We study the difference between quantum and classical behavior in a pair of nonidentical cavities with second-harmonic generation. In the classical limit, each cavity has a limit-cycle solution, in which the photon number oscillates periodically in time. Coupling between the cavities leads to synchronization of the oscillations and classical correlations between the cavities. In the quantum limit, there are quantum correlations due to entanglement. The quantum correlations persist even when the cavities are far off resonance with each other, in stark contrast with the classical case. We also find that the quantum and classical limits are connected by an intermediate regime of almost no correlations. Our results can be extended to a wide variety of quantum models.

In a multipartite setting, it is possible to distinguish quantum states that are genuinely $n$-way entangled from those that are separable with respect to some bipartition. Similarly, the nonlocal correlations that can arise from measurements on entangled states can be classified into those that are genuinely $n$-way nonlocal, and those that are local with respect to some bipartition. Svetlichny introduced an inequality intended as a test for genuine tripartite nonlocality. This work introduces two alternative definitions of $n$-way nonlocality, which we argue are better motivated both from the point of view of the study of nature, and from the point of view of quantum information theory. We show that these definitions are strictly weaker than Svetlichny's, and introduce a series of suitable Bell-type inequalities for the detection of three-way nonlocality. Numerical evidence suggests that all three-way entangled pure quantum states can produce three-way nonlocal correlations.

Bohr's principle of complementarity (BPC) is the cornerstone of quantum mechanics. According to this principle, the total wavelike and particlelike information of a particle is limited by the Englert-Greenberger (EG) duality relation. Here, by introducing a quantum detecting device into the experiment, we find that the limit of the EG duality relation is exceeded because of the interference between the wave and particle properties of the photon. A generalized EG duality relation is further developed. Our work provides a generalization of BPC and gives new insights into quantum mechanics.

The sensitivity of the PbF molecule to the electron electric dipole moment has motivated detailed microwave spectroscopy. Previous theoretical approaches cannot fully explain the spectra. In turn, the explanation from “first principles” is very important both for molecular theory and for confirmation of the correctness of the interpretation of experimental data obtained with high precision. All of these issues are decisively addressed here. We have determined centrifugal correction parameters for hyperfine structure constants, both on lead and fluorine nuclei, of the $X{}^2{\Pi }_{1/2}$ state of lead monofluoride. These parameters were obtained by fitting experimentally observed pure rotational transitions and from ab initio calculations. We show that taking this correction into account is required to reproduce the experimental transition energies obtained by R. J. Mawhorter et al. [Phys. Rev. A 84, 022508 (2011)].

We propose a scheme to investigate the time scale of wave-function collapse by using polarization-entangled photon pairs. The setup is similar to those employed to study quantum correlations, but in the present case, synchronization is essential at all stages. We find that it is possible to discriminate between the scenarios of instantaneous collapse and finite-time reduction via a large number of double measurements of polarization. The quantities to be recorded present distinct behaviors in each scenario, the deviations being small but distinguishable from pure statistical fluctuations. The connection between the presented formalism and the theory of weak measurements is discussed as well as the possible consequences for experimental tests of nonlocality.

In this paper we address the problem of translating one-way quantum computation (1WQC) into the circuit model. We start by giving a straightforward circuit representation of any 1WQC, at the cost of introducing many ancilla wires. We then propose a set of simple circuit identities that explore the relationship between the entanglement resource and correction structure of a 1WQC, allowing one to obtain equivalent circuits acting on fewer qubits. We conclude with some examples and a discussion of open problems.

If entanglement is available, the error-correcting ability of quantum codes can be increased. We show how to optimize the minimum distance of an entanglement-assisted quantum error-correcting (EAQEC) code, obtained by adding ebits to a regular quantum stabilizer code, over different encoding operators. By this encoding optimization procedure, we found a number of EAQEC codes, including a family of entanglement-assisted quantum repetition codes and several optimal EAQEC codes.

