), where $N$ is the number of photons in the resonator.

The entanglement evolution of two qubits under local, single-, and two-sided noisy channels is investigated. It is found that for all pure initial states, the entanglement under a one-sided noisy channel is completely determined by the maximal trace distance which is the main element to construct the measure of non-Markovianity. For the two-sided noisy channel case, when the qubits are initially prepared in a general class of states, either pure or mixed, the entanglement can be expressed as the product of the initial entanglement and the channels’ action on the maximally entangled state.

Although quantum computers have the potential to efficiently solve certain problems considered difficult by known classical approaches, the design of a quantum circuit remains computationally difficult. It is known that the optimal gate-design problem is equivalent to the solution of an associated optimal-control problem; the solution to which is also computationally intensive. Hence, in this article, we introduce the application of a class of numerical methods (termed the max-plus curse of dimensionality-free techniques) that determine the optimal control, thereby synthesizing the desired unitary gate. The application of this technique to quantum systems has a growth in complexity that depends on the cardinality of the control-set approximation rather than the much larger growth with respect to spatial dimensions in approaches based on gridding of the space, which is used in previous research. This technique is demonstrated by obtaining an approximate solution for the gate synthesis on SU(4)—a problem that is computationally intractable by grid-based approaches.

We reveal a close relationship between quantum metrology and the Deutsch-Jozsa algorithm on continuous-variable quantum systems. We develop a general procedure, characterized by two parameters, that unifies parameter estimation and the Deutsch-Jozsa algorithm. Depending on which parameter we keep constant, the procedure implements either the parameter-estimation protocol or the Deutsch-Jozsa algorithm. The parameter-estimation part of the procedure attains the Heisenberg limit and is therefore optimal. Due to the use of approximate normalizable continuous-variable eigenstates, the Deutsch-Jozsa algorithm is probabilistic. The procedure estimates a value of an unknown parameter and solves the Deutsch-Jozsa problem without the use of any entanglement.

Using convex optimization, we propose entanglement-assisted quantum error-correction procedures that are optimized for given noise channels. We demonstrate through numerical examples that such an optimized error-correction method achieves higher channel fidelities than existing methods. This improved performance, which leads to perfect error correction for a larger class of error channels, is interpreted in at least some cases by quantum teleportation, but for general channels this interpretation does not hold.

For many quantum information protocols such as state transfer, entanglement transfer, and entanglement generation, standard notions of controllability for quantum systems are too strong. We introduce the weaker notion of accessible pairs and prove an upper bound on the achievable fidelity of a transformation between a pair of states based on the symmetries of the system. A large class of spin networks is presented for which this bound can be saturated. In this context, we show how the inaccessible dark states for a given excitation-preserving evolution can be calculated and illustrate how some of these can be accessed using extra catalytic excitations. This emphasizes that it is not sufficient for analyses of state transfer in spin networks to restrict to the single excitation subspace. One class of symmetries in these spin networks is exactly characterized in terms of the underlying graph properties.

We present a protocol for deterministically producing continuous-variable Einstein-Podolsky-Rosen- (EPR-) entangled optical beams in macroscopic atomic ensembles. In contrast to previous cavity-based schemes, our protocol requires only a single pass of two coherent light beams through two spin-polarized free-space atomic vapors, which greatly simplifies the experimental implementation and makes the scheme very efficient. The two entangled modes have a frequency difference of about twice the Zeeman frequency of the atomic vapors, which can be widely tuned via magnetic-field control. Under realistic experimental conditions, we show that large entanglement could be created even with room-temperature atomic vapors.

We study multipartite correlations and nonlocality in an isotropic Ising ring under transverse magnetic field at both zero and finite temperature. We highlight parity-induced differences between the multipartite Bell-like functions used in order to quantify the degree of nonlocality within a ring state and reveal a mechanism for the passive protection of multipartite quantum correlations against thermal spoiling effects that is clearly related to the macroscopic properties of the ring model.

We investigated the nonadiabatic transition of an electron spin-polarized ${}^4\mathrm{He}$${}^{+}$ ion beam in the energy range of 0.2–2 keV. We successfully evaluated the spin polarization of the ${\mathrm{He}}^{+}$ ion beam passing through a solenoid coil by measuring the elastically scattered ion intensity from the magnetized Fe(100) target. We observed that the beam polarization exhibited sinusoidal magnetic oscillation after passing through the coil. Furthermore, the beam polarization almost completely reversed with single periodicity in this oscillation. By comparison with the theory of the double-passage nonadiabatic transition, we concluded that this oscillation corresponds to the Stückelberg oscillation in beam polarization. Thus, we propose that the quantum interference between the nonadiabatic transitions using a single solenoid coil acts as an effective spin flipper.

We study the dynamics of neutral cold atoms in an L-shaped crossed-beam optical waveguide formed by two perpendicular red-detuned lasers of different intensities and a blue-detuned laser at the corner. The motion in one sense is optimized, and the motion in the other sense may be suppressed even if it is energetically allowed. Quantum and classical simulations are performed and give similar results. Complemented with a vibrational cooling process we find a range of parameters for which this setting works as a one-way device or “atom diode.”

