We experimentally study the breakdown of hyperfine coupling for an atom in a deep optical-dipole trap. One-color laser spectroscopy is performed at the resonance lines of a single ${}^{87}\mathrm{Rb}$ atom for a trap wavelength of 1064 nm. Evidence of hyperfine breakdown comes from three observations, namely, a nonlinear dependence of the transition frequencies on the trap intensity, a splitting of lines which are degenerate for small intensities, and the ability to drive transitions which would be forbidden by selection rules in the absence of hyperfine breakdown. From the data, we infer the hyperfine interval of the $5P_{1/2}$ state and the scalar and tensor polarizabilities for the $5P_{3/2}$ state.

We develop a scattering theory to investigate the multiphoton transmission in a one-dimensional waveguide in the presence of quantum emitters. It is based on a path integral formalism, uses displacement transformations, and does not require the Markov approximation. We obtain the full time evolution of the global system, including the emitters and the photonic field. Our theory allows us to compute the transition amplitude between arbitrary initial and final states, as well as the $S$ matrix of the asymptotic in and out states. For the case of few incident photons in the waveguide, we also rederive a generalized master equation in the Markov limit. We compare the predictions of the developed scattering theory and that with the Markov approximation. We illustrate our methods with five examples of few-photon scattering: (i) by a two-level emitter, (ii) in the Jaynes-Cummings model; (iii) by an array of two-level emitters; (iv) by a two-level emitter in the half-end waveguide; and (v) by an array of atoms coupled to Rydberg levels. In the first two, we show the application of the scattering theory in the photon scattering by a single emitter, and examine the correctness of our theory with the well-known results. In the third example, we analyze the condition of the Markov approximation for the photon scattering in the array of emitters. In the fourth one, we show how a quantum emitter can generate entanglement of outgoing photons. Finally, we highlight the interplay between the phenomenon of electromagnetic-induced transparency and the Rydberg interaction, and show how this results in a rich variety of possibilities in the quantum statistics of the scattering photons.

In most models and theoretical calculations describing multiphoton ionization by infrared light, the dipole approximation is used. This is equivalent to setting the very small photon momentum to zero. Using numerical solutions of the two-dimensional (2-D) time-dependent Schrödinger equation for one electron (H-like) systems, we show that, for linear polarization, the radiation pressure on photoelectrons is very sensitive to the details of the ionization mechanism. The directly ionized photoelectrons, those that never recollide with the parent ion, are driven in the direction of the laser photon momentum, whereas a fraction of slower photoelectrons are pushed in the opposite direction, leading to the counterintuitive shifts observed in recent experiments [Phys. Rev. Lett. 113, 243001 (2014)]. This complex response is due to the interplay between the Lorentz force and the Coulomb attraction from the ion. On average, however, the photoelectron momentum is in the direction of the photon momentum as in the case of circular polarization. The influence of the photon momentum is shown to be discernible in the holographic patterns of time-resolved atomic and molecular holography with photoelectrons, thus suggesting a new research subject in multiphoton ionization.

We report an experimental measurement of a light wavelength at which the ac electric polarizability equals zero for ${}^{87}\mathrm{Rb}$ atoms in the $F=2$ ground hyperfine state. The experiment uses a condensate interferometer both to find this “tune-out” wavelength and to accurately determine the light polarization for it. The wavelength lies between the $D1$ and $D2$ spectral lines at 790.032388(32) nm. The measurement is sensitive to the tensor contribution to the polarizability, which has been removed so that the reported value is the zero of the scalar polarizability. The precision is 50 times better than previous tune-out wavelength measurements. Our result can be used to determine the ratio of matrix elements $|\langle 5P_{3/2}\left|\right|d\left|\right|5S_{1/2}\rangle /\langle 5P_{1/2}\left|\right|d\left|\right|5S_{1/2}{\rangle |}^2=1.99221\left(3\right)$, a 100-fold improvement over previous experimental values. New theoretical calculations for the tune-out wavelength and matrix element ratio are presented. The results are consistent with the experiment, with uncertainty estimates for the theory about an order of magnitude larger than the experimental precision.

Magneto-optical trapping forces for molecules are far weaker than for alkali-metal atoms because the photon scattering rate is reduced when there are multiple ground states, and because of optical pumping into dark states. The force is further reduced when the upper state has a much smaller Zeeman splitting than the lower state. We use a rate model to estimate the strength of the trapping and damping forces in a magneto-optical trap (MOT) of CaF molecules, using either the $A{}^2{\mathrm{\Pi }}_{1/2}–X{}^2{\mathrm{\Sigma }}^{+}$ transition or the $B{}^2{\mathrm{\Sigma }}^{+}–X{}^2{\mathrm{\Sigma }}^{+}$ transition. We identify a mechanism of magneto-optical trapping that arises when, in each beam of the MOT, two laser components with opposite polarizations and different detunings address the same transition. This mechanism produces a strong trapping force even when the upper state has little or no Zeeman splitting. It is the main mechanism responsible for the trapping force when the $A{}^2{\mathrm{\Pi }}_{1/2}–X{}^2{\mathrm{\Sigma }}^{+}$ transition is used.

