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Acta Physica Sinica

Experimental generation of two EPR entangled states with classical coherence

Xiaojun Jia, SU Xiao-long, Qing Pan, Changde Xie, Kunchi Peng, 量子光学与光量子器件国家重点实验室，山西大学光电研究所，太原 030006

Published Jan 1, 2005• 1 citation
Publisher

Abstract

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.

DOI
10.7498/aps.54.2717

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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 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.
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.
Quantum entanglement shared by more than two partites is the essential bases for developing quantum communication networks and quantum computation. The multipartite entanglement of continuous variables can be generated by distributing or combining squeezed states of light using linear optics. We experimentally obtained a tripartite entangled state of bright optical field by means of a nondegenerate optical parametric amplifier (NOPA) and optical beam-splitters. The NOPA is operated in the state of parametric deamplification by locking the relative phase between the pump laser and the injected seed wave to (2n+1)π, so the generated Einstein-Podolsky-Rosen (EPR) beam for continuous variables is an entangled state of light with the quantum correlation of phase quadratures and the quantum anticorrelation of amplitude quadratures [1]. The output EPR beam consists of two frequency-degenerate optical modes with horizontal and vertical polarizations which are distributed to three parties with two linear beam-splitters. We have theoretically and experimentally demonstrated the three parties are in a tripartite entangled state with three-mode “position” (amplitude quadrature) correlation and relative “momentum” (phase quadrature) correlation [2]-[4]. The obtained GHZ-like state is a fully inseparable tripartite continuous-variables state violating the three inequalities used for the criterion of the inseparability [5][6]. The controlled dense coding quantum communication is experimentally accomplished by exploiting the tripartite entanglement. The three entangled modes are sent to a sender (Alice), a receiver (Bob) and a controller (Claire), respectively. The information transmission capacity of the quantum channel between Alice and Bob is controlled by Claire. The channel capacity completed under Claire’s help is always larger than that without her help.
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.
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.
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.