We experimentally study the effect of a slight nonorthogonality in a two-dimensional optical lattice onto resolved-sideband Raman cooling. We find that when the trap frequencies of the two lattice directions are equal, the trap frequencies of the combined potential exhibit an avoided crossing and the corresponding eigenmodes are rotated by ${45}^{\circ }$ relative to the lattice beams. Hence, tuning the trap frequencies makes it possible to rotate the eigenmodes such that both eigenmodes have a large projection onto any desired direction in the lattice plane, in particular, onto the direction along which Raman cooling works. Using this, we achieve two-dimensional Raman ground-state cooling in a geometry where this would be impossible, if the eigenmodes were not rotated. Our experiment is performed with a single atom inside an optical resonator but this is inessential and the scheme is expected to work equally well in other situations.

Producing positronium (Ps) in the metastable $2{}^{3}S$ state is of interest for various applications in fundamental physics. We report here on an experiment in which Ps atoms are produced in this long-lived state by spontaneous radiative decay of Ps excited to the $3{}^{3}P$ level manifold. The Ps cloud excitation is obtained with a UV laser pulse in an experimental vacuum chamber in presence of guiding magnetic field of $25\mathrm{mT}$ and an average electric field of $300\mathrm{V}{\mathrm{cm}}^{-1}$. The evidence of the $2{}^{3}S$ state production is obtained to the $3.6\sigma$ level of statistical significance using a novel analysis technique of the single-shot positronium annihilation lifetime spectra. The dynamic of the Ps population on the involved levels has been studied with a rate equation model.

We investigate Landau-Zener-Stückelberg-Majorana (LZSM) interferometry in a nonlinear two-level system subjected to a sinusoidal driving field. A comprehensive analysis for the interference patterns is shown from which we can analytically obtain the condition of constructive and destructive interferences in some cases. In the presence of nonlinear interaction, the level structure is changed, which can lead to an asymmetric deformation of the interference patterns and a significant position shift of the interference fringes. Our findings suggest an application of the nonlinear LZSM interferometry in accurately calibrating the parameters characterizing a nonlinear two-level system, as well as its coupling with the driven fields.

In this work, we develop a general formalism that categorizes the action of broken scale invariance on the nonequilibrium dynamics of nonrelativistic quantum systems. This approach is equally applicable to both strongly and weakly interacting systems. We show that any small deviation from the strongly interacting fixed point, in three spatial dimensions, leads to nonpertubative effects in the long-time dynamics, dramatically altering the dynamics observed at the scale-invariant fixed point. As a concrete example, we apply this approach to the nonequilibrium dynamics for the interacting two-body problem and for a noninteracting quantum gas in the presence of an impurity, both in three spatial dimensions. Slightly away from the resonantly interacting scale invariant fixed point, we show that the dynamics are altered by a nonperturbative log-periodic beat. The presence of the beat depends only on deviating from the resonant fixed point, while the frequency depends on the microscopic parameters of the system.

Input-output theory is invaluable for treating superconducting and photonic circuits connected by transmission lines or waveguides. However, this theory cannot in general handle situations in which retroreflections from circuit components or configurations of beam splitters create loops for the traveling-wave fields that connect the systems. Here, building upon the network-contraction theory of Gough and James [Commun. Math. Phys. 287, 1109 (2009)], we provide a compact and powerful method to treat any circuit that contains such loops so long as the effective cavities formed by the loops are sufficiently weak. Essentially all present-day on-chip superconducting and photonic circuits will satisfy this weakness condition so long as the reflectors that form the loops are not especially highly reflecting. As an example, we analyze the problem of transmitting entanglement between two qubits connected by a transmission line with imperfect circulators, a problem for which our method is essential. We obtain a full solution for the optimal receiver given that the sender employs a simple turn-on–turn-off procedure. This solution shows that near-perfect transmission is possible even with significant retroreflections.

We show that linearly polarized light in a suspension of gyrotropic nanoparticles can experience modulation instability, which leads to spatial separation of right- and left-circularly polarized waves. Such a separation preserves zero full angular momentum of the electromagnetic field; thus, the time-reversal symmetry is broken only locally. Eventually, this instability results in the formation of the lattice of spatial solitons with alternating right and left circular polarization. As magnetic moments of nanoparticles tend to reorient parallel to the wave vector in circularly polarized light, every spatial soliton captures a static magnetic field directed along or opposite to the wave vector depending on the direction of the field angular momentum; therefore, the soliton lattice altogether looks like an array of magnetic domains with antiferromagnetic ordering.

We produce Bose-Einstein condensates of ${}^{162}\mathrm{Dy}$ atoms employing an innovative technique based on a resonator-enhanced optical trap that allows efficient loading from the magneto-optical trap. We characterize the scattering properties of the ultracold atoms for magnetic fields between 6 and 30 G. In addition to the typical chaotic distribution of narrow Feshbach resonances in lanthanides, we discover two rather isolated broad features at around 22 G and 27 G. A characterization using the complementary measurements of losses, thermalization, anisotropic expansion, and molecular binding energy points towards resonances of predominant $s$-wave character. Such resonances will ease the investigation of quantum phenomena relying on the interplay between dipole and contact interactions.

