Interference of two independently prepared ideal Bose gases is discussed, on the basis of the idea of measurement-induced interference. It is known that, even if the number of atoms in each gas is individually fixed finite and the symmetry of the system is not broken, an interference pattern is observed on each single snapshot. The key role is played by the Hanbury Brown and Twiss effect, which leads to an oscillating pattern of the cloud of identical atoms. Then, how essential is the Bose-Einstein condensation to the interference? In this work, we describe two ideal Bose gases trapped in two separate three-dimensional harmonic traps at a finite temperature $T$, using the canonical ensembles (with fixed numbers of atoms). We compute the full statistics of the snapshot profiles of the expanding and overlapping gases released from the traps. We obtain a simple formula valid for finite $T$, which shows that the average fringe spectrum (average fringe contrast) is given by the purity of each gas. The purity is known to be a good measure of condensation, and the formula clarifies the relevance of the condensation to the interference. The results for $T=0$, previously known in the literature, can be recovered from our analysis. The fluctuation of the interference spectrum is also studied, and it is shown that the fluctuation is vanishingly small only below the critical temperature $T_c$, meaning that interference pattern is certainly observed on every snapshot below $T_c$. The fact that the number of atoms is fixed in the canonical ensemble is crucial to this vanishing fluctuation.

We report on the generation of narrow-bandwidth and frequency-modulated cascaded emission of two photons from a collimated Yb atomic beam. Efficient population transfer from the ground state ($6s^2$ ${}^1{S}_0$) to upper state ($6s7s$ ${}^1{S}_0$), of which direct transition at 291.1 nm is dipole forbidden, is achieved through a resonant two-photon excitation enhanced by the electromagnetically induced transparency mediated by the intermediate state ($6s6p$ ${}^1{P}_1$). Then cascaded emission of two photons with a bandwidth of 54 MHz at 611.3 nm (idler) and 555.8 nm (signal) occurs in sequence from the upper state via the spin triplet state ($6s^2$ ${}^3{P}_1$). Numerical calculations of the density matrix equations taking into account the residual Doppler effect and strong driving fields successfully explain the experimental results for the idler and signal beam intensities depending on the various parameters of the driving fields. Synchronized optical switching and frequency-modulation characteristics of the idler and signal beams are also reported.

We calculate and investigate the relativistic correlation function for bipartite systems of spin-$1/2$ particles in vector states and spin-$1$ particles in tensor states. We show that the relativistic correlation function, which depends on particle momenta, may have local extrema. Furthermore, the momentum dependence of the correlation functions for two choices of the relativistic spin operator may be significantly different.

Using a Hartree-Fock-like partitioning of the two-matrix in terms of the true correlated one-matrix, the corresponding approximate ground-state energy $E_0$ is calculated for an exactly solvable closed-shell system proposed earlier by Moshinsky [Am. J. Phys. 36, 52 (1968)]. Comparisons of $E_0$ with the Hartree-Fock ground-state energy $E_{\mathrm{HF}}$ and the exact ground-state energy $E_{\mathrm{ex}}$ are made. These comparisons provide valuable insight into the usefulness of a Hartree-Fock-like partitioning of the two-matrix with repulsive interparticle potentials. In particular, an explicit quantification for the applicability limit of Lieb’s [Phys. Rev. Lett. 46, 457 (1981)] bound $E_0⩾E_{\mathrm{HF}}$ with unbounded oscillator potentials is given.

It is demonstrated that with appropriately arranged dielectric nanoparticle structures an extremely low loss guiding of light is achievable at the nanoscale. These dielectric nanoparticle waveguides (DNPWs) exhibit subwavelength transverse confinement of light comparable with metallic plasmon particle waveguides (MPPWs). Different from the MPPWs subject to inevitable intrinsic absorption loss, the material loss for some DNPWs is negligible, enabling long-distance light transport and offering an alternative choice for integrated photonics.

We present a quantum description of electromagnetic waves propagating through time-dependent homogeneous nondispersive conducting and nonconducting linear media without charge sources. Based on the Coulomb gauge and the quantum invariant method, we find the exact wave functions for this problem. In addition, we construct coherent and squeezed states for the quantized electromagnetic waves and evaluate the quantum fluctuations in coordinate and momentum space as well as the uncertainty product for each mode of the electromagnetic field.

Several attacks have been proposed on quantum key distribution systems with gated single-photon detectors. The attacks involve triggering the detectors outside the center of the detector gate, and/or using bright illumination to exploit classical photodiode mode of the detectors. Hence a secure detection scheme requires two features: The detection events must take place in the middle of the gate, and the detector must be single-photon sensitive. Here we present a technique called bit-mapped gating, which is an elegant way to force the detections in the middle of the detector gate by coupling detection time and quantum bit error rate. We also discuss how to guarantee single-photon sensitivity by directly measuring detector parameters. Bit-mapped gating also provides a simple way to measure the detector blinding parameter in security proofs for quantum key distribution systems with detector efficiency mismatch, which up until now has remained a theoretical, unmeasurable quantity. Thus if single-photon sensitivity can be guaranteed within the gates, a detection scheme with bit-mapped gating satisfies the assumptions of the current security proofs.

