We use the Bose-Hubbard Hamiltonian to study quantum fluctuations in canonical equilibrium ensembles of bosonic Josephson junctions at relatively high temperatures, comparing the results for finite particle numbers $N$ to the classical limit that is attained as $N$ approaches infinity. We consider both attractive and repulsive atom-atom interactions, with special focus on the behavior near the $T=0$ quantum phase transition that occurs, for large enough $N$, when attractive interactions surpass a critical level. Differences between Bose-Hubbard results for small $N$ and those of the classical limit are quite small even when $N\sim 100$, with deviations from the limit diminishing as $1/N$.

A correlated many-body calculation is presented to characterize the Shannon information entropy of trapped interacting bosons. We reformulate the one-body Shannon information entropy in terms of the one-body probability density. The minimum limit of the entropy uncertainty relation is approached by making $N$ very small in our numerical work. We examine the effect of correlations in the calculation of information entropy. Comparison with the mean-field result shows that the correlated basis function is indeed required to characterize the important features of the information entropies. We also accurately calculate the point of critical instability of an attractive BEC, which is in close agreement with the experimental value. Next, we calculate two-body entropies in position and momentum spaces and study quantum correlations in the attractive BEC.

By engineering appropriate artificial gauge potentials, a Bose-Einstein condensate (BEC) can be adiabatically loaded into a current-carrying state that is analogous to a vortex lattice of a rotating uniform Bose gas. We give two explicit, experimentally feasible protocols by which vortex lattices can be smoothly formed from a condensate initially at rest. In the first example we show how this can be achieved by adiabatically loading a uniform BEC into an optical flux lattice, formed from coherent optical coupling of internal states of the atom. In the second example we study a tight-binding model that is continuously manipulated in parameter space such that it smoothly transforms into the Harper-Hofstadter model with $1/3$ flux per plaquette.

We study the stability of a Bose-Fermi system loaded into an array of coupled one-dimensional (1D) “tubes,” where bosons and fermions experience different dimensions: Bosons are heavy and strongly localized in the 1D tubes, whereas fermions are light and can hop between the tubes. Using the ${}^{174}$Yb-${}^6$Li system as a reference, we obtain the equilibrium phase diagram. We find that, for both attractive and repulsive interspecies interaction, the exact treatment of 1D bosons via the Bethe ansatz implies that the transitions between pure fermion and any phase with a finite density of bosons can only be first order and never continuous, resulting in phase separation in density space. In contrast, the order of the transition between the pure boson and the mixed phase can either be second or first order depending on whether fermions are allowed to hop between the tubes or they also are strictly confined in 1D. We discuss the implications of our findings for current experiments on ${}^{174}$Yb-${}^6$Li mixtures as well as Fermi-Fermi mixtures of light and heavy atoms in a mixed-dimensional optical-lattice system.

The geometric phase is studied for quantum polarized light described by an SU(2) coherent state. This is revealed by using quantum interference. The SU(2) coherent state is constructed from the quantized Stokes parameters. The quantum interferometry is arranged by the evolution operator implemented by the Hamiltonian inspired from the birefringence of the Faraday and Kerr effect. The geometric phase is given by the coherent state path integral. In particular, by considering a resonant Hamiltonian, the geometric phase is extracted from the total factor that consists of both geometric and dynamical phases. This extraction forms the basis of an experimental way to detect the geometric phase.

Dual-cavity optoelectromechanical systems (OEMS) are those where two electromagnetic cavities are connected by a common mechanical spring. These systems have been shown to facilitate high-fidelity quantum-state transfer from one cavity to another. In this paper, we explicitly calculate the effect on the fidelity of state transfer when an additional spring is attached to only one of the cavities. Our quantitative estimates of loss of fidelity highlight the sensitivity of a dual-cavity OEMS when it couples to additional mechanical modes. We show that this sensitivity can be used to design an effective mechanical switch for inhibition or high-fidelity transmission of quantum states between the cavities.

We report on high-order-harmonic generation in nanoparticle-containing laser ablation plasmas using a Ti:sapphire driving laser at a repetition rate of 1 kHz and on the relation of the harmonic signals with the morphological and spectroscopic characteristics of the ablation deposits. The study of the deposits has allowed the definition of ablation conditions, leading to the generation of a plasma containing nanoparticles of similar sizes to the ones used as target material. The corresponding plasma plumes lead to efficient conversion of the IR photons towards the short-wavelength range through high-order harmonic generation and show superior performance to those corresponding to bulk targets of the same metallic element.

