Credit: © 2010 NPG

A mechanical device has been cooled into its quantum ground state for the first time in a landmark experiment that could shed new light on the boundary between quantum and classical mechanics. Andrew Cleland and co-workers at the University of California at Santa Barbara (UCSB) cooled a mechanical oscillator to a temperature of 25 mK with a dilution refrigerator, and then used a superconducting quantum bit (qubit) to measure the quantum state of the device. The UCSB team was also able to create a superposition state in the qubit and transfer it to the oscillator so that the latter was both moving and not moving at the same time (Nature 464, 697–703; 2010).

The oscillator, which was visible to the naked eye, consisted of a thin layer of aluminium nitride sandwiched between two aluminium electrodes (left). When a voltage was applied to the electrodes, the device expanded and contracted because aluminium nitride is a piezoelectric material. The oscillator — also known as a 'quantum drum' — was coupled to the qubit, which acted as a quantum thermometer, by a capacitor.

The temperature at which quantum effects can be observed in a mechanical oscillator is proportional to its fundamental resonant frequency. At just over 6 GHz, the resonant frequency of the oscillator made by the UCSB team was relatively high for a mechanical device, which meant that quantum effects kicked in at the relatively high temperature of 0.1 K. Other groups are still trying to achieve the much lower temperatures needed to reach the quantum ground state in mechanical devices that have considerably lower resonant frequencies.

Reaching the quantum ground state in such an experiment involves removing all the thermal vibrations from the system. Cleland and co-workers estimated that their oscillator contained a maximum of 0.07 phonons (the quanta of vibrational energy in a solid), which means that the probability of the oscillator being in the ground state was greater than 93%. They also transferred a single quantum excitation from the qubit to the oscillator and back again multiple times, and created coherent phonon states in the oscillator (right).