High-precision interferometric vibration measurement with squeezed light

In a recent paper, we experimentally realised a squeezed-light enhanced heterodyne Laser Doppler Vibrometer (LDV) for vibration measurements and demonstrated a phase-modulated signal with a carrier-to-noise ratio exceeding the shot noise limit. For this, we characterised the non-classical sensitivity enhancement with a spectrum analyser. However, to obtain the actual displacement values, the signal must be demodulated and digitised without adding significant noise. Here we show that clipping in combination with zero-crossing synchronous sampling can avoid quantisation noise during digital demodulation. For a signal with a frequency of 2 MHz, we improved the shot-noise limited resolution of a 50 mu W measurement beam from 10 fm/root Hz to 8 fm/root Hz. Our results show for the first time the effect of squeezed light enhancement in a demodulated signal of a heterodyne LDV.