Theoretical ${\mathrm{H}}_2\mathrm{O}–{\mathrm{H}}_2$ pressure broadening and line shifts are compared with experimental values for three water rotational transitions. These transitions, which occur at terahertz frequencies, are primary radiant coolants for collapsing interstellar clouds that contain water. They are observed by the submillimeter/FIR Herschel space observatory. Systematic effects in previous pressure-broadening measurements that were due to ortho-hydrogen to para-hydrogen conversion have been overcome, and the present results follow the expected behavior predicted by collision theory. The systemic error, discovered through comparisons between theory and experiments, is due to conversion of ortho hydrogen to para hydrogen by water ice below 40 K. This process occurs on a time scale very short compared to astrophysical processes and may be pertinent to ice-grain interactions in the interstellar medium.

We investigate the dynamics of a single two-level atom, which interacts with pulses propagating in two spatial modes (right and left) and frequency continuum. Using Heisenberg equations of motion, we present the explicit analytical derivations and general formalisms for atomic excitation with two-spatial-mode multiphoton pulses in both the Fock and coherent states. Based on those formalisms, we show that perfect atomic excitation by a single-photon Fock state pulse can only be realized when it is rising exponentially shaped in the even mode, a balanced superposition of the two spatial modes. A single photon from a single spatial mode can only give half of the maximal atomic excitation probability. We also show that the maximum atomic excitation probability with multiphoton pulses in the even mode is a monotonic function of the average photon number for the coherent state, but not for the Fock states. Furthermore, we demonstrate that the atomic dynamics can be controlled by the initial relative phase between the two counterpropagating coherent state pulses incident on the atom, which is not the case with the two Fock state pulses.

We study the spin-squeezing property of weighted graph states, which can be used to improve the sensitivity in interferometry. Decoherence reduces the spin-squeezing property but the result remains superior over other reference schemes with Greenberger-Horne-Zeilinger (GHZ) type maximally entangled states and product states. We study the time evolution of spin squeezing of weighted graph states coupled to different decoherence channels. Based on the analysis, the spin squeezing of the weighted graph states is robust in the presence of decoherence, and the decoherence limit in the improvement of the interferometric sensitivity is still achievable.

We derive analytic expressions for the spectral amplitude of high-order harmonic generation from a single atom using the strong-field approximation (SFA). We demonstrate good agreement with time-dependent Schrödinger equation calculations, including the dependence on the drive wavelength across the range 566–2260 nm. Previous claims of a discrepancy in the drive wavelength scaling ignore changes in the timing of trajectories corresponding to a fixed harmonic photon energy. Under this condition, tunnel ionization is shown to play the most important role for the short trajectories. The established agreement enables us to use the SFA to predict that the intensity at the classical cutoff scales inversely with the ninth power of the drive wavelength in the high photon energy limit when the Coulomb singularity dominates the recombination amplitude.

We calculate corrections of order ${\alpha }^3\left(Z\alpha \right)E_F$ to hyperfine splitting in muonium generated by the gauge invariant set of diagrams with polarization insertions in the light-by-light scattering diagrams. This nonrecoil contribution turns out to be $-2.63$ Hz. The total contribution of all known corrections of order ${\alpha }^3\left(Z\alpha \right)E_F$ is equal to $-4.28$ Hz.

We describe the formation of fermionic NaLi Feshbach molecules from an ultracold mixture of bosonic ${}^{23}$Na and fermionic ${}^6$Li. Precise magnetic field sweeps across a narrow Feshbach resonance at 745 G result in a molecule conversion fraction of $5\%$ for our experimental densities and temperatures, corresponding to a molecule number of $5\times {10}^4$. The observed molecular decay lifetime is $1.3$ ms after removing free Li and Na atoms from the trap. Due to its extremely low reactivity, NaLi molecules in the ground state will have a much longer lifetime than KRb.

