Spin-exchange optical hyperpolarization of ${}^{129}\mathrm{Xe}$ gas enhances the signal-to-noise ratio in nuclear magnetic resonance experiments. The governing parameter of the Rb-Xe spin-exchange process, the so-called enhancement factor, was recently reevaluated experimentally. However, the underlying hyperfine coupling and atomic interaction potential as functions of the internuclear distance of the open-shell Rb-Xe dimer have not been accurately determined to date. We present a piecewise approximation based on first-principles calculations of these parameters contributing to the NMR and EPR frequency shifts in the low-density Rb-Xe gas mixture of relevance to hyperpolarization experiments. Both Rb electron and ${}^{129}\mathrm{Xe}$ nuclear spin polarizations are estimated based on a combination of electronic-structure calculations, observed frequency shifts, and an estimate of the Rb number density. Finally, an expression for the enhancement factor in terms of modern electronic-structure theory is obtained.

We consider the basic collision dynamics of single ionization of helium by 1-MeV protons using distorted-wave models and the first Born approximation. A good agreement is found between results of the distorted-wave models and recent experimental data [H. Gassert, O. Chuluunbaatar, M. Waitz et al., Phys. Rev. Lett. 116, 073201 (2016)] for electron emission into the azimuthal plane. However, unexpected discrepancies are observed between the theoretical results and the experimental data for the emission into the collision plane. Our consideration also shows that even though these collisions belong to the regime of weak perturbations, substantial differences exist between the results of the distorted-wave models and the first Born approximation.

We investigate the energy dependence and the internal-state dependence of the charge-exchange collision cross sections in a mixture of ${}^6\mathrm{Li}$ atoms and ${{}^{40}\mathrm{Ca}}^{+}$ ions. Deliberately excited ion micromotion is used to control collision energy of atoms and ions. The energy dependence of the charge-exchange collision cross section obeys the Langevin model in the temperature range of the current experiment, and the measured magnitude of the cross section is correlated to the internal state of the ${{}^{40}\mathrm{Ca}}^{+}$ ions. Revealing the relationship between the charge-exchange collision cross sections and the interaction potentials is an important step toward the realization of the full quantum control of the chemical reactions at an ultralow-temperature regime.

We study inelastic two-body relaxation in a spin-polarized ultracold Fermi gas in the presence of a $p$-wave Feshbach resonance. It is shown that in reduced dimensionalities, especially in the quasi-one-dimensional case, the enhancement of the inelastic rate constant on approach to the resonance is strongly suppressed compared to three dimensions. This may open promising paths for obtaining novel many-body states.

We theoretically analyze angle-resolved photoelectron spectra (ARPES) generated by the interaction of ${\text{C}}_{60}$ with intense, short laser pulses. In particular, we focus on the impact of the carrier-envelope phase (CEP) onto the angular distribution. The electronic dynamics is described by time-dependent density functional theory, and the ionic background of ${\text{C}}_{60}$ is approximated by a particularly designed jellium model. Our results show a clear dependence of the angular distributions onto the CEP for very short pulses covering only very few laser cycles, which disappears for longer pulses. For the specific laser parameters used in recent experiments, a very good agreement is obtained. Furthermore, the asymmetry is found to depend on the energy of the emitted photoelectrons. The strong influence of the angular asymmetry of electron emission onto the CEP and pulse duration suggests using this sensitivity as a means to analyze the structure of few-cycle laser pulses.

The majority of quantum simulators treat simplified one-band strongly correlated models, whereas multiple bands are needed to describe materials with intermediate correlation. We investigate the sensitivity of multiband quantum simulators to: (1) the form of optical lattices, (2) the interactions between electron analogs. Since the kinetic-energy terms of electron analogs in a quantum simulator and electrons in a solid are identical, by examining both periodic potential and interaction we explore the full problem of many-band quantum simulators within the Born–Oppenheimer approximation. Density functional calculations show that band structure is highly sensitive to the form of optical lattice, and it is necessary to go beyond sinusoidal potentials to ensure that the bands closest to the Fermi surface are similar to those in real materials. Analysis of several electron analog types finds that dressed Rydberg atoms (DRAs) have promising interactions for multiband quantum simulation. DRA properties can be chosen so that interaction matrices approximate those in real systems and decoherence effects are controlled, albeit with parameters at the edge of currently available technology. We conclude that multiband quantum simulators implemented by using the principles established here could provide insight into the complex processes in real materials.

