Cryogenic and hermetically sealed packaging of photonic chips for optomechanics.

We demonstrate a hermetically sealed packaging system for integrated photonic devices at cryogenic temperatures with plug-and-play functionality. This approach provides the ability to encapsulate a controlled amount of gas into the optical package allowing helium to be used as a heat-exchange gas to thermalize photonic devices, or condensed into a superfluid covering the device. This packaging system was tested using a silicon-on-insulator slot waveguide resonator which fills with superfluid 4He below the transition temperature. To optimize the fiber-to-chip optical integration 690 tests were performed by thermally cycling optical fibers bonded to various common photonic chip substrates (silicon, silicon oxide and HSQ) with a range of glues (NOA 61, NOA 68, NOA 88, NOA 86H and superglue). This showed that NOA 86H (a UV curing optical adhesive with a latent heat catalyst) provided the best performance under cryogenic conditions for all the substrates tested. The technique is relevant to superfluid optomechanics experiments, as well as quantum photonics and quantum optomechanics applications.

Squeezed-Light Interferometry on a Cryogenically Cooled Micromechanical Membrane.

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. Squeezed states of light have high potential to complement cryogenically cooled sensors, whose thermal noise is suppressed below the quantum noise of light by operation at low temperature. They allow us to reduce the optical heat load on cooled devices by lowering the light power without losing measurement precision. Here, we demonstrate the squeezed-light position sensing of a cryo-cooled micromechanical membrane. The sensing precision is improved by up to 4.8 dB below photon counting noise, limited by optical loss, at a membrane temperature of about 20 K. We prove that realizing 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-cooled mirror test masses.