Photon-echo based optical quantum memory schemes often employ intermediate steps to transform optical coherences to spin coherences for longer storage times. We analyze a scheme that uses three identical chirped control pulses for coherence rephasing in an inhomogeneously broadened ensemble of three-level $\Lambda$ systems. The pulses induce a cyclic permutation of the atomic populations in the adiabatic regime. Optical coherences created by a signal pulse are stored as spin coherences at an intermediate time interval, and are rephased for echo emission when the ensemble is returned to the initial state. Echo emission during a possible partial rephasing when the medium is inverted can be suppressed with an appropriate choice of control pulse wave vectors. We demonstrate that the scheme works in an optically dense ensemble, despite control pulse distortions during propagation. It integrates conveniently the spin-wave storage step into memory schemes based on a second rephasing of the atomic coherences.

Studying out-of-equilibrium physics in quantum systems under quantum quench is of vast experimental and theoretical interest. Using periodic quantum quenches, we present an experimentally accessible scheme to simulate the quantum Zeno and anti-Zeno effects in an $\mathit{open}$ quantum system of a single superconducting qubit interacting with an array of transmission line resonators. The scheme is based on the following two observations: First, compared with conventional systems, the short-time nonexponential decay in our superconducting circuit system is readily observed; and second, a quench-off process mimics an ideal projective measurement when its time duration is sufficiently long. Our results show the active role of quantum quench in quantum simulation and control.

Using numerical solutions to the quantum-field theoretical Dirac equation, we study the space-time evolution of the vacuum's polarization charge and current densities induced by an external electric force in one spatial dimension. We discuss a simple analytical model that can predict the dynamics of the polarization dynamics for arbitrary external force configurations. We then study the corrections to these predictions when the external force becomes supercritical and real electron-positron pairs can be created. By coupling the Dirac equation to the Maxwell equations, we examine how the dynamics of the polarization density is affected, if we allow the corresponding induced charges to interact with each other.

We show that the phenomenon of quantum contextuality can be used to certify lower bounds on the dimension accessed by the measurement devices. To prove this, we derive bounds for different dimensions and scenarios of the simplest noncontextuality inequalities. Some of the resulting dimension witnesses work independently of the prepared quantum state. Our constructions are robust against noise and imperfections, and we show that a recent experiment can be viewed as an implementation of a state-independent quantum dimension witness.

Implementing nonadiabatic holonomic quantum computation in decoherence-free subspaces may consolidate the advantages of both strategies and thus leads to suppression of both local and collective noises. In a recent paper, Xu et al. [Phys. Rev. Lett. 109, 170501 (2012)] proposed an interesting implementation of such a scheme; however, four-body interaction, which is hard to achieve in a realistic system, is required there. In this paper we propose a simplified scheme to implement such single-qubit and two-qubit logical operations by utilizing only two-body interactions, which is much easier to realize in a trapped ion system.

The standard perturbative expansion of the $S$ matrix cannot generally be used in the treatment of atomic scattering processes, involving atomic bound states, due to the special type of singularity that can appear here. It is demonstrated in the present paper that as a consequence of the optical theorem for free particles the effective Hamiltonian is closely related to the total cross section, a relation that is valid also when bound states are present. This implies that methods designed for structure calculations also can be used in dynamical processes. We have found that the covariant-evolution-operator technique that we have developed for structure calculations is here particularly useful. This is a consequence of the fact that the regular part of this operator (the Green's operator), running over all times, is essentially equal to the effective Hamiltonian and therefore directly related to the scattering cross section. In this paper the procedure is demonstrated for the case of radiative recombination.

Manipulation of a quantum system can be viewed in the framework of a control landscape defined as the physical objective as a functional of the control. Control landscape analyses have thus far considered linear quantum dynamics. This paper extends the analysis of control landscape topology to nonlinear quantum dynamics with the objective of steering a finite-level quantum system from an initial state to a final target state. The analysis rests on the assumptions that (i) the final state is reachable from the initial state, (ii) the differential mapping from the control to the state is surjective, and (iii) the control resources are unconstrained. Under these assumptions, landscape critical points (i.e., where the slope vanishes) for nonlinear quantum dynamics only appear as the global maximum and minimum; thus, the landscape is free of traps. Moreover, the landscape Hessian (i.e., the second derivative with respect to the control) at the global maximum has finite rank, indicating the presence of a large level set of optimal controls that preserve the value of the maximum. Extensive numerical simulations on finite-level models of the Gross-Pitaevskii equation confirm the trap-free nature of the landscape as well as the Hessian rank analysis, using either an applied electric field or a tunable condensate two-body interaction strength as the control. In addition, the control mechanisms arising in the numerical simulations are qualitatively assessed. These results are a generalization of previous findings for the linear Schrödinger equation, and show promise for successful control in a wide range of nonlinear quantum dynamics applications.