We show that quantum optimal control theory (OCT) and time-dependent density-functional theory (TDDFT) can be combined to provide realistic femtosecond laser pulses for an enhanced ionization yield in few-electron systems. Using a one-dimensional model H${}_2$ molecule as a test case, the optimized laser pulse from the numerically exact scheme is compared to pulses obtained from OCT+TDDFT within the TD exact-exchange (TDEXX) and the TD local-density approximation (TDLDA). We find that the TDDFT pulses produce an ionization yield of up to $50\%$ when applied to the exact system. In comparison, pulses with a single frequency but the same fluence typically reach to yields around 5$\%$–15$\%$, unless the frequency is carefully tuned into a Fano-type resonance that leads to $\sim$30$\%$ yield. On the other hand, optimization within the exact system alone leads to yields higher than $80\%$, demonstrating that correlation effects beyond the TDEXX and TDLDA can give rise to even more efficient ionization mechanisms.

The first- and second-order wave-particle dualities for two-photon states are investigated in a Mach-Zehnder interferometer. These duality relations are experimentally tested through two-photon pure states and two-photon mixed states. The experimental results are in full agreement with the theoretical predictions. The optical fields exhibiting the same first-order particlelike and wavelike behaviors may carry entirely different second-order duality information, which implies that the higher-order duality relations are a useful tool for analyzing the wave-particle duality for multiphoton states.

We show that the extremely blueshifted dispersive wave emitted in Kerr media owing to the coupling with the negative-frequency branch [E. Rubino et al., Phys. Rev. Lett. 108, 253901 (2012)] can be observed in quadratic media via second-harmonic generation. Not only is such a phenomenon thus independent of the specific nonlinear mechanism, but it is shown to occur regardless of the fact that the process is pumped by a pulse which exhibits soliton-like features or vice versa undergoes wave breaking. A simple unified formula gives the frequencies of the emitted dispersive waves in both cases.

We analyze the general nonclassicality of correlations of a composite quantum system as measured by the negativity of quantumness. The latter corresponds to the minimum entanglement, as quantified by the negativity that is created between the system and an apparatus that is performing local measurements on a selection of subsystems. The negativity of quantumness thus quantifies the degree of nonclassicality on the measured subsystems. We demonstrate a number of possible different interpretations for this measure and for the concept of quantumness of correlations in general. In particular, for general bipartite states in which the measured subsystem is a qubit, the negativity of quantumness acquires a geometric interpretation as the minimum trace distance from the set of classically correlated states. This can be further reinterpreted as minimum disturbance, with respect to trace norm, due to a local measurement or a nontrivial local unitary operation. We calculate the negativity of quantumness in closed form for Werner and isotropic states and for all two-qubit states for which the reduced state of the system that is locally measured is maximally mixed; this includes all Bell diagonal states. We discuss the operational significance and potential role of the negativity of quantumness in quantum information processing.

The nonradiative charge-transfer processes in Na${}^{+}$+${}^{87}$Rb(5$s$) collisions have been investigated by using the quantum-mechanical molecular-orbital close-coupling method and the two-center atomic-orbital close-coupling method for the energy range of 10${}^{-4}$–5 and 0.3–100 keV/u, respectively. The radiative charge-transfer, radiative-decay, and radiative-association processes have been investigated by using the fully quantum, optical-potential, and semiclassical methods for the energy range of 10${}^{-18}$–0.2 eV/u. The nonradiative charge-transfer processes dominate the collisions for energies above 0.2 eV/u and radiative-decay processes dominate in the lower-energy region. At the very low collision energies of 10${}^{-18}$–10${}^{-3}$ eV/u, the radiative-association process is more important than the radiative charge-transfer process. Most importantly, it is found that the radiative cross sections exhibit Langevin behavior as $E^{-1/2}$ for energies less than 10${}^{-2}$ eV/u.

