Phonon counting and intensity interferometry of a nanomechanical resonator

A silicon nanometre-scale mechanical resonator, patterned to couple optical and mechanical resonances, is found to emit photons when optically pumped; photon emission corresponds directly to phonon emission, enabling the phonons to be counted. Individual phonons counted Nanoscale mechanical resonators offer high precision in various detection and sensing applications. So far it has not been possible to count individual phonons — the quanta of mechanical motion — in such systems, an advance that could open new applications in quantum information schemes. Oskar Painter and colleagues study a silicon nanobeam that is patterned in such a way that optical and mechanical resonances are coupled. When the device is optically pumped, emitted photons, corresponding directly to the number of phonons, are detected using a standard technique. The measurement reveals a transition, after a threshold of pumping power, to laser-like phonon emission — that is, self-sustained oscillations of the nanomechanical resonator. This experiment was performed at room temperature, but by extending to low temperature, quantum behaviour of mechanical systems can be tested in detail. In optics, the ability to measure individual quanta of light (photons) enables a great many applications, ranging from dynamic imaging within living organisms 1 to secure quantum communication 2 . Pioneering photon counting experiments, such as the intensity interferometry performed by Hanbury Brown and Twiss 3 to measure the angular width of visible stars, have played a critical role in our understanding of the full quantum nature of light 4 . As with matter at the atomic scale, the laws of quantum mechanics also govern the properties of macroscopic mechanical objects, providing fundamental quantum limits to the sensitivity of mechanical sensors and transducers. Current research in cavity optomechanics seeks to use light to explore the quantum properties of mechanical systems ranging in size from kilogram-mass mirrors to nanoscale membranes 5 , as well as to develop technologies for precision sensing 6 and quantum information processing 7 , 8 . Here we use an optical probe and single-photon detection to study the acoustic emission and absorption processes in a silicon nanomechanical resonator, and perform a measurement similar to that used by Hanbury Brown and Twiss to measure correlations in the emitted phonons as the resonator undergoes a parametric instability formally equivalent to that of a laser 9 . O