Abstract
Balanced homodyne detection relies on a beam splitter to superpose the weak signal input and strong local oscillator. However, recent investigation shows that a high gain phase sensitive amplifier (PSA) can be viewed as homodyne detector, in which the strong pump of PSA serves as the local oscillator [1]. Here, we analyze a new method of measuring the continuous variable entanglement by assisting a balanced homodyne detector with the PSA and implement it experimentally. Before measuring quadrature amplitude with the balanced homodyne detectors, two entangled fields generated from a pulse pumped fiber optical parametric amplifier are simultaneously coupled into the PSA. We find that the normalized noise for both the difference and sum of the quadrature amplitudes of the two entangled fields fall below the shot noise limit by about 4.6 dB, which is the record degree of entanglement measured in optical fiber systems. The experimental results illustrate that the advantages of the new measurement method include but not limit to tolerance to detection loss and characterizing entanglement with only one homodyne detector. The influence of mode-mismatching due to multi-mode property of entanglement on the measured noise reduction can also be greatly mitigated, indicating the new method is advantageous over the traditional measurement in multi-mode case.
Highlights
Quantum entanglement of continuous variables (CV) is an essential resource for quantum information science and quantum metrology
Quantum teleportation, quantum dense coding, quantum image, quantum logic gates, and quantum enhanced precision measurement have been experimentally realized [2,3,4,5,6,7]. Among these proof-of-principle experimental demonstrations, joint measurement realized by two sets of balanced homodyne detection (HD) is widely used for measuring the CV entanglements
The quantum fields measured by balanced HD, which relies on a 50/50 beam splitter to superpose the weak signal and the strong local oscillator (LO), is prone to losses, such as propagation loss, less-than-unit quantum efficiency of detectors and imperfect mode matching efficiency in HD [2,3,4,5,8]
Summary
Quantum entanglement of continuous variables (CV) is an essential resource for quantum information science and quantum metrology. We find that both the noise variances for the difference and sum of the quadrature amplitudes of the signal and idler twin beams, ∆2(X 1 − X 2) and ∆2(Y1 + Y2) , fall below the shot noise limit (SNL) by about 4.6 dB, corresponding to the inseparability I = 0.68 (lower than the classical limit of I = 2), which is better than previous results for entanglement generated from optical fibers [13,24] This is because the influences of detection loss and mode-mismatching due to multi-mode property on the measured degree of entanglement are greatly mitigated
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