This paper analyzes the decoherence induced on a single qubit by the interaction with a spin chain with nontrivial internal dynamics ($XY$-type interactions). The aim of the paper is to study the existence and properties of the so-called universal regime, in which the decoherence time scale becomes independent of the strength of the coupling with the environment. It is shown that, although such a regime does exist, as previously established by Cucchietti et al. [Phys. Rev. A 75, 032337 (2007)], it is not a clear signature of a quantum phase transition in the environment. In fact, this kind of universality also exists in the absence of quantum phase transitions. A universal regime can be related to the existence of an energy scale separation between the Hamiltonian of the environment and the one characterizing the system-environment interaction. The results presented also indicate that in the strong-coupling regime the quantum phase transition does not produce an enhancement of decoherence (as opposed to what happens in the weak-coupling regime).

We study the importance of interference for the performance of Shor’s factoring algorithm and Grover’s search algorithm using a recently proposed interference measure. To this aim we introduce systematic unitary errors, random unitary errors, and decoherence processes in these algorithms. We show that unitary errors which destroy the interference destroy the efficiency of the algorithm, too. However, unitary errors may also create useless additional interference. In such a case the total amount of interference can increase, while the efficiency of the quantum computation decreases. For decoherence due to phase flip errors, interference is destroyed for small error probabilities, and converted into destructive interference for error probabilities approaching 1, leading to success probabilities which can even drop below the classical value. Our results show that in general, interference is necessary in order for a quantum algorithm to outperform classical computation, but large amounts of interference are not sufficient and can even lead to destructive interference with worse than classical success rates.

In this paper, for the decoy state method using a finite number of decoy light intensities, we present improved upper and lower bounds for the asymptotic yield $y_n$ for $n$-photon states. In particular if all the light intensities are less than or equal to 1, they are not only a lower or upper bound, but in fact are the exact minimum or maximum.

We investigate the disentanglement dynamics of a two-qubit system in the non-Markovian approach. It is shown that only for weak coupling between the system and environment does an exponential decay of entanglement appear, for certain classes of two-qubit entangled states. When the coupling between qubit and the environment becomes stronger, entanglement sudden death always appears even if the dissipation environment is at zero temperature.

Nonstatistical enhancements for formation of the metastable $1s2s2p{}^{4}P$ state compared to the similarly configured $1s2s2p{}^{2}P_{-}$ and $1s2s2p{}^{2}P_{+}$ states are observed following single electron transfer to ${\mathrm{C}}^{4+}\left(1s2s{}^{3}S\right)$ and double electron transfer to ${\mathrm{C}}^{5+}\left(1s\right)$ ions, respectively. Previously, similar enhancements were observed in the population of the ${}^{4}P$ metastable state resulting from electron transfer in $\sim 1\mathrm{MeV}∕\mathrm{u}$ ${\mathrm{F}}^{7,8+}+\mathrm{He}$ and Ne collisions [J. A. Tanis et al., Phys. Rev. Lett. 92, 133201 (2004)]. The enhancements were attributed to a dynamical Pauli exchange mechanism involving projectile and target electrons having the same spin alignment. Recently, it was suggested that the enhancement might be due to cascade effects following electron transfer to states with $n>3$. To further understand the observed enhancement of the ${}^{4}P$ state, new investigations of Auger emission spectra in collisions of $0.5–1\mathrm{MeV}∕\mathrm{u}$ ${\mathrm{C}}^{4,5+}$ with He and Ne targets have been conducted. Experimental ratios of the resulting ${}^{4}P$ intensities to the sum of the ${}^{2}P_{-}$ and ${}^{2}P_{+}$ intensities for the collision systems studied give values three to four times larger than expected based solely on spin statistics for single transfer to ${\mathrm{C}}^{4+}\left(1s2s{}^{3}S\right)$ and double transfer to ${\mathrm{C}}^{5+}\left(1s\right)$. Theoretical calculations suggest that about half of the observed enhancement is caused by cascade effects.

We experimentally demonstrate one- and two-photon phase sensitive coherent control over the excitation and ionization of Rydberg states of Sr atom in the presence of a static homogeneous electric field. Without the field this excitation scheme can only be employed for the manipulation of angular distributions of photoelectrons and molecular photofragmentation products. Total atomic excitation-ionization yields cannot be modulated because the final states excited by each pathway are orthogonal to each other. When, however, a static electric field is applied, $s$, $p$ and $d$ character is admixed into the final Rydberg state, which has no definite parity anymore. Hence, the excitation of the target Stark state from the ground state is possible with either two (fundamental laser frequency) photons or one (second harmonic frequency) photon and its population and further ionization can be controlled by varying the relative phase between the two radiation fields. The concept is successfully tested below as well as above the classical saddle point and with either mutually crossed or parallel linear laser beam polarizations (while the second harmonic beam polarization vector is always parallel to the static field direction). We examine the behavior of the obtained photoionization signal modulation depth $V$ as a function of the static field strength $F$ for otherwise identical experimental conditions. The $V\left(F\right)$ curve exhibits a maximum (typically $\sim 65\%–85\%$) at a field strength value that is dictated by the interplay between the employed laser power densities, one- and two-photon transition dipole moments, and relative amounts of field-dependent $s$, $p$ and $d$ character. It is therefore shown that the static field strength may serve as an additional, experimentally adjustable, control parameter in a fashion complementary to both the intensities and relative phase of the two light beams.

