We develop a temperature measurement of an atomic cloud based on the temporal correlations of fluorescence photons evanescently coupled into an optical nanofiber. We measure the temporal width of the intensity-intensity correlation function due to atomic transit time and use it to determine the most probable atomic velocity, hence the temperature. This technique agrees well with standard time-of-flight temperature measurements. We confirm our results with trajectory simulations.

The x-ray near-field speckle-scanning concept is an approach recently introduced to obtain absorption, phase, and dark-field images of a sample. In this paper, we present ways of recovering from a sample its ultrasmall-angle x-ray scattering distribution using numerical deconvolution. We also show how to access the 2D phase gradient signal from random step scans, the latter having the potential to elude the flat-field correction error. Each feature is explained theoretically and demonstrated experimentally at a synchrotron x-ray facility.

In the spin Hall effect of light (SHEL) with an incident linear polarization, the left circularly polarized (LCP) and the right circularly polarized (RCP) spin components are transversely shifted to opposite sides of the incident plane. We find that each spin component of the reflected or the refracted light is actually the coherent superposition of a pair with the same handedness. The two parts in a pair are generated by both the incident LCP and RCP components, respectively, and have different transverse shifts. Their weighted shifts lead to the overall displacement of the corresponding spin component. This perspective can successfully elucidate the dependence of the SHEL spin separations on the angle of polarization of the incident electric field.

We say that two (or more) state assignments for one and the same quantum system are compatible if they could represent the assignments of observers with differing information about the system. A criterion for compatibility was proposed in [Phys. Rev. A 65, 032315 (2002)]; however, this leaves unanswered the question of whether there are degrees of compatibility which could be represented by some quantitative measure, and whether there is a straightforward procedure whereby the observers can pool their information to arrive at a unique joint state assignment. We argue that such measures are only sensible given some assumption about what kind of information was used in making the state assignments in the first place, and that in general state assignments do not represent all of the information possessed by the observers. However, we examine one particular measure and show that it has a straightforward interpretation, assuming that the information was acquired from a particular type of measurement, and that in this case there is a natural rule for pooling information. We extend this measure to compatibility of states for $k$ observers and show that the value is the solution to a semidefinite program. Similar compatibility measures can be defined for alternative notions of state compatibility, including post-Peierls and equal support compatibilities.

The quasistability of highly exicited H and He atoms in a strong infrared (IR) field is studied based on three-dimensional quantum calculations. The spectra of atoms that survive the IR field show a series of IR-frequency-modulated peaks that extend from deeply bound states all the way to the ionization threshold and above. The atoms that survive mainly consist of highly excited Rydberg states, even after hundreds of the intense IR cycles. Also, they tend to have initial energies that allow emission or absorption of an integer number of IR photons to reach these quasistable states. Peaks above the ionization threshold in the survival spectra indicate the existence of multiphoton-assisted recombination in the intense IR field.

We present a theoretical model allowing precise calculation of the phase shift induced by atom-atom interactions in a light-pulse atom interferometer based on two-photon Raman atom optics. This model is in good agreement with numerical simulations based on solving the Gross-Pitaevskii equation. The atom interferometer exhibits an atom-atom-interaction-induced phase shift when there is asymmetry between the two arms of the interferometer. In the case of a Ramsey-Bordé atom interferometer $(\{\pi /2–\pi /2\}–\{\pi /2–\pi /2\}$ pulse configuration), the asymmetry comes from the fact that the number of atoms in each arm of the interferometer is not constant. In the case of a Mach-Zehnder $(\{\pi /2–\pi –\pi /2\}$ pulse sequence), if the pulses are perfect, the number of atoms is constant in the interferometer. We study the effect due to imperfections of the light pulses as well as the effect due to the expansion of the cloud. Our model leads to precise and simple formulas of the mean-field phase shift as a function of the experimental parameters.

We present a compact atomic clock interrogating ultracold ${}^{87}\mathrm{Rb}$ magnetically trapped on an atom chip. Very long coherence times sustained by spin self-rephasing allow us to interrogate the atomic transition with 85% contrast at 5-s Ramsey time. The clock exhibits a fractional frequency stability of $5.8\times {10}^{-13}$ at 1 s and is likely to integrate into the ${10}^{-15}$ range in less than a day. A detailed analysis of seven noise sources explains the measured frequency stability. Fluctuations in the atom temperature (0.4 nK shot-to-shot) and in the offset magnetic field $(5\times {10}^{-6}$ relative fluctuations shot-to-shot) are the main noise sources together with the local oscillator, which is degraded by the 30% duty cycle. The analysis suggests technical improvements to be implemented in a future second generation setup . The results demonstrate the remarkable degree of technical control that can be reached in an atom chip experiment.

