High frequency squeezing and detection

Many of the experiments conducted in our group use non-classical (“squeezed”) light as a resource, or as the fundamental object under investigation. Mostly we use squeezed light at high frequencies, generated at higher free spectral ranges (FSR) of the optical resonator of the sub-threshold optical parametric oscillator. As the resonator length is on the order of meters we produce a so-called squeezing comb of light, which shows squeezing at each FSR of resonator. With our large bandwidth (~ 1 GHz) photodetectors we can then measure several “teeth” in the comb of squeezing.

Precision spectroscopy with squeezing-enhanced sensitivity at high frequencies 

For precision metrology the signal-to-noise ratio (SNR) of a measurement is a fundamental figure of merit. As an example, stabilisation of an optical resonator to a laser requires a clean, steep error signal with sufficient SNR. If the signal strength is small and fixed, the only way to increase the SNR is to reduce the noise floor. In many of our cases, however, the system under investigation is already quantum noise limited. Hence the only way to reduce the noise floor is to surpass the quantum limit, i.e. go below shot noise. We propose an enhanced-sensitivity phase measurement using non-classical light at high frequencies.

Schematic of the experimental setup (all frequency locks are achieved via homodyne locking):



  • “Squeezing comb” for measurement with reduced noise floor on each FSR
  • Intermodulation product of low-frequency shifts (cavity length changes) and EOM-induced sidebands at the FSR frequencies
  • Observe „up-shifted“ DC signals on squeezed noise floor without the need to eliminate spurious technical noise

We recently published a paper on the demonstration of a new pump-phase locking technique  for non-classical light sources, called weak pump depletion locking (WPD locking).  WPD locking can be used to generate an error signal suitable for locking the pump phase, without the need for modulation techniques, and without deterioration of the produced non-classical state.


This work was published in Optics Express (T. Denker, D. Schütte, M. H. Wimmer, T. A. Wheatley, E. H. Huntington, and M. Heurs, “Utilizing weak pump depletion to stabilize squeezed vacuum states“, Opt. Express 23, 16517-16528 (2015))

We have also designed and built a new high-frequency homodyne detector. This is a comparison of different configurations of the detector: