A candidate for converting quantum information from microwave to optical frequencies is the use of a single atom that interacts with a superconducting microwave resonator on one hand and an optical cavity on the other. The large electric dipole moments and microwave transition frequencies possessed by Rydberg states allow them to couple strongly to superconducting devices. Lasers can then be used to connect a Rydberg transition to an optical transition to realize the conversion. Since the fundamental source of noise in this process is spontaneous emission from the atomic levels, the resulting control problem involves choosing the pulse shapes of the driving lasers so as to maximize the transfer rate while minimizing this loss. Here we consider the concrete example of a cesium atom, along with two specific choices for the levels to be used in the conversion cycle. Under the assumption that spontaneous emission is the only significant source of errors, we use numerical optimization to determine the likely rates for reliable quantum communication that could be achieved with this device. These rates are on the order of a few megaqubits per second.

We report on the extreme-value statistics of output intensities in a one-dimensional cw-pumped erbium-doped random fiber laser, with a strongly scattering disordered medium consisting of randomly spaced Bragg gratings. The experimental findings from the analysis of a large number of emission spectra are well described by the Gumbel distribution below and above the laser threshold, whereas the Fréchet distribution, typical of strongly fluctuating extreme events with heavy power-law probability tails, provides a nice support to the data near the threshold. We establish a close connection, relying on theoretical arguments, between the reported extreme-value statistics and the shifts in the statistics of intensity fluctuations, from the Gaussian to the Lévy distribution at the threshold and back to the Gaussian well above threshold.

We examine the Green function and the density of states for fermions moving in three-dimensional Dirac materials with interfaces which affect the propagation properties of particles. Motivation for our research comes from interest in materials that exhibit quasirelativistic dispersion relations. By modifying Dirac-type contributions to the Hamiltonian in an interface we are able to calculate the Green function and the density of states. The density of states inside the interface exhibits interpolating behavior between two and three dimensions, with two-dimensional behavior at high energies and three-dimensional behavior at low energies, provided that the shift in the mass parameter in the interface is small. We also discuss the impact of the interpolating density of states on optical absorption in Dirac materials with a two-dimensional substructure.

We study the problem of designing optical receivers to discriminate between multiple coherent states using coherent processing receivers—i.e., one that uses arbitrary coherent feedback control and quantum-noise-limited direct detection—which was shown by Dolinar to achieve the minimum error probability in discriminating any two coherent states. We first derive and reinterpret Dolinar's binary-hypothesis minimum-probability-of-error receiver as the one that optimizes the information efficiency at each time instant, based on recursive Bayesian updates within the receiver. Using this viewpoint, we propose a natural generalization of Dolinar's receiver design to discriminate $M$ coherent states, each of which could now be a codeword, i.e., a sequence of $N$ coherent states, each drawn from a modulation alphabet. We analyze the channel capacity of the pure-loss optical channel with a general coherent-processing receiver in the low-photon number regime and compare it with the capacity achievable with direct detection and the Holevo limit (achieving the latter would require a quantum joint-detection receiver). We show compelling evidence that despite the optimal performance of Dolinar's receiver for the binary coherent-state hypothesis test (either in error probability or mutual information), the asymptotic communication rate achievable by such a coherent-processing receiver is only as good as direct detection. This suggests that in the infinitely long codeword limit, all potential benefits of coherent processing at the receiver can be obtained by designing a good code and direct detection, with no feedback within the receiver.

Entangled quantum states, such as NOON states, are of major importance for quantum technologies due to their quantum-enhanced performance. At the same time, their quantum correlations are relatively vulnerable when they are subjected to imperfections. Therefore, it is crucial to determine under which circumstances their distinct quantum features can be exploited. In this paper, we study the entanglement property of noisy NOON states. This class of states is a generalization of NOON states including various attenuation effects, such as mixing, constant or fluctuating losses, and dephasing. To verify their entanglement, we pursue two strategies: detection-based entanglement witnesses and entanglement quasiprobabilities. Both methods result from our solution of so-called separability eigenvalue equations. In particular, the entanglement quasiprobabilities allow for a full entanglement characterization. As examples of our general treatment, the cases of NOON states subjected to Gaussian dephasing and fluctuating atmospheric losses are explicitly studied. In any correlated fluctuating loss channel, entanglement is found to survive for nonzero transmissivity. In addition, an extension of our approach to multipartite systems is given, and the relation to the quantum-optical nonclassicality in phase space is discussed.

