The sideband cooling of a single ^{40}Ca^{+ }ion has been achieved recently in a home-built linear ion trap by the group of Bound System Quantum Information, led by Professor Mang Feng. After the cooling, the average phonon number in the system is <n>=0.056 and 95% population occupies the ground motional state of the ion. This success paves the way towards experimental realization of quantum information processor using trapped ultracold ions.

Cooling the trapped ions down to the ground motional state is the prerequisite of making quantum computers, which is a highly difficult job. Mang Feng , Fei Zhou and other researchers have taken long-time hard work,and finally made the breakthrough that efficiently cooled a single ion down to the ground motional state. The cooling consists of two steps: The first is the Doppler cooling of the ion down to the Lamb-Dicke regime by dipole transitions, and then cooling of the ion by quadrupole transitions using the sidebands, where each cooling cycle eliminates a single phonon until reaching zero phonon.

The trapped ion in ground motional state is an ideal quantum system, with which we may not only deepen our understanding of the quantum world, but also work for ultra-precision measurement, quantum computation and quantum simulation.

The research work is supported by National Fundamental Research Program of China entitled ”Quantum state engineering in the coupled systems of single trapped atoms (ions) and light”, and by National Natural Science Foundation of China.

Fig. 1 The sideband absorption spectra of S_{1/2} (m=+1/2)←→D_{5/2} (m=-1/2)_{ }transitions. (a) The first red-sideband: comparison of the effect before the cooling (circles) with after the cooling (black dots). The solid curve is the simulation. (b) The first blue sideband: The black dots are experimental data and the solid curve is the simulation. Comparison of the occupations in the first red-sideband with in the first blue-sideband yields the average motional quantum number <n>=0.056.