In this article, we discuss the one-excitation dynamics of a quantum system consisting of two two-level atoms each interacting with one of two coupled single-mode cavities via spontaneous emission. When the atoms and cavities are tuned into resonance, a wide variety of time-evolution behaviors can be realized by modulating the atom-cavity coupling strength $g$ and the cavity-cavity hopping strength $\lambda$. The dynamics is solved rigorously via the eigenproblem of an ordinary coupled linear system and simple analytical solutions are derived at several extreme situations of $g$ and $\lambda$. In the large hopping limit where $g\ll \lambda$, the behavior of the system is the linear superposition of a fast and slow periodic oscillation. The quantum state transfers from one atom to the other atom accompanied with weak excitation of the cavity mode. In the large coupling limit where $g\gg \lambda$, the time-evolution behavior of the system is characterized by the usual slowly varying carrier envelope superimposed upon a fast and violent oscillation. At a certain instant, the energy is fully transferred from the one quantum subsystem to the other. When the two interaction strengths are comparable in magnitude, the dynamics acts as a continuous pulse having irregular frequency and line shape of peaks and valleys, and the complicated time-evolution behaviors are ascribed to the violent competition between all the one-excitation quantum states. The coupled quantum system of atoms and cavities makes a good model to study cavity quantum electrodynamics with great freedoms of many-body interaction.

We report theoretical and experimental studies of the propagation of a square-modulated laser pulse through a laser-cooled atomic ensemble of two-level absorbers, as well as through a three-level system with electromagnetically induced transparency. We find that the transmission characteristics over a wide range of optical depth can be fully accounted for as optical coherent transients excited by the wide spectral content of the square-modulated optical pulse. We show that both time-domain atom-field coupled equations and frequency-domain linear dispersion theory give precisely the same description of the optical transients at a weak power limit. At low optical depth with moderate absorption, resonant excitation dominates and free-induction decay (FID) contributes mostly to the transient field. At high optical depth when absorption and propagation effects become significant, lossless optical precursors start to dominate the transient response. By varying the optical depth from 0 to 45, we observe that optical transients evolve gradually from FID to optical precursors. We thus show that FID and optical precursors, which have been considered as two different optical transients for many decades, can be unified within a single theoretical frame and they are the manifestations of the same physical process in two different regimes.

This article reports an experimental demonstration on nondegenerate, two-color, biphoton ghost imaging which reproduced a ghost image with enhanced angular resolving power by means of a greater field of view compared with that of classical imaging. With the same imaging magnification, the enhanced angular resolving power and field of view compared with those of classical imaging are $1.25:1$ and $1.16:1$, respectively. The enhancement of angular resolving power depends on the ratio between the idler and the signal photon frequencies, and the enhancement of the field of view depends mainly on the same ratio and also on the distances of the object plane and the imaging lens from the two-photon source. This article also reports the possibility of reproducing a ghost image with the enhancement of the angular resolving power by means of a greater imaging amplification compared with that of classical imaging.

We investigate transition of a one-dimensional interacting Bose gas from a strongly repulsive regime to a strongly attractive regime, where a stable highly excited state known as the super Tonks-Girardeau gas was experimentally realized very recently. By solving exact dynamics of the integrable Lieb-Liniger Bose gas, we demonstrate that such an excited gas state can be a very stable dynamic state. Furthermore we calculate the breathing mode of the super Tonks-Girardeau gas which is found to be in good agreement with experimental observation. Our results show that the highly excited super Tonks-Girardeau gas phase can be well understood from the fundamental theory of the solvable Bose gas.

We measure the two-point density correlation function of freely expanding quasicondensates in the weakly interacting quasi-one-dimensional (1D) regime. While initially suppressed in the trap, density fluctuations emerge gradually during expansion as a result of initial phase fluctuations present in the trapped quasicondensate. Asymptotically, they are governed by the thermal coherence length of the system. Our measurements take place in an intermediate regime where density correlations are related to near-field diffraction effects and anomalous correlations play an important role. Comparison with a recent theoretical approach described by Imambekov et al. yields good agreement with our experimental results and shows that density correlations can be used for thermometry of quasicondensates.

We study the quantum correlations in a two-dimensional system that possesses a topological quantum phase transition. The quantumness of two-body correlations is measured by quantum discord. We calculate both the correlation of two local spins and that between an arbitrary spin and the rest of the lattice. It is notable that local spins are classically correlated, while the quantum correlation is hidden in the global lattice. This is different from other systems which are not topologically ordered. Moreover, the mutual information and global quantum discord show critical behavior in the topological quantum phase transition.

