Nature 480, 351–354 (2011)

Any device that amplifies a signal inevitably adds noise, and quantum mechanics prevents this added noise being reduced below a certain value. It is possible to approach this quantum limit by using superconducting devices to amplify electrical signals, but these devices are complex. Now Francesco Massel and co-workers at Aalto University and the VTT Technical Research Centre of Finland have shown that nanomechanical resonators can amplify microwave signals, and that it may be possible to reach the quantum limit with this approach.

The Finnish team start by using lithography and focused ion-beam etching to define a mechanical resonator and a microwave cavity in a 150-nm-thick layer of aluminium on a silica surface. When a pump signal is fed into this system, energy is transferred from the cavity to the resonator if the pump frequency is higher than the resonance frequency of the cavity, and vice versa. And if a weak probe signal is sent into the system when energy is being transferred to the resonator, this probe can also be amplified. Massel and co-workers show that approximately 20 noise quanta are added to the signal, and predict that it should be possible to reach the quantum limit of adding just half a quantum of noise.