Noninteracting bosons were proposed to be used for a demonstration of quantum-computing supremacy in a boson-sampling setup. A similar demonstration with fermions would require that the fermions are initially prepared in an entangled state. I suggest that pairwise entanglement of fermions would be sufficient for this purpose. Namely, it is shown that computing multiparticle scattering amplitudes for fermions entangled pairwise in groups of four single-particle states is #P-hard. In linear algebra, such amplitudes are expressed as exterior products of two-forms of rank 2. In particular, a permanent of a $N\times N$ matrix may be expressed as an exterior product of $N^2$ two forms of rank 2 in dimension $2N^2$, which establishes the #P-hardness of the latter.

We investigate roles of electron correlation effects in the determination of the $g_j$ factors of the $4s{}^2{S}_{1/2}$, $4p{}^2{P}_{1/2}$, $4p{}^2{P}_{3/2}$, $3d{}^2{D}_{3/2}$, and $3d{}^2{D}_{5/2}$ states, representing different parities and angular momenta, of the ${\mathrm{Ca}}^{+}$ ion. Correlation contributions are highlighted with respect to the mean-field values evaluated using the Dirac-Hartree-Fock method, relativistic second-order many-body theory, and relativistic coupled-cluster (RCC) theory with the single- and double-excitation approximation considering only the linear terms and also accounting for all the nonlinear terms. This shows that it is difficult to achieve results below ${10}^{-5}$ precision employing an approximate perturbative approach. We also find that contributions through the nonlinear terms and higher-level excitations such as triple excitations, estimated perturbatively in the RCC method, are found to be crucial to attain precise values of the $g_j$ factors in the considered states of the ${\mathrm{Ca}}^{+}$ ion.

Vibrational state-resolved photoelectron anisotropy parameters, $\beta$, for the $\tilde{X}{}^2{B}_1$, $\tilde{B}{}^2{B}_2$, and $\tilde{C}{}^2{B}_1$ state ionizations of bromobenzene have been recorded at photon energies ranging from 20.5 to 94 eV, thus spanning the region of the expected bromine Cooper minimum (CM). The $\tilde{X}$ state displays no CM and its $\beta$ value is also independent of vibrational level, in accord with the Franck-Condon approximation. The $\tilde{B}$ and $\tilde{C}$ state $\beta$ values display the CM to differing degrees, but both show a vibrational dependence that extends to energies well below the obvious CM dip. Calculations are presented that replicate these observations. We thus demonstrate a wide-ranging Franck-Condon approximation breakdown detected in the $\beta$ anisotropy parameter in the absence of any resonance. Measured and calculated vibrational branching ratios for these states are also presented. Although the $\tilde{B}$ state branching ratios remain constant, in accord with Franck-Condon expectations, the $\tilde{X}$ and (especially) the $\tilde{C}$ state ratios display weak, quasilinear variations across the studied range of photon energy, but with no apparent correlation with the CM position.

We study photoelectron angular distributions (PADs) near the ionization threshold with a newly developed Coulomb quantum-orbit strong-field approximation (CQSFA) theory. The CQSFA simulations exhibit an excellent agreement with the result from the time-dependent Schrödinger equation. We show that the low-energy fan-shaped pattern in the PADs corresponds to a subcycle time-resolved holographic structure and stems from the significant influence of the Coulomb potential on the phase of the forward-scattered electron trajectories, which affects different momenta and scattering angles unequally. Our work provides a direct explanation of how the fan-shaped structure is formed, based on the quantum interference of direct and forward-scattered orbits. Moreover, our work shows that the fan-shaped pattern can be used to extract information on the target structure, as the number of fringes in the PADs depends strongly on the symmetry of the electronic bound state.

