Raul Coto

Propagation and distribution of quantum correlations in a cavity QED network

We study the propagation and distribution of quantum correlations through two chains of atoms inside cavities joined by optical fibres. This system is interesting because it can be used as a channel for quantum communication or as a network for quantum computation. In order to quantify those correlations, we used two different measurements: entanglement and quantum discord. We also use tangle for multipartite entanglement. We consider an effective Hamiltonian for the system and cavity losses, in the dressed atom picture, using the generalized master equation. We found a case where the quantum discord and the classical correlation are almost constant, and we also found multipartite entanglement, starting with only one excitation per chain. Finally, we propose a way to select the initial condition so that we can optimize the results for different purposes.

Determination of the maximum global quantum discord via measurements of excitations in a cavity QED network

Multipartite quantum correlation is one of the most relevant indicators of the quantumness of a system in many body systems. This remarkable feature is in general difficult to characterize and the known definitions are hard to measure. Besides the efforts dedicated to solve this problem, the question of which is the best approach remains open. In this paper, we study the global quantum discord (GQD) as a bipartite and multipartite measure. We also check the limits of this definition and present an experimental scheme to determine the maximum of the GQD via the measurements of the system’s excitations, during the time evolution of the present system.

Quantum correlations in cavity QED networks

We present a review of the dynamical features such as generation, propagation, distribution, sudden transition and freezing of the various quantum correlation measures, as Concurrence, Entanglement of Formation, Quantum Discord, as well their geometrical measure counterparts within the models of Cavity Quantum Electrodynamics Networks. Dissipation and thermal effects are discussed both in the generation of quantum correlations as well as their effect on the sudden changes and freezing of the classical and quantum correlations in a cavity quantum electrodynamical network. For certain initial conditions, double transitions in the Bures geometrical discord are found. One of these transitions tends to disappear at a critical temperature.

Generation of quantum correlations for two qubits through a common reservoir

Recently, besides entanglement, an entropy-based measurement called quantum discord has been used to describe the quantum correlations in separable and non-separable states. Here we compare the dynamics of entanglement with that of quantum discord, in the case of two qubits in a common thermal or squeezed reservoir. We focus on the generation of quantum correlations, starting from an uncorrelated system.

The power of a control qubit in weak measurements

In the late 80 s, a curious effect suggested by Aharanov et al. was found to lead to an anomalous amplification based on quantum measurements of weakly coupled systems. In this paper, we investigate the quantum control of the weak value amplification of a qubit system coupled to a meter. For the qubit system, the coupling occurs via a second non-interacting qubit, initially quantum correlated with the first one. We show that for weak measurements, the control can be remotely realized via the post-selected state of the second qubit or the degree of squeezing of the meter. In our study of the quantum control of the amplification, we can easily manipulate the degree of quantum correlations between the initially correlated qubits. We find that the degree of entanglement has no effect on the quantum control of the amplification. However, we also found a clear connection between the amplification and quantum discord like measurements as well as classical correlations between the qubits. Finally, we suggest an application of the amplification control on the enhancement of the quantum measurement accuracy, e.g. measuring the relative phase of the post-selected control qubit in a more precise way, as opposed to the non-amplified case.

Macroscopic nonclassical-state preparation via postselection

Macroscopic mechanical qubits are fundamental both to test the classical-quantum boundary and present suitable candidates for quantum information processing. Motivated by these, we propose a feasible probabilistic scheme to generate an on-demand mechanical qubit, as well as Schrodingers cat and Fock number states. In order to accomplish this proposal, we study an open dispersive spin-mechanical system in the absence of any external driving. The procedure is solely based on spin post-selection in the weak coupling regime. Through this scheme we demonstrate that the achieved superposition is closely related to the amplification of the mean values of the mechanical quadratures as they are associated to the maximum quantum coherence.

Generation and protection of a maximally entangled state between many modes in an optical network with dissipation

We present a three-cavity network model with two modes in each cavity and a non-linear medium that generates a Kerr type interaction via both self-phase and cross-phase modulation processes. We have two main goals. The first one is to generate a multipartite Maximally Entangled State (MES), starting from the ground state of the system. We address the problem both without and with dissipation. Secondly, we want to protect the MES from decoherence. While studying the MES, we analyze different bipartite and multipartite entanglement measures. We also study the effect of an Avoided Level Crossing (ALC) identified by the critical behavior of the entanglement measures, thus showing that the quantum correlations act as a witness for such phenomena. Our findings provide the quantum tools to perform the operation of generation and protection of a maximally entangled state in a cavity QED environment.

Self-trapping triggered by losses in cavity QED

In a coupled cavity QED network model, we study the transition from a localized super fluid like state to a delocalized Mott insulator like state, triggered by losses. Without cavity losses, the transition never takes place. Further, if one measures the quantum correlations between the polaritons via the negativity, we find a critical cavity damping constant, above which the negativity displays a single peak in the same time region where the transition takes place. Additionally, we identify two regions in the parameter space, where below the critical damping, oscillations of the initial localized state are observed along with a multipeaked negativity, while above the critical value, the oscillations die out and the transition is witnessed by a neat single peaked negativity.

New definition of the polariton ladder operators leads to interesting detuning dependent effects

Dressed States or Polaritons have proved to be an important resource for quantum information, especially because this hybrid light-matter state sometimes simplify the time evolution of the system. For a many body systems, like a cavity QED array, we derive a different hopping energy for the polaritons. This is because a complete formulation of the ladder operators for these states is missing. In this work, we present a new definition for the polariton ladder operators, which satisfy the boson commutation relations, including transitions between higher levels beyond the ground and first excited state. Finally, for a three cavity system, we show how this new insight can affect the entanglement and the Mott-Superfluid quantum phase transition.