Representative sections of the quasienergy spectrum of argon and of alkali-metal atoms are presented for wavelengths between $250\mathrm{nm}$ and $800\mathrm{nm}$ and excursion amplitudes up to ${\alpha }_0=20\text{a.}\mathrm{u}$. The variation of the quasienergy spectrum with the wavelength of the laser field is discussed. It is shown that large-scale changes in its topology arise from laser-induced degeneracies with states without physical high-intensity or low-intensity limits.

In this paper we discuss Bose-Einstein spinor condensates for $F=1$ atoms in the context of ${}^{87}\text{R}\text{b}$, as studied experimentally by the Stamper-Kurn group [L. E. Sadler et al., Nature (London) 443, 312 (2006)]. The dynamical quantum fluctuations of a sample that starts as a condensate of $N$ atoms in a pure $F=1$, $m_F=0$ state are described in analogy to the two-mode squeezing of quantum optics in terms of an $\mathfrak{s}\mathfrak{u}\left(1,1\right)$ algebra. In this system the initial $m_F=0$ condensate acts as a source (pump) for the creation pairs of $m_F=1,-1$ atoms. We show that even though the system as a whole is described by a pure state with zero entropy, the reduced density matrix for the $m_F=+1$ degree of freedom, obtained by tracing out the $m_F=-1,0$ degrees of freedom, corresponds to a thermal state. Furthermore, these quantum fluctuations of the initial dynamics of the system provide the seeds for the formation of domains of ferromagnetically aligned spins.

Polaritons are the collective excitations of many atoms dressed by resonant photons, which can be used to explain the slow light propagation with the mechanism of electromagnetically induced transparency. As quasiparticles, these collective excitations possess the typical feature of the matter particles, which can be reflected and deflected by the inhomogeneous medium in its spatial motion with some velocity. In this paper we develop a quantum theory to systematically describe the spatial motion of polaritons in inhomogeneous magnetic and optical fields. This theoretical approach treats these quasiparticles through an effective Schrödinger equation with anisotropic dispersion that the longitudinal motion is similar to an ultrarelativistic motion of a “slow light velocity” while the transverse motion is of nonrelativity with certain effective mass. We find that, after passing through the EIT medium, the light ray bends due to the spatial-dependent profile of external field. This phenomenon explicitly demonstrates the exotic corpuscular and anisotropic property of polaritons.

We study a four-level double-$\Lambda$ atomic configuration working as a two photon linear amplifier where two atomic transitions independently interact with cavity mode, while the other transitions are driven by a strong pump field. It is found that our system always works as a phase sensitive linear amplifier with no window for a phase insensitive linear amplifier. We also investigate that the system behaves as a two-photon correlated-emission laser under certain conditions.

A genuine tripartite entanglement monotone is presented for $\left(2\otimes 2\otimes n\right)$-dimensional tripartite pure states, obtained by introducing an entanglement measure for bipartite pure states. As an application, we consider the genuine tripartite entanglement of the ground state of the exactly solvable isotropic spin-$1/2$ chain with three-spin interaction. It is shown that the singular behavior of the genuine tripartite entanglement exactly signals a quantum phase transition.

The progress in quantum operations of continuous-variable (CV) schemes can be reduced to that in CV quantum teleportation. The fidelity of quantum teleportation of an optical setup is limited by the finite degree of quantum correlation that can be prepared with a pair of finitely squeezed states. Reports of improvement of squeezing level have appeared recently, and we adopted the improved methods in our experimental system of quantum teleportation. As a result, we teleported a coherent state with a fidelity $F=0.83\pm 0.01$, which is better than any other figures reported to date, to our knowledge. In this paper, we introduce a measure $n_s$, the number of teleportations expected to be carried out sequentially. Our result corresponds to $n_s=5.0\pm 0.4$. It suggests that our improvement would enable us to proceed toward more advanced quantum operations involving multiple steps.