The common objective of the application of adiabatic techniques in the field of quantum control is to transfer a quantum system from one discrete energy state to another. These techniques feature both high efficiency and insensitivity to variations in the experimental parameters, e.g., variations in the driving field amplitude, duration, frequency, and shape, as well as fluctuations in the environment. Here we explore the potential of adiabatic techniques for creating arbitrary predefined coherent superpositions of two quantum states. We show that an equally weighted coherent superposition can be created by temporal variation of the ratio between the Rabi frequency $\mathrm{\Omega }\left(t\right)$ and the detuning $\mathrm{\Delta }\left(t\right)$ from 0 to $\infty$ (case 1) or vice versa (case 2), as it is readily deduced from the explicit adiabatic solution for the Bloch vector. We infer important differences between cases 1 and 2 in the composition of the created coherent superposition: The latter depends on the dynamical phase of the process in case 2, while it does not depend on this phase in case 1. Furthermore, an arbitrary coherent superposition of unequal weights can be created by using asymptotic ratios of $\mathrm{\Omega }\left(t\right)/\mathrm{\Delta }\left(t\right)$ different from 0 and $\infty$. We supplement the general adiabatic solution with analytic solutions for three exactly soluble models: two trigonometric models and the hyperbolic Demkov-Kunike model. They allow us not only to demonstrate the general predictions in specific cases but also to derive the nonadiabatic corrections to the adiabatic solutions.

The anticrossing structure of the $1s7l$ manifold of the helium atom in combined dc electric and magnetic fields is studied using a broadband photoexcitation with synchrotron light. The anticrossing signal is provided by the yield of atoms in the metastable $1s2s$ states to which the $1s7l$ states cascade. The mapping resolution depends solely on the homogeneity of the two fields in the target region, which is formed by the intersection of the synchrotron beam and the helium atom beam. The measured positions, as well as anticrossing intensities and widths, measured in the region of 1–1.5 kV/cm and 0–10 mT are in excellent agreement with the results of our extended theoretical simulations based on highly accurate zero-field wave functions. By centering the photoexcitation window to 65.110 and 65.130 eV, the same technique is applied to look for the anticrossings in the vicinity of the $7^{+}{}^1{P}_1^o–7d{}^3{D}_1^o$ and $7d{}^1{P}_1^o–8^{-}{}^1{P}_1^o$ pairs of doubly excited states, respectively.

Efficient cooling of trapped ions is a prerequisite for various applications of the ions in precision spectroscopy, quantum information, and coherence control. Raman sideband cooling is an effective method to cool the ions to their motional ground state. We investigate both numerically and experimentally the optimization of Raman sideband cooling strategies and propose an efficient one, which can simplify the experimental setup as well as reduce the number of cooling pulses. Several cooling schemes are tested and compared through numerical simulations. The simulation result shows that the fixed-width pulses and varied-width pulses have almost the same efficiency for both the first-order and the second-order Raman sideband cooling. The optimized strategy is verified experimentally. A single ${{}^{25}\mathrm{Mg}}^{+}$ ion is trapped in a linear Paul trap and Raman sideband cooled, and the achieved average vibrational quantum numbers under different cooling strategies are evaluated. A good agreement between the experimental result and the simulation result is obtained.

We consider a binary dipolar Bose-Einstein condensate with repulsive contact and dipolar interactions under rotation. Our results show that the interplay among short-range interaction, long-range interaction, and rotation can give rise to a rich variety of topological configurations, including giant skyrmions with multiply topological charges and skyrmion-vortex lattices. In particular, we find that for fixed rotation frequencies, tuning the short- and long-range interactions can derive novel ground-state phases, such as a meron pair composed of two fractional skyrmions and a skyrmion with topological charge $Q=2$ centered in giant skyrmions.

We present a general, Gaussian spatial-mode propagation formalism for describing the generation of higher-order multi-spatial-mode beams generated during nonlinear interactions. Furthermore, to implement the theory, we simulate optical angular momentum transfer interactions and show how one can optimize the interaction to reduce the undesired modes. Past theoretical treatments of this problem have often been phenomenological, at best. Here we present an exact solution for the single-pass no-cavity regime, in which the nonlinear interaction is not overly strong. We apply our theory to two experiments, with very good agreement, and give examples of several more configurations, easily tested in the laboratory.

