Quantum enhanced optomechanics

Optomechanics is the study of how light interacts with mechanical systems via radiation pressure forces. As a photon carries momentum, it exerts a mechanical force while bouncing on an object. On the other way around, a moving object modifies the characteristics (phase, momentum, etc.) of the light reflected off it. The idea behind quantum optomechanics is to use this interaction to place a mechanical system into a quantum state such as a Shrödinger cat.

Background

Studies of micromechanical resonators have attracted much attention in recent years. In particular microtoroids, integrating high optical and mechanical quality factors, have proven to be a versatile platform for phase measurements when combined with ultra-sensitive optical interferometry. Both optical and electrostatic gradient force cooling techniques have been applied to such structures, in the endeavour to reach the quantum mechanical ground state of a macroscopic oscillator. However, a prerequisite for sensing the quantum ground state is the achievement of measurement sensitivities below the zero-point motion of the microresonator. Interferometric measurements of the optomechanically transduced phase shift are ultimately limited by the number of available photons. But since the optomechanical coupling is mediated by radiation pressure, one cannot simply increase the probe power since that would lead to an increased quantum back-action on the mechanical oscillator. For a given optical probe power, the best possible sensitivity is achieved when optical shot noise and quantum back-action noise contribute equally to the total measurement imprecision. This limit is known as the standard quantum limit.

 

Our research

Motivation

Revolutionary new quantum technologies are being developed worldwide with the aim of controlling the quantum behavior of massive mechanical oscillators. The ultimate aim is to cool the motion of a micron-sized mechanical oscillator (e.g. a cantilever) towards its quantum mechanical ground state and to prepare quantum superposition states where the oscillator appears to be at two positions at the same time. The key requirements for achieving quantum control in those systems are to be able to transduce the mechanical motion at the level of zero-point motion, i.e. measuring the displacement of a mechanical oscillator in its quantum ground state, and to be able to actuate the oscillator at the quantum level, i.e. mapping quantum states onto its mechanical degree of freedom. Several proposals have been made on the use of mechanical oscillators for quantum computation and communication technologies such as quantum transducers, memories or buffers. Applications are also found in ultra-sensitive measurement technologies and test of novel gravity theories.

 

Methods

Different systems have been studied to achieve quantum control of massive mechanical oscillators, e.g. nano-electromechanical systems (NEMS) where the mechanical oscillator is coupled to an electrical circuit, and cavity optomechanical systems (COMS) where it is coupled to a circulating light field inside a high quality optical cavity. In NEMS and COMS, both transduction and actuation is realized by electrical and optical means, respectively. Actuation is easily achieved by the use of electrical forces in NEMS whereas radiation pressure forces from an optical field are weak, thus it requires a very high finesse cavity and a high optical power to achieve efficient actuation in COMS. On the other hand, COMS enable transduction very close to the zero-point motion whereas transduction capabilities of NEMS are limited by technical constraints. In our lab, we combine the technologies of COMS and NEMS in order to achieve transduction via optical fields and actuation via electrical forces. The resulting system, called a cavity opto-eclectromechanical system (COEMS), consists of a micron-sized mechanical oscillator coupled to an optical cavity and control by electrical forces. Due to the motion of the cavity, the phase of the light field is modulated which is subsequently measured via homodyne detection to provide the transduction signal. By probing the mechanical motion of the oscillator with squeezed light, we expect to measure a transduction signal with much higher signal-to-noise ratio (SNR) which will allow us to perform measurements at the zero-point motion of the mechanical oscillator. Electrical actuation provides a mean to cool the mechanical oscillator towards its quantum ground state. Via feedback of the transduction signal with appropriate amplification and phase delay, we can generate electrical forces opposing the mechanical motion, and thus damp the mechanical oscillations. It has been shown theoretically that providing a higher SNR by using squeezed light will enhance the achievable cooling. Nevertheless, to achieve ground state cooling of the mechanical oscillator, we will implement additional cooling via cryogenic pre-cooling or laser cooling. Once ground state cooling is achieved, we will investigate the coherent coupling of optical and mechanical modes which enables the transfer of quantum states from the optical field onto the mechanical oscillator and vice versa.

Our team

  • Jan Bílek
  • Clemens Schäfermeier
  • Tobias Gehring
  • Ulrich Busk Hoff
  • (formerly) Hugo Kerdoncuff
  • Ulrik Lund Andersen 

Collaborations

This research project is carried out in collaboration with the group of Ass. Prof. Warwick Bowen at the University of Queensland, Brisbane, Australia. They share with us their knowledge on microcavities and electrical feedback scheme while we provide our expertise on squeezed light.

Funding

The project is supported by the Lundbeck Foundation.