Localized electron dynamics in attosecond-pulse-excited molecular systems: Probing the time-dependent electron density by sudden photoionization

in Physical Review. A (2012)

Ultrafast UV excitation can prepare a nonstationary coherent superposition of molecular electronic states. The purely electronic dynamics before the onset of nuclear motion can be probed by a sudden XUV ... [more ▼]

Ultrafast UV excitation can prepare a nonstationary coherent superposition of molecular electronic states. The purely electronic dynamics before the onset of nuclear motion can be probed by a sudden XUV ionization of the electronic wave packet. Dynamical computations at the many-electron level on the LiH and 1-azabicyclo[3.3.3]undecane (C10H19N) molecules showthat molecular frame photoelectron angular distributions reflect the spatial localization and undulations of the electronic coherent superposition accessed by the initial ultrafast UV excitation. The sudden ionization is sensitive to interference effects. [less ▲]

Multipartite-entanglement monotones and polynomial invariants

in Physical Review. A (2012), 85

We show that a positive homogeneous function that is invariant under determinant 1 stochastic local operations and classical communication (SLOCC) transformations defines an N-qubit entanglement monotone ... [more ▼]

We show that a positive homogeneous function that is invariant under determinant 1 stochastic local operations and classical communication (SLOCC) transformations defines an N-qubit entanglement monotone if and only if the homogeneous degree is not larger than four. We then describe a common basis and formalism for the N-tangle and other known invariant polynomials of degree four. This allows us to elucidate the relation of the four-qubit invariants defined by Luque and Thibon [Phys. Rev. A 67, 042303 (2003)] and the reduced two-qubit density matrices of the states under consideration, thus giving a physical interpretation for those invariants. We demonstrate that this is a special case of a completely general law that holds for any multipartite system with bipartitions of equal dimension, e.g., for an even number of qudits. [less ▲]

Scattering of twisted particles: Extension to wave packets and orbital helicity

in Physical Review. A (2011), 84(3), 033804

High-energy photons and other particles carrying nonzero orbital angular momentum (OAM) emerge as a new tool in high-energy physics. Recently, it was suggested to generate high-energy photons with nonzero ... [more ▼]

High-energy photons and other particles carrying nonzero orbital angular momentum (OAM) emerge as a new tool in high-energy physics. Recently, it was suggested to generate high-energy photons with nonzero OAM (twisted photons) by the Compton backscattering of twisted optical photons on relativistic electron beams. Twisted electrons in the intermediate energy range have also been demonstrated experimentally; twisted protons and other particles can, in principle, be created in a similar way. Collisions of energetic twisted states can offer a new look at particle properties and interactions. A theoretical description of twisted particle scattering developed previously treated them as pure Bessel states and ran into difficulty when describing the OAM of the final twisted particle at nonzero scattering angles. Here we develop further this formalism by incorporating two additional important features. First, we treat the initial OAM state as a wave packet of a finite transverse size rather than a pure Bessel state. This realistic assumption allows us to resolve the existing controversy between two theoretical analyses for nonforward scattering. Second, we describe the final twisted particle in terms of the orbital helicity: the OAM projection on its average direction of propagation rather than on the fixed reaction axis. Using this formalism, we determine to what extent the twisted state is transferred from the initial to final OAM particle in a generic scattering kinematics. As a particular application, we prove that in the Compton backscattering the orbital helicity of the final photon stays close to the OAM projection of the initial photon. [less ▲]

Entanglement equivalence of N-qubit symmetric states

in Physical Review. A (2010), 81

We study the interconversion of multipartite symmetric N-qubit states under stochastic local operations and classical communication (SLOCC). We demonstrate that if two symmetric states can be connected ... [more ▼]

We study the interconversion of multipartite symmetric N-qubit states under stochastic local operations and classical communication (SLOCC). We demonstrate that if two symmetric states can be connected with a nonsymmetric invertible local operation (ILO), then they belong necessarily to the separable, W, or Greenberger-Horne-Zeilinger (GHZ) entanglement class, establishing a practical method of discriminating subsets of entanglement classes. Furthermore, we prove that there always exists a symmetric ILO connecting any pair of symmetric N-qubit states equivalent under SLOCC, simplifying the requirements for experimental implementations of local interconversion of those states. [less ▲]

Operational multipartite entanglement classes for symmetric photonic qubit states

in Physical Review. A (2010), 81

We present experimental schemes that allow us to study the entanglement classes of all symmetric states in multiqubit photonic systems. We compare the efficiency of the proposed schemes and highlight the ... [more ▼]

We present experimental schemes that allow us to study the entanglement classes of all symmetric states in multiqubit photonic systems. We compare the efficiency of the proposed schemes and highlight the relation between the entanglement properties of symmetric Dicke states and a recently proposed entanglement scheme for atoms. In analogy to the latter, we obtain a one-to-one correspondence between well-defined sets of experimental parameters and multiqubit entanglement classes inside the symmetric subspace of the photonic system. [less ▲]

Multiqubit symmetric states with high geometric entanglement

in Physical Review. A (2010), 81(6), 0623471-6

We propose a detailed study of the geometric entanglement properties of pure symmetric N-qubit states, focusing more particularly on the identification of symmetric states with a high geometric ... [more ▼]

We propose a detailed study of the geometric entanglement properties of pure symmetric N-qubit states, focusing more particularly on the identification of symmetric states with a high geometric entanglement and how their entanglement behaves asymptotically for large N. We show that much higher geometric entanglement with improved asymptotical behavior can be obtained in comparison with the highly entangled balanced Dicke states studied previously. We also derive an upper bound for the geometric measure of entanglement of symmetric states. The connection with the quantumness of a state is discussed. [less ▲]

Controlled full adder or subtractor by vibrational quantum computing

in Physical Review. A (2009), 80(2),

A controlled full addition or subtraction can be realized by a unitary transformation on a register of four qubits. The fourth qubit is then used as a control qubit to enforce the addition or the ... [more ▼]

A controlled full addition or subtraction can be realized by a unitary transformation on a register of four qubits. The fourth qubit is then used as a control qubit to enforce the addition or the subtraction of two binary digits and a carry or a borrow. The transformation can be decomposed into six elementary gates. The network differs from the adder network of four elementary gates by including two new controlled-NOT gates. The scheme is general and its implementation using vibrational computing has the advantage that the single global transformation that connects the inputs to the outputs can be driven in one step by a single laser shot. This decreases the time of operation and allows for a better use of the optical resources and for an improvement of the fidelity. The laser pulses are optimized by optimal control theory. [less ▲]

Heralded entanglement of arbitrary degree in remote qubits

in Physical Review. A (2009), 80

Incoherent scattering of photons off two remote atoms with a -level structure is used as a basic Young-type interferometer to herald long-lived entanglement of an arbitrary degree. The degree of ... [more ▼]

Incoherent scattering of photons off two remote atoms with a -level structure is used as a basic Young-type interferometer to herald long-lived entanglement of an arbitrary degree. The degree of entanglement, as measured by the concurrence, is found to be tunable by two easily accessible experimental parameters. Fixing one of them to certain values unveils an analog to the Malus’ law. An estimate of the variation in the degree of entanglement due to uncertainties in an experimental realization is given. [less ▲]

Generation of total angular momentum eigenstates in remote qubits

in Physical Review. A (2009), 79

We propose a scheme enabling the universal coupling of angular momentum of N remote noninteracting qubits using linear optical tools only. Our system consists of N single-photon emitters in a ... [more ▼]

We propose a scheme enabling the universal coupling of angular momentum of N remote noninteracting qubits using linear optical tools only. Our system consists of N single-photon emitters in a configuration that are entangled among their long-lived ground-state qubits through suitably designed measurements of the emitted photons. In this manner, we present an experimentally feasible algorithm that is able to generate any of the 2N symmetric and nonsymmetric total angular momentum eigenstates spanning the Hilbert space of the N-qubit compound. [less ▲]

Rovibrational controlled-NOT gates using optimized stimulated Raman adiabatic passage techniques and optimal control theory

in Physical Review. A (2009), 80