The photonic de Broglie wave, in which an ensemble of $N$ identical photons with wavelength $\lambda$ reveals $\lambda /N$ interference fringes, has been known to be a unique feature exhibited by the photon-number-path–entangled $|N,0\rangle +|0,N\rangle$ state or the $N00N$ state. Here, we report the observation of the photonic de Broglie wave for a pair of photons, generated by spontaneous parametric down-conversion, that are not photon-number-path entangled. We also show that the photonic de Broglie wave can even be observed for a pair of photons that are completely separable (i.e., no entanglement in all degrees of freedom) and distinguishable. The experimental and theoretical results suggest that the photonic de Broglie wave is, in fact, not related to the entanglement of the photons, rather it is related to the indistinguishable pathways established by the measurement scheme. The phase sensitivity surpassing the standard quantum limit, however, is shown to be closely related to the $N00N$ state.

We study the statistics of the fluorescence decay rate of a dipole emitter embedded in a strongly scattering medium. In the multiple-scattering regime, the probability of observing a decrease in the decay rate is substantial, as predicted by exact numerical simulations. The decrease originates from a reduction of the local density of optical states and is due to collective interactions and interferences. In the strong-scattering regime, signatures of recurrent scattering are visible in the behavior of the average decay rate.

We introduce a scheme for efficiently describing pure states of strongly correlated fermions in higher dimensions using unitary circuits featuring a causal cone. A local way of computing local expectation values is presented. We formulate a dynamical reordering scheme, corresponding to time-adaptive Jordan-Wigner transformation, that avoids nonlocal string operators. Primitives of such a reordering scheme are highlighted. Fermionic unitary circuits can be contracted with the same complexity as in the spin case. The scheme gives rise to a variational description of fermionic models not suffering from a sign problem. We present numerical examples in a $9\times 9$ and $6\times 6$ fermionic lattice model to show the functioning of the approach.

The classical and quantum correlation sharing between modes of Dirac fields in the noninertial frame are investigated. It is shown that (i) the classical correlation for the Dirac fields decreases as the acceleration increases, which is different from the result in a scalar field, where the classical correlation is independent of the acceleration; (ii) there is no simple dominating relation between the quantum correlation and entanglement for the Dirac fields, which is unlike the scalar case, where the quantum correlation is always over and above the entanglement; (iii) as the acceleration increases, the correlations between modes I and II and between modes $A$ and II increase, but the correlations between modes $A$ and I decrease.

The long time accumulation of the random actions of a single particle “reservoir” on its coupled system can transfer some temperature information of its initial state to the coupled system. This dynamic process can be referred to as a quantum thermalization in the sense that the coupled system can reach a stable thermal equilibrium with a temperature equal to that of the reservoir. We illustrate this idea based on the usual micromaser model, in which a series of initially prepared two-level atoms randomly pass through an electromagnetic cavity. It is found that, when the randomly injected atoms are initially prepared in a thermal equilibrium state with a given temperature, the cavity field will reach a thermal equilibrium state with the same temperature as that of the injected atoms. As in two limit cases, the cavity field can be cooled and “coherently heated” as a maser process, respectively, when the injected atoms are initially prepared in ground and excited states. Especially, when the atoms in equilibrium are driven to possess some coherence, the cavity field may reach a higher temperature in comparison with the injected atoms. We also point out a possible experimental test for our theoretical prediction based on a superconducting circuit QED system.

An approach to the construction of the controlled-not quantum logic gate for a four-dimensional coupled-qubit model with weak but otherwise arbitrary coupling has been given recently [M. R. Geller et al., Phys. Rev. A 81, 012320 (2010)]. How does the resulting fidelity depend on the form of qubit-qubit coupling? In this paper we calculate intrinsic fidelity curves (fidelity in the absence of decoherence versus total gate time) for a variety of qubit-qubit interactions, including the commonly occurring isotropic Heisenberg and $\mathit{XY}$ models, as well as randomly generated ones. For interactions not too close to that of the Ising model, we find that the fidelity curves do not significantly depend on the form of the interaction, and we calculate the resulting interaction-averaged fidelity curve for the non-Ising-like cases and a criterion for determining its applicability.

We show that vacuum-induced coherence in three-level atomic systems can lead to preservation of bipartite entanglement when two such atoms are prepared as two initially entangled qubits, each independently interacting with their respective vacuum reservoirs. We explicitly calculate the time evolution of concurrence for two different Bell states and show that a large amount of entanglement can survive in the long time limit. The amount of entanglement left between the two qubits depends strongly on the ratio of the nonorthogonal transitions in each qubit and can be more than $50\%$. Moreover, we find that as a consequence of vacuum-induced coherence, sudden death of entanglement is prevented for an initial mixed entangled state of the qubits.

Many protocols and experiments in quantum information science are described in terms of simple measurements on qubits. However in a real implementation the exact description is more difficult and more complicated observables are used. The question arises whether a claim of entanglement in the simplified description still holds, if the difference between the realistic and simplified models is taken into account. We show that a positive entanglement statement remains valid if a certain positive linear map connecting the two descriptions—a so-called squashing operation—exists; then lower bounds on the amount of entanglement are also possible. We apply our results to polarization measurements of photons using only threshold detectors, and derive procedures under which multiphoton events can be neglected.

Real-time dynamics in electron-nucleus coupled systems in molecules is studied using the path-integral formalism, with a special emphasis on nonadiabatic interactions. We first establish a formal path-integral description of the entire system. Applying the stationary phase approximation, we then derive coupled equations for the mixed quantum-classical treatment of the system: the equations of motion for electron wave-packet dynamics and those for nuclear dynamics driven by what we call the force form. Thus the present theory also serves as a general theory for dynamics in mixed quantum and classical systems. On this theoretical foundation, we analyze two theories of nonadiabatic electron-nucleus coupled systems from the viewpoint of path branching: the semiclassical Ehrenfest theory and the recently developed method of phase-space averaging and natural branching [T. Yonehara, S. Takahashi, and K. Takatsuka, J. Chem. Phys. 130, 214113 (2009)]. We give a unified account of the essential feature of their physical implications and limitations. Path-integral formalism leads to further refinement of the idea of path branching caused by nonadiabatic coupling, thus giving deeper insight into the nonadiabatic dynamics. Further, we study the conservation laws for energy, linear momentum, and angular momentum in the general mixed quantum-classical representation. We also extend the present path-integral formulation so as to handle nonadiabatic dynamics in laser fields.

Configuration interaction method is used to calculate transition amplitudes and other properties of the low states of dysprosium which are used in cooling and in the study of the time variation of the fine structure constant and violation of fundamental symmetries. The branching ratio for the cooling state to decay to states other than ground states is found to be smaller than ${10}^{-4}$. The matrix element of the weak interaction between degenerate states at $E=19797.96$ cm${}^{-1}$ is about $4$ Hz which is consistent with the experimental limit $\left|H_W\right|=|2.3\pm 2.9(\mathrm{stat}.)\pm 0.7(\mathrm{syst}.)|$ Hz [A. T. Nguyen, D. Budker, D. DeMille, and M. Zolotorev, Phys. Rev. A 56, 3453 (1997)] and points to feasibility of its experimental measurement. Applications include the search for physics beyond the standard model using the parity nonconservation (PNC) isotopic chain approach.

The Huang-Yang multipolar pseudopotential [see Huang and Yang, Phys. Rev. 105, 767 (1957); later revised by Huang, Statistical Mechanics (Wiley, New York, 1963)] is derived in a mathematically rigorous way using distribution theory in arbitrary dimension. For higher partial waves (i.e., all except $s$ waves), the original pseudopotential contained wrong numerical factors in dimension 3, which were corrected only recently [see Derevianko, Phys. Rev. A 72, 044701 (2005); Stampfer and Wagner, J. Math. Anal. Appl. 342, 202 (2008)]. Here, we generalize the expression for the pseudopotential, in particular to even dimension, enabling the study of scattering processes in a large class of systems, especially in dimension 2.

