We experimentally demonstrate that violations of Bell’s inequalities for two-photon polarization-entangled states with colored noise are extremely robust, whereas this is not the case for states with white noise. Controlling the amount of noise by using the timing compensation scheme introduced by Kim et al. [Phys. Rev. A 67, 010301(R) (2003)], we have observed violations even for states with very high noise, in excellent agrement with the predictions of Cabello et al. [Phys. Rev. A 72, 052112 (2005)].

The time evolution of a quantum system with at most quadratic Hamiltonian is described with the help of different methods, namely the time-dependent Schrödinger equation, the time propagator or Feynman kernel, and the Wigner function. It is shown that all three methods are connected via a dynamical invariant, the so-called Ermakov invariant. This invariant introduces explicitly the quantum aspect via the position uncertainty and its possible time dependence. The importance of this aspect, also for the difference between classical and quantum dynamics, and in particular the role of the initial position uncertainty is investigated.

Universal quantum information processing requires single-qubit rotations and two-qubit interactions as minimal resources. A possible step beyond this minimal scheme is the use of three-qubit interactions. We consider such three-qubit interactions and show how they can reduce the time required for a quantum state transfer in an $XY$ spin chain. For the experimental implementation, we use liquid-state nuclear magnetic resonance, where three-qubit interactions can be implemented by sequences of radio-frequency pulses.

This paper investigates the dynamical generation of entanglement in scattering systems, in particular two spin systems that interact via rotationally invariant scattering. The spin degrees of freedom of the in states are assumed to be in unentangled, pure states, as defined by the entropy of entanglement. Because of the restriction of rotationally symmetric interactions, perfectly entangling $S$ matrices, i.e., those that lead to a maximally entangled out state, only exist for a certain class of separable in states. Using Clebsch-Gordan coefficients for the rotation group, the scattering phases that determine the $S$ matrix are determined for the case of spin systems with $\sigma =1∕2$, 1, and $3∕2$.

We study the problem of driving an unknown initial mixed quantum state onto a known pure state without using unitary transformations. This can be achieved, in an efficient manner, with the help of sequential measurements on at least two unbiased bases. However here we found that, when the system is affected by a decoherence mechanism, only one observable is required in order to achieve the same goal. In this way the decoherence can assist the process. We show that, depending on the sort of decoherence, the process can converge faster or slower than the method implemented by means of two complementary observables.

We have investigated an extension of the buffer gas cooling technique to a non-neutral buffer gas. The proposed scheme will allow efficient mass-selective centering of ions confined in a Penning trap in situations where the use of a neutral damping agent is not possible. The present paper reviews the principle of the technique and reports on evidence for sideband cooling of antiprotons in an electron gas, obtained with the ATHENA apparatus at CERN’s Antiproton Decelerator facility.

Using lattice effective field theory, we study the ground state binding energy of $N$ distinct particles in two dimensions with equal mass interacting weakly via an attractive $\mathrm{SU}\left(N\right)$-symmetric short range potential. We find that in the limit of zero range and large $N$, the ratio of binding energies $B_N∕B_{N-1}$ approaches the value 8.3(6).

The quantum control of the ionization of potassium atoms using shaped intense femtosecond laser pulses is investigated. We use sinusoidal phase modulation as a prototype for complex shaped pulses to investigate the physical mechanism of the strong-field quantum control by shaped femtosecond light fields. The influence of all parameters characterizing the sinusoidal phase modulation on strong-field-induced dynamics is studied systematically in experiment and theory. Our results are interpreted in terms of the selective population of dressed states (SPODS) which gives a natural physical picture of the dynamics in intense laser fields. We show that modulated femtosecond pulses in combination with photoelectron spectroscopy are a versatile tool to prepare and to probe SPODS. The decomposition of the excitation and ionization process induced by shaped pulses into elementary physically transparent steps is discussed.

We analyze quantum entanglement of Stokes light and atomic electronic polarization excited during single-pass, linear-regime, stimulated Raman scattering in terms of optical wave-packet modes, and atomic-ensemble spatial modes. The output of this process is confirmed to be decomposable into multiple discrete, Bosonic mode pairs, each pair undergoing independent evolution into a two-mode squeezed state. For this we extend the Bloch-Messiah reduction theorem, previously known for discrete linear systems [S. L. Braunstein, Phys. Rev. A 71, 055801 (2005)]. We present typical mode functions in the case of one-dimensional scattering in an atomic vapor. We find that in the absence of dispersion, one mode pair dominates the process, leading to a simple interpretation of entanglement in this continuous-variable system. However, many mode pairs are excited in the presence of dispersion-induced temporal walkoff of the Stokes, as witnessed by the photon-count statistics. We also consider the readout of the stored atomic polarization using the anti-Stokes scattering process. We prove that the readout process can also be decomposed into multiple mode pairs, each pair undergoing independent evolution analogous to a beam-splitter transformation. We show that this process can have unit efficiency under realistic experimental conditions. The shape of the output light wave packet can be predicted. In the case of unit readout efficiency it contains only excitations originating from a specified atomic excitation mode.