Using a magnetic-field-insensitive two-photon transition, the phase shift of atomic interference fringes and the frequency shift of the resonance spectrum were examined for a noncyclic rotation of a magnetic field during the interrogation time of an atom interferometer. We observed a phase shift proportional to the rotation angle of the magnetic field when the initial state and the final state were subjected to interference. We confirmed that the phase shift is equivalent to the scalar Aharonov-Bohm phase due to the geometrical potential. On the other hand, we demonstrated that the noncyclic Berry phase defined by the geodesic gauge is measurable using a strategy that the magnetic field rapidly returns to the initial direction during the interrogation time of the atom interferometer.

Calculation of the energies, magnetic dipole hyperfine structure constants, $E$1 transition amplitudes between the low-lying states, and nuclear spin-dependent parity-nonconserving amplitudes for the ${}^2{S}_{1/2}-{}^2{D}_{3/2,5/2}$ transitions in ${}^{171}$Yb${}^{+}$ ion is performed using two different approaches. First, we carried out many-body perturbation theory calculation considering Yb${}^{+}$ as a monovalent system. Additional all-order calculations are carried out for selected properties. Second, we carried out configuration interaction calculation considering Yb as a 15-electron system and compared the results obtained by two methods. The accuracy of different methods is evaluated. We find that the monovalent description is inadequate for evaluation of some atomic properties due to significant mixing of the one-particle and the hole-two-particle configurations. Performing the calculation by such different approaches allowed us to establish the importance of various correlation effects for Yb${}^{+}$ atomic properties for future improvement of theoretical precision in this complicated system.

This work explores the relationship between optimal control theory and adiabatic passage techniques in quantum systems. The study is based on a geometric analysis of the Hamiltonian dynamics constructed from Pontryagin's maximum principle. In a three-level quantum system, we show that the stimulated Raman adiabatic passage technique can be associated to a peculiar Hamiltonian singularity. One deduces that the adiabatic pulse is solution of the optimal control problem only for a specific cost functional. This analysis is extended to the case of a four-level quantum system.

We examine the magnetostrictive position and momentum space distortions that occur in harmonically confined dipolar Bose and Fermi gases. Direct interactions give rise to position space magnetostriction and exchange interactions give rise to momentum space magnetostriction. While the position space magnetostriction is similar in Bose and Fermi systems, the momentum space magnetostriction is markedly different: The Bose gas momentum distribution distorts in the opposite sense to that of the Fermi gas. By relating exchange effects to short-range correlations between the particles we discuss the energetic origin of this difference. Our main calculations are based on Hartree-Fock theory, but we also provide analytic approximations for the magnetostriction effects at zero and finite temperature. Our predictions should be verifiable in current experiments with ultracold polar molecules.

We propose an experimental scheme to favor both the realization and the detection of the Néel state in a two-component gas of ultracold fermions in a three-dimensional simple-cubic optical lattice. By adding three compensating Gaussian laser beams to the standard three pairs of retroreflected lattice beams, and adjusting the relative waists and intensities of the beams, one can significantly enhance the size of the Néel state in the trap, thus increasing the signal of optical Bragg scattering. Furthermore, the additional beams provide for adjustment of the local chemical potential and the possibility to evaporatively cool the gas while in the lattice. Our proposals are also relevant to efforts to realize other many-body quantum phases in optical lattices.

We consider a gas of bosons in a bichromatic optical lattice at finite temperatures. As the amplitude of the secondary lattice grows, the single-particle eigenstates become localized. We calculate the canonical partition function using exact methods for the noninteracting and strongly interacting limits and analyze the statistical properties of the superfluid phase, localized phase, and the strongly interacting gas. We show that those phases may be distinguished in experiment using off-resonant light scattering.

We theoretically study the normal phase properties of strongly interacting two-component Fermi gases in two spatial dimensions. In the limit of weak attraction, we find that the gas can be described in terms of effective polarons. As the attraction between fermions increases, we find a crossover from a gas of noninteracting polarons to a pseudogap regime. We investigate how this crossover is manifested in the radio-frequency (rf) spectroscopy. Our findings qualitatively explain the differences in the recent rf spectroscopy measurements of two-dimensional Fermi gases [Sommer et al., Phys. Rev. Lett. 108, 045302 (2012) and Zhang et al., Phys. Rev. Lett. 108, 235302 (2012)].

Bessel beams possess a nondiffractive property that makes them suitable for propagating optical energy over long distances. We have utilized an axicon-generated Bessel beam to carry out a systematic study of the physics that governs propagation of intense, femtosecond-long pulses in a transparent crystal (barium fluoride). Filamentation and supercontinuum generation have been studied using axicon lenses of different cone angles. Emission induced by six-photon absorption of 800-nm photons is utilized to visualize the filaments within the crystal. Our results show that higher incident energy, $E_{\text{in}}$ ($\ge$ 600 $\mu$J), is required to generate filaments inside the crystal when an axicon is used, as compared to a spherical lens (6 $\mu$J). Increase in cone angles increases the value of $E_{\text{in}}$ for filament formation. The size of the filaments is found to be independent of whether an axicon or a spherical lens is used. For a given axicon cone angle, variation of the input energy enables us to observe the transition from steady Bessel filamentation to a nonlinear, unbalanced, Bessel regime. Varying the separation distance between the axicon and the crystal enables differences in periodicity (distance between two focusing-refocusing events) in the filaments to be quantified. Supercontinuum generation is also probed as a function of distance of sample from the tip of the axicon and with different incident beam sizes. The input beam diameter is shown to have a distinct effect on axicon-generated white light.

We experimentally study the photonic properties of a cold-atom sample trapped in a one-dimensional optical lattice under the conditions of electromagnetically induced transparency. We show that such a medium has two photonic band gaps. One of them is in the transparency window and gives rise to a Bragg mirror, which is spectrally very narrow and dynamically tunable. We discuss the advantages and the limitations of this system. As an illustration of a possible application we demonstrate a two-port all-optical switch.

Exact solutions are obtained for a collective model of two identical two-level atoms interacting with a quantized bimodal field with intensity-dependent coupling terms in a lossless cavity. A unitary transformation method is used to solve the time-dependent problem that also gives the eigensolutions of the interaction Hamiltonian. The atomic population dynamics and the dynamics of the photon statistics of the two cavity modes are studied. We present evidence of cooperative effects.

We show that the current laser technology used for field-free molecular alignment via impulsive Raman rotational excitation allows for observing long-discussed nonlinear quantum phenomena in the dynamics of the periodically kicked rotor. This includes the scaling of the absorbed energy near the conditions of quantum resonance and Anderson-like localization in the angular momentum. Based on this, we show that periodic trains of short laser pulses provide an efficient tool for selective rotational excitation and alignment in a molecular mixture. We demonstrate the efficiency of this approach by applying it to a mixture of two nitrogen isotopologues (${}^{14}$N${}_2$ and ${}^{15}$N${}_2$), and show that strong selectivity is possible even at room temperature.

We consider the evaluation of the quantum discord and other related measures of quantum correlations in a system formed by a spin-1 and a complementary spin system. A characterization of general projective measurements in such system in terms of spin averages is thereby introduced, which allows one to easily visualize their deviation from standard spin measurements. It is shown that the measurement optimizing these measures corresponds in general to a nonspin measurement. The important case of states that commute with the total $S_z$ spin-parity is discussed in detail, and the general stationary measurements for such states (parity preserving measurements) are identified. Numerical and analytical results for the quantum discord, the geometric discord, and the one way information deficit in the relevant case of a mixture of two aligned spin-1 states are also presented.

Sideband transitions have been shown to generate controllable interaction between superconducting qubits and microwave resonators. Up to now, these transitions have been implemented with voltage drives on the qubit or the resonator, with the significant disadvantage that such implementations only lead to second-order sideband transitions. Here we propose an approach to achieve first-order sideband transitions by relying on controlled oscillations of the qubit frequency using a flux-bias line. Not only can first-order transitions be significantly faster, but the same technique can be employed to implement other tunable qubit-resonator and qubit-qubit interactions. We discuss in detail how such first-order sideband transitions can be used to implement a high fidelity controlled-not operation between two transmons coupled to the same resonator.