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Unconditional entanglement swapping of continuous variables——quantum teleportation of entangled state

X.-Y. Chang

Published Jan 1, 2005• 1 citation
OpenAlex

Abstract

A pair of entangled states of light with classical coherence and independent quantum fluctuations has been produced from two home-designed and home-made non-degenerate optical parametric amplifiers. Utilizing these entangled states we have experimentally demonstrated unconditional entanglement swapping for continuous variables, i.e. quantum teleportation of an entangled state. Based on the direct detection of Bell states and entanglement collapse, two optical beams which have never directly interacted with each other are entangled together. The degrees of quantum correlation resulting from the entanglement swapping have been determined by direct measurement to be 1.23dB and 1.12dB below the shot noise limit for the amplitude and phase quadratures, respectively. The theoretical calculation and experimental measurements are in reasonable agreement.

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A pair of entangled states of light with classical coherence and independent quantum fluctuations has been produced from two home-designed and home-made non-degenerate optical parametric amplifiers. Utilizing these entangled states we have experimentally demonstrated unconditional entanglement swapping for continuous variables, i.e. quantum teleportation of an entangled state. Based on the direct detection of Bell states and entanglement collapse, two optical beams which have never directly interacted with each other are entangled together. The degrees of quantum correlation resulting from the entanglement swapping have been determined by direct measurement to be 1.23dB and 1.12dB below the shot noise limit for the amplitude and phase quadratures, respectively. The theoretical calculation and experimental measurements are in reasonable agreement.
The unconditional entanglement swapping for continuous variables is experimentally demonstrated. Two initial entangled states are produced from two nondegenerate optical parametric amplifiers operating at de-amplification. Through implementing the direct measurement of the Bell-state between two optical beams from each amplifier the remaining two optical beams, which have never directly interacted with each other, are entangled. The quantum correlation degrees of 1.23 and 1.12 dB below the shot noise limit for the amplitude and phase quadratures resulting from the entanglement swapping are measured straightly.
Quantum entanglement swapping, i.e. the teleportation of entanglement state, i s a basic method used for developing quantum communication and quantum informati on network. It is necessary to produce two independent entangled states for impl ementing the quantum entanglement swapping. To experimentally demonstrate the en tanglement swapping of continuous variables, we should firstly generate two elec tron paramagnetic resonance(EPR) entangled states with classical coherence and w ithout any correlation of quantum fluctuation. Using a home_made continuous Nd3+YAP/KTP laser to pump two nondegenarate optical parametric amplifiers with the same configuration, we obtained a pair of independent EPR entangled states with classical coherence. The experimental technology of generating two sets of EPR beams is introduced and the influence of mode match on the detection of quan tum correlation is discussed.
A scheme achieving quantum entanglement swapping between two pairs of bright EPR optical beams is theoretically proposed. Two pairs of coherent EPR beams are produced from two independent nondegenerate optical parametric amplifiers. The entanglement swapping is demonstrated by means of a half of each EPR beam. The measuring process entangles other two halves of EPR beams that have never directly interacted with each other. In the proposed system, the direct detection of Bell-states is applied instead of usual homodyne detetion. Due to that the local oscillator is not needed, the technic trouble for mode-matching is overcome and the efficiency of the detection can be increased in experiments.
The experimental demonstration of quantum teleportation for continuous variables using the squeezed_state entanglement generated by a type II phase matching opt ical parametric amplifier(OPA) is reported. Type II OPA operating at deamplification can generate amplitude_anticorrelated and phase_correlated entanglements, which can be used in quantum teleportation scheme by direct Bell_ state measurement. The correlation of 2dB was obtained with a pair of KTP crystals in the cavity. The inferred teleportation fidelity is 060,which when correlated for the e fficiency of detection by the output observer, gives a fidelity of 057 The i mperfect factors in detection are also discussed.
Results of experiments recently performed are reported, in which two optical parametric amplifiers were set up to generate two independently quadrature squeezed continuous wave laser beams. The transformation of quadrature squeezed states into polarization squeezed states and into states with spatial quantum correlations is demonstrated. By utilizing two squeezed laser beams, a polarization squeezed state exhibiting three simultaneously squeezed Stokes operator variances was generated. Continuous variable polarization entanglement was generated and the Einstein-Podolsky-Rosen paradox was observed. A pair of Stokes operators satisfied both the inseparability criterion and the conditional variance criterion. Values of 0.49 and 0.77, respectively, were observed, with entanglement requiring values below unity. The inseparability measure of the observed quadrature entanglement was 0.44. This value is sufficient for a demonstration of quantum teleportation, which is the next experimental goal of the authors.