  The cross-correlation sensitivity of two identical balanced photodiode
heterodyne receivers is characterized. Both balanced photodiodes receive the
same weak signal split up equally, a situation equivalent to an astronomical
spatial interferometer. A common local oscillator (LO) is also split up equally
and its phase difference between both receivers is stabilized. We show by
semi-classical photon deletion theory that the post-detection laser shot noise
contributions on both receivers must be completely uncorrelated in this case of
passing three power splitters. We measured the auto- and cross-correlation
outputs as a function of weak signal power (system noise temperature
measurement), and obtain a cross-correlation system noise temperature up to 20
times lower than for the auto-correlation system noise temperature of each
receiver separately. This is supported by Allan plot measurements showing
cross-correlation standard deviations 30 times lower than in auto-correlation.
Careful calibration of the source power shows that the auto-correlation
(regular) noise temperature of the single balanced receivers is already very
near to the quantum limit as expected, which suggests a cross-correlation
system noise temperature below the quantum limit. If validated further, this
experimentally clear finding will not only be relevant for astronomical
instrumentation but also for other fields like telecommunications and medical
imaging.
