We investigate the dynamics of classical and quantum correlations between two qubits. Each qubit is implemented by a pair of phosphorous impurities embedded in a silicon substrate. The main decoherence mechanism affecting these types of qubits is provided by the coupling of the phosphorous impurities to the acoustical vibrations of the silicon lattice. We find that depending on the temperature of the substrate and the initial state, three different dynamics can be found. These are characterized by the number of abrupt changes in both classical and quantum correlations. We also show that the correlations do not disappear. Moreover, before the classical correlations reach a constant value, they may experience successive abrupt changes associated with the apparition of a metastable pointer-states basis. Then a constant value for the classical correlations is reached when the preferred basis is established.

Dynamics of a two-level open system transversally coupled to a single zero-temperature bosonic reservoir may be Markovian or non-Markovian, depending on whether the system-reservoir coupling is weak or strong. In this paper, we show that when the system is simultaneously coupled to $N$ reservoirs its dynamics is always non-Markovian, provided that $N\ge N_{\mathrm{cr}},$ with $N_{\mathrm{cr}}$ a critical number depending on the reservoirs' parameters. Quantitatively, the non-Markovianity $\mathcal{N}$ is shown proportional to the number of contributed reservoirs. We explain our results in terms of the pseudomode theory, finding out that when $N\ge N_{\mathrm{cr}}$ the pseudomodes of all the reservoirs can always partially return information back to the system, no matter how strong the couplings between the system and each individual reservoir.

We investigate the Diósi-Penrose (DP) proposal for connecting the collapse of the wave function to gravity. The DP model needs a free parameter, acting as a cutoff to regularize the dynamics, and the predictions of the model highly depend on the value of this cutoff. The Compton wavelength of a nucleon seems to be the most reasonable cutoff value since it justifies the nonrelativistic approach. However, with this value, the DP model predicts an unrealistically high rate of energy increase. Thus, either one is forced to choose a much larger cutoff, which is not physically justified and totally arbitrary, or one needs to include dissipative effects in order to tame the energy increase. Taking the analogy with dissipative collisional decoherence seriously, we develop a dissipative generalization of the DP model. We show that even with dissipative effects, the DP model contradicts known physical facts, unless either the cutoff is kept artificially large or one limits the applicability of the model to massive systems. We also provide an estimation for the mass range of this applicability.

We investigate continuous-time quantum walks of two indistinguishable particles [two bosons, or two fermions, or two hard-core bosons (HCBs)] in one-dimensional lattices with nearest-neighbor interactions. The results for two HCBs are consistent with the recent experimental observation of two-magnon dynamics [Fukuhara et al., Nature (London) 502, 76 (2013)]. The two interacting particles can undergo independent walking and/or co-walking depending on both quantum statistics and interaction strength. Two strongly interacting particles may form a bound state and then co-walk like a single composite particle with a statistics-dependent walk speed. Analytical solutions for the scattering and bound states, which appear in the two-particle quantum walks, are obtained by solving the eigenvalue problem in the two-particle Hilbert space. In the context of degenerate perturbation theory, an effective single-particle model for the quantum co-walking is analytically derived and the walk speed of bosons is found to be exactly three times that of fermions and HCBs. Our result paves the way for experimentally exploring quantum statistics via two-particle quantum walks.

We consider the problem of controlling in minimum time a two-level quantum system which can be subject to a drift. The control is assumed to be bounded in magnitude and to affect two or three independent generators of the dynamics. We describe the time optimal trajectories in SU(2), the Lie group of possible evolutions for the system, by means of a particularly simple parametrization of the group. A key ingredient of our analysis is the introduction of the optimal front line. This tool allows us to fully characterize the time evolution of the reachable sets and to derive the worst-case operators and the corresponding times. The analysis is performed in any regime—controlled dynamics stronger than, of the same magnitude as, or weaker than the drift term—and gives a method to synthesize quantum logic operations on a two-level system in minimum time.

The unambiguous detection and quantification of entanglement is a hot topic of scientific research, though it is limited to low dimensions or specific classes of states. Here we identify an additional class of quantum states, for which bipartite entanglement measures can be efficiently computed, providing rigorous results. Such states are written in arbitrary $d\times d$ dimensions, where each basis state in the subsystem A is paired with only one state in B. This class, to which we refer as pair basis states, is remarkably relevant in many physical situations, including quantum optics. We find that negativity is a necessary and sufficient measure of entanglement for mixtures of states written in the same pair basis. We also provide analytical expressions for a tight lower-bound estimation of the entanglement of formation, a central quantity in quantum information.

The theory of stabilizer quantum error correction allows us to actively stabilize quantum states and simulate ideal quantum operations in a noisy environment. It is critical to correctly diagnose noise from its syndrome and nullify it accordingly. However, hardware that performs quantum error correction itself is inevitably imperfect in practice. Here, we show that stabilizer codes possess a built-in capability to correct errors not only on quantum information but also on faulty syndromes extracted by themselves. Shor's syndrome extraction for fault-tolerant quantum computation is naturally improved. This opens a path to realizing the potential of stabilizer quantum error correction hidden within an innocent-looking choice of generators and stabilizer operators that have been deemed redundant.

