We report the observation of photoassociation to the $2{}^1{\Sigma }_g^{+}$ state of ${}^{85}$Rb${}_2$. We have observed two vibrational levels ($v^{\prime }=98,99$) below the $5s_{1/2}+5p_{1/2}$ atomic limit and 11 vibrational levels ($v^{\prime }=102$–112) above it. The photoassociation—and subsequent spontaneous emission—occur predominantly between 15 and 20 bohrs in a region of internuclear distance best described as a transition between Hund's case (a) and Hund's case (c) coupling. The presence of a $g$-wave shape resonance in the collision of two ground-state atoms affects the photoassociation rate and line shape of the $J^{\prime }=3$ and 5 rotational levels.

We characterize the immiscibility-miscibility transition (IMT) of a two-component Bose-Einstein condensate (BEC) with dipole-dipole interactions. In particular, we consider the quasi-two-dimensional geometry, where a strong trapping potential admits only zero-point motion in the trap direction, while the atoms are more free to move in the transverse directions. We employ the Bogoliubov treatment of the two-component system to identify both the well-known long-wavelength IMT in addition to a rotonlike IMT, where the transition occurs at finite-wave number and is reminiscent of the roton softening in the single-component dipolar BEC. Additionally, we verify the existence of the roton IMT in the fully trapped, finite systems by direct numerical simulation of the two-component coupled nonlocal Gross-Pitaevskii equations.

The binding energy of Feshbach molecules from a two-component Fermi gas of ${}^{40}$K atoms has been experimentally measured with the momentum-resolved Raman spectroscopy. Comparing with the radio-frequency spectroscopy, in the present experiment the signal of unpaired (free atoms) and the bound molecules can be directly observed and the binding energy can be simultaneously determined in a single running experiment. The energy-momentum dispersion spectra of the ultracold Fermi gas in the BEC side are also measured and reconstructed. The present experimental technology of the momentum-resolved Raman spectroscopy can be easily extended to perform spatially momentum-resolved Raman spectroscopy and to obtain the response spectra of a homogeneous system in the local density approximation.

We investigate scattering bright solitons off a potential using both analytical and numerical methods. Our paper focuses on low kinetic energies for which differences between the mean-field description via the Gross-Pitaevskii equation (GPE) and the quantum behavior are particularly large. On the $N$-particle quantum level, adding an additional harmonic confinement leads to a simple signature to distinguish quantum superpositions from statistical mixtures. While the nonlinear character of the GPE does not allow quantum superpositions, the splitting of GPE solitons takes place only partially. When the potential strength is increased, the fraction of the soliton which is transmitted or reflected jumps noncontinuously. We explain these jumps via energy conservation and interpret them as indications for quantum superpositions on the $N$-particle level. On the GPE level, we also investigate the transition from this stepwise behavior to the continuous case.

We have investigated the interaction of two counterpropagating focused laser beams with vacuum at electro-magnetic-field strengths far below the Schwinger limit. Unlike the case of a single beam, the polarization and magnetization in vacuum that interacts with the two laser beams are mainly generated by electro-magnetic-field components parallel to the electromagnetic field of the incident laser light. This results in much greater polarization and magnetization being generated than for a single beam. The number of photons radiated from a vacuum when counterpropagating beams were used was more than 3 orders of magnitude higher than when a single beam was used for the same laser power.

We propose two alternative entanglement concentration protocols (ECPs) using the Faraday rotation of photonic polarization. Through the single-photon input-output process in cavity QED, it is shown that the maximally entangled atomic (photonic) state can be extracted from two partially entangled states. The distinct feature of our protocols is that we can concentrate both atomic and photonic entangled states via photonic Faraday rotation, and thus they may be universal and useful for entanglement concentration in the experiment. Furthermore, as photonic Faraday rotation works in low-$Q$ cavities and only involves virtual excitation of atoms, our ECPs are insensitive to both cavity decay and atomic spontaneous emission.

The ideal (i.e., noninteracting), homogeneous Fermi gas, with its characteristic sharp Fermi surface in the momentum distribution, is a fundamental concept relevant to the behavior of many systems. With trapped Fermi gases of ultracold atoms, one can realize and probe a nearly ideal Fermi gas; however, these systems have a nonuniform density due to the confining potential. We show that the effect of the density variation, which typically washes out any semblance of a Fermi surface step in the momentum distribution, can be mitigated by selectively probing atoms near the center of a trapped gas. With this approach, we have directly measured a Fermi surface in momentum space for a nearly ideal gas, where the average density and temperature of the probed portion of the gas can be determined from the location and sharpness of the Fermi surface.

