Measurement-based ground-state cooling of a trapped-ion oscillator

Measurement-based cooling is a method by which a quantum system, initially in a thermal state, can be prepared probabilistically in its ground state through some sort of measurement. This is done by making a measurement that heralds the system being in the desired state. Here we demonstrate the application of a measurement-based cooling technique to a trapped atomic ion. The ion is precooled by Doppler laser cooling to a thermal state with a mean excitation of n approximate to 18 and the measurement-based cooling technique selects those occasions when the ion happens to be in the motional ground state. The fidelity of the heralding process is greater than 95%. This technique could be applied to other systems that are not as amenable to laser cooling as trapped ions.