The collective excitation of a gas of highly excited atoms confined to a large spacing ring lattice is studied, where the ground and the excited states are resonantly coupled via a laser field. Attention is focused on the regime where the interaction between the highly excited atoms is very weak in comparison to the Rabi frequency of the laser. In this case, the many-body excitations of the system can be expressed in terms of free spinless fermions. The complex many-particle states arising in this regime are characterized and their properties, for example their correlation functions, are studied. Additional investigation into how some of these many-particle states can actually be experimentally accessed by a temporal variation of the laser parameters is performed.

We present a theoretical analysis of a new scheme for temporal retardation combined with temporal compression of weak laser pulses, optically controlled by the presence of one additional light pulse. A realistic realization of this scheme in hot Rb is discussed, showing that a large range of values of the optically controllable delay-bandwidth product can be obtained with a limited pulse distortion.

I derive unidirectional wave equations for fields propagating in materials with both electric and magnetic dispersion and nonlinearity. The derivation imposes no conditions on the pulse profile except that the material modulates the propagation slowly, that is, that loss, dispersion, and nonlinearity have only a small effect over the scale of a wavelength. It also allows a direct term-to-term comparison of the exact bidirectional theory with its approximate unidirectional counterpart.

We revisit recent claims about the instability of nonrotating tunnel coupled annular Bose-Einstein condensates leading to the emergence of angular momentum Josephson oscillation [Phys. Rev. Lett. 98, 050401 (2007)]. It was predicted that all stationary states with uniform density become unstable in certain parameter regimes. By careful analysis, we arrive at a different conclusion. We show that there is a stable nonrotating and uniform ground state for any value of the tunnel coupling and repulsive interactions. The instability of an excited state with $\pi$ phase difference between the condensates can be interpreted in terms of the familiar snake instability. We further discuss the sign of the tunnel coupling through a separating barrier, which carries significance for the nature of the stationary states. It is found to always be negative for physical reasons.

We propose to use ferroelectric $(\mathrm{Eu},\mathrm{Ba}){\mathrm{TiO}}_3$ ceramics just above their magnetic ordering temperature for a sensitive electron electric-dipole-moment search. We have synthesized a number of such ceramics with various europium concentrations and measured their properties relevant for such a search: permeability, magnetization noise, and ferroelectric hysteresis loops. We also identify and estimate the likely systematics: the magnetoelectric effect, hysteresis-loop heating, and ferroelectric relaxation currents. Our measurements and estimates indicate that a search for the electron electric dipole moment with ${\mathrm{Eu}}_{0.5}{\mathrm{Ba}}_{0.5}{\mathrm{TiO}}_3$ could lead to an order of magnitude improvement on the current best limit, if the systematic effects can be controlled.

We analyze a system in which, due to entanglement between the spin and spatial degrees of freedom, the reduced transmitted state has the shape of the freely propagating pulse translated in the complex coordinate plane. In the case an apparently “superluminal” advancement of the pulse, the delay amplitude distribution is found to be a peculiar approximation to the Dirac $\delta$ function, and the transmission coefficient exhibits a well-defined superoscillatory window. Analogies with potential tunneling and Wheeler’s delayed choice experiment are highlighted.

We propose a scheme to simulate one-dimensional $\mathit{XXZ}$-type Heisenberg spin models with competing interactions between nearest neighbors (NNs) and next NNs in photon-coupled microcavities. Our scheme exploits the rich resources and flexible controls available in such a system to realize arbitrarily adjustable ratios between the effective NN and next-NN coupling strengths. Such a powerful capability allows us to simulate frustration phenomena and disorder behaviors in one-dimensional systems arising from next-NN interactions, a large class of problems of great importance in condensed-matter physics. Our scheme is robust due to the lack of atomic excitations, which suppresses spontaneous emission and cavity decay strongly.

We investigate low-temperature coherence properties of the $Z_2$ quantum memory which is capable of storing the information of a single logical qubit. We show that the memory has superposition of macroscopically distinct states for some values of a control parameter and at sufficiently low temperature and that the code states of this memory have no instability except for the inevitable one. However, we also see that the coherence power of this memory is limited by space and time. We also briefly discuss the resonating valence bond memory, which is an improvement of the $Z_2$ quantum memory, and the relations of our results to the obscured symmetry breaking in statistical physics.

