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Acta Sinica Quantum Optica

Experimental Testing of Time-domain Quantum Correlation of EPR Entangled Beams

Kunchi Peng

Published Jan 1, 2009• 0 citations
OpenAlex

Abstract

We measured the quantum fluctuations of the quadratures of signal and idler beams of EPR entangled beams in time domain directly by using the homodyne detection system and data acquisition system,and demonstrated intuitively the quantum correlations between two optical beams.By delaying one of the EPR entangled beams,we verify that if the measurements on the signal and the idler beams are not synchronal,the correlations between them will decrease and finally vanish.The presented experimental method can be utilized in developing quantum key distribution with continuous variables,experimental preparation of non-Gaussian state and entanglement purification.

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We measured the quantum fluctuations of the quadratures of signal and idler beams of EPR entangled beams in time domain directly by using the homodyne detection system and data acquisition system,and demonstrated intuitively the quantum correlations between two optical beams.By delaying one of the EPR entangled beams,we verify that if the measurements on the signal and the idler beams are not synchronal,the correlations between them will decrease and finally vanish.The presented experimental method can be utilized in developing quantum key distribution with continuous variables,experimental preparation of non-Gaussian state and entanglement purification.
For measurements describable by a discrete set of possibilities, numerous demonstrations of the violation of Bell's inequalities have been reported in optical physics based on observations of polarization correlations for photon pairs. By contrast for observables with a continuous spectrum, there appears to be no general inequalities corresponding to those of Bell and no experimental realization of the original Einstein-Podolsky-Rosen paradox. Following Reid and Drummond,1 we are attempting to demonstrate the EPR paradox in the context of the measurement of continuous quantum variables with a subthreshold optical parametric oscillator operated in a frequency degenerate but polarization nondegenerate mode. In this case, the EPR paradox is formulated in terms of two sets of quadrature phase amplitudes for the spatially separated signal and idler fields. As the parametric gain is increased toward threshold, the quadrature amplitudes of the signal and idler beams become increasingly faithful quantum copies of one another. Our measurements of quadrature amplitudes are carried out using independent balanced homodyne detectors at two sites. When operated for the detection of squeezing at one site, noise reductions of 4 dB relative to the vacuum state limit are observed, which is most encouraging since noise reductions >3 dB in the difference photocurrent from the two sites are sufficient for inference at a distance of a signal observable from a measurement of the corresponding idler variable.
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.
We experimentally demonstrate the creation and characterization of Einstein-Podolsky-Rosen (EPR) correlation between optical beams in a time-gated fashion. The correlated beams are created with two independent continuous-wave optical parametric oscillators and a half beam splitter. We define the temporal modes using a square temporal filter with duration TTT and make time-resolved measurements on the generated state. We observe correlations between the relevant conjugate variables in the temporal mode which correspond to EPR correlation. Our scheme is extendable to continuous-variable quantum teleportation of a non-Gaussian state defined in the time domain such as a superposition of coherent states.
Abstract Heisenberg’s original uncertainty relation is related to measurement effect, which is different from the preparation uncertainty relation. However, it has been shown that Heisenberg’s error disturbance uncertainty relation is not valid in some cases. We experimentally test the error-tradeoff uncertainty relation by using a continuous-variable Gaussian Einstein–Podolsky–Rosen (EPR)-entangled state. Based on the quantum correlation between the two entangled optical beams, the errors on amplitude and phase quadratures of one EPR optical beam coming from joint measurement are estimated, respectively, which are used to verify the error–tradeoff relation. Especially, the error–tradeoff relation for error-free measurement of one observable is verified in our experiment. We also verify the error–tradeoff relations for nonzero errors and mixed state by introducing loss on one EPR beam. Our experimental results demonstrate that Heisenberg’s error–tradeoff relation is violated in some cases for a continuous-variable system, while the Ozawa’s and Branciard’s relations are valid.
Quantum cryptography with the predetermined key was experimentally realized using Einstein-Podolsky-Rosen(EPR) correlations of continuously bright optical beams. Only one of two EPR correlated beams is transmitted with the signals modulated on quadrature phases and amplitudes, and the other one is retained by the authorized receiver. The modulated small signals are totally submerged in the large quantum noise of the signal beam, therefore nobody except the authorized receiver can decode the signals. Usability of imperfect quantum correlation, high transmission and detection efficiencies, and security provided by quantum mechanics are the favorable features of the presented scheme.
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.