We use a near-concentric optical cavity at 1064 nm to generate trapping potentials for cold atoms. The cavity exhibits nondegenerate Hermite-Gaussian modes. Using just a few-milliwatt trap laser power, the cavity readily generates one- and higher-dimensional optical traps that replicate the mode functions. We spectroscopically characterize the optical trapping potentials and laser-cooling limits under continuous loading conditions. We use absorption images to measure atom densities and to compare the spatial profiles of trapped-atom samples with calculated mode functions. Steady-state fluorescence images reveal bright radiation emerging from the ends of the elongated, cavity-trapped atom clouds, providing evidence for radiation guiding.

We present a procedure for nonlinearity management of metal-dielectric nanocomposites (MDNCs). Varying the volume fraction occupied by silver nanoparticles (NPs) suspended in acetone we could investigate the nonlinear (NL) response of the MDNC. In particular, we could cancel the NL refractive index related to the effective third-order susceptibility, ${\chi }_{\mathrm{eff}}^{\left(3\right)}$, and thus the NL refractive behavior of the MDNC was due to the effective fifth-order susceptibility, ${\chi }_{\mathrm{eff}}^{\left(5\right)}$. Hence, in a cross-phase modulation experiment, we demonstrated the effect of spatial-modulation instability due to ${\chi }_{\mathrm{eff}}^{\left(5\right)}$. The experimental results are corroborated with numerical calculations based on a generalized Maxwell-Garnet model that includes the direct contributions of the NPs and the host medium as well as cascade contributions due to the third- and fifth-order susceptibilities of the NPs.

We present experimental data on quantum interferences in resonantly enhanced frequency up-conversion towards the vacuum-ultraviolet spectral regime. The process is driven in xenon atoms by ultrashort (picosecond) laser pulses. We use two simultaneous frequency conversion pathways via an excited intermediate state, i.e., fifth harmonic generation of the fundamental wavelength and four-wave mixing of the fundamental and two photons of its second harmonic wavelength. Both conversion pathways yield radiation at 102 nm. The two pathways interfere, depending on the relative phase of the fundamental and second harmonic. By appropriate choice of the phase we get constructive interference (resulting in increased conversion efficiency) or destructive interference (resulting in reduced conversion efficiency). The total conversion yield shows very pronounced constructive and destructive quantum interference with a visibility of roughly 90%. A stable and highly accurate phase control setup enables such strong quantum interferences for more than 260 oscillation cycles. In an extension of the experiment, simultaneously to frequency conversion we also monitor laser-induced fluorescence as a measure for the excitation probability to the excited intermediate state. Also, in the excitation probability we observe strong quantum interferences. As an interesting feature, a small phase lag occurs between the quantum interference patterns of frequency conversion and population transfer. This is due to an additional atomic phase acquired during frequency conversion.

We examine the influence of environmental interactions on simple quantum systems by obtaining the exact reduced dynamics of a qubit coupled to a one-dimensional spin bath. In contrast to previous studies, both the qubit-bath coupling and the nearest-neighbor intrabath couplings are taken as the spin-flip $XX$ type. We first study the Rabi oscillations of a single qubit with the spin bath prepared in a spin coherent state, finding that nonresonance and finite intrabath interactions have significant effects on the qubit dynamics. Then we discuss the bath-induced decoherence of the qubit when the bath is initially in the ground state and show that the decoherence properties depend on the internal phases of the spin bath. By considering two independent copies of the qubit-bath system, we finally probe the disentanglement dynamics of two noninteracting entangled qubits. We find that entanglement sudden death appears when the spin bath is in its critical phase. We show that the single-qubit decoherence factor is an upper bound for the two-qubit concurrence.

Quantum illumination is a protocol where quantum resources are utilized to detect a low-reflectivity object embedded in a bright thermal noise bath. For example, quantum illumination with a two-mode squeezed state (TMSS) provides a 6 dB advantage in the error-probability exponent over the optimal classical illumination. We here consider quantum illumination with the photon-subtracted two-mode squeezed state (PSTMSS). Our result is twofold. First, we show that a much smaller error probability $P_{\mathrm{err}}$ could be obtained, meaning that much smaller resources will be required in quantum illumination for a fixed $P_{\mathrm{err}}$. Second, quantum illumination with PSTMSS appreciably outperforms its classic correspondence in both low- and high-noise operating regimes, extending the regimes in which quantum illumination is optimal for target detection.

