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Quantum Optomechanics using Monolithic Micro-Resonators

Final Report Summary - QUOM (Quantum optomechanics using monolithic micro-resonators)

The work performed during the project has successfully achieved most of the initial goals. Furthermore interesting novel aspects appeared during this period, opening interesting future prospects.

The cryogenic apparatus has been developed successfully during this period, allowing coupling light to the micro-cavities at cryogenic temperatures, with the same efficiency as demonstrated at room temperature. A very compact setup was designed, weekly sensitive to parasitic vibrations. The ultra-high optical quality factors of the microcavities could be preserved at low temperatures and an optical technique was developed to preserve them in the cryogenic environment.

The mechanical properties and quality factors of the microstructures was studied first down to 1.4 K, by monitoring the Brownian motion of the micro-oscillator with a new technique allowing to measure its optomechanical properties at very low light intensities. This revealed the coupling between phonons and structural defects states of the amorphous material, inducing non trivial temperature dependence of the intrinsic material limitation on its mechanical damping.

Importantly at this occasion we did realise that this coupling opens a bright new horizon for the amorphous optomechanical systems since it is possible to enter a regime where the dominant contribution to the mechanical damping is dominated by phonon resonant coupling to TLS states presenting an energy splitting close to the mechanical quantum of energy. A first observation of this regime was performed.

An ultra sensitive detection scheme has been developed, based on a homodyne detection of the phase of the optical beam transmitted by the fibre, on which is encoded the mechanical vibrations of the resonator. Sensitivities limited by the quantum nature of the light meter were achieved, at the level of ca. 10-18 m/Hz1/2, already beyond the SQL with a unprecedently achieved sensor ideality.

In addition, a different approach has been developed in parallel to extend the ultrahigh displacement sensitivities down to the nanoscale. This was allowed by inserting the nanoresonator under investigation in the evanescent fields of a microcavity optical mode, rendering its optical resonances sensitive to the nano-resonator motion. Sensitivities at the SQL were reported, for sub wavelength size, 1-picogram oscillators.

The combination of optical and cryogenic cooling in the Helium 4 cryostat (1.7 K) allowed to reduce the mean phonon number down to 60 phonons, while starting at 80 000 at room temperature. The system developed was found to be the position sensor presenting the closest approach to the Heisenberg uncertainty principle to date.

The experimental efforts were subsequently focussed on developing a Helium 3 experiment in order to achieve a lower initial cryogenic temperature (600 mK), that brings the double benefit of increasing the initial mechanical quality factor by a factor of 10, and lowering the initial occupation by a factor of three so that lower mean phonon occupancy could be achieved.

At the end of the project, occupancies bellow 30 could be reproducibly achieved, while the last remaining phonon were induced by direct light scattering from the optical fibre region, A problem that can be solved by working in an environment with a reduced dust concentration.

The Helium 3 experiment allowed the observation of a new effect: the optomechanically induced transparency where the transmission of the microcavity depends on the dynamical behaviour of the oscillator and can be governed by an external laser field. The effect has been observed and characterised.

Furthermore, the helium 3 experiment allowed to enter for the first time in a regime where the mechanical damping of a macroscopic mechanical oscillator is governed by the resonant interaction with structural defect states. The mechanical damping of a high frequency mode, thermally populated with less than 100 phonons at 700 mK, was shown to depend on the number of phonons injected within the toroid, exhibiting saturation due to the finite number of structural defect states the phonon mode is coupled to. This saturation of the phonon absorption occurred at extremely small driving amplitudes, ca 700 am, an amplitude that could only be detected thanks to the ultrahigh sensitivity setup developed.

As a conclusion the progress realised in the domain of optomechanical systems at low phonon number renders possible a new kind of experiments where a macroscopic mechanical oscillator can be actively cooled down to its ground state. The observation of the quantum radiation pressure noise, or measurements beyond the standard quantum limit should in principle be possible. Other quantum signatures like the observed quantum friction, can now be realistically envisioned in the next coming years.

In parallel with the optomechanical project, the project coordinator was involved in the supervision of the frequency comb generation project. Control was gained over the microcavity repetition rate, allowing telecom compatible combs, with a 80 GHz repetition rate. A new technique has been developed to characterise the microcavity's dispersion, whose understanding represents a keypoint for generating an octave spanning comb.