The extremely small probability of tunneling through an almost classical potential barrier may become not small under the action of the specially adapted nonstationary signal which selects the certain particle energy $E_R.$ For particle energies close to this value, the tunneling rate is not small during a finite interval of time and has a very sharp peak at the energy $E_R.$ After entering inside the barrier, the particle emits electromagnetic quanta and exits the barrier with a lower energy. The signal amplitude can be much less compared to the field of the static barrier. This phenomenon can be called the Euclidean resonance since the under-barrier motion occurs in imaginary time. The resonance may stimulate chemical and biochemical reactions in a selective way by adapting the signal to a certain particular chemical bond. The resonance may be used in search of the soft-α-particle decay for which a conventional observation is impossible due to an extremely small decay rate.

A proposal for a physical implementation of a quantum-state discrimination protocol using an ion in a linear trap is studied, where two nonorthogonal quantum states are codified using two electronic states of the ion. In addition, a protocol is given for discriminating superpositions of nonorthogonal entangled states between ions inside widely separated optical cavities. The discrimination protocol is extended to the case of N linearly independent nonorthogonal quantum states lying in a space of $2N-1$ dimensions.

The first small doubly charged negative cluster ions ${\mathrm{OC}}_n^{2-}$ (with $n=7\mathrm{}–14)$ were produced by sputtering of a graphite specimen with a 14.5 keV ${\mathrm{Cs}}^{+}$ ion beam at an elevated oxygen partial pressure in the vicinity of the sample’s surface. These dianions and the corresponding singly charged ${\mathrm{OC}}_n^{-}$ $(n<~21)$ cluster ions were detected in a double-focusing mass spectrometer. The yields of doubly and of singly charged mixed oxygen-carbon ions increase with the ratio of the ${\mathrm{O}}_2$ arrival rate to the ${\mathrm{Cs}}^{+}$ flux density; they tend to saturate with this ratio approaching unity. For $n>~11,$ the ion yields of the ${\mathrm{OC}}_n^{2-}$ dianions are comparable to those of the singly charged ${\mathrm{OC}}_n^{-}$ ions. For both types of ion species, the abundance distributions exhibit even-odd alternations with the number of C atoms in the cluster. The flight time through the mass spectrometer of ∼15–20 μs establishes a lower limit with respect to the intrinsic lifetimes of the doubly charged ions.

We consider the problem of cold atomic collisions in tight traps, where the absolute scattering length may be larger than the trap size. As long as the size of the trap ground state is larger than a characteristic length of the van der Waals potential, the energy eigenvalues can be computed self-consistently from the scattering phase shift for untrapped atoms. By comparing with the exact numerical eigenvalues of the trapping plus interatomic potentials, we verify that our model gives accurate eigenvalues up to milliKelvin energies for single-channel s-wave scattering of ${}^{23}\mathrm{Na}$ atoms in an isotropic harmonic trap, even when outside the Wigner threshold regime. Our model works also for multichannel scattering, where the scattering length can be made large due to a magnetically tunable Feshbach resonance.

We analyzed the influence of the thermal motion from the environment on the internal states of trapped ions and the center-of-mass mode phonon using a Green’s-function method. The deviation of the internal states and the variational rules of the phonon number with different trap parameters changing with the temperature are given. The upper limit of the temperature at which the gate operation can be performed safely is convenient to be found out according to different error correction capability.

We describe the bifurcation set for a class-$A$ laser with an external injected signal in terms of the amplitude and the frequency of the applied field. We explain the dynamical behavior of this kind of lasers in terms of a Lyapunov potential in the case where such a description is possible. In particular, a full description for the deterministic and nondeterministic dynamics can be given by using the Lyapunov potential for some of the external parameters. Depending also on the value of these parameters, the phase of the electric field drifts with time in the stochastic case.

The quantum-mechanical two-body problem with hard-sphere interaction in any-dimensional space is studied by means of Fermi’s pseudopotential. A unified pseudopotential operator is obtained for the interactions in different space dimensionalities.

We describe how a quasibound state that is in Feshbach resonance with an incoming atom wave and is coupled to a nondecaying bound state by an electromagnetic field will exhibit a quantum interference that controls the collision dynamics. By varying the frequency of a coupling field, the elastic cross section may be modulated. By varying the amplitude of a coupling field, the bound diatomic population may be increased by several orders of magnitude.

Recently quantum tomography has been proposed as a fundamental tool for prototyping a few qubit quantum device. It allows the complete reconstruction of the state produced from a given input into the device. From this reconstructed density matrix, relevant quantum information quantities such as the degree of entanglement and entropy can be calculated. Generally, orthogonal measurements have been discussed for this tomographic reconstruction. In this paper, we extend the tomographic reconstruction technique to two new regimes. First, we show how nonorthogonal measurements allow the reconstruction of the state of the system provided the measurements span the Hilbert space. We then detail how quantum-state tomography can be performed for multiqudits with a specific example illustrating how to achieve this in one- and two-qutrit systems.

We investigate analytically and numerically Bloch waves for a Bose-Einstein condensate in a sinusoidal external potential. At low densities the dependence of the energy on the quasimomentum is similar to that for a single particle, but at densities greater than a critical one the lowest band becomes triple valued near the boundary of the first Brillouin zone and develops the structure characteristic of the swallowtail catastrophe. We comment on the experimental consequences of this behavior.

