We demonstrate the ability to control spontaneous emission from a superconducting qubit coupled to a cavity. The time domain profile of the emitted photon is shaped into a symmetric truncated exponential. The experiment is enabled by a qubit coupled to a cavity, with a coupling strength that can be tuned in tens of nanoseconds while maintaining a constant dressed state emission frequency. Symmetrization of the photonic wave packet will enable use of photons as flying qubits for transferring the quantum state between atoms in distant cavities.

Practical implementations of quantum information-theoretic applications rely on phase coherences between systems. These coherences are disturbed by misalignment between phase references. Quantum states carrying phase-asymmetry act as resources to counteract this misalignment. In this paper, we construct protocols for interconverting these resources by first estimating the misalignment. Our method achieves sublinear, but otherwise arbitrarily high, asymptotic rate of conversion of pure resource states to mixed ones. No other method is known for asymptotic pure-to-mixed state conversion. Our method also has the advantage of achieving phase reference alignment in addition to resource conversion.

We investigate the possibility of generating nonlinear surface polaritons at the interface between a negative-index metamaterial and a dielectric doped with two-level quantum emitters by means of coherent population oscillation (CPO) working at room temperature. Based on the CPO and the strong confinement of the optical field near the interface, we find that the system can possess a giant Kerr nonlinearity. We demonstrate that it is possible to obtain a type of surface polaritonic solitons which is not only robust during propagation but also has ultraweak generation power and superluminal propagating velocity.

The propagation of macroscopic entangled states over large distances in the presence of loss is of fundamental interest and may have practical applications as well. Here we describe two different techniques in which state discrimination can be used to violate Bell's inequality with macroscopic phase-entangled coherent states. We find that Bell's inequality can be violated by these macroscopic states over a distance of approximately 400 km in commercially available optical fibers.

The paper considers Bose-Einstein condensation (BEC) of light in a cavity with a medium. In the framework of a two-level model we show the effect of gaseous medium on the critical temperature of light condensation in the system. Transition of the system to the state with released light condensate is illustrated in consequent stages. Analytical expressions for a typical spatial extent of the condensed cloud of photons, as well for spectral characteristics of the condensate peak are derived. Energy and heat capacity of photons as functions of temperature are obtained. Finally, we demonstrate that the energy of light can be accumulated in the BEC state.

We study experimentally and theoretically the temporal dynamics of laser pulses propagating under conditions of spatial self-focusing in quadratic nonlinear waveguide arrays made from periodically poled lithium niobate. We observed temporal pulse breakup and temporal pulse narrowing and studied the dynamics of these effects in different waveguides. We investigated the influence of the frequency dependence of the mode indices as a limiting factor for soliton formation. Our experimental results are in good agreement with the theoretical model developed from coupled-mode theory, providing a detailed understanding of pulse dynamics and beam distribution in waveguide arrays with quadratic nonlinearity.

We consider quantum-information division, which is characterized by a channel whose outputs have no correlation and are not completely randomized. We show that the quantum-information division is possible in a probabilistic manner by optimizing the average fidelity in the channel with $M$ outputs in both deterministic and probabilistic cases. Moreover, we show that the optimal fidelities drastically change depending on the condition imposed on the outputs (symmetric and asymmetric), which is quite in contrast to the case of imperfect cloning.

We study how photon emission of a two-level system is modified if the superposition principle is violated. We solve the relevant equations of motion. We quantify the magnitude of the new spectral effects for relevant collapse models to illustrate our theoretical results. We show how these effects can be distinguished from those of standard environmental decoherence. We apply our result to physically interesting systems and suggest that accurate-enough spectroscopic experiments are within reach with current technology.

We investigate the implementation of a controlled-$Z$ gate on a pair of Rydberg atoms in spatially separated dipole traps where the joint excitation of both atoms into the Rydberg level is strongly suppressed (the Rydberg blockade). We follow the adiabatic gate scheme of Jaksch et al. [D. Jaksch, J. I. Cirac, P. Zoller, S. L. Rolston, R. Côté, and M. D. Lukin, Phys. Rev. Lett. 85, 2208 (2000)], where the pair of atoms is coherently excited using lasers, and apply it to the experimental setup outlined by Gaëtan et al. [A. Gaëtan, Y. Miroshnychenko, T. Wilk, A. Chotia, M. Viteau, D. Comparat, P. Pillet, A. Browaeys, and P. Grangier, Nat. Phys. 5, 115 (2009)]. We apply optimization to the experimental parameters to improve gate fidelity and consider the impact of several experimental constraints on the gate success.

Spectroscopic labels for a few particles with spin that are harmonically trapped in one dimension with effectively zero-range interactions are provided by quantum numbers that characterize the symmetries of the Hamiltonian: permutations of identical particles, parity inversion, and the separability of the center-of-mass. The exact solutions for the noninteracting and infinitely repulsive cases are reduced with respect to these symmetries. This reduction explains how states of single-component and multicomponent fermions and bosons transform under adiabatic evolution from noninteracting to strong hard-core repulsion. These spectroscopic methods also clarify previous analytic and numerical results for intermediate values of interaction strength. Several examples, including adiabatic mapping for two-component fermionic states in the cases $N=3–5$, are provided.