We study the superfluorescence (SF) from a gas of rubidium atoms. The atoms of a dense vapor are excited to the $5D$ state from the $5S$ state by a two-photon process driven by 100-fs laser pulses. The atoms decay to the $6P$ state and then to the $5S$ state. The SF emission at 420 nm on the $6P$–$5S$ transition is recorded by a streak camera with picosecond time resolution. The time duration of the generated SF is tens of picoseconds, which is much shorter than the time scale of the usual relaxation processes, including spontaneous emission and atomic coherence dephasing. The dependence of the time delay between the reference input pulse and SF is measured as a function of laser power. The experimental data are described quantitatively by a simulation based on the semiclassical atom-field interaction theory. The observed change in scaling laws for the peak intensity and delay time can be elucidated by an SF theory in which the sample length is larger than the cooperation length.

We demonstrate that the electronic density distribution of a Rydberg state can be probed directly using a broadband harmonic spectrum in a few-cycle laser pulse. Taking advantage of the large orbital radius of a Rydberg state, we show that the Rydberg electron can be accelerated directly toward the core under the influence of a few-cycle laser pulse, leading to recombination and emission of harmonic photons. In this case the initial position of the electron and the harmonic order form a one-to-one correspondence, resulting in a realization of direct probing of the density distribution of the initial Rydberg state.

We present the design of a type of compact toroidal, or “halo,” ion trap. Such traps may be useful for mass spectrometry, studying small Coulomb cluster rings, quantum-information applications, or other quantum simulations where a ring topology is of interest. We present results from a Monte Carlo optimization of the trap design parameters using finite-element analysis simulations that minimize higher-order anharmonic terms in the trapping pseudopotential, while maintaining complete control over ion placement at the pseudopotential node in three dimensions using static bias fields. These simulations are based on a practical electrode design using readily available parts, yet can be easily scaled to any size trap with similar electrode spacings. We also derive the conditions for a crystal structure transition for two ions in the compact halo trap, the first nontrivial transition for Coulomb crystals in this geometry.

A correlated quantum many-body method is applied to describe resonance states of atomic Bose-Einstein condensates (BEC) in a realistic shallow trap (as opposed to the infinite traps commonly used). The realistic van der Waals interaction is adopted as the interatomic interaction. We calculate experimentally measurable decay rates of the lowest quasibound state in the shallow trap. The most striking result is the observation of a metastable branch besides the usual one for attractive BEC in a pure harmonic trap. As the particle number increases the metastable branch appears and then gradually disappears, and finally the usual metastable branch (associated with the attractive BEC in a harmonic trap) appears, eventually leading to the collapse of the condensate.

We present a detailed study of light dynamics in passive nonlinear resonators with shallow intracavity periodic modulation of the refractive index in both longitudinal and transverse directions of the resonator. Specifically, we concentrate on the nonlinear envelopes of dissipative Bloch modes, localized in the transverse plane of the resonator, the so-called Bloch cavity solitons, predicted recently in K. Staliunas et al. [Phys. Rev. Lett. 101, 153903 (2008)]. Bloch cavity solitons, being dissipative structures, are attractors, therefore they can be excited from a wide range of initial conditions (the attractor basin) depending on the system’s parameters. A unique property of Bloch cavity solitons is that they are envelopes of waves with tailored diffraction. Using the round-trip model for forward- and backward-propagating waves we reveal different types of Bloch cavity solitons supported by both focusing (at normal diffraction) and defocusing (at anomalous diffraction) nonlinearities. We show also the coexistence of solitons bifurcating from different Bloch wave dispersion branches. In order to analyze the properties of Bloch cavity solitons and to obtain an analytical access we develop a modified mean-field model and prove its validity. In particular, we demonstrate substantial narrowing of Bloch cavity solitons near the zero-diffraction regime.

We show with numerical simulations that for certain simple choices of parameters, the waveguides induced by colliding dark solitons in a Kerr medium yield a complete family of beam splitters for trapped linear waves, ranging from total transmission to total deflection. The way energy is transferred from one waveguide to another is similar to that of a directional coupler, but no special fabrication is required. Dark soliton beam splitters offer potential advantages over their bright soliton counterparts: Their transfer characteristics do not depend on the relative phase or speed of the colliding solitons; dark solitons are generally more robust than bright solitons; and the probe peaks at nulls of the pump, enhancing the signal-to-noise ratio for probe detection. The last factor is especially important for possible application to quantum information processing.

The effect of Kerr nonlinearity on an Airy beam is investigated by using the nonlinear Schrödinger equation. Based on the moments method, the evolution of the Airy beam width in the rms sense is analytically described. Numerical simulations indicate that the central parts of the major lobe of the Airy beam initially give rise to radial compression during propagation in a focusing medium, even though the rms beam width broadens. The partial collapse of the center parts of the major lobe of the beam appear below the threshold for a global collapse. The evolutions of the field distributions of the Airy beams are different during propagation in different Kerr media while the beams still travel along the parabolic trajectory just as the beam propagates in free space.