On the possibility of breaking the heterodyne detection quantum noise limit with cross-correlation

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.