James DiGuglielmo

Preparation of distilled and purified continuous-variable entangled states

Two independent experiments demonstrate that quantum entanglement that has been lost in decoherence processes can be recovered. For the first time such ’entanglement distillation’ has been achieved for states of light that are entangled in continuous variables, which should help to increase the distance over which quantum information can be distributed.

Experimental Demonstration of Continuous Variable Purification of Squeezed States

We report on the first experimental demonstration of purification of nonclassical continuous variable states. The protocol uses two copies of phase-diffused states overlapped on a beam splitter and provides Gaussified, less mixed states with the degree of squeezing improved. The protocol uses only linear optical devices such as beam splitters and homodyne detection, thereby proving these optical elements can be used for successful purification of this type of state decoherence which occurs in optical transmission channels.

On the distillation and purification of phase-diffused squeezed states

Recently it was discovered that non-Gaussian decoherence processes, such as phase-diffusion, can be counteracted by purification and distillation protocols that are solely built on Gaussian operations. Here, we make use of this experimentally highly accessible regime, and provide a detailed experimental and theoretical analysis of several strategies for purification/distillation protocols on phase-diffused squeezed states. Our results provide valuable information for the optimization of such protocols with respect to the choice of the trigger quadrature, the trigger threshold value and the probability of generating a distilled state.

Experimental Unconditional Preparation and Detection of a Continuous Bound Entangled State of Light

J. DiGuglielmo,1 A. Samblowski,1 B. Hage,1 C. Pineda,2, 3 J. Eisert,4, 3 and R. Schnabel1 Institut für Gravitationsphysik, Leibniz Universität Hannover, 30167 Hannover, Germany and Max-Planck Institut für Gravitationsphysik, 30167 Hannover, Germany Instituto de Fı́sica, Universidad Nacional Autónoma de México, México Institute of Physics and Astronomy, University of Potsdam, 14476 Potsdam, Germany Dahlem Center for Complex Quantum Systems, Freie Universität Berlin, 14195 Berlin, Germany (Dated: February 22, 2012)

Experimental characterization of Gaussian quantum-communication channels

We present a full experimental characterization of continuous-variable quantum-communication channels established by shared entanglement together with local operations and classical communication. The resulting teleportation channel was fully characterized by measuring all elements of the covariance matrix of the shared two-mode squeezed Gaussian state. From the experimental data we determined the lower bound to the quantum channel capacity, the teleportation fidelity of coherent states, and the logarithmic negativity and purity of the shared state. Additionally, a positive secret key rate was obtained for two of the established channels.

Experimental test of nonclassicality criteria for phase-diffused squeezed states

We experimentally examine the nonclassical character of a class of non-Gaussian states known as phase-diffused squeezed states. These states may show no squeezing effect at all and therefore provide an interesting example to test nonclassicality criteria. The characteristic function of the Glauber-Sudarshan representation (

Markov chain Monte Carlo estimation of quantum states

P

Multiple-copy distillation and purification of phase-diffused squeezed states

function) proves to be a powerful tool to detect nonclassicality. Using this criterion we find that phase-diffused squeezed states are always nonclassical, even if the squeezing effect vanishes. Testing other criteria of nonclassicality based on higher-order squeezing and the positive semidefiniteness of special matrices of normally ordered moments, it is found that these criteria fail to reveal the nonclassicality for some of the prepared phase-diffused squeezed states.

Iterative Entanglement Distillation: Approaching the Elimination of Decoherence

We apply a Bayesian data analysis scheme known as the Markov Chain Monte Carlo (MCMC) to the tomographic reconstruction of quantum states. This method yields a vector, known as the Markov chain, which contains the full statistical information concerning all reconstruction parameters including their statistical correlations with no a priori assumptions as to the form of the distribution from which it has been obtained. From this vector can be derived, e. g. the marginal distributions and uncertainties of all model parameters and also of other quantities such as the purity of the reconstructed state. We demonstrate the utility of this scheme by reconstructing the Wigner function of phase-diffused squeezed states. These states posses non-Gaussian statistics and therefore represent a non-trivial case of tomographic reconstruction. We compare our results to those obtained through pure maximum-likelihood and Fisher information approaches.