We calculate the single-particle spectral functions and quasiparticle dispersions for a Bose-Fermi mixture when the boson-fermion attraction is sufficiently strong to suppress completely the condensation of bosons at zero temperature. Within a T-matrix diagrammatic approach, we vary the boson-fermion attraction from the critical value where the boson condensate first disappears to the strongly attractive (molecular) regime and study the effect of both mass and density imbalance on the spectral weights and dispersions. An interesting spectrum of particle-hole excitations mixing two different Fermi surfaces is found. These unconventional excitations could be produced and explored experimentally with radio-frequency spectroscopy.

We employ the inhomogeneous Helmholtz equation to explore the influence of the fractional degeneracy and the pump distribution on the resonant lasing mode. Theoretical analyses clearly reveal the relationship between the fractional degeneracy and the emergence of the ray-wave duality. Furthermore, we perform thorough laser experiments to confirm the theoretical exploration that the resonant modes near the degenerate cavities are well localized on the ray trajectories under the condition of the off-axis pumping. We also exploit the derived wave functions to calculate the resonant strengths that can noticeably manifest the enhancements of the output powers in the degenerate cavities.

Quantum discord is a measure of quantum correlation beyond entanglement. Computing quantum discord for simple quantum states is a basic problem. An analytical formula of quantum discord for two-qubit $X$ states is first claimed in [Ali, Rau, and Alber, Phys. Rev. A 81, 042105 (2010)], but later found to be not always correct. I observe numerically that the formula is valid with worst-case absolute error 0.0021. For symmetric two-qubit $X$ states, I give a counterexample to the analytical formula derived in [F. F. Fanchini et al., Phys. Rev. A 81, 052107 (2010)], but observe that the formula is valid with worst-case absolute error 0.0006. The formula has been used in many research papers. The results in all these works are approximately correct, even if they may not be exactly correct.

This paper describes the modeling of quantum-dot lasers with the aim of assessing the conditions for stable cw dual-mode operation when the mode separation lies in the THz range. Several possible models suited for InAs quantum dots in InP barriers are analytically evaluated, in particular quantum dots electrically coupled through a direct exchange of excitation by the wetting layer or quantum dots optically coupled through the homogeneous broadening of their optical gain. A stable dual-mode regime is shown possible in all cases when quantum dots are used as active layer whereas a gain medium of quantum well or bulk type inevitably leads to bistable behavior. The choice of a quantum-dot gain medium perfectly matched the production of dual-mode lasers devoted to THz generation by photomixing.

We describe an implementation of Doppler broadening thermometry based upon a room-temperature continuous-wave quantum cascade laser at 9.07 $\mu$m, coherently phase locked to a thulium optical frequency comb centered at 2 $\mu$m. The thermodynamic temperature is retrieved from the analysis of multiple-line absorption spectra of the NH${}_3$ ${\nu }_2$ band, adopting semiclassical line-shape models and taking into full account the speed dependence of collisional broadening. We demonstrate that a precision of approximately five parts over 10${}^5$ can be reached as a result of the analysis of only 90 absorption spectra, as acquired as a function of the gas pressure.

We show a method to protect quantum states from the disturbance due to the random potential using successive transport of the quantum states. The quantum states are kept undisturbed for a longer time than the case of the simple trapping with a stationary potential because the effective potential, which the quantum states feel, becomes uniform when the velocity of the transport is sufficiently large. It is shown that a Bose-Einstein condensate is protected from the disturbance by the alternating transport using the fast forward scaling theory [Masuda and Nakamura, Proc. R. Soc. A 466, 1135 (2010)].

We present a method for measuring magnetic field gradients with macroscopic singlet states realized with ensembles of spin-$j$ particles. While the singlet state is completely insensitive to homogeneous magnetic fields, the variance of its collective spin components is highly sensitive to field gradients. We compute the dynamics of this variance analytically for a chain of spins and also for an ensemble of particles with a given density distribution. We find an upper bound on how precisely the field gradient can be estimated from the measured data. Based on our calculations, differential magnetometry can be carried out with cold atomic ensembles using a multipartite singlet state obtained via spin squeezing. On the other hand, comparing the metrological properties of the experimentally prepared state to that of the ideal singlet can be used as further evidence that a singlet state has indeed been created.