Quantum mechanics predicts the existence of a quantum ground state, with minimal energy E. For a 1 MHz harmonic oscillator, this limit is reached when E=½ hω is equal to the thermal energy quantum kBT. This corresponds to 25 µK, which cannot be obtained by cryogenic techniques. To cool down small objects, such as atoms to µK temperatures, laser cooling was successfully used. In 2004, Höhberger & Karrai [Nature 432, 1002 (2004)] has shown that the fundamental mode of a cantilever could be cooled from room temperature down to 18K by photon-induced forces in a low-finesse optical interferometer. By active feedback cooling, Kleckner & Bouwmeester [Nature 444, 75 (2006)] cooled the fundamental mode of a cantilever to sub-K levels. Although encouraging, there are still 3 orders of magnitude to improve.
We have recently taken into operation a home-built, UHV low-temperature scanning force microscope (LTSFM) that uses a Fabry-Pérot interferometer with a finesse of about 20 for ultra sensitive deflection detection [Hoogenboom et al., APL 86, 074101 (2005)]. The photon shot noise-limited noise level was measured to be 3fm/√Hz at 1MHz.
We performed laser cooling experiments at RT, 77K and 4.3K. For different laser powers, the laser-induced cooling was determined from the reduction of the area under the resonance curve of the cantilever. For a hard cantilever (stiffness k=40N/m, resonance frequency fo~300kHz) typically used for atomic resolution AFM, a temperature decrease from 4.3K down to 200mK was observed.
Starting from 320K with a softer cantilever (k=0.59N/m, fo~ 44.9 kHz), the final temperature was 3.8K. At 77K, we measured the maximum cooling of the lever to be 44mK, a record with passive damping. Cooling effects starting from 4.3K were so strong that no resonance peak could be reliably detected, suggesting that the peaks were below the electronic noise level.
In order to investigate the quantum limit with a macroscopic object, further reduction of the cantilever temperature at its fundamental mode must be reached. With the present cantilevers this is hardly possible because of their low fo. Further detection improvement and experiments with ultra small cantilevers (k=0.1 N/m, fo~1MHz) are planned.
|