We observed continuous-variable entanglement between the bright beams emitted above threshold by an ultrastable optical parametric oscillator, classically phase-locked at a frequency difference of 161.8273240(5) MHz. The amplitude-difference squeezing is -3 dB and the phase-sum one is -1.35 dB. Besides proving entanglement in a new physical system, the phase-locked OPO, such unprecedented frequency-difference stability paves the way for transferring entanglement between different optical frequencies and densely implementing continuous-variable quantum information in the frequency domain.
@article{arxiv.quant-ph/0604134,
title = {Experimental continuous-variable entanglement from a phase-difference-locked optical parametric oscillator},
author = {Jietai Jing and Sheng Feng and Russell Bloomer and Olivier Pfister},
journal= {arXiv preprint arXiv:quant-ph/0604134},
year = {2009}
}