We study the spreading of density-density correlations in Bose-Hubbard models after a quench of the interaction strength, using time-dependent variational Monte Carlo simulations. It gives access to unprecedented long propagation times and to dimensions higher than one. In both one and two dimensions, we find ballistic light-cone spreading of correlations and extract accurate values of the light-cone velocity in the superfluid regime. We show that the spreading of correlations is generally supersonic, with a light-cone propagating faster than sound modes but slower than the maximum group velocity of density excitations, except at the Mott transition, where all the characteristic velocities are equal. Further, we show that in two dimensions the correlation spreading is highly anisotropic and presents nontrivial interference effects.

We describe how a universal set of dynamically corrected quantum gates can be implemented using sequences of shaped decoupling pulses on any qubit network forming a sparse bipartite graph with always-on Ising interactions. These interactions are constantly decoupled except when they are needed for two-qubit gates. We analytically study the error operators associated with the constructed gates up to third order in the Magnus expansion, analyze these errors numerically in the unitary time evolution of small qubit clusters, and give a bound on high-order errors for qubits on a large square lattice. We prove that with a large enough toric code the present gate set can be used to implement fault-tolerant quantum memory.

Quantum walks have been shown to be fruitful tools in analyzing the dynamic properties of quantum systems. This article proposes using quantum walks as an approach to quantum neural networks (QNNs). QNNs replace binary McCulloch-Pitts neurons with a qubit in order to use the advantages of quantum computing in neural networks. A quantum walk on the firing states of such a QNN is supposed to simulate the central properties of the dynamics of classical neural networks, such as associative memory. It is shown that a biased discrete Hadamard walk derived from the updating process of a biological neuron does not lead to a unitary walk. However, a stochastic quantum walk between the global firing states of a QNN can be constructed, and it is shown that it contains the feature of associative memory. The quantum contribution to the walk accounts for a modest speedup in some regimes.

We propose a general technique to retrieve the information of dipole-forbidden resonances in the autoionizing region. In the simulation, a helium atom is pumped by an isolated attosecond pulse in the extreme ultraviolet (EUV) combined with a few-femtosecond laser pulse. The excited wave packet consists of the ${}^{1}S$, ${}^{1}P$, and ${}^{1}D$ states, including the background continua, near the $2s2p{(}^{1}P)$ doubly excited state. The resultant electron spectra with various laser intensities and time delays between the EUV and laser pulses are obtained by a multilevel model and an ab initio time-dependent Schrödinger equation calculation. By taking the ab initio calculation as a “virtual measurement,” the dipole-forbidden resonances are characterized by the multilevel model. We found that in contrast to the common assumption, the nonresonant coupling between the continua plays a significant role in the time-delayed electron spectra, which shows the correlation effect between photoelectrons before they leave the core. This technique takes the advantages of ultrashort pulses uniquely and would be a timely test for the current attosecond technology.

The primary purpose of this investigation is to determine the effective coherent extent of the neutron wave packet transverse to its mean propagation vector $\mathbf{k}$ when it is prepared in a typical instrument used to study the structure of materials in thin film form via specular reflection. There are two principal reasons for doing so. One has to do with the fundamental physical interest in the characteristics of a free neutron as a quantum object, while the other is of a more practical nature, relating to the understanding of how to interpret elastic scattering data when the neutron is employed as a probe of condensed-matter structure on an atomic or nanometer scale. Knowing such a basic physical characteristic as the neutron's effective transverse coherence can dictate how to properly analyze specular reflectivity data obtained for material film structures possessing some amount of in-plane inhomogeneity. In this study we describe a means of measuring the effective transverse coherence length of the neutron wave packet by specular reflection from a series of diffraction gratings of different spacings. Complementary nonspecular measurements of the widths of grating reflections were also performed, which corroborate the specular results. (This paper principally describes measurements interpreted according to the theoretical picture presented in a companion paper.) Each grating was fabricated by lift-off photolithography patterning of a nickel film (approximately 1000 Å thick) formed by physical vapor deposition on a flat silicon crystal surface. The grating periods ranged from 10 $\mu$m (5 $\mu$m Ni stripe, 5 $\mu$m intervening space) to several hundred microns. The transverse coherence length, modeled as the width of the wave packet, was determined from an analysis of the specular reflectivity curves of the set of gratings.

We present a general approach to analyzing elastic scattering for those situations where the incident beam is prepared as an incoherent ensemble of wave packets of a given arbitrary shape. Although wave packets, in general, are not stationary solutions of the Schrödinger equation, the analysis of elastic scattering data treats the scattering as a stationary-state problem. We thus must gate the wave packet, coherently distorting its shape in a manner consistent with the elastic condition. The resulting gated scattering amplitudes (e.g., reflection coefficients) thus are weighted coherent sums of the constituent plane-wave scattering amplitudes, with the weights determined by the shape of the incident wave packet as “filtered” by energy gating. We develop the gating formalism in general and apply it to the problem of neutron scattering from ruled gratings described by Majkrzak et al. in a companion paper. The required exact solution of the associated problem of plane-wave reflection from gratings also is derived.

A Lagrangian formalism is used toderive the Hamiltonian for a $\lambda$/4 resonator shunted by a current-biased Josephson junction. The eigenstates and the quantum dynamics of the system are analyzed numerically, and we show that this quantum system can function as an efficient detector of weak incident microwave fields.