We investigate the relaxation of a superconducting qubit for the case when its detector, the Josephson bifurcation amplifier, remains latched in one of its two (meta)stable states of forced vibrations. The qubit relaxation rates are different in different states. They can display strong dependence on the qubit frequency and resonant enhancement, which is due to quasienergy resonances. Coupling to the driven oscillator changes the effective temperature of the qubit.

Single-differential and integral cross sections for electron-impact ionization of the C atom and its ions were calculated with the binary-encounter-dipole model [Y.-K. Kim and M. E. Rudd, Phys. Rev. A 50, 3954 (1994)] to study the distribution of free-electron energies in carbon clusters after being irradiated with an x-ray free-electron laser (XFEL). The averaged energies of the secondary electrons for the C atom, and ${\mathrm{C}}^{1+}$, ${\mathrm{C}}^{2+}$, and ${\mathrm{C}}^{3+}$ ions were about $20$, $70$, $160$, and $200$ eV, respectively, when incident electron energy was about $20$ keV, while those energies were in the order of tens of electronvolts when the incident electron energy was about $250$ eV. The damage to carbon clusters irradiated with the XFEL was also investigated with time-dependent rate equations, considering photoionization, Compton scattering, Auger decay, and electron-impact ionization of the C atom and its ions. The results show that the electron-impact ionization becomes a more important process as the x-ray flux decreases, while the effect of Auger decay gradually appears as the x-ray flux increases. The energy dependence of the incident x ray was also investigated to evaluate the resolution of the diffraction pattern. These results indicate that we should make the XFEL pulse a few fs and about $16$ keV to suppress damage and obtain desired resolution of the diffraction pattern.

By analyzing vortex lattices, re-entrant Cooper pairing, and Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states in a single theoretical framework, we explore how vortices and spin textures join to protect superconductivity against large magnetic fields. We use a rapidly rotating ultracold gas of fermionic atoms near unitarity as a model system amenable to experimental exploration and discover a hierarchy of spin-polarized and FFLO phases in which a metal or a band insulator of unpaired particles coexists with a spatially modulated superfluid hosting a vortex lattice. Quantum fluctuations can transform these phases into strongly correlated “vortex-liquid” metals and insulators, respectively. We argue that vortex lattices significantly enhance the stability of FFLO states and discuss prospects for observing these states in cold-atom experiments.

We study the Fermi liquid properties of the cold atomic dipolar Fermi gases with the explicit dipolar anisotropy using perturbative approaches. Due to the explicit dipolar anisotropy, Fermi surfaces exhibit distortions of the $d_{r^2-3z^2}$ type in three dimensions and of the $d_{x^2-y^2}$ type in two dimensions. The fermion self-energy, effective mass, and Fermi velocity develop the same anisotropy at the Hartree-Fock level proportional to the interaction strength. The Landau interaction parameters in the isotropic Fermi liquids become the tridiagonal Landau interaction matrices in the dipolar Fermi liquids which renormalize thermodynamic susceptibilities. With large dipolar interaction strength, the Fermi surface collapses along directions perpendicular to the dipole orientation. The dynamic collective zero sound modes exhibit an anisotropic dispersion with the largest sound velocity propagating along the polar directions. Similarly, the longitudinal $p$-wave channel spin mode becomes a propagating mode with an anisotropic dispersion in multicomponent dipolar systems.

Motivated by a recent demonstration of cyclic addition of quantized vorticity into a Bose-Einstein condensate, the vortex pump, we study dynamical instabilities and core sizes of giant vortices. The core size is found to increase roughly as a square-root function of the quantum number of the vortex, whereas the strength of the dynamical instability either saturates to a fairly low value or increases extremely slowly for large quantum numbers. Our studies suggest that giant vortices of very high angular momenta may be achieved by gradually increasing the operation frequency of the vortex pump.

Feedback control protocols can stabilize and enhance the operation of quantum devices, however, unavoidable delays in the feedback loop adversely affect their performance. We introduce a quantum control methodology, combining open-loop control with quantum filtering, which is not constrained by feedback delays. For the problems studied (rapid purification and rapid measurement) we analytically derive lower bounds on the control performance that are comparable with the best corresponding bounds for feedback protocols.