We study the decay of superflow via thermally activated phase slips in one-dimensional Bose gases in a shallow optical lattice. By using the Kramers formula, we numerically calculate the nucleation rate of a thermally activated phase slip for various values of the filling factor and flow velocity in the absence of a harmonic trapping potential. Within the local density approximation, we derive a formula connecting the phase-slip nucleation rate with the damping rate of a dipole oscillation of the Bose gas in the presence of a harmonic trap. We use the derived formula to directly compare our theory with the recent experiment done by the LENS group [L. Tanzi et al., Sci. Rep. 6, 25965 (2016)]. From the comparison, the observed damping of dipole oscillations in a weakly correlated and small velocity regime is attributed dominantly to thermally activated phase slips rather than quantum phase slips.

The shear viscosity has been shown to be equal to the product of the pressure and relaxation time in normal scale-invariant fluids, but the presence of superfluidity at low temperatures can alter the relation. Using the mean-field BCS-Leggett theory with a gauge-invariant linear response theory for unitary Fermi superfluids, we present an explicit relation between thermodynamic quantities, including the pressure and chemical potential, and transport coefficients, including the shear viscosity, superfluid density, and anomalous shear viscosity from momentum transfer via Cooper pairs. The relation is modified when pairing fluctuations associated with noncondensed Cooper pairs are considered. Within a pairing fluctuation theory consistent with the BCS-Leggett ground state, we found an approximate relation for unitary Fermi superfluids. The exact mean-field relation and the approximate one with pairing fluctuations advance our understanding of relations between equilibrium and transport quantities in superfluids, and they help determine or constrain quantities which otherwise can be difficult to measure.

Light propagating in $\mathcal{P}\mathcal{T}$-symmetric photonic crystals (PhCs) under Bragg diffraction in the Laue geometry has been studied theoretically using the spectral method. The $\mathcal{P}\mathcal{T}$-symmetric solutions describing propagating modes have been found in the PhCs with gain and loss beyond paraxial approximation. We described the pendulum effect—the periodical spatial localization of the total field intensity in a PhC—near the $\mathcal{P}\mathcal{T}$-symmetry-breaking point. It is shown that, due to $\mathcal{P}\mathcal{T}$-symmetric properties of the medium, an asymmetric change in the amplitudes of the diffracted waves in PhCs is observed when the sign of the Bragg incidence angle is changed from positive to negative. Thus, the intensity of a spatially periodic field in a medium radically alters under the pendulum effect. Moreover, when the sign of the Bragg incidence angle changes, a PhC of a certain thickness is turned from an absorbing structure into an amplifying one, also a PhC of any thickness evolves from completely transparent into amplifying in the vicinity of the $\mathcal{P}\mathcal{T}$-symmetry-breaking point. Under a small change of the imaginary part of permittivity, the light switching from a transmitted wave into a gain or loss diffracted wave is possible in a diffraction-thick PhC.

This paper theoretically investigates the optical-rectification (OR) effect in metallic structures with periodic modulation. The nonlinear surface and bulk currents responsible for the OR effect are formulated based on the Drude model with electron-gas pressure. The new formulation clarifies the generation mechanism of the OR effect and reveals that the OR effect is strongly enhanced by a combination of spatial variation of the metallic structure and asymmetry of electric-field strength. In order to consider the validity of our theory, the OR effect in the metallic structure is numerically calculated. The calculation results show fairly well agreement with the experiments.

We construct electromagnetically induced transparency (EIT) by dynamically coupling a superradiant state with a subradiant state. The superradiant and subradiant states with enhanced and inhibited decay rates act as the excited and metastable states in EIT, respectively. Their energy difference determined by the distance between the atoms can be measured by the EIT spectra, which renders this method useful in subwavelength metrology. The scheme can also be applied to many atoms in nuclear quantum optics, where the transparency point due to counter-rotating wave terms can be observed.

We describe the attractive optical nature of highly discrete coherent spectra. We show that the relative amplitude and phase of such spectra can be almost arbitrarily manipulated by simply placing three fundamental optical elements—namely, a waveplate, polarizer, and dispersive plate—on an optical axis and then controlling their thicknesses. We also describe the relevant physical mechanism. Furthermore, as a typical application of this optical nature, we demonstrate arbitrary optical waveform generation in a numerical experiment, and we discuss its limitations as an optical-wave manipulation technology and how we can overcome these limitations.