Christandl et al. have noted that the $d$-dimensional hypercube can be projected to a linear chain with $d+1$ sites so that, by considering fixed but different couplings between the qubits assigned to the sites, the perfect state transfer (PST) can be achieved over arbitrarily long distances in the chain [Phys. Rev. Lett. 92, 187902 (2004); Phys. Rev. A 71, 032312 (2005)]. In this work we consider distance-regular graphs as spin networks and note that any such network (not just the hypercube) can be projected to a linear chain and so can allow PST over long distances. We consider some particular spin Hamiltonians which are the extended version of those of Christandl et al. Then, by using techniques such as stratification of distance-regular graphs and spectral analysis methods, we give a procedure for finding a set of coupling constants in the Hamiltonians so that a particular state initially encoded on one site will evolve freely to the opposite site without any dynamical control, i.e., we show how to derive the parameters of the system so that PST can be achieved. It is seen that PST is only allowed in distance-regular spin networks for which, starting from an arbitrary vertex as reference vertex (prepared in the initial state which we wish to transfer), the last stratum of the networks with respect to the reference state contains only one vertex; i.e., stratification of these networks plays an important role which determines in which kinds of networks and between which vertices of them, PST can be allowed. As examples, the cycle network with even number of vertices and $d$-dimensional hypercube are considered in details and the method is applied for some important distance-regular networks.

We present a study of multistate Feshbach resonances mediated by high-order couplings. Our analysis focuses on a system with one open scattering state and multiple bound states. The scattering state is coupled to one off-resonant bound state and multiple Feshbach resonances are induced by a sequence of indirect couplings between the closed channels. We derive a general recursive expression that can be used to fit the experimental data on multistate Feshbach resonances involving one continuum state and several bound states and present numerical solutions for several model systems. Our results elucidate general features of multistate Feshbach resonances induced by high-order couplings and suggest mechanisms for controlling collisions of ultracold atoms and molecules with external fields.

We study ionization in laser-assisted high-energy nonrelativistic ion-atom collisions and show that the low-energy angular differential electron spectrum may be enhanced by five orders of magnitude by an external field of strength less than 1/100 of the atomic field. With increasing strength of the assisting field, the energy spectrum develops a plateau with a characteristic cutoff. In the plateau region we predict distinct multiphoton peaks separated by the photon energy of the assisting field. In the present laser-assisted continuum-distorted-wave eikonal-initial-state theory, this effect may be related to the dynamics in the two-body electron-projectile subsystem. The laser-assisted distorted wave Born and first Born approximation do not account for the phase-distortion of the target electron by the incoming projectile and consequently the associated plateau, cutoff, and multiphoton features are not predicted by these latter theories.

We consider the problem of suppression of noise acting on atomic ensembles trapped in optical lattices in the low-energy limit. Noise affecting external degrees of freedom of each atom independently and noise influencing only the center-of-mass (c.m.) mode of the ensemble are addressed. Taking into account the quantum character of the atomic motion, we show that negative feedback loop acting on the c.m. coordinate of the atomic ensemble is able to partially compensate both noise sources mentioned above.

We propose to encode quantum information in rotational excitations in a molecular ensemble. Using a stripline cavity field for quantum-state transfer between the molecular ensemble and a Cooper-pair-box two-level system, our proposal offers a linear scaling of the number of qubits in our register with the number of rotationally excited states available in the molecules.

Temperatures below the Doppler limit of $1.9\mathrm{mK}$ have been observed in magnesium. The strong cooling transition was modified by a coherent two-color excitation exploiting the longer lifetime of an upper level. We developed a theoretical model to describe the light forces and cooling originating from the induced quantum interference in a three-level system. Time-of-flight measurements verified temperatures of $500\mu \mathrm{K}$ in a one-dimensional (1D) molasses in accordance with our theoretical model. By implementing this scheme in a 3D magneto-optical trap with a single ir beam, temperatures as low as $1\mathrm{mK}$ could be realized. For ideal conditions we extrapolate to temperatures of $50\mu \mathrm{K}$. With cooling times of about $1\mathrm{ms}$, a fast and efficient cooling scheme was realized, particularly attractive for optical frequency standards.

We elaborate an approach to quantum fluctuations of angular momentum based on the diagonalization of the covariance matrix in two versions: real symmetric and complex Hermitian. At difference with previous approaches this is SU(2) invariant and avoids any difficulty caused by nontrivial commutators. Meaningful uncertainty relations are derived which are nontrivial even for vanishing mean angular momentum. We apply this approach to some relevant states.

Consequences of uniform coordinate scaling of the independent particle kinetic energy functional $T_s\left[n\right]$ of density functional theory are investigated. A scaling relation for the functional derivative of $T_s\left[n\right]$ is obtained which leads to a scaling relation for the Kohn-Sham potential, while the scaling of the Kohn-Sham equations also provides information on the scaling of the eigenvalues. An expression is deduced for the virial in terms of the ground-state density and the corresponding Kohn-Sham potential. It is shown that the virial is the kinetic energy functional for a two-electron systems while for $N$-electron systems the external potential as a functional of the density preclude us from having an explicit form for the noninteracting kinetic energy functional. An approximated kinetic energy functional for $N$-electron systems, based on the virial, is presented and some numerical results discussed.