"Such sensors could operate with the best sensitivity allowed by quantum mechanics," team leader Konrad Lehnert of the National Institute of Standards and Technology (NIST) and the University of Colorado, Boulder, told nanotechweb.org.

As micro and nanoscale mechanical resonators become ever smaller, they are fast approaching the size where a quantum description of these devices is needed. Such a description is not without certain challenges: we need to be able to "freeze out" thermomechanical motion so that only zero-point quantum fluctuations remain; and we need to make a "Heisenberg-limited" displacement detector.

Reaching the quantum limit
Now, Lehnert and co-workers have developed a detector that is, in principle, quantum limited. Moreover, it can efficiently couple to the motion of nanoscale objects with tiny masses that have zero-point motion.

The technique works by measuring the displacement of a nanomechanical beam using a superconducting transmission-line microwave cavity. At its heart, this cavity is an electrical circuit with a sharp resonance frequency. "We arrange for the motion of a nanomechanical beam to change the capacitance in that circuit, and therefore the resonance frequency," explained Lehnert. "By monitoring the circuit's resonance frequency, we can thus infer the beam's motion."

The team succeeded in achieving an excellent mechanical force sensitivity for the beam of 3 aNHz-1/2, detected thermal motion at ultralow temperatures of millikelvins and achieved a measurement imprecision of 30 times the standard quantum limit.

"We hope that our technique will yield a new class of ultrasensitive force detector well suited for studying nanoscale objects," added Lehnert. "It is a compact, electrical method that is naturally multiplexible – that is, multiple beams can be measured at the same time with no more microwave electronics than it requires to measure one beam."

The researchers are now working on extending their research in two ways: first, they will also cool the beam with microwave "light", much as atomic physicists cool atoms or ions with lasers. Second, they will reach the quantum limit of measurement by developing quantum mechanically ideal microwave amplifiers.

"Our measurement technique efficiently imprints the motion of a nanomechanical beam onto microwave signals, but reaching the quantum limit using this method requires a quantum mechanically ideal amplifier of that microwave signal," explained Lehnert. "The amplifiers that we currently use, which are the best available, still fall far short of this ideal."

The results were reported in Nature Physics.