Quantum machine

In order to demonstrate the quantum mechanical behaviour, the team first needed to cool the mechanical resonator until it was in its quantum ground state (the state with the lowest possible energy). Specifically, a temperature ${\displaystyle T\ll {\frac {hf}{k}}}$ was required, where ${\displaystyle h}$ is the Planck Constant, ${\displaystyle f}$ is the frequency of the resonator and ${\displaystyle k}$ is the Boltzmann constant.^[a] Previous teams of researchers had struggled with this stage, as a 1 MHz resonator, for example, would need to be cooled to the extremely low temperature of 50 μK.[7] O'Connell's team constructed a different type of resonator, a film bulk acoustic resonator,[5] with a much higher resonant frequency (6 GHz) which would hence reach its ground state at a (relatively) higher temperature (~0.1 K); this temperature could then be easily reached with a dilution refrigerator.[5] In the experiment, the resonator was cooled to 25 mK.[5]

The film bulk acoustic resonator was made of piezoelectric material, so that as it oscillated its changing shape created a changing electric signal, and conversely an electric signal could affect its oscillations. This property enabled the resonator to be coupled with a superconducting phase qubit, a device used in quantum computing whose quantum state can be accurately controlled.

In quantum mechanics, vibrations are made up of elementary vibrations called phonons. Cooling the resonator to its ground state can be seen as equivalent to removing all of the phonons. The team was then able to transfer individual phonons from the qubit to the resonator. The team was also able to transfer a superposition state, where the qubit was in a superposition of two states at the same time, onto the mechanical resonator.[8] This means the resonator "literally vibrated a little and a lot at the same time", according to the American Association for the Advancement of Science.[9] The vibrations lasted just a few nanoseconds before being broken down by disruptive outside influences.[10] In the Nature paper, the team concluded "This demonstration provides strong evidence that quantum mechanics applies to a mechanical object large enough to be seen with the naked eye."[5]