events in steps of 12.5 ns (the time between adjacent oscillator
pulses) and counting coincident detections. Results are shown in
Fig. 2d,e for blue- and red-shifted cases, respectively. We quantify
this using the two-mode intensity cross-correlation function
between output signal and herald fields given by
g
ð Þ2
s;h ¼ Ps;h=ð Þ PsPh . Here, Ps(Ph) is the probability of detecting
a photon in the signal (herald) mode, and Ps,h is the
probability of measuring a joint detection. A measurement of
g
ð2Þ
s;h 42 indicates non-classical correlation36,37 (see Methods),
whereas uncorrelated photon detections, for example, from noise,
give g
ð2Þ
s;h ¼1. We calculate the values of g
ð2Þ
s;h at the peak of the blueand
red-shifted spectra to be 2.7±0.2 and 3.4±0.3, respectively