The sensitivity of rotational transitions in CH and CD to a possible variation of fundamental constants has been investigated. Largely enhanced sensitivity coefficients are found for specific transitions which are due to accidental degeneracies between the different fine-structure manifolds. These degeneracies occur when the spin-orbit coupling constant is close to four times the rotational constant. CH and particularly CD match this condition closely. Unfortunately, an analysis of the transition strengths shows that the same condition that leads to an enhanced sensitivity suppresses the transition strength, making these transitions too weak to be of relevance for testing the variation of fundamental constants over cosmological time scales. We propose a test in CH based on the comparison between the rotational transitions between the $e$ and $f$ components of the ${\Omega }^{\prime }=1/2,J=1/2$ and ${\Omega }^{\prime }=3/2,J=3/2$ levels at 532 and 536 GHz and other rotational or $\Lambda$-doublet transitions in CH involving the same absorbing ground levels. Such a test, to be performed by radioastronomy of highly redshifted objects, is robust against systematic effects.

Ultracold collisions between metastable bosonic ${}^{88}$Sr [$\left(5s5p\right){}^3{P}_2$] and fermionic ${}^{87}$Sr [$\left(5s5p\right){}^3{P}_2$ $(i=9/2)$] atoms are investigated based on an approach of tensorial analysis. The scattering physics of two atoms in fully polarized $s$-wave entrance channels is studied in detail. In the Born-Oppenheimer approximation, the strong anisotropic interatomic interaction is demonstrated to induce formations of long-range molecular potential wells. However, besides significantly modifying the elastic scattering, the anisotropic interatomic interaction leads to the strong multichannel coupling between different partial waves, which triggers a high rate of inelastic losses. By applying an external static magnetic field ${\mathbf{B}}_0={\mathcal{B}}_0{\mathbf{e}}_{\mathrm{z}}$, the inelastic scattering can be suppressed while the elastic scattering is significantly enhanced. Additionally, the energy-dependent complex $s$-wave scattering lengths at a given relative collision energy strongly depend on the strength of magnetic field. We, moreover, self-consistently investigate ultracold collisions of two atoms at low temperatures in an optical lattice site. In the $\delta$-function pseudopotential approximation, we derive the effective $s$-wave scattering lengths, energy eigenvalues, elastic and inelastic scattering rates, and their dependence on the external magnetic field. We find that (i) the effective scattering lengths of ultracold atoms in entrance channels of interest display resonance behavior at certain values of magnetic field and (ii) the extra Zeeman interaction not only leads to the suppression of inelastic scattering but also enlarges the elastic scattering rates.

We design, by invariant-based inverse engineering, resonant laser pulses to perform fast population transfers in three-level systems. The laser intensities to improve the fidelity or to achieve a perfect transfer are examined for different protocols. They can be reduced by populating the intermediate state and by multimode driving.

Erasing quantum-mechanical distinguishability is of fundamental interest and also of practical importance, particularly in subject areas related to quantum information processing. We demonstrate a method applicable to optical systems in which single-mode filtering is used with only linear optical instruments to achieve quantum indistinguishability. Through “heralded” Hong-Ou-Mandel interference experiments we measure and quantify the improvement of indistinguishability between single photons generated via spontaneous four-wave mixing in optical fibers. The experimental results are in excellent agreement with predictions of a quantum-multimode theory we develop for such systems, without the need for any fitting parameter.

In the quasi-one-dimensional lattice system with a band gap, the effective magnitude of the gap may be reduced, and even collapsed, by applying a monochromatic oscillating external field. The internal relationship between this dynamical band gap collapse (BGC) and the well-known coherent destruction of tunneling (CDT) in the periodically driven two-state model is clarified from a unified point of view. The destructive interference between the resonant scattering paths plays an essential role in both the dynamical BGC and the CDT. This leads to the suppression of the Bragg reflection in the momentum space for the dynamical BGC, and to the suppression of tunneling in the coordinate space for the CDT.

