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Acta Physica Sinica

Direct Bell state measurement for optical beams with correlated amplitude quadratures and anticorrelated phase quadratures

Dong Wang

Published Jan 1, 2010• 1 citation
Publisher

Abstract

A new method is produced of directly measuring EPR beams. This measurement scheme can be used for measuring the correlation degree of EPR beams with correlated amplitude quadratures and anticorrelated phase quadratures without using local oscillators. Then a new scheme for quantum dense coding is proposed based on this measurement scheme.

DOI
10.7498/aps.59.7596

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A new method is produced of directly measuring EPR beams. This measurement scheme can be used for measuring the correlation degree of EPR beams with correlated amplitude quadratures and anticorrelated phase quadratures without using local oscillators. Then a new scheme for quantum dense coding is proposed based on this measurement scheme.
In this work, we propose a new simple entanglement measurement scheme, which can be utilized to take a measurement for the bright EPR beams generated by a nondegenerate optical parametric amplifier. Owing to the output signal and idler modes being frequency degenerate and in phase, one need not take a measurement for the signal and idler modes, respectively. By virtue of a quarter-wave plate and a half-wave plate, and then inserting a polarizing beam splitter, one can realize the measurements of the correlated phase quadratures and anti-correlated amplitude quadratures, simultaneously. Therefore, phase-locking and local oscillators are avoided.
Highly efficient quantum dense coding for continuous variables has been experimentally accomplished by means of exploiting a bright Einstein-Podolsky-Rosen (EPR) beam with anticorrelation of amplitude quadratures and correlation of phase quadratures. Two bits of classical information are encoded on two quadratures of a half of a bright EPR beam, then are simultaneously decoded by the other half of the EPR beam with the sensitivities beyond that of the shot noise limit. A high degree of immunity to unauthorized eavesdropping has been experimentally demonstrated.
We propose a simple scheme using an unbalanced Mach-Zehnder interferometer to directly measure the entangled beams with correlation of amplitude quadratures and anticorrelation of phase quadratures. In the experiment, we use two fibers with lengths of 2 and 50 m to construct the interferometer. The correlation variances of amplitude difference and phase sum of the entangled beams from a nondegenerate optical parametric amplifier are simultaneously measured to be 1.79 and 1.62 dB below the shot noise limit, respectively. Such a simple and convenient method has potential applications in quantum measurements.
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.