Publications from 2008

A Classification of Entanglement in Three-Qubit Systems.

C. Sabín, G. García-Alcaine.
We present a classification of three-qubit states based in their three-qubit and reduced two-qubit entanglements. For pure states these criteria can be easily implemented, and the different types can be related with sets of equivalence classes under local unitary operations. For mixed states characterization of full tripartite entanglement is not yet solved in general; some partial results will be presented here.

Entanglement Swapping Between Spacelike-Separated Atoms.

J. León, C. Sabín.
We show a mechanism that projects a pair of neutral two-level atoms from an initially uncorrelated state to a maximally entangled state while they remain spacelike separated. The atoms begin both excited in a common electromagnetic vacuum, and the radiation is collected with a partial Bell-state analyzer. If the interaction time is short enough and a certain two-photon Bell state is detected after the interaction, a high degree of entanglement, even maximal, can be generated while one atom is outside the light cone of the other, for arbitrary large interatomic distances.

Sequential Implementation of Global Quantum Operations.

L. Lamata, J. León, D. Pérez-García, D. Salgado, E. Solano.
We consider a general quantum computation that can be described as a global unitary operation acting simultaneously on several qubits, performing a prescribed task without measurements. Though the design of such operations grows in difficulty with the system size, they can be implemented with universal sets of one- and two-qubit gates acting in a convenient order. Here, we study the possibility for a global unitary applied on an arbitrary number of qubits to be decomposed in a sequential unitary procedure, where an ancillary system is allowed to interact only once with each qubit. Surprisingly, we prove that sequential unitary decompositions are in general impossible for genuine entangling operations, even with an infinite-dimensional ancilla, being the paradigmatic controlled-NOT gate a striking example. Nevertheless, we find particular nontrivial operations in quantum information that can be performed in a sequential unitary manner, as is the case of quantum error correction and quantum cloning.

Strong Dissipation Inhibits Losses and Induces Correlations in Cold Molecular Gases.

N. Syassen, D. M. Bauer, M. Lettner, T. Volz, D. Dietze, J. J. García-Ripoll, J. I. Cirac, G. Rempe, S. Dürr.
Atomic quantum gases in the strong-correlation regime offer unique possibilities to explore a variety of many-body quantum phenomena. Reaching this regime has usually required both strong elastic and weak inelastic interactions because the latter produce losses. We show that strong inelastic collisions can actually inhibit particle losses and drive a system into a strongly correlated regime. Studying the dynamics of ultracold molecules in an optical lattice confined to one dimension, we show that the particle loss rate is reduced by a factor of 10. Adding a lattice along the one dimension increases the reduction to a factor of 2000. Our results open the possibility to observe exotic quantum many-body phenomena with systems that suffer from strong inelastic collisions.

Tunnelling of Plane Waves Through a Square Barrier.

J. Julve, F. J. de Urríes.
The time evolution of plane waves in the presence of a one-dimensional square quantum barrier is considered. Comparison is made between the cases of an infinite and a cut-off (shutter) initial plane wave. The difference is relevant when the results are applied to the analysis of the tunnelling regime. This work is focused on the analytical calculation of the time-evolved solution and highlights the contribution of the resonant (Gamow) states.