Microscopic calculations show that CdSe-based semiconductor quantum dots with confined exciton and biexciton states are suitable candidates for Zeno-logic applications. The frequencies of the control and signal fields are chosen to guarantee very high transmission of the individual beams. If both fields are present simultaneously, they are strongly absorbed due to efficient ground-state-to-biexciton transitions. The optical Bloch equations for a three-level quantum-dot model with self-consistent light-matter coupling are solved numerically. The influence of dephasing and/or inhomogeneous dot distributions is analyzed and the conditions for satisfactory device operation are identified.

The transient solution and delay time for a $\delta$ potential scatterer with a movable quantum shutter is calculated by solving analytically the time-dependent Schrödinger equation. The delay time is analyzed as a function of the distance between the shutter and the potential barrier and also as a function of the distance between the potential barrier and the detector. In both cases, it is found that the delay time exhibits a dynamical behavior and that it tends to a saturation value $\Delta t_{\mathrm{sat}}$ in the limit of very short distances, which represents the maximum delay produced by the potential barrier near the interaction region. The phase time ${\tau }_{\theta }$, on the other hand, is not an appropriate time scale for measuring the time delay near the interaction region, except if the shutter is moved far away from the potential. The role played by the antibound state of the system on the behavior of the delay time is also discussed.

Ichikawa et al. [Phys. Rev. A 78, 052105 (2008)] showed that exchange symmetry gives rise to a simple characterization of whether multipartite pure quantum states being either globally entangled or fully separable. In this Brief Report, we provide a simple alternative approach and some extension to their conclusions.

It is known that arrays of trapped ions can be used to efficiently simulate a variety of many-body quantum systems. Here we show how it is possible to build a model representing a spin chain interacting with bosons that is exactly solvable. The exact spectrum of the model at zero temperature and the ground-state properties are studied. We show that a quantum phase transition occurs when the coupling between spins and bosons reaches a critical value, which corresponds to a level crossing in the energy spectrum. Once the critical point is reached, the number of bosonic excitations in the ground state, which can be assumed as an order parameter, starts to be different from zero. The population of the bosonic mode is accompanied by a macroscopic magnetization of the spins. This double effect could represent a useful resource for phase transition detection since a measure of the phonon can give information about the phase of the spin system. A finite-temperature phase diagram is also given in the adiabatic regime.

It was recently pointed out how differences in preparation procedures for quantum experiments can lead to nontrivial differences in the results of the experiment. The differences arise from the initial correlations between the system and the environment. Therefore, any quantum experiment that is prone to influences from the environment must be prepared carefully. We study quantum-process tomography in light of this. We suggest several experimental setups, where preparation of the initial state plays a role in the final outcome of the experiment. By studying the linearity and the positivity of the resulting maps, the experimenter can determine the nature of the initial correlations between the system and the environment.

Phase contrast imaging is used to observe Bose-Einstein condensates at finite temperature in situ. The imaging technique is used to accurately derive the absolute phase shift of a probe laser beam due to both the condensate and the thermal cloud. The accuracy of the method is enhanced by using the periodicity of the intensity signal as a function of the accumulated phase. The measured density profiles can be described using a two-relevant-parameter fit, in which only the chemical potential and the temperature are to be determined. This allows us to directly compare the measured density profiles to different mean-field models in which the interaction between the condensed and the thermal atoms is taken into account to various degrees.

Code-word-stabilized (CWS) codes are, in general, nonadditive quantum codes that can correct errors by an exhaustive search of different error patterns, similar to the way that we decode classical nonlinear codes. For an $n$-qubit quantum code correcting errors on up to $t$ qubits, this brute-force approach consecutively tests different errors of weight $t$ or less and employs a separate $n$-qubit measurement in each test. In this article, we suggest an error grouping technique that allows one to simultaneously test large groups of errors in a single measurement. This structured error recovery technique exponentially reduces the number of measurements by about $3^t$ times. While it still leaves exponentially many measurements for a generic CWS code, the technique is equivalent to syndrome-based recovery for the special case of additive CWS codes.