We show that two initially nonresonant quantum dots may be brought into resonance by the application of a single detuned laser. This allows for control of the interdot interactions and the generation of highly entangled excitonic states on the picosecond time scale. Along with arbitrary single-qubit manipulations, this system would be sufficient for the demonstration of a prototype excitonic quantum computer.

The features of the concurrences between the nearest-neighbors and that of the next-nearest-neighbors for the one-dimensional Heisenberg model with next-nearest-neighbor coupling $J$ are studied as functions of temperature and $J$. The two concurrences exhibit a different dependence on $J$ at the ground state, which could be interpreted from the point of view of the correlation functions. The threshold temperature at which the concurrence is zero and the temperature effect on the two concurrences for systems up to 12 sites are studied numerically.

We give a detailed analysis of an indirect method used to obtain the photon distribution of a single-mode field using only on/off avalanche photodetectors. The method is based on measuring the field at different quantum efficiencies and then inferring the photon distribution by maximum-likelihood estimation. We address the case when only a limited range of quantum efficiency is available and when these values are not precisely known. The convergence of the method and its robustness against fluctuations are illustrated by means of numerically simulated experiments.

We study dynamical decoupling of a qubit from non-Gaussian quantum noise due to discrete sources, as bistable fluctuators and $1∕f$ noise. We obtain analytic and numerical results for generic operating points. For very large pulse frequency, where dynamic decoupling compensates decoherence, we found universal behavior. At intermediate frequencies noise can be compensated or enhanced, depending on the nature of the fluctuators and on the operating point. Our technique can be applied to a larger class of non-Gaussian environments.

We propose a way to manifestly reduce the entropy of a finite system of atoms to arbitrarily small values. First, the locations of vacancies of laser-cooled atoms in a deep optical lattice are measured. Then, the distribution is efficiently compacted using a combination of site-specific atomic state flips and state-sensitive lattice site translations. In the final state, the central region of the lattice has exactly one atom per site in its vibrational ground state. This is a good initial state for a quantum computer. The process can be understood to be an experimentally viable Maxwell demon with a memory.

We have measured the relative cross sections for quartet and doublet satellite states of ${\mathrm{Ar}}^{+}$ formed in photoionization. Quartet states are dipole forbidden in the nonrelativistic limit and result from spin-orbit interactions in the continuum [H. W. Van der Hart and C. H. Greene, J. Phys. B 32, 4029 (1999)]. Because of this, the cross sections for their formation are generally thought to be smaller than the dipole-allowed doublet states. We find that the cross sections for the $3p^4\left[{}^{3}P\right]4p{}^{4}D_{1∕2}^o$ and $3p^4\left[{}^{3}P\right]4p{}^{4}D_{3∕2}^o$ quartet states over certain energy regions are 16 to 30 times larger than doublet states. This counterintuitive result remains to be theoretically explained.

We study the multiparticle generalized GHZ states. It has been shown that for an odd number of qubits and for a specific range of parameters, they do not violate any Bell inequality for correlation functions. We show here both analytically and numerically that, nevertheless, such states violate local realism, once a more detailed analysis of the correlations is made than the one allowed by correlation functions. The results imply that multiparticle Clauser-Horne-type inequalities involving probabilities are stronger tools for analyzing violations of local realism in multiparticle systems than inequalities involving the correlation functions.

We present a numerical study of the robustness of a specific class of non-Abelian holonomic quantum gates. We take into account the parametric noise due to stochastic fluctuations of the control fields which drive the time-dependent Hamiltonian along an adiabatic loop. The performance estimator used is the state fidelity between noiseless and noisy holonomic gates. We carry over our analysis with different correlation times and we find out that noisy holonomic gates seem to be close to the noiseless ones for “short” and “long” noise correlation times. This result can be interpreted as a consequence of the geometric nature of the holonomic operator. Our simulations have been performed by using parameters relevant to the excitonic proposal for the implementation of holonomic quantum computation [P. Solinas et al., Phys. Rev. B 67, 121307 (2003)].

Secondary-ion emission from III-V semiconductive chemical compounds (InP, InAs, and InSb) has been experimentally studied at heavy-ion energies from $0.5\text{to}5.0\mathrm{MeV}$, where electronic collision is a dominant process. Various secondary ions such as large cluster ions and atomic ions were observed. Yields of In atomic and cluster ions depend scantly on the incident energy, and those of group-V atomic ions and of cluster ions containing group-V elements can be expressed by an exponential function of $S_e^{-1}$, where $S_e$ is the electronic stopping power. This fact shows that the ionization probabilities of the atoms and the clusters whose ionization potentials are higher than the work functions of target materials are increased by transient electronic excitation induced by ion bombardment. The energy distributions of the atomic ions show that the singly charged atomic ions are emitted through the linear collision cascade process even at MeV incident energies, and the multiply charged ions are produced by a projectile-induced simultaneous process of ionization and recoiling of atoms on the target surface. The yield dependences of the cluster ions on the electronic stopping power and on the cluster size are so much different from those for $\mathrm{Si}{\mathrm{O}}_2$. This fact precludes the multiple-bond-breaking process applied to the insulating material. Structural instabilities caused by high-density electronic excitations, which are known to take place in GaAs irradiated by slow multiply charged ions or lasers, are a possible cause of the cluster-ion emission from the semiconductive compounds at the MeV energy range.

