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 $161.8273240\left(5\right)\mathrm{MHz}$. The amplitude-difference squeezing is $-3\mathrm{dB}$ and the phase-sum one is $-1.35\mathrm{dB}$. 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.