April 15, 2008, Garching, Germany--Scientists at the Max Planck Institute of Quantum Optics have already succeeded in the damping of mechanical oscillations of a microresonator by applying the method of laser cooling which has been developed for single quantum particles. Now they have shown that even "resolved-sideband cooling"--a special kind of laser cooling--is applicable to an object consisting of about 10exp14 molecules.
This experiment is an important step towards attaining the ultimate quantum ground state of a mesoscopic object. The effective cooling process demonstrated here may be of practical interest as well, since it may be used to improve techniques such as scanning probe microscopy.
When a trapped ion oscillates with a certain frequency, its absorption spectrum consists of a series of sidebands that are displaced from the original resonance frequency by multiples of the oscillation frequency. Now cooling can be achieved by exciting the ion with laser light that is tuned to one of the energetically lower-lying sidebands. This way the photons that are absorbed by the ion are, on average, of lower energy than the photons that are emitted. This is how cooling proceeds.
In analogy to trapped ions, resolved sidebands also occur in the absorption spectra of mesoscopic optomechanical systems. Reaching this regime requires however that the mechanical oscillator frequency exceeds the optical dissipation rate of the optical resonator, that is, photons must be stored in the resonator for many mechanical oscillation periods. Only in this case, the cooling effect can outbalance the heating induced by the fluctuations of the light force. To this end, the researchers lithographically fabricated silica microtoroids (60 micrometer diameter, 70 MHz resonance frequency) and highly efficient cooling at unprecedented cooling rates was demonstrated. If the ground state can be achieved remains to be proven; after all researchers worldwide have been working on this already for more than a decade. But with the new method at hand--which has removed a fundamental roadblock--the way towards the ground state is now boldly signposted and should enable some exciting science over the coming years.
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