Proc. Natl Acad. Sci. USA 108, 16182–16187 (2011)

Despite recent progress in the field of optomechanics, probing the quantum state of mechanical resonators remains a significant challenge. Now, using short optical pulses, Michael Venner and colleagues from Austria, the UK, Germany and Australia have shown that they can fully reconstruct an oscillator's quantum states of mechanical motion. In their experimental implementation, a pulse of duration much less than the mechanical period of the oscillator, which is an optomechanical Fabry–Pérot cavity operating at 1,064 nm with a length of 4λ and an amplitude decay rate of 2.5 GHz, is incident upon the oscillator. The radiation–pressure interaction between the pulse and the cavity generates entanglement, resulting in a correlation between optical phase and mechanical position and thus transferring momentum from the optical pulse to the mechanical oscillator. Access to the system's quantum-mechanical states is obtained by observing the distribution of phase noise for strong light pulses at various times throughout a mechanical period. A time-domain homodyne detection scheme determines the phase — and thus the mechanical position — of the field emerging from the cavity. The researchers say that the scheme can also be applied to other systems, such as trapped ions and superconducting resonators.