Entanglement of isolated elementary particles other than photons has not yet been achieved. We show how building blocks demonstrated with one trapped electron might be used to make a model system and method for entangling two electrons. Applications are then considered, including two-qubit gates and more precise quantum metrology protocols.

We propose a probabilistic measurement-induced amplification for coherent states. The amplification scheme uses a counterintuitive architecture: a thermal noise addition (instead of a single-photon addition) followed by a feasible multiple-photon subtraction using a realistic photon-number-resolving detector. It allows one to substantially amplify weak coherent states and simultaneously reduce their phase uncertainty, which is impossible when using a deterministic Gaussian amplifier.

This paper reports on multiconfiguration Dirac-Hartree-Fock calculations for both allowed and intercombination transitions and fine structure referring to the levels of a term in highly charged aluminum like ions. Results for fine-structure energy levels, the term splitting, the wavelengths, transition rates, and thereby the branching ratios and lifetimes for the Al-like $3s^{2}3p$-$3s3p^2$ transitions in the ions Fe XIV–Au LXVII are reported and compared with other theories and experiments, using the codes GRASP2K. Our calculated fine-structure energy levels are in excellent agreement with the experimental results and the experimentally compiled energy values of the National Institute for Standards and Technology wherever available. The calculated values including core–valence correlation are found to be similar and to compare very well with other theoretical and experimental values for medium-$Z$ ions. For higher $Z$ the inclusion of the valence correlation gives results in excellent agreement with those from many-body perturbation theory. We believe that our extensive calculated values can guide experimentalists in identifying the fine-structure levels in their future work. From our radiative decay rates we have also calculated radiative lifetimes of some fine-structure levels. In this calculation we also predict new data for several fine-structure levels where no other theoretical and/or experimental results are available.

We use the quantum threshold laws combined with a classical capture model to provide an analytical estimate of the chemical quenching cross sections and rate coefficients of two colliding particles at ultralow temperatures. We apply this quantum threshold model (QT model) to indistinguishable fermionic polar molecules in an electric field. At ultracold temperatures and in weak electric fields, the cross sections and rate coefficients depend only weakly on the electric dipole moment $d$ induced by the electric field. In stronger electric fields, the quenching processes scale as $d^{4(L+\frac{1}{2})}$ where $L>0$ is the orbital angular-momentum quantum number between the two colliding particles. For $p$-wave collisions ($L=1$) of indistinguishable fermionic polar molecules at ultracold temperatures, the quenching rate thus scales as $d^6$. We also apply this model to pure two-dimensional collisions and find that chemical rates vanish as $d^{-4}$ for ultracold indistinguishable fermions. This model provides a quick and intuitive way to estimate chemical rate coefficients of reactions occuring with high probability.

The collision cross sections of sodium from the ground state to the first four excited states at the incident energy ranging from 0 to 5.4 eV are calculated using the $R$-matrix method. The convergences of the cross sections are checked systematically by using four sets of high-quality target states, i.e., 5, 9, 14, and 19 physical target states. The influence of the Rydberg target states on the collision cross sections is also elucidated at higher incident energies; i.e., the amplitude of resonance structures will decrease with respect to the effective quantum number $\upsilon$ of the Rydberg target states. This result is very useful for the calculations of these cross sections at intermediate energy with finite target states by combining the partial-wave-expansion methods valid at low energy with the first Born approximation method valid at high energy, which would be of great importance in obtaining complete cross-section data for related scientific fields.

We present detailed analyses of our recent experiment on the three-dimensional (3D) optical lattice clock with bosonic ${}^{88}\mathrm{Sr}$ atoms in which the collisional frequency shift was suppressed by applying a single-occupancy lattice. Frequency shifts in magnetically induced spectroscopy on the ${}^1{S}_0–{}^3{P}_0$ clock transition ($\lambda =698$ nm) of ${}^{88}\mathrm{Sr}$ were experimentally investigated by referencing a one-dimensional (1D) lattice clock based on spin-polarized ${}^{87}\mathrm{Sr}$ atoms. We discuss that the clock stability is limited by the current laser stability as well as the experimental sequence of the clock operation, which may be improved to ${\sigma }_y(\tau )=2\times {10}^{-16}/\sqrt{\tau }$ by optimizing the cycle time of the clock operation.