We propose a scheme for driving a dimer of spatially separated qubits into a maximally entangled nonequilibrium steady state. A photon-mediated retarded interaction between the qubits is realized by coupling them to two tunnel-coupled leaky cavities where each cavity is driven by a coherent microwave tone. The proposed cooling mechanism relies on striking the right balance between the unitary and driven-dissipative dynamics of the qubit subsystem. We map the dimer to an effective transverse-field isotropic $XY$ model coupled to a nonequilibrium bath that can be suitably engineered through the choice of drive frequencies and amplitudes. We show that both singlet and triplet states can be obtained with remarkable fidelities. The proposed protocol can be implemented with a superconducting circuit architecture that was recently experimentally realized and paves the way to achieving large-scale entangled systems that are arbitrarily long lived.

We provide strong evidence for the strong monogamy inequality of multiqubit entanglement recently proposed [B. Regula et al., Phys. Rev. Lett. 113, 110501 (2014)]. We consider a large class of multiqubit generalized $W$-class states and analytically show that the strong monogamy inequality of multiqubit entanglement is saturated by this class of states.

We report the implementation of the four-component spinor relativistic equation-of-motion (EOM) coupled-cluster method within the single- and double-excitation approximation to calculate the ionization potential of molecules. We have applied this method to calculate vertical ionization potentials of the molecules $XH$ ($X=\text{F}$, Cl, Br, I) along with ${\mathrm{Cl}}_2$ and ${\mathrm{Br}}_2$ in their closed-shell configuration. We have also presented intermediate results using the second-order many-body perturbation-theory level in the EOM framework to understand the role of electron correlation. All the calculated values are compared with the available experimental results. Our results are found to be in good agreement with sophisticated experiments, and relative deviation of less than $1\%$ is achieved for all the considered systems.

Inelastic cross sections for the excitation, deexcitation, ion-pair formation, and mutual neutralization processes in cesium-hydrogen collisions $\mathrm{Cs}(6s,6p,5d,7s)+\mathrm{H}$ and $\mathrm{Cs}{}^{+}+{\mathrm{H}}^{-}$ are calculated by means of the recently proposed branching-probability-current method and the recently calculated accurate ab initio adiabatic potential energies. Scattering calculations are performed in the low-energy range from 0.01 eV to 1 keV. It is shown that among the endothermic processes, the highest values of the partial cross sections correspond to the ion-pair formation processes with the maximum values up to $23{\text{Å}}^2$. Among the exothermic processes in the low-energy range, the largest partial cross section corresponds to the mutual neutralization process into the $\mathrm{Cs}\left(5d\right)+\mathrm{H}$ final state.

The magnetic-sublevel population of the autoionization state formed in dielectronic recombination (DR) of highly charged heavy ions has been experimentally investigated by combining two types of measurements with an electron beam ion trap. The two different measurements are the differential x-ray measurement at ${90}^{\circ }$ with respect to the electron beam and the integral resonance-strength measurement. The alignment parameter, which denotes the magnetic-sublevel population distribution, has been obtained for the autoionization states ${\left[1s2s^{2}2p_{1/2}\right]}_1$ formed in DR of Li-like Pr (atomic number $Z=59$) and Ho ($Z=67$). This work is a complement to our previous work [Phys. Rev. Lett. 108, 073002 (2012)] on Li-like Au ($Z=79$). The experimental results are in reasonable agreement with theoretical predictions, and a strong $Z$-dependent tendency due to the Breit interaction has been confirmed.

We present a formalism for cold and ultracold atom-diatom chemical reactions that combines a quantum close-coupling method at short range with quantum defect theory at long range. The method yields full state-to-state rovibrationally resolved cross sections as in standard close-coupling (CC) calculations but at a considerably less computational expense. This hybrid approach exploits the simplicity of MQDT while treating the short-range interaction explicitly using quantum CC calculations. The method, demonstrated for ${\mathrm{D}+\mathrm{H}}_2\to \mathrm{HD}+\mathrm{H}$ collisions with rovibrational quantum state resolution of the HD product, is shown to be accurate for a wide range of collision energies and initial conditions. The hybrid CC-MQDT formalism may provide an alternative approach to full CC calculations for cold and ultracold reactions.

I propose a method to remove the axial motional excitation from an ion qubit within a few oscillation periods of a harmonic trap without the need for laser illumination. The principle is to prepare another coolant ion in its motional ground state, and then apply a phonon beam splitter to swap the motional state between the ions without affecting the internal state. This method requires only the precise control of the trapping potential, thus its performance does not suffer from the limitations of current laser-cooling schemes. With sufficient coolant ions preprepared, this method can rapidly recool ion qubits during quantum information processing.