We study the repulsive van der Waals interaction of cold rubidium $70S_{1/2}$ Rydberg atoms by analysis of time-delayed pair-correlation functions. After excitation, Rydberg atoms are allowed to accelerate under the influence of the van der Waals force. Their positions are then measured using a single-atom imaging technique. From the average pair-correlation function of the atom positions we obtain the initial atom-pair separation and the terminal velocity, which yield the van der Waals interaction coefficient $C_6$. The measured $C_6$ value agrees well with calculations. The experimental method has been validated by simulations. The data hint at anisotropy in the overall expansion, caused by the shape of the excitation volume. Our measurement implies that the interacting entities are individual Rydberg atoms, not groups of atoms that coherently share a Rydberg excitation.

We analyze a quantum force sensor that uses coherent quantum-noise cancellation (CQNC) to beat the standard quantum limit. This sensor, which allows for the continuous, broadband detection of feeble forces, is a hybrid dual-cavity system composed of a mesoscopic mechanical resonator optically coupled to an ensemble of ultracold atoms. In contrast to the stringent constraints on dissipation typically associated with purely optical schemes of CQNC, the dissipation rate of the mechanical resonator only needs to be matched to the decoherence rate of the atomic ensemble—a condition that is experimentally achievable even for the technologically relevant regime of low-frequency mechanical resonators with large quality factors. The modular nature of the system further allows the atomic ensemble to aid in the cooling of the mechanical resonator, thereby combining atom-mediated state preparation with sensing deep in the quantum regime.

An efficient technique for producing a periodic sequence of short nearly bandwidth-limited pulses of recoilless $\gamma$ radiation via its transmission through an optically thick vibrating resonant absorber was demonstrated recently [Nature (London) 508, 80 (2014)]. In this paper we extend the theoretical analysis to a case of multiple absorbers. We analyze a simple physical model describing control of spectral content of a frequency modulated $\gamma$ radiation by adjusting the amplitudes and initial phases of spectral components, using the resonant absorption and dispersion in a set of several sequentially placed resonant absorbers. On the basis of analytical solutions, we determine the ultimate possibilities of the proposed technique.

We report linear polarization measurements of x rays emitted due to dielectronic recombination into highly charged krypton ions. The ions in the He-like through O-like charge states were populated in an electron-beam ion trap with the electron-beam energy adjusted to recombination resonances in order to produce $K\alpha$ x rays. The x rays were detected with a newly developed Compton polarimeter using a beryllium scattering target and 12 silicon x-ray detector diodes sampling the azimuthal distribution of the scattered x rays. The extracted degrees of linear polarization of several dielectronic recombination transitions agree with results of relativistic distorted-wave calculations. We also demonstrate a high sensitivity of the polarization to the Breit interaction, which is remarkable for a medium-$Z$ element like krypton. The experimental results can be used for polarization diagnostics of hot astrophysical and laboratory fusion plasmas.

We study the many-body localization transition in one-dimensional Hubbard chains using exact diagonalization and quantum chaos indicators. We also study dynamics in the delocalized (ergodic) and localized phases and discuss thermalization and eigenstate thermalization, or the lack thereof, in such systems. Consistently within the indicators and observables studied, we find that ergodicity is very robust against disorder, namely, even in the presence of weak Hubbard interactions the disorder strength needed for the system to localize is large. We show that this robustness might be hidden by finite size effects in experiments with ultracold fermions.

Dynamic nuclear polarization (DNP) is used in nuclear magnetic resonance to transfer polarization from electron spins to nuclear spins. The resulting nuclear polarization enhancement can, in theory, be two or three orders of magnitude depending on the sample. In solid-state systems, however, there are competing mechanisms of DNP, which, when occurring simultaneously, reduce the net polarization enhancement of the nuclear spin. We present a simple quantum description of DNP and apply optimal control theory (OCT) with an open-quantum-system framework to design pulses that select one DNP process and suppress the others. We demonstrate experimentally an order of magnitude improvement in the DNP enhancement using OCT pulses.

We report the realization of a quantum degenerate mixture of bosonic ${}^{87}\mathrm{Rb}$ and fermionic ${}^{171}\mathrm{Yb}$ atoms in a hybrid optical dipole trap with a tunable, species-dependent trapping potential. ${}^{87}\mathrm{Rb}$ is shown to be a viable refrigerant for the noninteracting ${}^{171}\mathrm{Yb}$ atoms, cooling up to $2.4\times {10}^5$ Yb atoms to a temperature of $T/T_F=0.16\left(2\right)$ while simultaneously forming a ${}^{87}\mathrm{Rb}$ Bose-Einstein condensate of $3.5\times {10}^5$ atoms. Furthermore we demonstrate our ability to independently tailor the potentials for each species, which paves the way for studying impurities immersed in a Bose gas.

