Ground-state cooling of mechanical oscillators in optomechanics

Short answer: Preparing a mechanical resonator in its quantum ground state is a prerequisite for quantum optomechanics, according to a 2023 review. A 2019 Physical Review Letters paper reports feedback cooling of a room-temperature mechanical oscillator to around 1 mK, near its motional ground state, and a 2021 Communications Physics paper proposes a scheme for cooling multiple resonators at once. The first is an experiment, the second a theory proposal.

Why is the ground state a prerequisite?

To enter the regime of quantum optomechanics, it is necessary to prepare a mechanical object in its ground state. This prerequisite enables the manipulation of the quantum state of a macroscopic object and the conversion of frequency in different domains for quantum information processing. The review says a macroscopic mechanical resonator is usually surrounded by a complex thermal bath, which makes it hard to stay in its quantum ground state, and that the radiation pressure between the optical field and the mechanical resonator is generally weak, insufficient to bring it to the ground state, which cavity optomechanics addresses by enhancing the light-matter interaction with high-finesse cavities.

How does intrinsic optomechanical cooling function?

Intrinsic optomechanical cooling relies on the interaction between the optical field and the mechanical resonator. The 2021 paper says that in standard laser cooling schemes coherence renders the motion of a resonator to its ground state, with resolved sideband cooling as an example, and that in a standard approach thermal energy is removed from a single mode of the resonator. The 2021 authors explain the mechanism through a direct connection between laser sideband cooling and "cooling by heating". They propose a scheme for simultaneous cooling of multiple degenerate or near-degenerate mechanical resonators. It uses an incoherent thermal source, enabled by quantum reservoir engineering, and could be realized in setups employing parametric coupling of a cooling agent with the target systems.

What role does active feedback play?

The review lists active feedback cooling and intrinsic optomechanical cooling as the two experimental routes. A 2019 paper by Guo, Norte and Gröblacher reports feedback cooling of a room temperature mechanical oscillator close to its motional ground state. This experiment combined integrated nanophotonics with phononic band gap engineering to overcome limitations in isolation from the surrounding environment and achievable mechanical frequencies. They report a single-photon cooperativity of 200, which lets them feedback cool of a mechanical resonator to around 1 mK, near its motional ground state, directly from room temperature. The authors note that laser cooling combined with cryogenic precooling had succeeded for various devices, while a similar demonstration starting from room temperature remained an outstanding goal.

What are the theoretical extensions for future cooling?

The 2023 review also discusses novel cooling approaches that may serve as guidance for future experiment design. These include cooling beyond the resolved-sideband regime and multimode cooling. The 2021 authors say ground-state cooling of multiple mechanical resonators becomes vital for applications from ultra-precise sensing to quantum information processing, and is otherwise a challenging goal. They also discuss using non-thermal baths to simulate ultra-high temperature thermal baths for cooling.

FAQ

Is ground-state cooling required for quantum information processing?

The 2021 paper says that for quantum information processing applications of optomechanical systems, mechanical resonators need to be close to their quantum ground state. This preparation is a prerequisite for manipulating the quantum state of macroscopic objects and converting frequency in different domains for quantum network and computing tasks.

Can feedback cooling operate from room temperature?

The 2019 authors report feedback cooling of a mechanical resonator to around 1 mK, near its motional ground state, starting from a room temperature environment. This was achieved by combining integrated nanophotonics with phononic band gap engineering to enhance isolation and mechanical frequency capabilities.

What is the role of phononic band gap engineering?

Phononic band gap engineering helps overcome prior limitations in the isolation of the mechanical resonator from the surrounding environment. It also addresses limitations in achievable mechanical frequencies and limited optomechanical coupling strength, enabling higher cooperativity necessary for effective feedback cooling from ambient conditions.

Are there proposals for cooling multiple resonators?

The 2021 paper proposes a scheme for the simultaneous cooling of multiple degenerate or near-degenerate mechanical resonators to their quantum ground state. These schemes utilize quantum reservoir engineering and parametric coupling, offering an alternative to standard laser cooling schemes that typically target single modes.

Is the radiation pressure force sufficient for cooling?

Generally, no. The radiation pressure between the optical field and a mechanical resonator is weak and insufficient to bring a mechanical resonator to its quantum ground state without enhancement. Cavity optomechanics achieves prominent enhancement of light-matter interaction by fabricating high-finesse cavities to facilitate the cooling process.