The effect of partial temporal coherence of free-electron laser (FEL) pulses in IR-laser-assisted short-pulse photoionization of atoms is theoretically analyzed. In a typical two-color photoionization experiment, when an atom is ionized by extreme ultraviolet photons in the presence of a strong IR laser field, several sidebands are produced in the electron spectrum at both sides of the photoline. The stochastic nature of the FEL radiation leads to a broadening of the sidebands. When the coherence time is short, shorter than the inverse frequency of the IR field, the sidebands cannot be observed. However, in angle-resolved experiments a certain structure in the spectrum can be seen as a remnant of the sideband gross structure.

The use of a Raman laser generated by modulation for a light-pulse atom interferometer allows for a laser system that is compact and robust. However, the additional laser frequencies generated can perturb the atom interferometer. In this article, we present a precise calculation of the phase shift induced by the additional laser frequencies. The model is validated by comparison with experimental measurements on an atom gravimeter. The uncertainty of the phase-shift determination limits the accuracy of our compact gravimeter to $8\times {10}^{-8}\text{m}/{\text{s}}^2$. We show that it is possible to reduce this inaccuracy considerably with a better control of experimental parameters or with particular interferometer configurations.

Transitions between different types of pulse solutions of the Ginzburg-Landau equation (GLE) have been studied experimentally in a mode-locked fiber laser. It is demonstrated that the different pulses corresponding to different solutions of the GLE can be generated in a single mode-locked laser. Dispersion-managed solitons, dissipative solitons, and similaritons can be emitted depending on parameters such as the pulse intensity, the linear cavity phase-delay bias, and the birefringence effect in the cavity. The three nonlinear waves show different features, especially the spectral shapes and dynamics accompanying pump power scaling. Also, our study implies that integrable systems and dissipative systems can be switched in a mode-locked fiber laser. This phenomenon reveals properties of the GLE which are not only scientifically interesting but also valuable for practical applications of mode-locked fiber lasers.

In this Brief Report we provide the physical interpretation of the success probability of an optimal tunable linear-optical controlled-phase gate as a function of the phase shift imposed by the gate. We analyze in detail the theoretical prediction of this success probability as identified by Kieling et al. [New J. Phys. 12, 013003 (2010)] and provide discussion revealing the reasons for its nonmonotonous behavior. Moreover, we show that the success probability can be increased if several requirements imposed on the gate can be lifted.

We show that hybrid local measurements combining homodyne measurements and photodetection provide violations of a Bell inequality with arbitrarily low photodetection efficiency. This is shown in two different scenarios: when one part receives an atom entangled to the field mode to be measured by the other part and when both parts make similar photonic measurements. Our findings promote the hybrid measurement scenario as a candidate for loophole-free Bell tests beyond previous expectations.

We propose a piecewise deterministic Markovian jump process in Hilbert space such that the covariance matrix of this stochastic process solves the thermodynamic quantum master equation. The proposed stochastic process is particularly simple because the normalization of the vectors in Hilbert space is preserved only on average. As a consequence of the nonlinearity of the thermodynamic master equation, the construction of stochastic trajectories involves the density matrix as a running ensemble average. We identify a principle of detailed balance and a fluctuation-dissipation relation for our Markovian jump process.

We simulate a piecewise deterministic Markovian jump process that represents an unraveling of a nonlinear quantum master equation describing the evolution of a quantum subsystem in contact with a heat bath. This process relies on running ensemble averages and state vectors normalized only on average. The latter causes the normalization of a small subset of trajectories to grow exponentially, resulting in numerical instability. We propose an efficient solution to this problem and illustrate the general ideas for a harmonic oscillator and a two-level system, each of them being weakly coupled to a heat reservoir. Our findings lay the foundation for a powerful simulation technique for dissipative quantum systems surrounded by general classical nonequilibrium environments.

We theoretically investigate polarization-entangled-photon generation by using a quantum dot embedded in a microcavity. In the absence of a biexciton, two of the four Bell states can be generated by applying cross-circularly polarized laser fields because of a cavity quantum electrodynamics effect [Ajiki et al., New J. Phys. 11, 033033 (2009)]. In the presence of the biexciton state, the generation of these Bell states can be forbidden under specific conditions. Furthermore, another type of Bell state can be generated by applying cross-circularly polarized fields. Our results indicate that one can generate three different types of Bell-state photon pairs just by changing the frequencies of the applied fields.