We discuss a method for generating Fock (number) states in a single-mode traveling-wave optical field, based on a method we recently proposed for performing a quantum nondemolition measurement of parity and for the generation of parity eigenstates [C. C. Gerry, A. Benmoussa, and R. A. Campos, Phys. Rev. A 72, 053818 (2005)]. The approach is a kind of “dial-up” scheme that is probabilistic but also depends partially on directed state reductive measurements.

We consider a massless scalar field in a two-dimensional space-time inside an oscillating cavity with mixed boundary conditions. In order to discuss particle creation phenomenon, we consider the situation of parametric resonance in which the oscillating frequency is twice the frequency of the first mode of the static cavity. Following the procedure developed by Dodonov and Klimov, we compute the number of created particles, the generation rate, and the energy in the cavity. We compare our results with those found in the literature for the Dirichlet-Dirichlet case.

We reexamine the Markovian approximation of local current in open quantum systems, discussed recently by Gebauer and Car. Our derivation is more transparent; the proof of the current conservation becomes explicit and easy.

We demonstrate a method of creating photonic two-dimensional cluster states that is considerably more efficient than previously proposed approaches. Our method uses only local unitaries and type-I fusion operations. The increased efficiency of our method compared to previously proposed constructions is obtained by identifying and exploiting local equivalence properties inherent in cluster states.

We show that the complex scaling method for resonances is equivalent to carrying out a gedanken experiment, where particles are scattered by a target with the incoming flux not being constant as usual but exponentially reduced in time at the rate $\gamma$. Resonances are examined at discrete energy levels, while a counterintuitive reverse dynamical control is demonstrated off these levels for $\gamma$ greater than the resonance decay rate ${\Gamma }_r$.

We determine thermal entanglement in mean-field clusters of $N$ spin one-half particles interacting via the anisotropic Heisenberg interaction, with and without external magnetic field. For the $xxx$ cluster in the absence of magnetic field we prove that only the $N=2$ ferromagnetic cluster shows entanglement. An external magnetic field $B$ can only entangle $xxx$ antiferromagnetic clusters in certain regions of the $B\text{−}T$ plane. On the other hand, the $xxz$ clusters of size $N>2$ are entangled only when the interaction is ferromagnetic. Detailed dependence of the entanglement on various parameters is investigated in each case.

We prove upper and lower bounds relating the quantum gate complexity of a unitary operation, $U$, to the optimal control cost associated to the synthesis of $U$. These bounds apply for any optimal control problem, and can be used to show that the quantum gate complexity is essentially equivalent to the optimal control cost for a wide range of problems, including time-optimal control and finding minimal distances on certain Riemannian, sub-Riemannian, and Finslerian manifolds. These results generalize the results of [Nielsen, Dowling, Gu, and Doherty, Science 311, 1133 (2006)], which showed that the gate complexity can be related to distances on a Riemannian manifold.

We propose a scheme to perform probabilistic quantum gates on remote trapped atom qubits through interference of optical frequency qubits. The method does not require localization of the atoms to the Lamb-Dicke limit, and is not sensitive to interferometer phase instabilities. Such probabilistic gates can be used for scalable quantum computation.

The dynamics of (de)excitation between highly excited Rydberg states ($15<n<60$ of Rb) in a magnetic field of 0.85 $\pm 0.05\mathrm{T}$ is studied with far-infrared pulses (90–110 and $50\mathrm{\mu }\mathrm{m}$) originating from a free electron laser. We measured the excitation spectrum to states around $n=40$ from a deeper bound state near $n=25$. Moreover, starting from a highly excited state $(30<n<60)$ below and in the $n$-mixing regime we investigated the efficiency of the deexcitation channel vs the ionization channel. We measured deexcitation efficiencies well above 50% for some of the states. However, starting deep in the $n$-mixing regime the deexcitation efficiency is less than 10%. The measurements were in good agreement with fully quantum mechanical calculations. Calculations for deexcitation of $n=35$ states in H found the largest amount of deexcitation for $m=0$ and almost none for $m=20$. In recent experiments at CERN, antihydrogen is produced in high $n$ states in a strong magnetic field with a wide distribution of $m$. Our measurements and calculations suggest that deexcitation stimulated by infrared photons is not an efficient method for accelerating cascade to the ground state.