Measurement-based cooling is a method by which a quantum system, initially in a thermal state, can be prepared probabilistically in its ground state.  This is done through some sort of measurement process that heralds the system being in the desired state, without changing it. Here we demonstrate the application of this technique to a trapped atomic calcium ion.

The ion is held in a linear RF blade trap. It is pre-cooled by Doppler laser cooling to a thermal state with a mean excitation of 𝑛≈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.