Alkaline-earth-metal atoms present an ideal platform for exploring magneto-optic trap (MOT) dynamics, enabling unique and definitive tests of laser cooling and trapping mechanisms. We have measured the trapping beam intensity, detuning, magnetic-field gradient, trap density, and lifetime dependence of the spring constant $\kappa$ and damping coefficient $\alpha$ for a ${}^1{S}_0{-}^1{P}_1$ ${}^{88}\mathrm{Sr}$ MOT by fitting the oscillatory response of the atom cloud to a step-function force. We find that the observed behavior of $\kappa$ and $\alpha$ provide a unified and consistent picture of trap dynamics that agrees with Doppler cooling theory at the level of $10\%.$ Additionally, we demonstrate that the trapped atom temperature can be determined directly from measured value of $\kappa$ and the trap size, in excellent agreement with free-expansion temperature measurements. However, the experimentally determined temperature is much higher than Doppler cooling theory, implying significant additional heating mechanisms.

We study optimal teleportation based on Bell measurements. An explicit expression for the quantum channel associated with the optimal teleportation with an arbitrary mixed state resource is presented. The optimal transmission fidelity of the corresponding quantum channel is calculated and shown to be related to the fully entangled fraction of the quantum resource, rather than the singlet fraction as in the standard teleportation protocol.

We study non-Markovian decoherence phenomena by employing projection-operator formalism when a quantum system (a quantum bit or a register of quantum bits) is coupled to a reservoir. By projecting out the degree of freedom of the reservoir, we derive a non-Markovian master equation for the system, which is reduced to a Lindblad master equation in Markovian limit, and obtain the operator sum representation for the time evolution. It is found that the system is decohered slower in the non-Markovian reservoir than the Markovian because the quantum information of the system is memorized in the non-Markovian reservoir. We discuss the potential importance of non-Markovian reservoirs for quantum-information processing.

Fourth-order many-body corrections to matrix elements for atoms with one valence electron are derived. The obtained diagrams are classified using coupled-cluster-inspired separation into contributions from n-particle excitations from the lowest-order wave function. The complete set of fourth-order diagrams involves only connected single, double, and triple excitations and disconnected quadruple excitations. Approximately half of the fourth-order diagrams are not accounted for by the popular coupled-cluster method truncated at single and double excitations (CCSD). Explicit formulas are tabulated for the entire set of fourth-order diagrams missed by the CCSD method and its linearized version, i.e., contributions from connected triple and disconnected quadruple excitations. A partial summation scheme of the derived fourth-order contributions to all orders of perturbation theory is proposed.

Energy-resolved electron-ion coincidence spectra of the deuteromethane molecule were measured following ionization by narrow-band synchrotron radiation. The ion mass spectra were recorded in coincidence with the outer and inner valence photoelectrons and with the normal Auger electrons from the decay of the carbon $1s$ core hole. Complementary noncoincidence ion mass spectra were measured below and above the C $1s$ threshold. The fragmentation patterns of the singly and doubly ionized deuteromethane under different ionization conditions are examined. Carbon core ionization is shown to open new photodissociation pathways not available in the valence ionization photon energy regime. With the aid of ab initio quantum chemistry calculations, a two-step model of the dissociation following core ionization is proposed, showing a good agreement with the experimental findings.

We investigate high-order harmonic generation in closed systems using the two-level atom as a simplified model. By means of a windowed Fourier transform of the time-dependent dipole acceleration, we extract the main contributions to this process within a cycle of the driving field. We show that the patterns obtained can be understood by establishing a parallel between the two-level atom and the three-step model. In both models, high-order harmonic generation is a consequence of a three-step process, which involves either the continuum and the ground state, or the adiabatic states of the two-level Hamiltonian. The knowledge of this physical mechanism allows us to manipulate the adiabatic states, and consequently the harmonic spectra, by means of a bichromatic driving field. Furthermore, using scaling laws, we establish sharp criteria for the invariance of the physical quantities involved. Consequently, our results can be extended to a broader parameter range, as, for instance, those characteristic of solid-state systems in strong fields.

Bifurcation behavior of effective Hamiltonians is investigated for coupled systems of Bose-Einstein condensates. Phase-space structure mapped on the Bloch sphere shows a variety of Josephson-related behavior, systematically classified in the bifurcation analysis, leading to a phase diagram with prediction of some physical effects.

Large scale numerical simulations of the Gross-Pitaevskii equation are used to elucidate the self-evolution of a Bose gas from a strongly nonequilibrium initial state. The stages of the process confirm and refine the theoretical scenario of Bose-Einstein condensation developed by Svistunov and co-workers [J. Mosc. Phys. Soc. 1, 373 (1991); Sov. Phys. JETP 75, 387 (1992); 78, 187 (1994)]: the system evolves from the regime of weak turbulence to superfluid turbulence via states of strong turbulence in the long-wavelength region of energy space.

We derive an analytic expression for the linewidth of electromagnetically induced transparency (EIT) resonance in a Doppler-broadened system. It is shown here that for relatively low intensity of the driving field the EIT linewidth is proportional to the square root of intensity and is independent of the Doppler width, similar to the laser-induced line narrowing effect described by Feld and Javan. In the limit of high intensity we recover the usual power-broadening case where the EIT linewidth is proportional to the intensity and inversely proportional to the Doppler width.

Quantum optics entails polarization properties that cannot be fully described by the classical Stokes parameters. In this work, we characterize the polarization of classical as well as quantum fields by means of a probability distribution on the Poincaré sphere. This serves to define the degree of polarization of a field state as the distance between the corresponding polarization distribution and the uniform distribution representing unpolarized light. We apply this definition to relevant quantum field states such as SU(2) coherent, squeezed, number, and phase states.