We apply quantum optimal control theory (QOCT) to an exactly solvable non-Markovian open quantum bit (qubit) system to achieve state-independent quantum control and construct high-fidelity quantum gates for moderate qubit decaying parameters. An important quantity, improvement $\mathcal{I}$, is proposed and defined to quantify the correction of gate errors due to the QOCT iteration when the environment effects are taken into account. With the help of the exact dynamics, we explore how the gate error is corrected in the open qubit system and determine the conditions for significant improvement. The model adopted in this paper can be implemented experimentally in realistic systems such as the circuit QED system.

We examine the behavior of non-Gaussian states of light under the action of probabilistic noiseless amplification and attenuation. Surprisingly, we find that the mean-field amplitude may decrease in the process of noiseless amplification—or may increase in the process of noiseless attenuation, a counterintuitive effect that Gaussian states cannot exhibit. This striking phenomenon could be tested with experimentally accessible non-Gaussian states, such as single-photon added coherent states. We propose an experimental scheme, which is robust with respect to the major experimental imperfections, such as inefficient single-photon detection and imperfect photon addition. In particular, we argue that the observation of mean-field amplification by noiseless attenuation should be feasible with current technology.

The transmission of 4.5 keV ${\text{Ar}}^{7+}$ ions through a straight cylindrical macrocapillary is simulated in accordance with the results of a recent experiment. A detailed comparison with the results for nanocapillaries is performed. The theoretical method is based on a code previously developed for a nanocapillary. The ion trajectories are affected by the electric field originating from the charges previously deposited on the capillary wall. The tilt angle of the capillary axis relative to the incident beam was varied within ${\text{0}}^{\circ }$–${\text{5}}^{\circ }$. Both the surface conductivity along the capillary wall and the conductivity into the capillary bulk were treated, providing clear evidence that the bulk conductivity is dominant, involving essentially a linear field dependence. By variation of this conductivity the mechanisms for the major experimental observations were revealed. At low conductivity the fluctuations of the mean ion emission angle are explained by the overloading of temporary charge patches, which are missing at high conductivity. Moreover, it is shown that the capillary guiding power decreases with increasing conductivity, in agreement with the experimental results.

We experimentally studied the dissociation dynamics of a highly charged ${\text{Ar}}_3$ cluster initiated by a high-energy electron. The dissociation patterns of the correlated ions from a two-body and a three-body Coulombic explosion (CE) of (${\text{Ar}}_3$)${}^{2+}$ suggest that predissociation alters the evolution of radiative charge transfer. The three-body CE in (${\text{Ar}}_3$)${}^{4+}$ and (${\text{Ar}}_3$)${}^{5+}$ is driven, after double ionization of one constituent Ar atom, by single ionization with a simultaneous interatomic Coulombic decay process.

Within the Born-Oppenheimer approximation, we build a multichannel model to simulate the dissociation of ${\mathrm{H}}_2{}^{+}$ in strong laser fields. Two applications of this model are presented: first, orthogonally polarized UV and infrared pulses are used to dissociate ${\mathrm{H}}_2{}^{+}$ into $2p{\pi }_u$ and $3d{\pi }_g$ states, whose superposition results in asymmetric electron localization after the dissociation; second, two orthogonally polarized UV pulses are introduced to trigger competing Rabi oscillations among $1s{\sigma }_g$, $2p{\sigma }_u$, $3d{\sigma }_g$ and $2p{\pi }_u$ states. This study shows the dissociation of ${\mathrm{H}}_2{}^{+}$ can be steered into selected electronic states with controllable probabilities with state-of-the-art laser technologies.

We study the two-photon double-ionization process of the helium atom by solving numerically the nonrelativistic, time-dependent Schrödinger equation in its full dimensionality. We investigate with intense chirped attosecond laser pulses of 23.5-nm wavelength the two-photon absorption near and above the sequential threshold. We show how it is possible by adjusting the chirp parameter to control the electronic transitions inside the atom, thereby reinforcing or weakening the ionization process. Attosecond chirped laser pulses offer a promising way to probe and control the two-photon double ionization of helium when compared with attosecond transform-limited pulses.

Rate equation models are extensively used to describe the many-body states of laser driven atomic gases. We show that the properties of the rate equation model used to describe nonlinear optical effects arising in interacting Rydberg gases can be understood by considering the excitation of individual superatoms. From this we deduce a simple semianalytical model that accurately describes the Rydberg density and optical susceptibility for different dimensionalities. We identify the previously reported universal dependence of the susceptibility on the Rydberg excited fraction as an intrinsic property of the rate equation model that is rooted in one-body properties. Benchmarking against exact master equation calculations, we identify regimes in which the semianalytical model is particularly reliable. The performance of the model improves in the presence of dephasing which destroys higher-order atomic coherences.