We analyze the properties of particles trapped in three-dimensional potentials formed from superimposed Gaussian beams, fully taking into account effects of potential anharmonicity and nonseparability. Although these effects are negligible in more conventional optical lattice experiments, they are essential for emerging ultracold-atom developments. We focus in particular on two potentials utilized in current ultracold-atom experiments: arrays of tightly focused optical tweezers and a one-dimensional optical lattice with transverse Gaussian confinement and highly excited transverse modes. Our main numerical tools are discrete variable representations (DVRs), which combine many favorable features of spectral and grid-based methods, such as the computational advantage of exponential convergence and the convenience of an analytical representation of Hamiltonian matrix elements. Optimizations, such as symmetry adaptations and variational methods built on top of DVR methods, are presented and their convergence properties discussed. We also present a quantitative analysis of the degree of nonseparability of eigenstates, borrowing ideas from the theory of matrix product states, leading to both conceptual and computational gains. Beyond developing numerical methodologies, we present results for construction of optimally localized Wannier functions and tunneling and interaction matrix elements in optical lattices and tweezers relevant for constructing effective models for many-body physics.

A quantum-eraser experiment is reported with photon pairs generated by two synchronously pumped parametric down-converters coupled via induced coherence. The complementarity between which-source information and two-photon interference fringe visibility is demonstrated explicitly. Changing the phase in a Mach-Zehnder interferometer allows a continuous transition from wavelike to particlelike behavior of photons.

Over the past decade, precision measurements of the helium $2^{3}P$ fine-structure intervals have been performed using a variety of experimental techniques. We have shown in previous work [Phys. Rev. A 89, 043403 (2014), and references therein] that these measurements can be significantly shifted by quantum interference with neighboring transitions, despite the fact that the neighboring transitions are more than a thousand natural widths away from the transition being measured. The shifts depend on the experimental technique used, since different techniques allow for different quantum-mechanical interference paths. Here we consider measurements using the saturated absorption technique, and find that quantum interference effects cause substantial shifts for the $2^3{P}_1–2^3{P}_2$ interval. We find that four independent measurements for this interval are now in agreement when interference shifts are taken into account.

We study the Anderson localization of atomic gases exposed to three-dimensional optical speckles by analyzing the statistics of the energy-level spacings. This method allows us to consider realistic models of the speckle patterns, taking into account the strongly anisotropic correlations which are realized in concrete experimental configurations. We first compute the mobility edge $E_c$ of a speckle pattern created using a single laser beam. We find that $E_c$ drifts when we vary the anisotropy of the speckle grains, going from higher values when the speckles are squeezed along the beam propagation axis to lower values when they are elongated. We also consider the case where two speckle patterns are superimposed, forming interference fringes, and we find that $E_c$ is increased compared to the case of idealized isotropic disorder. We discuss the important implications of our findings for cold-atom experiments.

We investigate quantum synchronization theoretically in a system consisting of two cold ions in microtraps. The ions' motion is damped by a standing-wave laser while also being driven by a blue-detuned laser which results in self-oscillation. Working in a nonclassical regime, where these oscillations contain only a few phonons and have a sub-Poissonian number variance, we explore how synchronization occurs when the two ions are weakly coupled using a probability distribution for the relative phase. We show that strong correlations arise between the spin and vibrational degrees of freedom within each ion and find that when two ions synchronize their spin degrees of freedom in turn become correlated. This allows one to indirectly infer the presence of synchronization by measuring the ions' internal state.

We present a multimode laser-linewidth theory for arbitrary cavity structures and geometries that contains nearly all previously known effects and also finds new nonlinear and multimode corrections, e.g., a correction to the $\alpha$ factor due to openness of the cavity and a multimode Schawlow-Townes relation (each linewidth is proportional to a sum of inverse powers of all lasing modes). Our theory produces a quantitatively accurate formula for the linewidth, with no free parameters, including the full spatial degrees of freedom of the system. Starting with the Maxwell-Bloch equations, we handle quantum and thermal noise by introducing random currents whose correlations are given by the fluctuation-dissipation theorem. We derive coupled-mode equations for the lasing-mode amplitudes and obtain a formula for the linewidths in terms of simple integrals over the steady-state lasing modes.

We study entanglement creation over global distances based on a quantum repeater architecture that uses low-Earth-orbit satellites equipped with entangled photon sources, as well as ground stations equipped with quantum nondemolition detectors and quantum memories. We show that this approach allows entanglement creation at viable rates over distances that are inaccessible via direct transmission through optical fibers or even from very distant satellites.

Optically levitated mechanical sensors promise isolation from thermal noise far beyond what is possible using flexible materials alone. One way to access this potential is to apply a strong optical trap to a minimally supported mechanical element, thereby increasing its quality factor $Q_m$. Current schemes, however, require prohibitively high laser power ($\sim 10$ W), and the $Q_m$ enhancement is ultimately limited to a factor of $\sim 50$ by hybridization between the trapped mode and the dissipative modes of the supporting structure. Here we propose a levitation scheme taking full advantage of an optical resonator to reduce the circulating power requirements by many orders of magnitude. Applying this scheme to the case of a dielectric disk in a Fabry-Perot cavity, we find a tilt-based tuning mechanism for optimizing both center-of-mass and torsional-mode traps. Notably, the two modes are trapped with comparable efficiency, and we estimate that a $10-\mu$m-diameter, 100-nm-thick Si disk could be trapped to a frequency of $\sim 10$ MHz with only 30 mW circulating in a cavity of (modest) finesse 1500. Finally, we simulate the effect that such a strong trap would have on a realistic doubly tethered disc. Of central importance, we find torsional motion is comparatively immune to $Q_m$-limiting hybridization, allowing a $Q_m$-enhancement factor of $\sim 1500$. This opens the possibility of realizing a laser-tuned 10 MHz mechanical system with a quality factor on the order of ${10}^9$.

Particle delocalization is a common feature of quantum random walks in arbitrary lattices. However, in the typical scenario a particle spreads over multiple sites and its evolution is not directly useful for controlled quantum interferometry, as may be required for technological applications. In this paper we devise a strategy to perfectly split the wave packet of an incoming particle into two components, each propagating in opposite directions, which reconstruct the shape of the initial wavefunction after a particular time $t^{*}$. Therefore, a particle in a $\delta$-like initial state becomes exactly delocalized between two distant sites after $t^{*}$. We find the mathematical conditions to achieve the perfect splitting, which are satisfied by viable example Hamiltonians with static site-dependent interaction strengths. Our results pave the way for the generation of peculiar many-body interference patterns in a many-site atomic chain (such as the Hanbury Brown and Twiss and quantum Talbot effects) as well as for the distribution of entanglement between remote sites. Thus, as for the case of perfect state transfer, the perfect wave-packet splitting can be a new tool for varied applications.

Due to electronic many-body effects, the ionization of a molecule can trigger ultrafast electron dynamics appearing as a migration of the created hole charge throughout the system. Here we propose a scheme to control the charge-migration dynamics with a single ultrashort laser pulse. We demonstrate by fully ab initio calculations on a molecule containing a chromophore and amine moieties that simple pulses can be used to stop the charge-migration oscillations and localize the charge on the desired site of the system. We argue that this control may be used to predetermine the follow-up nuclear rearrangement and thus the molecular reactivity.

We investigate fluorescence detection using a standing wave of blue-detuned light of one or more atoms held in a deep, microscopic dipole trap. The blue-detuned standing wave realizes a Sisyphus laser cooling mechanism so that an atom can scatter many photons while remaining trapped. When imaging more than one atom, the blue detuning limits loss due to inelastic light-assisted collisions. Using this standing-wave probe beam, we demonstrate that we can count from 1 to the order of 100 atoms in the microtrap with sub-Poissonian precision.

The amplitude and phase noises of a dual-frequency vertical-external-cavity surface-emitting laser (DF-VECSEL) operating at telecom wavelength are theoretically and experimentally investigated in detail. In particular, the spectral behavior of the correlation between the intensity noises of the two modes of the DF-VECSEL is measured. Moreover, the correlation between the phase noise of the radio-frequency beat note generated by optical mixing of the two laser modes with the intensity noises of the two modes is investigated. All these spectral behaviors of noise correlations are analyzed for two different values of the nonlinear coupling between the laser modes. We find that to describe the spectral behavior of noise correlations between the laser modes, it is of utmost importance to have precise knowledge about the spectral behavior of the pump noise, which is the dominant source of noise in the frequency range of interest (10 kHz to 35 MHz). Moreover, it is found that the noise correlation also depends on how the spatially separated laser modes of the DF-VECSEL intercept the noise from a multimode fiber-coupled laser diode used for pumping both the laser modes. To this aim, a specific experiment is reported which aims at measuring the correlations between different spatial regions of the pump beam. The experimental results are in excellent agreement with a theoretical model based on modified rate equations.

We investigate ways to optimize adiabaticity and diabaticity in the Landau-Zener model with nonuniform sweeps. We show how diabaticity can be engineered with a pulse consisting of a linear sweep augmented by an oscillating term. We show that the oscillation leads to jumps in populations whose value can be accurately modeled using a model of multiple, photon-assisted Landau-Zener transitions, which generalizes work by Wubs et al. [New J. Phys. 7, 218 (2005)]. We extend the study on diabaticity using methods derived from optimal control. We also show how to preserve adiabaticity with optimal pulses at limited time, finding a nonuniform quantum speed limit.