Quantum leap forward in measuring mechanical motion

The precise measurement of position has a distinguished place in physics; about a 100 years ago, debate surrounding the atomic hypothesis was settled by the careful observation of Brownian motion. Today, researchers in the field of cavity quantum optomechanics hope to settle similar fundamental questions.

Cavity optomechanics, a field that has emerged over the past ten years, relies on applying radiation pressure – a force that is not perceptible at the macroscopic scale – to measure and control nano- and micromechanical oscillators, making it possible to achieve measurements of mechanical motion with unprecedented sensitivity.

The EU-funded QCDOM project's key contribution to this field has been to develop a chip-scale nano-optomechanical system that enables extremely precise measurements. The system consists of an optical micro resonator – a device that confines lights in microscale volumes for a long period of time on a chip – coupled to a small nano-mechanical oscillator – a mechanical vibrating string.

‘By efficiently coupling the nano-mechanical oscillator to the optical cavity field, we were able to achieve one of the most sensitive measurements of mechanical motion to date,’ explains project coordinator Prof Tobias Kippenberg from the Ecole Polytechnique Federale de Lausanne (EPFL) in Switzerland. This degree of sensitivity is sufficient to resolve the equivalent of the quantum mechanical vacuum fluctuations of a mechanical oscillator within its thermal decoherence time; that is, the time it takes for its quantum state to be destroyed by the environment. In doing so, the researchers were able to catch a glimpse of the quantum effects arising due to the act of measurement.

Kippenberg and his team were able to achieve this advance by bringing into the team a quantum optics expert, Dr. Dal Wilson, from the prestigious California Institute of Technology (Caltech), Pasadena US, with an EU Marie Skłodowska-Curie International Incoming Fellowship grant. The two year project was completed at the end of November 2015, and the results published in the leading science journal Nature.

The ability to achieve such a precise readout of mechanical motion is interesting as it provides evidence of limits that quantum mechanics enforces on making precise position measurements. The position and momentum of a mechanical oscillator cannot be known with arbitrary precision. This leads to a very fundamental measurement ‘quantum’ backaction, which is appreciable in the reported experiments.

Practical applications

While fundamental in nature, the project may also have implications for actual technology. Indeed measurements of precise motion are already used in commercial micro/nano-electromechanical systems (MEMS) sensors to measure acceleration or rotation. Cell phones use piezo-mechanical resonators to filter radio frequency signals, and mechanical quartz oscillators are used in timekeeping and navigation, and can be found in automotive and aeroplane controls.

The nano-optomechanical sensors developed through the QCDOM project also offer other potential functions, such as accurately measuring temperature and for amplifying weak radio frequency signals.

‘It is noteworthy that in just ten years an entirely new paradigm for how to actuate, read out, and control micro- and nano-mechanical oscillators has emerged,’ says Kippenberg. ‘This has already led to remarkable progress in experimental quantum physics and scientific progress has been staggering. Experimental studies like ours indicate the significant technological potential that such opto- and electromechanical systems could unlock.’

With the ability to measure something as fundamental as the position of a macroscopic object with ever more precision, it is likely that the unusual predictions of quantum theory will be witnessed and verified at unprecedented size scales.

Further information is available on the project coordinator lab website.