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. O'Connell's team constructed a different type of resonator, a film bulk acoustic resonator, 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. In the experiment, the resonator was cooled to 25 mK.