Showcasing new tools for developing quantum circuits made of mechanical parts, scientists from the National Institute of Standards and Technology (NIST) have demonstrated a flexible, broadly applicable technique for steadily damping the vibrations of a mechanical object down to the quantum "ground state," the lowest possible energy level.
Described in a Nature paper posted online July 6,* the NIST experiments nearly stop the beating motion of a microscopic aluminum drum made of about 1 trillion atoms, damping its motion below a single quantum, or unit of energy, and so placing the drum in a realm governed by quantum mechanics. Like a plucked guitar string that plays the same tone while the sound dissipates, the drum continues to beat 11 million times per second, but its range of motion approaches zero. The cooling technique and drum device together promise new machinery for quantum computing and tests of quantum theory, and could help advance the field of quantum acoustics exploring the quantum nature of mechanical vibrations.
NIST scientists used the pressure of microwave radiation to calm the motion of the drum, which is embedded in a superconducting circuit.** The circuit is designed so that the drum motion can influence the microwaves inside an electromagnetic cavity. The cooling method takes advantage of the microwave light's tendency to change frequency, if necessary, to match the frequency, or tone, at which the cavity naturally resonates.
"I put in the light at the wrong frequency, and it comes out at the right frequency, and it does that by stealing energy from the drum motion," says John Teufel, a NIST research affiliate who designed the drum. Teufel led the cooling experiments in NIST physicist and co-author Konrad Lehnert's lab at JILA, a joint institute of NIST and the University of Colorado Boulder.
The NIST drum can store individual packets of energy, or quanta, for about 100 microseconds without change, much longer than conventional superconducting quantum bits can maintain information. The drum, thus, might serve as a short-term memory device for a quantum computer as well as a platform for exploring complex mechanical and quantum states for tests of theories such as quantum gravity. The NIST apparatus also allows researchers to measure the position of the drum directly, which is useful for force detection, with a precision closer than ever to the ultimate limit allowed by quantum mechanics.
To make engineered bulk objects obey the rules of quantum mechanics, typically observed only in atoms and smaller particles, scientists must lower an object's temperature beyond the reach of conventional refrigeration. The NIST drum experiments used a technique analogous to the way lasers are used to cool individual atoms to near absolute zero, lowering the drum temperature to below 400 microKelvin (millionths of a degree above absolute zero), or just one-third of 1 quantum.
The research was supported in part by the Defense Advanced Research Projects Agency. For more details, see the NIST news announcement at www.nist.gov/pml/quantum/drum-070611.cfm.