Cross sections for single-electron capture in collisions of He${}^{2+}$, Li${}^{q+}$ ($q$ = 1,2,3), ${\mathrm{C}}^{6+}$, and ${\mathrm{O}}^{8+}$ ions with helium atoms at incident energy ranging from 50 to 5000 keV/amu have been calculated in the framework of four-body boundary-corrected continuum intermediate state (BCCIS-4B) approximation in both prior and post forms. In this formalism, distortion in the final channel related to the Coulomb continuum states of the projectile ion and the active electron in the field of residual target ion are included. In all cases, total single-electron-capture cross sections have been calculated by summing over all contributions up to $n$ = 3 shells and subshells, respectively. It has been observed that the contribution of the capture cross section into the excited states is significant for asymmetric collision ($Z_P>Z_T$) and is insignificant for symmetric collision. Numerical results for the total cross sections show good agreement with the available experimental findings, particularly in the post form. Post-prior discrepancy has been found to be within 30% except for Li${}^{+}$ + He interactions below 150 keV/amu.

Luminescence from UV to visible light (200–570 nm) was observed in the ${\mathrm{H}}^{+}$$+$ ${\mathrm{O}}_2$, ${\mathrm{H}}_2$${}^{+}$$+$ ${\mathrm{O}}_2$, and ${\mathrm{H}}_3$${}^{+}$$+$ ${\mathrm{O}}_2$ collisions in the 10–1000 eV${}_{\text{lab}}$ energy range. For all systems a luminescent charge-exchange process leading to the ${\mathrm{O}}_2$${}^{+}$($A$$\hspace{0.5em}{}^2{\Pi }_u$, $b\hspace{0.5em}{}^4{\Sigma }_g$) and ${\mathrm{H}}^{*}$ products dominates. Computer simulation of the spectra was used to estimate relative cross sections for the reactions.

We present an experimental study on electron scattering by methanol (CH${}_3$OH). More specifically, differential, integral, and momentum-transfer cross sections for elastic electron scattering in the 100- to 1000-eV energy range are reported. Experimentally, angular distributions of the energy-selected electrons are measured and converted to absolute cross sections using the relative-flow technique. Relative-flow rates measured using the method of pressure decrease are applied for this purpose. The influence of the adsorption-desorption of methanol vapor on the surfaces inside of the gas-handling manifold is also studied. The possibility of dimerization of methanol at low pressures is investigated and disregarded based on the experimental evidence and also on a molecular-dynamics-simulation study. Electron-methanol collision cross sections are also calculated using the independent-atom model at the static-exchange-polarization and static-exchange-polarization-absorption levels of approximation. The comparison of our measured results of cross sections with other experimental and theoretical data available in the literature and with the present calculated data is encouraging.

We develop a coherent-state Ehrenfest trajectory (CSET) approach for the nonperturbative study of full electronic and nuclear dynamics of molecules interacting with intense laser fields. In this approach, electrons and nuclei are characterized by CSETs, dynamic processes are identified by the properties of the CSETs, and the transition probability of a process is calculated from the number of CSETs in this process. We apply this approach to simulate the full dynamics of ${\mathrm{H}}_2$ in intense linearly polarized laser fields. In the tunneling regime, the photoelectron energy spectra show a pronounced low-energy structure (LES) and the predicted scaling law of the LES with respect to the Keldysh parameter is in very good agreement with that observed in the latest experiments. Moreover, the photoelectron angular distribution is found to be squeezed along the laser field direction with the increase of laser wavelength and/or intensity.

The dipole forces on rotating diatomic molecules are worked out in detail for optical as well as microwave radiation fields. The objective is in particular to investigate how the dipole forces and potentials depend on the subtle internal structure of the molecule, with special emphasis on hyperfine and Zeeman states. Dipole potentials are obtained from computations of the real part of the complex molecular polarizability, whereas the imaginary part yields the scattering force. Numerical examples are presented for ${}^{23}{\mathrm{Na}}_2$ and OH for optical (laser) fields related to strong electronic transitions and for microwave fields for the Λ doubling in the OH ground state.

As a means of realizing oscillatory pairing between fermions, we study superfluid pairing between two fermion “spin” species that are confined to adjustable spin-dependent trapping potentials. Focusing on the one-dimensional limit, we find that with increasing separation between the spin-dependent traps, the fermions exhibit distinct phases, including a fully paired phase, a spin-imbalanced phase with oscillatory pairing, and an unpaired fully spin-polarized phase. We obtain the phase diagram of fermions in such a spin-split trap and discuss signatures of these phases in cold-atom experiments.