It was recently found that the information-to-work conversion in a quantum Szilard engine can be increased by using a working medium of bosons with attractive interactions. In the original scheme, the work output depends on the insertion and removal position of an impenetrable barrier that acts like a piston, separating the chambers of the engine. Here, we show that the barrier removal process can be made fully reversible, resulting in a full information-to-work conversion if we also allow for the interaction strength to change during the cycle. Hence, it becomes possible to reach the maximum work output per cycle dictated by the second law of thermodynamics. These findings can be experimentally verified, for instance, with ultracold atoms as a working medium, where a change of interaction strength can be controlled by Feshbach resonances.

We theoretically analyze a scheme for fast stabilization of arbitrary qubit states with high fidelities, extending a protocol recently demonstrated experimentally [Lu et al., Phys. Rev. Lett. 119, 150502 (2017)]. That experiment utilized red and blue sideband transitions in a system composed of a fluxonium qubit, a low-$Q$ $LC$ oscillator, and a coupler enabling us to tune the interaction between them. Under parametric modulations of the coupling strength, the qubit can be steered into any desired pure or mixed single-qubit state. For realistic circuit parameters, we predict that stabilization can be achieved within $100\mathrm{ns}$. By varying the ratio between the oscillator's damping rate and the effective qubit-oscillator coupling strength, we can switch between underdamped, critically damped, and overdamped stabilization and find optimal working points. We further analyze the effect of thermal fluctuations and show that the stabilization scheme remains robust for realistic temperatures.

A gradient ascent method for optimal quantum control synthesis is presented that employs a gradient derived with respect to the coefficients of a functional basis expansion of the control. Restricting the space of allowable controls to weighted sums of the Slepian sequences efficiently parametrizes the control in terms of bandwidth, resolution, and pulse duration. A bound showing minimum time evolutions scaling with the inverse of the control bandwidth [S. Lloyd and S. Montangero, Phys. Rev. Lett. 113, 010502 (2014)] is recovered and the method is shown to achieve the scaling predicted by the bound on entangling two-qubit quantum gates.

According to the Beer-Lambert law, the spectral density of weak electromagnetic radiation is attenuated exponentially, with the characteristic length depending on atomic density and the frequency-dependent photoabsorption cross section of the target. We discuss an analog of the Beer-Lambert law in the time domain, showing that the temporal profile of the initial light pulse enters explicitly into each term of expansion of the exponential function as a multiconvolution with the corresponding temporal response of an atom. The form invokes a description of pulse propagation in terms of the temporal base functions, which may be precalculated for a chosen initial pulse shape, allowing us to obtain the attenuated profile at a given target depth simply by summing up a sufficient number of weighted base functions. The variant with a 100-fs initial Gaussian pulse is worked out in detail, with the atomic response exhibiting either pure exponential or mixed prompt-exponential decay, characteristic of a photoexcited bound state or a Fano resonance. The method is shown to be valid for an arbitrary temporal profile of the initial electric field and is illustrated for pulses resonantly tuned to the low-lying doubly excited states of the He atom.

The length standard for atom gravimeters is defined by Raman laser wavelength, which, in turn, depends on the absolute laser frequency. Here we present a method to measure Raman laser frequency based on an atom gravimeter itself, which has an advantage of calibrating the length standard on-site. The calibration utilizes the $D_2$ line of ${}^{87}\mathrm{Rb}$ atoms as a reference and takes advantage of the cold atom free fall as well as developed techniques of evaluating hyperfine-level shift within atom gravimeters. A calibration accuracy of 1 part in ${10}^{10}$ is achieved for the Raman laser frequency, which constrains the corresponding error below 1 $\mu \mathrm{Gal}$ in our gravity measurement.

A major goal within the field of optomechanics is to achieve the single-photon strong-coupling regime, wherein even a mechanical displacement as small as the zero-point uncertainty is enough to shift an optical cavity resonance by more than its linewidth. This goal is difficult, however, due to the small zero-point motion of conventional mechanical systems. Here, we show that an atom trapped in and coupled to a cavity constitutes an attractive platform for realizing this regime. In particular, while many experiments focus on achieving strong coupling between a photon and the atomic internal degree of freedom, this same resource also naturally enables one to obtain optomechanical strong coupling, in combination with the low mass of an atom and the isolation of its motion from a thermal environment. As an example, we show that an optomechanically induced photon blockade can be realized in realistic setups, and provide signatures of how this effect can be distinguished from the conventional Jaynes-Cummings blockade associated with the two-level nature of the atomic transition.