In the recent work of Herath et al. [T. Herath, L. Yan, S. K. Lee, and W. Li, Phys. Rev. Lett. 109, 043004 (2012)] the first experimental observation of a dependence of strong-field ionization rate on the sign of the magnetic quantum number $m$ [of the initial bound state $\left(n,l,m\right)]$ was reported. The experiment with nearly circularly polarized light could not distinguish which sign of $m$ favors faster ionization. We perform ab initio calculations for the hydrogen atom initially in one of the four bound substates with the principal quantum number $n=2$, and irradiated by a short circularly polarized laser pulse of $800\mathrm{nm}$. In the intensity range of ${10}^{12}-{10}^{13}\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$ excited bound states play a very important role, but also up to some ${10}^{15}\mathrm{W}/\mathrm{c}{\mathrm{m}}^2$ they cannot be neglected in a full description of the laser-atom interaction. We explore the region that with increasing intensity switches from multiphoton to over-the-barrier ionization and we find, unlike in tunneling-type theories, that the ratio of ionization rates for electrons initially counter-rotating and corotating (with respect to the laser field) may be higher or lower than 1.

We present the observations of the phase differences $\Delta {\varphi }_{\mathrm{HH}}^{\left(2n\right)}$ between adjacent high-order harmonics generated from Ar and ${\mathrm{N}}_2$ at the near-threshold region. The $\Delta {\varphi }_{\mathrm{HH}}^{\left(2n\right)}$'s are extracted from the photoelectron signals resulting from two-color two-photon ionization of rare-gas atoms, which are produced by high-order harmonics to be measured and a part of the fundamental pulse for probing. An analysis method is employed to remove the inevitable modulations in high-order-harmonic intensities based on the underlying mechanism of the production of photoelectrons. We find a significant difference in the $\Delta {\varphi }_{\mathrm{HH}}^{\left(2n\right)}$ at the nearest-threshold order between Ar and ${\mathrm{N}}_2$. This difference cannot be reproduced by the model calculation by using the saddle-point method within the strong-field approximation. To elucidate the origin of the difference between the $\Delta {\varphi }_{\mathrm{HH}}^{\left(2n\right)}$ for Ar and that for ${\mathrm{N}}_2$, we note the fact that the phase difference $\Delta {\varphi }_{\mathrm{HH}}^{\left(2n\right)}$ contains information both on the recombination time $t_{\mathrm{r}}$ of the freed electron and on the phase of the recombination dipole moment $d^{*}$. With the help of some numerical calculations, we discuss the effect of the potential created by the parent ion on $t_{\mathrm{r}}$ and $d^{*}$ which are neglected in the strong-field approximation.

We investigate the scaling behavior of a very large magneto-optical trap (VLMOT) containing up to $1.4\times {10}^{11}{\mathrm{Rb}}^{87}$ atoms, loaded from a background vapor. By varying the diameter of the trapping beams, we are able to change the number of trapped atoms by more than 5 orders of magnitude. We then study the scaling laws of the loading and size of the VLMOT, and analyze the shape of the density profile in this regime where the Coulomb-like, light-mediated repulsive interaction between atoms is expected to play an important role.

The dissociative ionization of ${{\text{H}}_2}^{+}$ interacting with intense, femtosecond extreme-ultraviolet laser pulses is investigated theoretically. This is done by numerical propagation of the time-dependent Schrödinger equation for a collinear one-dimensional model of ${{\text{H}}_2}^{+}$, with electronic and nuclear motion treated exactly within the limitations of the model. The joint energy spectra (JESs) are extracted for the fragmented electron and nuclei by means of the time-dependent surface flux method. The dynamic interference effect, which was first observed in one-electron atomic systems, is in the present work observed for ${{\text{H}}_2}^{+}$, emerging as interference patterns in the JESs. The photoelectron and the nuclear energy spectra are obtained by integration of the JESs. Without the JESs, the photoelectron spectrum itself is shown to be inadequate for the observation of the dynamic interference effect. The resulting JESs are analyzed in terms of two models. In one model the wave function is expanded in terms of the “essential” states of the system, consisting of the ground state and the continuum states, allowing for the interpretation of the main features in the JESs in terms of dynamic interference. In the second model the photoelectron spectra from fixed-nuclei calculations are used to reproduce some features of the JESs using simple reflection arguments. The range of validity of these models is discussed and it is shown that the consideration of the population of excited vibrational states is crucial for understanding the structures of the JESs.

We present measurements of the absolute photoionization cross section of the aluminumlike ${\mathrm{Si}}^{+}$ ion over the 94–137 eV photon energy range. The measurements were performed using the merged-beam setup on the PLEIADES beamline at the SOLEIL synchrotron radiation facility. Signals produced in the ${\mathrm{Si}}^{2+}$ and ${\mathrm{Si}}^{3+}$ photoionization channels of the $2p$ subshell of the ${\mathrm{Si}}^{+}$ ion from both the $1s^{2}2s^{2}2p^{6}3s^{2}3p{}^2{P}_{1/2,3/2}$ ground levels and the $1s^{2}2s^{2}2p^{6}3s3p^2{}^{4}P$ metastable levels were observed. Absolute cross sections were determined. Calculations of the $2p$ inner-shell photoionization cross sections were carried out using the multiconfiguration Dirac-Fock and Dirac-Coulomb R-matrix theoretical approaches and are compared with experiment.