YQI Colloquium - Tobias Kippenberg - École Polytechnique Fédérale de Lausanne - Measurement and control of a nanomechanical oscillator at the thermal decoherence rate

In real-time quantum feedback protocols, the record of a continuous measurement is used to stabilize a desired quantum state. Recent years have seen spectacular advances in a variety of well-isolated micro-systems, including microwave photons and superconducting qubits. By contrast, the ability to stabilize the quantum state of a tangibly massive object, such as a nano-mechanical oscillator, remains a difficult challenge. The main obstacle is environmental decoherence, which places stringent requirements on the timescale in which the state must be measured. Using cavity optomechanical coupling we report on a position sensor that is capable of resolving the zero-point motion of a solid-state, 4.3 MHz frequency nanomechanical oscillator in the timescale of its thermal decoherence, a basic requirement for preparing its ground-state using feedback as well as (Markovian) quantum feedback. The sensor is based on evanescent coupling to a high-Q optical microcavity, and achieves an imprecision 40 dB below that at the standard quantum limit for a weak continuous position measurement, a 100-fold improvement over previous reports, while maintaining an imprecision-back-action product within a factor of 5 of the Heisenberg uncertainty limit. As a demonstration of its utility, we use the measurement as an error signal with which to feedback cool the oscillator. Using radiation pressure as an actuator, the oscillator is cold-damped with unprecedented efficiency: from a cryogenic bath temperature of 4.4 K to an effective value of 1.1 mK, corresponding to a mean phonon number of 5 (i.e., a ground state probability of 16%). The measurement reveals strong backaction-imprecision correlations, which we observe as quantum mechanical sideband asymmetries, as well as pondermotivesqueezing of the light field. Our results set a new benchmark for the performance of a linear position sensor, and signal the emergence of mechanical oscillators as practical subjects for measurement-based quantum control.