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L. Roa

Publications and source records attributed to L. Roa.

10 recordsLinked to original sources
arXiv2011

Entanglement of formation for a class of $(2\otimes d)$-dimensional systems

Currently the entanglement of formation can be calculated analytically for mixed states in a $(2\otimes2)$-dimensional Hilbert space. For states in higher dimensional Hilbert space a closed formula for quantifying entanglement does not exist. In this regard only entanglement bounds has been found for estimating it. In this work, we find an analytical expression for evaluating the entanglement of formation for bipartite ($2\otimes d$)-dimensional mixed states.

quant-ph↗
arXiv2009

Unambiguous modification of nonorthogonal single and two photon polarization states

In this work we propose a probabilistic method which allows an unambiguous modification of two non-orthogonal quantum states. We experimentally implement this protocol by using two-photon polarization states generated in the process of spontaneous parametric down conversion. In the experiment, for codifying initial quantum states, we consider single photon states and heralded detection. We show that the application of this protocol to entangled states, it allows a fine control of the amount of entanglement of the initial state.

quant-ph↗
arXiv2006

Narrow entanglement beats

We study how the entanglement between two atoms can be created or modified even when they do not interact but when each of them interacts dispersively, i.e., weak and far from the resonance with a single mode of the field. Considering that regime we apply a method which makes use of a small nonlinear deformation of the usual SU(2) algebra in order to obtain the effective Hamiltonian describing correctly the dynamics for any initial states. In particular we study two cases: In the first one we consider each atom initially in a pure state and in the second case we assume that they start in a Werner state. We find that both atoms can reach, periodically, maximum entanglement if each of them starts in any eigenstate of the x-componet of the Pauli oprator, independent of the initial Fock state of the mode. Thus we find that a dispersive vacuum can generate entanglement between two two-level atoms. In the second case and when the field mode is initially in a coherent or thermal state, we find that in the high energy limit, in general, there is no entanglement between the two atoms however at well defined moments the initial entanglement is as suddenly recovered as removed. This time behavior looks like narrow beats separated by the so called entanglement dead valleys.

quant-ph↗
arXiv2006

Measurement-driven quantum evolution from a known state

We study the problem of driving a known initial quantum state onto a known pure state without using a unitary evolution. This task can be achieved by means of von Neumann measurement processes, introducing N observables which are consecutively measured in order to approach the state of the system to the target state. We proved that the probability of projecting onto the target state can be increased meaningfully by adding suitable observables to the process, that is, it converges to 1 when N increases. We also discuss a physical implementation of this scheme.

quant-ph↗
arXiv2006

Quantum state stability against decoherence

We study the stability of the coherence of a state of a quantum system under the effect of an interaction with another quantum system at short time. We find an expression for evaluating the order of magnitude of the time scale for the onset of instability as a function of the initial state of both involved systems and of the sort of interaction between them. As an application we study the spin-boson interaction in the dispersive interaction regime, driven by a classical field. We find, for this model, that the behavior of the time scale for the onset of instability, with respect to the boson bath temperature, changes depending on the intensity of the classical field.

quant-ph↗
arXiv2005

Decoherence assisting a measurement-driven quantum evolution process

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.

quant-ph↗
arXiv2005

Measurement driven quantum evolution

We study the problem of mapping an unknown mixed quantum state onto a known pure state without the use of unitary transformations. This is achieved with the help of sequential measurements of two non-commuting observables only. We show that the overall success probability is maximized in the case of measuring two observables whose eigenstates define mutually unbiased bases. We find that for this optimal case the success probability quickly converges to unity as the number of measurement processes increases and that it is almost independent of the initial state. In particular, we show that to guarantee a success probability close to one the number of consecutive measurements must be larger than the dimension of the Hilbert space. We connect these results to quantum copying, quantum deleting and entanglement generation.

quant-ph↗
arXiv2003

Optimal conclusive teleportation of quantum states

Quantum teleportation of qudits is revisited. In particular, we analyze the case where the quantum channel corresponds to a non-maximally entangled state and show that the success of the protocol is directly related to the problem of distinguishing non-orthogonal quantum states. The teleportation channel can be seen as a coherent superposition of two channels, one of them being a maximally entangled state thus, leading to perfect teleportation and the other, corresponding to a non-maximally entangled state living in a subspace of the d-dimensional Hilbert space. The second channel leads to a teleported state with reduced fidelity. We calculate the average fidelity of the process and show its optimality.

quant-ph↗
arXiv1997

Investigation of acceptor levels and hole scattering mechanisms in p-gallium selenide by means of transport measurements under pressure

The effect of pressure on acceptor levels and hole scattering mechanisms in p-GaSe is investigated through Hall effect and resistivity measurements under quasi-hydrostatic conditions up to 4 GPa. The pressure dependence of the hole concentration is interpreted through a carrier statistics equation with a single (nitrogen) or double (tin) acceptor whose ionization energies decrease under pressure due to the dielectric constant increase. The pressure effect on the hole mobility is also accounted for by considering the pressure dependencies of both the phonon frequencies and the hole-phonon coupling constants involved in the scattering rates.

cond-mat.mtrl-sci↗