Entanglement between degrees of freedom, namely between the spin, path, and (total) energy degrees of freedom, in a single-neutron system is exploited. We implemented a triply entangled Greenberger-Horne-Zeilinger (GHZ)–like state and coherently manipulated relative phases of two-level quantum subsystems. An equality derived by Mermin was applied to analyze the generated GHZ-like state: We determined the four expectation values and finally obtained $M=2.558\pm 0.004⩽̸2$. This demonstrates a violation of Mermin-like inequality for triply entangled GHZ-like state in a single-particle system, which, in turn, clearly contradicts the noncontextual assumption and confirms quantum contextuality.

We address the problem of learning an unknown unitary transformation from a finite number of examples. The problem consists in finding the learning machine that optimally emulates the examples, thus reproducing the unknown unitary with maximum fidelity. Learning a unitary is equivalent to storing it in the state of a quantum memory (the memory of the learning machine) and subsequently retrieving it. We prove that, whenever the unknown unitary is drawn from a group, the optimal strategy consists in a parallel call of the available uses followed by a “measure-and-rotate” retrieving. Differing from the case of quantum cloning, where the incoherent “measure-and-prepare” strategies are typically suboptimal, in the case of learning the “measure-and-rotate” strategy is optimal even when the learning machine is asked to reproduce a single copy of the unknown unitary. We finally address the problem of the optimal inversion of an unknown unitary evolution, showing also in this case the optimality of the “measure-and-rotate” strategies and applying our result to the optimal approximate realignment of reference frames for quantum communication.

In quantum information systems it is of particular interest to consider the best way in which to use the nonclassical resources consumed by that system. Quantum communication protocols are integral to quantum information systems and are among the most promising near-term applications of quantum information science. Here we show that a multiplexed, digital quantum communications system supported by a comb of vacuum squeezing has a greater channel capacity per photon than a source of broadband squeezing with the same analog band width. We report on the time-resolved, simultaneous observation of the first dozen teeth in a 2.4-GHz comb of vacuum squeezing produced by a subthreshold optical parametric oscillator, as required for such a quantum communications channel. We also demonstrate multiplexed communication on that channel.

The location of quantum information in various subsets of the qudit carriers of an additive graph code is discussed using a collection of operators on the coding space which form what we call the information group. It represents the input information through an encoding operation constructed as an explicit quantum circuit. Partial traces of these operators down to a particular subset of carriers provide an isomorphism of a subgroup of the information group, and this gives a precise characterization of what kinds of information they contain. All carriers are assumed to have the same dimension $D$, an arbitrary integer greater than 1.

For highly charged ions with 3 to 10 electrons, we derive explicit, closed-form quantum states which become exact in the high charge limit. When combined with suitably renormalized experimental data across isoelectronic sequences, these quantum states provide a widely applicable route to predicting electronic configurations and term values for neutral atom energy levels. Moreover, our findings allow the prediction of missing levels, suggest that certain current term assignments for five-electron ions are incorrect, and provide insight into the rare failure of Hund’s rules in excited states.

The effects of Coulomb interaction screening on electron–hydrogen-atom elastic and excitation scattering around the $n=2$ threshold have been investigated by using the $R$-matrix method with pseudostates. The elastic and excitation collision strengths show dramatic changes when the interaction screening length $D$ varies from $\infty$ to 3.8 a.u., as a result of the convergence of ${}^{1,3}S$ Feshbach resonances to the varying 2$s$ threshold and of the transformation of ${}^{1,3}P$ and ${}^{1}D$ Feshbach resonances into shape-type resonances when they pass across the 2$s$ and 2$p$ threshold at certain critical value of $D$, respectively [S. B. Zhang et al., Phys. Rev. Lett. 104, 023203 (2010)]. The resonance parameters for a large number of $D$ in the range $D=\infty$–3.8 a.u. are presented. It is observed that the ${}^{1,3}P$ and ${}^{1}D$ resonance contributions to the elastic and excitation collision strengths decrease rapidly with decreasing $D$ after the resonance passes the critical $D$ value. The contribution of a ${}^1{S}^e$ Feshbach resonance to the elastic or excitation collision strength changes into a cusp after the resonance merges into its parent 2$s$ state and immerses into the background with the further decrease of $D$.

