Experimental continuous-variable entanglement from a phase-difference-locked optical parametric oscillator

We observed continuous-variable entanglement between the bright beams emitted above threshold by an ultrastable optical parametric oscillator (OPO), classically phase locked at a frequency difference of 161.827 324 0(5) MHz. The amplitude-difference squeezing is -3 dB and the phase-sum one is -1.35 dB. Besides proving entanglement in a phase-locked OPO, such outstanding frequency-difference stability paves the way for transferring entanglement between different optical frequencies and densely implementing continuous-variable quantum information in the frequency domain.