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 $161.827\phantom{\rule{0.2em}{0ex}}324\phantom{\rule{0.2em}{0ex}}0(5)\phantom{\rule{0.3em}{0ex}}\mathrm{MHz}$. The amplitude-difference squeezing is $\ensuremath{-}3\phantom{\rule{0.3em}{0ex}}\mathrm{dB}$ and the phase-sum one is $\ensuremath{-}1.35\phantom{\rule{0.3em}{0ex}}\mathrm{dB}$. 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.