We calculate photoelectron angular distributions (PADs) resulting from single-photon (43 eV) ionization of molecules that have been transiently aligned with a short laser pulse. The total ionization cross sections of ${\mathrm{N}}_2$ and ${\mathrm{CO}}_2$ vs the time delay between the aligning laser pulse and the soft x-ray photon are calculated and compared to experimental results reported by I. Thomann et al. [J. Phys. Chem. A 112, 9382 (2008)]. We present the PADs from these aligned molecules in the laboratory frame which can be compared directly with future experiments from aligned ${\mathrm{N}}_2$ and ${\mathrm{CO}}_2$. The alignment dependence of single-photon ionization, multiphoton ionization, and high-order harmonic generation are also analyzed.

We demonstrate an efficient numerical method to calculate the three-tangle of general mixed states. We construct an “energy function” (target function) for the three-tangle of the mixed state under certain constraints. The energy function (target function) is then optimized via the replica exchange Monte Carlo method. We have extensively tested the method for the examples with known analytical results, showing remarkable agreement. The method can be applied to other optimization problems in quantum information theory.

A significant feature of the one-dimensional super Tonks-Girardeau gas is its metastable gas-like state with a stronger Fermi-like pressure than for free fermions which prevents a collapse of atoms. This naturally suggests a way to search for such strongly correlated behavior in systems of interacting fermions in one dimension. We thus show that the strongly attractive Fermi gas without polarization can be effectively described by a super Tonks-Girardeau gas composed of bosonic Fermi pairs with attractive pair-pair interaction. A natural description of such super Tonks-Girardeau gases is provided by Haldane generalized exclusion statistics. In particular, they are equivalent to ideal particles obeying more exclusive statistics than Fermi-Dirac statistics.

In this article we study decoherence in the discrete-time quantum walk on the line. We generalize the method of decoherent coin quantum walk, introduced by Brun et al. [Phys. Rev. A 67, 32304 (2003)]. Our analytical expressions are applicable for all kinds of decoherence. As an example of the coin-position decoherence, we study the broken line quantum walk and compare our results with the numerical one. We also show that our analytical results reduce to the Brun formalism when only the coin is subjected to decoherence.

A two-qubit quantum gate is realized using electronically excited states in a single ion with an energy separation on the order of a terahertz times the Planck constant as a qubit. Two phase-locked lasers are used to excite a stimulated Raman transition between two metastable states $D_{3/2}$ and $D_{5/2}$ separated by $1.82$ THz in a single trapped ${}^{40}\mathrm{Ca}$${}^{+}$ ion to construct a qubit, which is used as the target bit for the Cirac-Zoller two-qubit controlled NOT gate. Quantum dynamics conditioned on a motional qubit is clearly observed as a fringe reversal in Ramsey interferometry.

We consider the entanglement manipulation capabilities of the universal covariant quantum cloner or quantum processor circuit for quantum bits. We investigate its use for cloning a member of a bipartite or a genuine tripartite entangled state of quantum bits. We find that for bipartite pure entangled states a nontrivial behavior of concurrence appears, while for GHZ entangled states a possibility of the partial extraction of bipartite entanglement can be achieved.

Three-step laser spectroscopy has been used to measure six additional fine-structure splittings in the $n{\hspace{0.5em}}^{2}F$ states of ${}^{87}\mathrm{Rb}$ for $11⩽n⩽16$. When combined with our previous measurements for $4⩽n⩽10$, they constitute a continuous sequence of $13$ measurements suitable for comparison to fine-structure calculations in heavy alkali-metal atoms where relativistic effects, core polarization, configuration mixing, and electron correlation are important.

Full quantum-scattering calculations are reported for low-energy near-threshold inelastic collision cross sections for $\mathrm{H}+\mathrm{Na}$. The calculations include transitions between all levels up to and including the ionic state (ion-pair production) for collision energies from the threshold up to 10 eV. These results are important for astrophysical modeling of spectra in stellar atmospheres. Results for the $3s-3p$ excitation are carefully examined using three different quantum chemistry input data sets, and large differences are found near the threshold. The differences are found to be predominantly due to differences in the radial coupling rather than potentials and are also found not to relate to differences in couplings in a simple manner. In fact, of the three input couplings, the two that are most similar give the cross sections with the largest differences. The $3s-3p$ cross sections show orbiting resonances which have been seen in earlier studies, while Feshbach resonances associated with closed channels were also found to be present in the low-energy cross sections for some transitions.