We compare several criteria for quantum Markovianities and illustrate the differences and similarities through explicit examples of random unitary qubit dynamics (Pauli channels). In particular, we demonstrate that the quantum Markovianity based on distinguishability [Breuer et al., Phys. Rev. Lett. 103, 210401 (2009)] is not equivalent to that based on correlations [Luo et al., Phys. Rev. A 86, 044101 (2012)].

We propose a convenient approach for managing orbital angular momentum (OAM) of second-harmonic photons generated by a quadratic nonlinear interaction under noncolinear type-I phase matching condition. Since the OAMs of two fundamental photons can be flexibly controlled by using computer-generated holograms, we were able to realize arbitrary combination of both OAMs and experimentally confirm the OAM conservation selection rule ${\ell }_{2\omega }={\ell }_{\omega }^{\prime }+{\ell }_{\omega }^{\prime \prime }$ in general cases. In particular, our approach could be used to generate the second-harmonic photon states with fractional and odd-order OAMs.

Dispersion and its cancellation in entanglement-based nonlocal quantum measurements are of fundamental and practical interests. We report a demonstration of cancellation of femtosecond-level dispersion by inverting the sign of the differential dispersion between the long and short paths in only one arm of a fiber-based Franson interferometer. We restore the otherwise limited quantum visibility to an unprecedented 99.6%, and put time-energy entanglement at the same level of quality as polarization entanglement for use in quantum information processing applications.

In quantum mechanics an incoming particle wave packet with sufficient energy will undergo both transmission and reflection when encountering a barrier of lower energy, but in classical mechanics there is no reflection, only transmission. In this paper we seek to explain the disappearance of quantum-mechanical reflection in the quasiclassical limit, using the standard machinery of decoherence through environmental interaction. We consider two models. In the first, the incoming particle is classicalized by coupling to an environment and modeled using a standard master equation of Lindblad form with Lindblad operator proportional to position (the simplest version of quantum Brownian motion). We find, however, that suppression of reflection is achieved only for environmental interaction so strong that large fluctuations in momentum are generated, which blurs the distinction between incoming and reflected wave packets. This negative conclusion also holds for a complex potential which has similar implications for attempts to understand the suppression of the Zeno effect using the same mechanism (discussed in more detail elsewhere). A different master equation with Lindblad operator proportional to momentum is shown to be successful in suppressing reflection without large fluctuations but such a master equation is unphysical. We consider a second model in which the barrier is modeled quantum mechanically by a massive target particle coupled to an environment to maintain it in a quasiclassical state. This avoids the fluctuations problem since the incoming particle is not coupled to the environment directly. We find that reflection is significantly suppressed as long as the decoherence time scale of the target particle is much smaller than certain characteristic scattering time scales of the incoming particle, or equivalently, as long as the velocity fluctuations in the target are larger than the velocity of the incoming particle.

We propose an experimental scheme for entangling two macroscopic mechanical resonators (movable mirrors) by their coupling to the two-mode fields of a correlated-emission laser inside a doubly resonant cavity. With this aim we investigate the quantum Langevin equations that describe the interaction of the field-mirror system in conjuction with the master equation of the correlated-emission laser. We show that the steady-state entanglement of two mirrors as well as that of two-mode fields can be obtained in the regime of strong field-mirror coupling when the input lasers are scattered at the anti-Stokes sidebands. Remarkably, the degree of entanglement for both the mirror pair and the field pair can be controlled by an external field driving the gain medium. Our scheme is able to entangle two macroscopic objects with current state-of-the-art experimental apparatus.

We consider the security of practical continuous-variable quantum-key-distribution implementation with the local oscillator (LO) fluctuating in time, which opens a loophole for Eve to intercept the secret key. We show that Eve can simulate this fluctuation to hide her Gaussian collective attack by reducing the intensity of the LO. Numerical simulations demonstrate that if Bob does not monitor the LO intensity and does not scale his measurements with the instantaneous intensity values of LO, the secret key rate will be compromised severely.

Quantum secret-sharing and quantum error-correction schemes rely on multipartite decoding protocols, yet the nonlocal operations involved are challenging and sometimes infeasible. Here we construct a quantum secret-sharing protocol with a reduced number of quantum communication channels between the players. We introduce a scheme based on embedding a classical linear code into a quantum error-correcting code and then mapping the latter to a quantum secret-sharing protocol. In contrast to the Calderbank-Shor-Steane construction, we do not impose any restriction on the classical code; our protocol works with any arbitrary linear code. Our work paves the way towards the more general problem of simplifying the decoding of quantum error-correcting codes.