We propose a scheme that allows us to Fourier-synthesize arbitrarily shaped periodic potentials for atoms. The method is based on the dispersion connected with higher order multiphoton Raman processes, where a suitable combination of laser frequencies eliminates unwanted standing wave effects. Future application of the scheme could range from blazed atomic beam splitters up to atomic quantum ratchets.

We present a quantitative study of roughness in the magnitude of the magnetic field produced by a current carrying microwire, i.e., in the trapping potential for paramagnetic atoms. We show that this potential roughness arises from deviations in the wire current flow due to geometric fluctuations of the edges of the wire: a measurement of the potential using cold trapped atoms agrees with the potential computed from the measurement of the wire edge roughness by a scanning electron microscope.

We study a confined mixture of bosons and fermions in the quantal degeneracy regime with attractive boson-fermion interaction. We discuss the effect that the presence of vortical states and the displacement of the trapping potentials may have on mixtures near collapse, and investigate the phase stability diagram of the K-Rb mixture in the mean-field approximation supposing in one case that the trapping potentials felt by bosons and fermions are shifted from each other, as it happens in the presence of a gravitational sag, and in another case, assuming that the Bose condensate sustains a vortex state. In both cases, we have obtained an analytical expression for the fermion effective potential when the Bose condensate is in the Thomas-Fermi regime, that can be used to determine the maxima of the Fermionic density. We have numerically checked that the values one obtains for the location of these maxima using the analytical formulas remain valid up to the critical boson and fermion numbers, above which the mixture collapses.

We use recapture of near-threshold photoelectrons by postcollision interaction with Auger electrons as an effective method for population of the high-Rydberg states of subvalence ionized Ne ions. The subsequent intermultiplet Auger transitions are detected by high-resolution electron spectrometry. The series of transitions $2p^4\left({}^{1}D\right)np{}^{2}D,{}^{2}F\to 2p^4{}^{3}P$ up to $n=20$ are observed and identified with the help of multiconfiguration Dirac–Fock ab initio calculations.

State-selective preparation and manipulation of discrete-level quantum systems such as atoms, molecules, or quantum dots is the ultimate tool for many diverse fields such as laser control of chemical reactions, atom optics, high-precision metrology, and quantum computing. Rabi oscillations are one of the simplest, yet potentially quite useful mechanisms for achieving such manipulation. Rabi theory establishes that in the two-level systems resonant drive leads to the periodic and complete population oscillations between the two system levels. In this paper an analytic optimization algorithm for producing Rabi-like oscillations in the general discrete many-level quantum system is presented.

We study the superfluid–Mott insulator (SF-MI) transition of spin-1 bosons interacting antiferromagnetically in an optical lattice. Starting from a Bose-Hubbard tight-binding model for spin-1 bosons, we obtain the zero-temperature phase diagram by a mean-field approximation. We find that the MI phase with an even number of atoms per site is a spin singlet state, while the MI phase with an odd number of atoms per site has spin 1 at each site in the limit of $t=0$, where $t$ is the hopping matrix element. We also show that the superfluid phase is a polar state as in the case for a spin-1 Bose condensate in a harmonic trap. It is found that the MI phase is strongly stabilized against the SF-MI transition when the number of atoms per site is even, due to the formation of singlet pairs. We derive the effective spin Hamiltonian for the MI phase with one atom per site and briefly discuss the spin order in the MI phase.

Entanglement between a quantum system and its environment leads to loss of coherence in the former. In general, the temporal fate of coherences is complicated. Here, we establish the connection between decoherence of a central system and fidelity decay in the environment for a variety of situations, including both energy conserving and dissipative couplings. We show how properties of unitary time evolution of the environment can be inferred from the nonunitary evolution of coherences in the central system. This opens up promising ways for measuring Loschmidt echoes in a variety of situations.

We present experimental measurements and theoretical calculations for the electron-impact single ionization cross section of ${\mathrm{Sc}}^{+}$ ions covering an energy range from threshold to $1000\mathrm{eV}$. An electron-ion crossed-beams setup was employed for the measurements of absolute cross sections as well as for a high-resolution energy scan to uncover fine details in the energy dependence of the cross section. Direct ionization is described by configuration-averaged distorted-wave theory and indirect ionization by $R$-matrix theory. Indirect processes contribute to the total ionization cross section by up to $\sim 40\%$. This finding is related to the existence of strong $3p\to 3d$ excitation channels in ${\mathrm{Sc}}^{+}\left(3p^{6}3d4s\right)$. The shape of the related cross-section feature is reminiscent of the very strong $4d\to 4f$ excitation, found in the ionization of xenon and its neighboring elements.

It is shown that the interplay of a confining potential with a periodic potential leads for free particles to states spatially confined on a fraction of the total extension of the system. A more complex “slicing” of the system can be achieved by increasing the period of the lattice potential. These results are especially relevant for fermionic systems, where interaction effects are in general strongly reduced for a single species at low temperatures.

We show that using the modified form of the Dirac Hamiltonian as suggested by Bekenstein does not affect the analysis of QSO data pertaining to a measurement of $\alpha$ variation. We obtain the present time limit on Bekenstein’s parameter, ${\tan }^2\chi =(0.2\pm 0.7)\times {10}^{-6}$, from the measurement of the hydrogen $2p$ fine structure using a value of $\alpha$ obtained from different experiments.