We investigate the effects of a trapping space-dependent potential on the low-temperature quasi-long-range order phase of two-dimensional particle systems with a relevant U(1) symmetry, such as quantum atomic gases. We characterize the universal features of the trap-size dependence using scaling arguments. The resulting scenario is supported by numerical Monte Carlo simulations of a classical two-dimensional $\mathit{XY}$ model with a space-dependent hopping parameter, the inhomogeneity of which is analogous to that arising from the trapping potential in experiments of atomic gases.

We introduce an approach to studying a driven qubit-oscillator system in the ultrastrong coupling regime, where the ratio $g/\Omega$ between coupling strength and oscillator frequency approaches unity or goes beyond, and simultaneously for driving strengths much bigger than the qubit energy splitting (extreme driving). Both qubit-oscillator coupling and external driving lead to a dressing of the qubit tunneling matrix element of different nature: the former can be used to suppress selectively certain oscillator modes in the spectrum, while the latter can bring the qubit’s dynamics to a standstill at short times (coherent destruction of tunneling) even in the case of ultrastrong coupling.

Fundamental aspects of ultracold collisions between identical bosonic or fermionic dipoles are studied under quasi-two-dimensional (Q2D) confinement. In the strongly dipolar regime, bosonic and fermion species are found to share important collisional properties as a result of the confining geometry, which suppresses the inelastic rates irrespective of the quantum statistics obeyed. A potential negative is that the confinement causes dipole-dipole resonances to be extremely narrow, which could make it difficult to explore Q2D dipolar gases with tunable interactions. Such properties are shown to be universal, and a simple WKB model reproduces most of our numerical results. In order to shed light on the many-body behavior of dipolar gases in Q2D we have analyzed the scattering amplitude and developed an energy-analytic form of the pseudopotentials for dipoles.

Recently we have presented a hidden-variable model of measurements for a qubit where the hidden-variable state-space dimension is one-half the quantum-state manifold dimension. The absence of a short memory (Markov) dynamics is the price paid for this dimensional reduction. The conflict between having the Markov property and achieving the dimensional reduction was proved by Montina [A. Montina, Phys. Rev. A 77, 022104 (2008)] using an additional hypothesis of trajectory relaxation. Here we analyze in more detail this hypothesis introducing the concept of invertible process and report a proof that makes clearer the role played by the topology of the hidden-variable space. This is accomplished by requiring suitable properties of regularity of the conditional probability governing the dynamics. In the case of minimal dimension the set of continuous hidden variables is identified with an object living an $N$-dimensional Hilbert space whose dynamics is described by the Schrödinger equation. A method for generating the economical non-Markovian model for the qubit is also presented.

We present an experimental and theoretical study of carrier-envelope-phase (CEP) effects on the population transfer between two bound atomic states interacting with pulses consisting of many cycles. Using intense radio-frequency pulse with Rabi frequency of the order of the atomic transition frequency, we investigate the influence of the CEP on the control of phase-dependent multiphoton transitions between the Zeeman sublevels of the ground state of ${}^{87}\mathrm{Rb}$. Our scheme has no limitation on the duration of the pulses. Extending the CEP control to longer pulses creates interesting possibilities to generate pulses with accuracy that is better than the period of optical oscillations.

Strong-field ionization generally produces ions in a superposition of ionic eigenstates. This superposition is generally not fully coherent and must be described in terms of a density matrix. A recent experiment [E. Goulielmakis et al., Nature (London) 466, 739 (2010)] employed attosecond transient absorption spectroscopy to determine the density matrix of strong-field-generated Kr${}^{+}$ ions. The experimentally observed degree of coherence of the strong-field-generated Kr${}^{+}$ ions is well reproduced by a recently developed multichannel strong-field-ionization theory, but there is significant disagreement between experiment and theory with respect to the degree of alignment of the Kr${}^{+}$ ions. In the present paper, the theory underlying attosecond transient absorption spectroscopy of strong-field-generated ions is developed. The theory is formulated in such a way that the nonperturbative nature of the strong-field-ionization process is systematically taken into account. The impact of attosecond pulse propagation effects on the interpretation of experimental data is investigated both analytically and numerically. It is shown that attosecond pulse propagation effects cannot explain why the experimentally determined degree of alignment of strong-field-generated Kr${}^{+}$ ions is much smaller than predicted by existing theory.

Ultracold atom experiments offer the unique opportunity to study mixing of different types of superfluid states. Our interest is in superfluid mixtures comprising of particles with different statistics—Bose and Fermi. Such scenarios occur naturally, for example, in dense quantum chromodynamics (QCD) matter. Interestingly, cold atomic experiments are performed in traps with finite spatial extent, thus critically destabilizing the occurrence of various homogeneous phases. Critical to this analysis is the understanding that the trapped system can undergo phase separation, resulting in a unique situation where phase transition in either species (bosons or fermions) can overlap with the phase separation between possible phases. In the present work, we illustrate how this intriguing interplay manifests in an interacting two-species atomic mixture—one bosonic and another fermionic with two spin components—within a realistic trap configuration. We further show that such interplay of transitions can render the nature of the ground state to be highly sensitive to the experimental parameters and the dimensionality of the system.