Sideband cooling technique is an example of a laser cooling. Traditionally it is used to cool atoms, confined in harmonic traps, to the ground state. The principle of sideband cooling is based on the key assumption (which is actually true in many cases) that if an excited atom emits a photon, the recoil is not enough to excite the translational degrees of freedom of the atom. The translational motion is quantized since the atom is bound in the trap. For the sideband cooling to work the following condition needs to be true: When an electron in the atom drops from a higher energy level to the ground state level, the quantum number describing the translational motion of the atom in the trap is not changed.
Cooling down the atom means reducing its quantum numbers describing the translational motion. To “freeze” the atom to its ground state the laser frequency is adjusted such that the energy of the photon is slightly insufficient to excite the electron in the atom. Thus a two-quanta process takes places such that a photon is absorbed and a phonon (i.e. a quantum of the kinetic energy of the atom) is absorbed, such that the total energy of the two quanta equals the energy needed to excite an electron (the laser has to be adjusted appropriately). Later, the electron decays from the excited state to the ground state. But, as was explained above, the emission of the photon does not lead to an excitation of the translational motion of the atom. Thus, after the process of the photon absorption and emission happens, the atom ends up in a state of lower kinetic energy. Eventually the atom migrates to its ground state.
Dr. Lehnert and his team have uses this principle to suck quanta of kinetic energy from rather macroscopic objects, composed of many atoms. They achieve sideband cooling of a 10 MHz micromechanical oscillator to the quantum ground state. This object cooled was a micromechanical cantilever. To make the interaction with the photon field stronger they put the cantilever into a microwave resonant circuit. They carry a near-Heisenberg-limited position measurement and thus confirm the result, i.e. that the cantilever is indeed in the quantum state with the lowest possible energy. Among unique features of the device is its strong coupling character, which allows a coherent exchange of microwave photons and mechanical phonons. The system will be used to test the theory of quantum behavior of macroscopic objects, advanced by Leggett around 1980. Another potential application of these results is for construction of new types of qubits. The work is published by as "Sideband cooling of micromechanical motion to the quantum ground state" by J. D. Teufel et al. in Nature (2011, posted online July 6).