We have carried out a detailed characterization of the temporal coherence and spectral bandwidth of the Ne-like Zn XUV laser, collisionally pumped in the quasi-steady-state (QSS) regime and operated at saturation. The measured coherence time ${\tau }_c=680\pm 70$ fs is significantly shorter than previously observed with other types of collisional XUV lasers, using the same measurement technique. Additional measurements performed below saturation were used to investigate gain narrowing of the spectral width of the lasing line. Our experimental data are well reproduced by calculations performed with a one-dimensional radiative transfer numerical code, taking the measured gain as an input. We show that our results are consistent with spectral measurements of QSS XUV lasers reported by different authors in the 1990s, although they were based on slightly different pumping conditions and measurement techniques.

The quantum Zeno effect freezes the evolution of a quantum system subject to frequent measurements. We apply a Fisher information analysis to show that because of this effect, a closed quantum system should be probed as rarely as possible, while a dissipative quantum system should be probed at specifically determined intervals to yield the optimal estimation of parameters governing the system dynamics. With a Bayesian analysis we show that a few frequent measurements are needed to identify the parameter region within which the Fisher information analysis applies.

As a test of the Kibble–Zurek mechanism (KZM) of defect formation, we simulate the Bose–Einstein condensation transition in a toroidally confined Bose gas by using the stochastic projected Gross–Pitaevskii equation, with and without the energy-damping reservoir interaction. Energy-damping alters the scaling of the winding-number distribution with the quench time—a departure from the universal KZM theory that relies on equilibrium critical exponents. Numerical values are obtained for the correlation-length critical exponent $\nu$ and the dynamical critical exponent $z$ for each variant of reservoir interaction theory. The energy-damping reservoir interactions cause significant modification of the dynamical critical exponent of the phase transition, while preserving the essential KZM critical scaling behavior. Comparison of numerical and analytical two-point correlation functions further illustrates the effect of energy damping on the correlation length during freeze-out.

Electrodynamics becomes nonlinear and permits the self-interaction of fields when the quantized nature of vacuum states is taken into account. The effect on a plane probe pulse propagating through a stronger constant crossed background is calculated using numerical simulation and by analytically solving the corresponding wave equation. The electromagnetic shock resulting from vacuum high-harmonic generation is investigated and a nonlinear shock parameter identified.

Motivated by several experimental efforts to understand spin diffusion and transport in ultracold fermionic gases, we study the spin dynamics of initially spin-polarized ensembles of harmonically trapped noninteracting spin-1/2 fermionic atoms, subjected to a magnetic field gradient. We obtain simple analytic expressions for spin observables in the presence of both constant and linear magnetic field gradients, with and without a spin-echo pulse and at zero and finite temperatures. The analysis shows the relevance of spin-motional coupling in the noninteracting regime, where the demagnetization decay rate at short times can be faster than the experimentally measured rates in the strongly interacting regime under similar trapping conditions. Our calculations also show that particle motion limits the ability of a spin-echo pulse to remove the effect of magnetic field inhomogeneity and that a spin-echo pulse can instead lead to an increased decay of magnetization at times comparable to the trapping period.

We analyze spontaneous parametric down-conversion in various experimentally feasible one-dimensional quadratic nonlinear waveguide arrays, with emphasis on the relationship between the lattice's topological invariants and the biphoton correlations. Nontrivial topology results in a nontrivial “winding” of the array's Bloch waves, which introduces additional selection rules for the generation of biphotons, independent of existing control using the pump beam's spatial profile and phase-matching conditions. In finite lattices, nontrivial topology produces single-photon edge modes, resulting in “hybrid” biphoton edge modes, with one photon localized at the edge and the other propagating into the bulk. When the single-photon band gap is sufficiently large, these hybrid biphoton modes reside in a band gap of the bulk biphoton Bloch wave spectrum. Numerical simulations support our analytical results.

The absolute configuration of chiral molecules in the gas phase is determined by femtosecond laser-induced Coulomb explosion and recording of the emission direction of critical ionic fragments. The spatial orientation of the $3,5-\mathrm{dibromo}-3^{\prime },5^{\prime }-\mathrm{diflouro}-4^{\prime }-\mathrm{cyanobiphenyl}$ molecules studied is determined by a combination of laser-induced alignment and time-of-flight measurement of ${\text{F}}^{+}$ ion recoils. Hereby, the enatiomeric state of the molecules could be uniquely identified through analysis of the correlation of the angular distribution of ${\text{F}}^{+}$ and ${\text{Br}}^{+}$ ions. The method will apply to a range of molecules and allow imaging of chiral changes on ultrafast time scales.

The characterization of the set of quantum correlations in Bell scenarios is a problem of paramount importance for both the foundations of quantum mechanics and quantum information processing in the device-independent scenario. However, a clear-cut (physical or mathematical) characterization of this set remains elusive and many of its properties are still unknown. We provide here simple and general analytical conditions that are necessary for an arbitrary bipartite behavior to be quantum. Although the conditions are not sufficient, we illustrate the strength and nontriviality of these conditions with a few examples. Moreover, we provide several applications of this result: we prove a quantitative separation of the quantum set from extremal nonlocal no-signaling behaviors in several general scenarios, we provide a relation to obtain Tsirelson bounds for arbitrary Bell inequalities and a construction of Bell expressions whose maximal quantum value is attained by a maximally entangled state of any given dimension.