  Squeezed states of light reduce the signal-normalized photon counting noise
of measurements without increasing the light power and enable fundamental
research on quantum entanglement in hybrid systems of light and matter.
Furthermore, the completion of squeezed states with cryo-cooling has high
potential. First, measurement sensitivities are usually limited by quantum
noise and thermal noise. Second, squeezed states allow for reducing the heat
load on cooled devices without losing measurement precision. Here, we
demonstrate squeezed-light position sensing of a cryo-cooled micro-mechanical
membrane. The sensing precision is improved by up to 4.8 dB below photon
counting noise, limited by optical loss in two Faraday rotators, at a membrane
temperature of about 20K, limited by our cryo-cooler. We prove that realising a
high interference contrast in a cryogenic Michelson interferometer is feasible.
Our setup is the first conceptual demonstration towards the envisioned European
gravitational-wave detector, the 'Einstein Telescope', which is planned to use
squeezed states of light together with cryo-cooling of its mirror test masses.