Single x-ray photons can be resonantly scattered and stored with the help of suitable transitions in the atomic nucleus. Here, we investigate theoretically means of mechanical-free modulation for the frequency spectra of such x-ray photons via periodic switching of an external magnetic field. We show that periodically switching on and off an external magnetic field generating hyperfine splitting of the nuclear transition leads to the generation of equidistant narrow sidebands of the resonantly scattered response. This frequency-comb-like structure depends on the magnitude and orientation of the applied magnetic field and on the switching period. An analytical approach for the characterization of the comblike frequency spectrum is presented. The feasibility of the external control on the frequency modulation of the x-ray response is discussed in view of possible applications in high-resolution spectroscopy or quantum technology.

We theoretically investigate strongly enhanced vibrational-mode-selective two-step excitation by ultrabroadband frequency-entangled photons. A diatomic molecule having three sets of vibronic states is used to evaluate the excitation efficiency of the two-step excitation. We show that the photon entanglement has little influence on the populations of intermediate vibrational modes of intermediate states but selectively increases the population of a single vibrational mode of excited states, resonant with the total energy of the entangled photons. For an ultrashort pulse close to a monocycle with high degree of entanglement, the population of the vibrational mode is enhanced 2500 times more than by uncorrelated photons, and the selectivity nearly reaches unity. Our results indicate that the two-step excitation with ultrabroadband frequency-entangled photons can achieve highly selective and strongly enhanced excitation of a single vibrational mode.

The complete relativistic $O\left({\alpha }^2\right)$ nuclear structure correction to the energy levels of ordinary (electronic) and muonic hydrogenlike atoms is investigated. The elastic part of the nuclear structure correction is derived analytically. The resulting formula is valid for an arbitrary hydrogenic system and is much simpler than analogous expressions previously reported in the literature. The analytical result is verified by high-precision numerical calculations. The inelastic $O\left({\alpha }^2\right)$ nuclear structure correction is derived for the electronic and muonic deuterium atoms. The correction comes from a three-photon exchange between the nucleus and the bound lepton and has not been considered in the literature so far. We demonstrate that in the case of deuterium, the inelastic three-photon-exchange contribution is of a similar size and of the opposite sign to the corresponding elastic part and, moreover, cancels exactly the model dependence of the elastic part. The obtained results affect the determination of nuclear charge radii from the Lamb shift in ordinary and muonic atoms.

Accurate evaluation of the $\mathcal{P},\mathcal{T}$-odd Faraday effect (rotation of the polarization plane of the light propagating through a medium in the presence of an external electric field) is presented. The magnitude of this rotation is directly proportional to the optical path length. A novel idea is to observe the $\mathcal{P},\mathcal{T}$-odd Faraday effect using intracavity absorption spectroscopy (ICAS) experiments. The modern ICAS experiments allow one to observe the rotation of the light polarization plane (natural or $\mathcal{P}$-odd) with the optical path length up to a hundred kilometers. This can be done mainly due to the work off-line in the resonance absorption experiments. For the Faraday rotation (ordinary or $\mathcal{P},\mathcal{T}$-odd) the maximum of the effect coincides with the maximum of absorption which usually prevents the work off-line and employment of the large optical path length. However, we propose to use the second rotation maximum that exists for the Faraday effect and would also allow for work off-line. The calculations are performed for the heavy metal atoms Cs, Tl, Pb, and Ra where the $\mathcal{P},\mathcal{T}$-odd effects are most pronounced. The results of the calculations demonstrate that with the large optical path length the ICAS experiments will be able to fix the possible $\mathcal{P},\mathcal{T}$-odd effects at a level comparable with other very advanced modern experiments.

We report Raman sideband cooling of a single sodium atom to its three-dimensional motional ground state in an optical tweezer. Despite a large Lamb-Dicke parameter, high initial temperature, and large differential light shifts between the excited state and the ground state, we achieve a ground-state population of 93.5(7)% after 53 ms of cooling. Our technique includes addressing high-order sidebands, where several motional quanta are removed by a single laser pulse, and fast modulation of the optical tweezer intensity. We demonstrate that Raman sideband cooling to the three-dimensional motional ground state is possible, even without tight confinement and low initial temperature.

We suggest a simple scheme for creating a NOON state of repulsively interacting Bose atoms in the double-well potential. The protocol consists of two steps. First, by setting atom-atom interactions to zero, the system is driven to the upper excited state. Second, the interactions are slowly increased and, simultaneously, the interwell tunneling is decreased to zero. We analyze fidelity of the final state to the NOON state depending on the number of atoms, ramp rate, and fluctuations of the system parameters. It is shown that for a given fidelity the ramp rate scales algebraically with the number of atoms.

The coupling between a quantum particle's intrinsic angular momentum and its center-of-mass motion gives rise to the so-called helicity states that are characterized by the projection of the spin onto the direction of momentum. In this paper, by unfolding the superfluid-density tensor into its intrahelicity and interhelicity components, we reveal that the latter contribution is directly linked with the total quantum metric of the helicity bands. We consider both Rashba and Weyl spin-orbit couplings across the BCS-BEC crossover and show that the geometrical interhelicity contribution is responsible for up to a quarter of the total superfluid density. We believe this is one of those elusive effects that may be measured within the highly tunable realm of cold Fermi gases.