In this paper, we analyze a chain-linked triple-cavity photonic molecule (TCPM) with controllable coupling strengths between the cavities on their spectral properties and field (energy) distributions by solving eigenvalues and eigenvectors of the Hamiltonian matrix based on coupled-mode theory. Phase transition is extended from double-cavity photonic molecules (DCPMs) to TCPMs, and evolutions of the supermode frequencies and linewidths are analyzed, which have synchronous relations with the degree of coherence between adjacent optical microcavities and energy distributions in the three cavities, respectively. We develop a superposition picture for the three supermodes of the TCPM, as interferences between supermodes of sub-DCPMs. In particular, we demonstrate the abnormal properties of the central supermode in TCPMs, such as the dark state in the middle cavity and phase shift when energy is flowing between the side cavities; these are promising for information processing and remote control of energy. General properties of TCPMs are summarized and limitations on linewidths are given. Finally, we make an interesting analog to intracavity electromagnetically induced transparency in multilevel atomic systems using the flexible TCPM platform under appropriate conditions.

We investigate the impact of a local random unitary noise on multipartite quantum states of an arbitrary dimension. We follow the dynamical approach, in which the single-particle unitaries are generated by local random Hamiltonians. Assuming short evolution time we derive a lower bound on the fidelity between an initial and the final state transformed by this type of noise. This result is based on averaging the Tamm-Mandelstam bound and holds for a wide class of distributions of random Hamiltonians fulfilling specific symmetry conditions. It is shown how the sensitivity of a given pure quantum state to the discussed type of noise depends on the properties of single-particle and bipartite reduced states.

Quantum digital signature (QDS) is an approach to guarantee the nonrepudiation, unforgeability, and transferability of a signature with information-theoretical security. Previous experimental realizations of QDS relied on an unrealistic assumption of secure channels and the longest distance is several kilometers. Here, we have experimentally demonstrated a recently proposed QDS protocol without assuming any secure channel. Exploiting the decoy state modulation, we have successfully signed a one-bit message through an up to 102-km optical fiber. Furthermore, we continuously run the system to sign the longer message “USTC” with 32 bits at the distance of 51 km. Our results pave the way towards the practical application of QDS.

Quantum annealing is guaranteed to find the ground state of optimization problems provided it operates in the adiabatic limit. Recent work [S. Muthukrishnan et al., Phys. Rev. X 6, 031010 (2016)] has found that for some barrier tunneling problems, quantum annealing can be run much faster than is adiabatically required. Specifically, an $n$-qubit optimization problem was presented for which a nonadiabatic, or diabatic, annealing algorithm requires only a constant run time, while an adiabatic annealing algorithm requires a run-time polynomial in $n$. Here we show that this nonadiabatic speedup is the direct result of a specific symmetry in the studied problem. In the more general case, no such nonadiabatic speedup occurs and we show why the special case achieves this speedup compared to the general case. We also prove that the adiabatic annealing algorithm has a necessary and sufficient run time that is quadratically better than the standard quantum adiabatic condition suggests. We conclude with an observation about the required precision in timing for the diabatic algorithm.

Using coherent states in optical quantum process tomography is a practically relevant approach. Here we develop a framework for complete characterization of quantum-optical processes in terms of normally-ordered moments by using coherent states as probes. We derive the associated superoperator tensors for several optical processes. We also show that our technique can be used to determine nonclassicality features of quantum-optical states and processes. Furthermore, we investigate identification of multimode Gaussian processes and show that the number of necessary probe coherent states scales linearly with the number of modes.

Despite its success, the composite fermion (CF) construction possesses some mathematical features that have not been fully understood until recently. In particular, this construction is known to produce wave functions that are not necessarily orthogonal, or even linearly independent, after projection to the lowest Landau level. While this is usually not a problem in practice in the quantum Hall regime, we have previously shown that it presents a technical challenge for rotating Bose gases with low angular momenta. These are systems where the CF approach yields surprisingly good approximations to the exact eigenstates of weak short-range interactions, and so solving the problem of linearly dependent wave functions is of interest. It can also be useful for studying higher bands of fermionic quantum Hall states. Here we present several ways of constructing a basis for the space of so-called “simple” CF states for two-component rotating Bose gases in the lowest Landau level, and prove that they all give sets of linearly independent wave functions that span the space. Using this basis, we study the structure of the lowest-lying state using so-called restricted wave functions. We also examine the scaling of the overlap between the exact and CF wave functions at the maximal possible angular momentum for simple states.