Quantum nanomechanics is an exciting new field enabled by recent advances in nanofabrication. However, a major challenge is finding transducers for displacing nanomechanical resonators.

Standard optical methods do not work because the devices are too small and optical coupling is very inefficient. Researchers have tried to use single electrons – for example, the single electron transistor – but such devices are intrinsically dissipative and therefore too noisy.

"In collaboration with Keith Schwab at Cornell University, we are focusing on importing the most recent innovations in circuit quantum electrodynamics to develop microwave transducers for nanomechanical motion," Gerard Milburn of the University of Queensland, told nanotechweb.org. The research team also includes scientists from the University of Camerino in Italy.

Building quantum transducers

Milburn explained that the first step in building a quantum transducer is to show how the motion of the device (in this case a superconducting microwave coplanar cavity) can itself become entangled. This is feasible because the mechanical resonant frequencies of nanomechanical resonators approach gigahertz frequencies, he says. This allows resonant interactions with electromagnetic wave fields.

"In our work, however, we use a much lower nanomechanical frequency to entangle the microwave field and the nanomechanical resonator," added Milburn. "The basic idea is that the resonator forms one plate of a capacitor, which couples the microwave cavity to the transmission line driving it from the outside world. As the resonator vibrates, it modulates the capacitive coupling, thus phase modulating the cavity resonance frequency."

The device has already been made in Schwab's group at Cornell.

Milburn says the main application for the entanglement will be to make a quantum limited transducer for nanomechanical motion. "Such devices will also enable ultrasensitive mass detection and very weak force measurements," he explained. The device could work on a non-quantum level too and be used as a nonlinear element in superconducting microwave circuits, as a substitute for Josephson junctions.

Entangling light

The researchers are now building a new quantum nanomechanics lab at the University of Queensland, which should open next June. "On the theory front, we have developed a way to make a bifurcation amplifier for nanomechanical motion using the nonlinearity of such devices," said Milburn. "We are also developing schemes for transducers based on nanooptics, such as plasmonic cavities and single quantum dots."

At their end, the Camerino researchers are focusing on light rather than microwaves. "We are investigating the limits down to which one can still efficiently couple light with a micromechanical resonator," explained lead author David Vitali. "If it could be used, light might be more robust than microwaves with respect to thermal noise," he said.

The work was reported in Physical Review A.