Magnetic quadrupole and electric octupole transitions from the configurations $4p^{5}4d^{N+1}$ and $4p^{6}4d^{N-1}4f$ were calculated along with the magnetic dipole, electric dipole, and electric quadrupole radiative transitions in the quasirelativistic Hartree-Fock approximation. Their significance in determining the metastable-level radiative lifetimes was investigated along several isoelectronic sequences for the ions from $Z=50$ to $Z=92$. Strontiumlike ions, zirconiumlike ions, molybdenumlike ions, and rhodiumlike ions were studied comprehensively. The remaining isoelectronic sequences with the ground configuration $4d^N$ ($N=1,3,5,7,8,10$) were also reviewed, although in less detail. Systematic trends of the determined total radiative lifetimes were studied. The importance of magnetic quadrupole and electric octupole transitions from metastable levels of ions belonging to these isoelectronic sequences was investigated and discussed. The inclusion of transitions of a higher multipole order can change theoretical radiative lifetime values for some levels by more than a factor of 10. These cases cannot be established in advance, without performing detailed calculations.

We present experimental results for dissociative electron attachment to carbon dioxide near the 8-eV Feshbach resonance. In particular, the dissociation channel leading to O${}^{-}$ production has been investigated with a momentum imaging technique that utilizes a supersonic gas jet to form a low-temperature, confined molecular target. Angular fragmentation and kinetic energy release distributions are compared to recent results by Slaughter et al. [J. Phys. B 44, 205203 (2012)] and Wu et al. [Phys. Rev. A 85, 052709 (2012)] using similar techniques. We show that careful attention to weighting of the O${}^{-}$ fragmentation momentum space is required to interpret the kinetic energy release observations and that there is no appreciable change in the angular distribution of O${}^{-}$ fragments at different energies around the resonance peak, as previously reported by Wu et al. The present O${}^{-}$ momentum distribution and kinetic energy release differ from previous results and provide alternative guidance for the theoretical consideration of potential energy surface dynamics that takes place during attachment.

The paper explores the prospects of observing the phenomenon of dynamical Anderson localization via nonresonant Raman-type rotational excitation of molecules by periodic trains of short laser pulses. We define conditions for such an experiment and show that current femtosecond technology used for nonadiabatic laser alignment of linear molecules is sufficient for this task. Several observables which can serve as indicators for Anderson localization are suggested for measurement, and the influence of experimental limitations imposed by the laser intensity noise, finite pulse duration, limited number of pulses in a train, and thermal effects is analyzed.

A joint experimental and theoretical study of four-photon ionization of Li is performed using a pump-probe setup with the laser frequency tuned in between the excitation energies of the $2s$–$4p$ and $2s$–$4f$ three-photon transitions. The three-dimensional photoelectron momentum pattern is recorded as a function of the delay time between two otherwise identical laser pulses. The process is modeled by numerically solving the time-dependent Schrödinger equation. While complicated interference patterns arise in the electron spectrum when the two pulses partially overlap in time, the expected quantum-beat phenomenon between Li atoms in either the $4p$ or the $4f$ state are seen when the fast oscillations involving the $2s$ initial state are averaged over. The qualitative agreement between the predicted and observed beating patterns is very good.

We demonstrate experimentally the evaporative cooling of a few hundred rubidium-87 atoms in a single-beam microscopic dipole trap. Starting with 800 atoms at a temperature of 125 $\mu$K, we produce an unpolarized sample of 40 atoms at 110 nK, within 3 s. The phase-space density at the end of the evaporation reaches unity, close to quantum degeneracy. The gain in phase-space density after evaporation is ${10}^3$. We find that the scaling laws used for much larger numbers of atoms are still valid despite the small number of atoms involved in the evaporative cooling process. We also compare our results to a simple kinetic model describing the evaporation process and find good agreement with the data.

We report on a simple oven-loaded magneto-optic trap (MOT) apparatus for the creation of both molecular Bose-Einstein condensates (mBECs) and degenerate Fermi gases (DFGs) of lithium. The apparatus does not require a Zeeman slower or a two-dimensional MOT, nor does it require any separation or differential pumping between the effusive atom source and the science chamber. The result is an exceedingly simple, inexpensive, and compact vacuum system ideal for miniaturization. We discuss our work in the context of other realizations of quantum degenerate gases by evaporation in optical dipole traps and illustrate that our apparatus meets the key requirements of atom number and trap lifetime. With this system, we also demonstrate the production of a mBEC, and we use it to observe the pairing gap of a strongly interacting two-component DFG in the BEC-BCS crossover regime.