We study analytically and numerically the optical analog of the Jackiw-Rebbi states in quantum-field theory. These solutions exist at the interface of two binary waveguide arrays, which are described by two Dirac equations with masses of opposite sign. We show that these special states are topologically robust not only in the linear regime, but also in the nonlinear one (with both focusing and defocusing nonlinearities). We also reveal that one can effectively generate Jackiw-Rebbi states starting from Dirac solitons.

An optical diode made of an asymmetric cavity containing a two-level atom is investigated. We focus on the quantum-statistical property of the transmitted field with nonclassical light input. Both coherent and squeezed light inputs have been considered. The results show that the transmitted contrast of such optical diode is independent of the statistical properties of the incident light but is only sensitive to its intensity. On the other hand, the quantum-statistical property, i.e., the squeezing, of the transmitted field strongly depends on the statistical properties and directions of the incident light. For squeezed light input, the degree of the squeezing of the transmitted field can be remarkably enhanced. Moreover, the squeezing of the amplitude quadrature of the incident light can be transferred to the phase quadrature due to the coupling of the light and the atom.

The problem of bounding the (Abelian) many-anyon ground-state energy from above, with a dependence on the statistics parameter which matches that of currently available lower bounds, is reduced to studying the correlation functions of Moore-Read (Pfaffian) and Read-Rezayi type clustering states.

A pivotal task in quantum metrology, and quantum parameter estimation in general, is to design schemes that achieve the highest precision with the given resources. Standard models of quantum metrology usually assume that the dynamics is fixed and that the highest precision is achieved by preparing the optimal probe states and performing optimal measurements. However, in many practical experimental settings, additional controls are usually available to alter the dynamics. Here we propose to use optimal control methods for further improvement of the precision limit of quantum parameter estimation. We show that, by exploring the additional degree of freedom offered by the controls, a higher-precision limit can be achieved. In particular, we show that the precision limit under the controlled schemes can go beyond the constraints put by the coherent time, which is in contrast with the standard scheme where the precision limit is always bounded by the coherent time.

We present measurements of interspecies thermalization between ultracold samples of ${}^{133}\mathrm{Cs}$ and either ${}^{174}\mathrm{Yb}$ or ${}^{170}\mathrm{Yb}$. The two species are trapped in a far-off-resonance optical dipole trap and ${}^{133}\mathrm{Cs}$ is sympathetically cooled by Yb. We extract effective interspecies thermalization cross sections by fitting the thermalization measurements to a kinetic model, giving ${\sigma }_{\mathrm{Cs}{}^{174}\mathrm{Yb}}=\left(5\pm 2\right)\times {10}^{-13}{\mathrm{cm}}^2$ and ${\sigma }_{\mathrm{Cs}{}^{170}\mathrm{Yb}}=\left(18\pm 8\right)\times {10}^{-13}{\mathrm{cm}}^2$. We perform quantum scattering calculations of the thermalization cross sections and optimize the CsYb interaction potential to reproduce the measurements. We predict scattering lengths for all isotopic combinations of Cs and Yb. We also demonstrate the independent production of ${}^{174}\mathrm{Yb}$ and ${}^{133}\mathrm{Cs}$ Bose-Einstein condensates using the same optical dipole trap, an important step toward the realization of a quantum-degenerate mixture of the two species.

We have studied $\mathrm{X}{\mathrm{e}}^{+}4d$ inner-shell photoionization in a direct experiment on $\mathrm{X}{\mathrm{e}}^{+}$ ions, merging an ion and a photon beam and detecting the ejected electrons with a cylindrical mirror analyzer. The measured $4d$ photoelectron spectrum is compared to the $4d$ core valence double ionization spectrum of the neutral Xe atom, obtained with a magnetic bottle spectrometer. This multicoincidence experiment gives access to the spectroscopy of the individual $\mathrm{X}{\mathrm{e}}^{2+}4d^{-1}5p^{-1}$ states and to their respective Auger decays, which are found to present a strong selectivity. The experimental results are interpreted with the help of ab initio calculations.