Measuring and slowing down
To achieve this, the researchers use a method that is well-known from slowing down a playground swing: just the right amount of pushing or pulling in the right direction, depending on where the swing happens to be. With a swing, taking a good look and acting accordingly will do the trick. In the case of a nanosphere, however, a more precise measurement has to be made. This measurement consists in superimposing the light reflected by the sphere onto another laser beam, which results in an interference pattern. From the position of that interference pattern it is possible to deduce where the sphere is located inside the laser trap. That information, in turn, is used to calculate how strongly the sphere has to be pushed or pulled in order to slow it down. The slowing itself is done by two electrodes, whose electric field exerts a precisely determined Coulomb force on the electrically charged nanosphere.
First quantum control in free space
“This is the first time that such a method has been used to control the quantum state of a macroscopic object in free space”, Novotny points out. Even though similar results have been obtained with spheres in optical resonators, Novotny’s approach has important advantages: it is less susceptible to disturbances, and by switching off the laser light one can, if required, examine the sphere in complete isolation.
Such an isolated examination becomes particularly relevant when trying to actually perform interference experiments, like those observed with light waves, with the nanosphere. This is because in order to see interference effects, the quantum mechanical wave of the sphere needs to be sufficiently large. One way to achieve this is to switch off the laser trap after cooling the sphere to its motional ground state, which allows its quantum wave to expand freely. Different parts of the wave can then fall through a double slit. As with molecules, also in this case the superposition of the matter waves is expected to result in a characteristic interference pattern.
Possible applications in sensors
“For now, however, that’s just a pipe dream”, Novotny cautions. Still, he also mentions that hovering nanospheres are of interest not only to basic research, but can also have practical applications. Nowadays there are already sensors that can measure the tiniest accelerations or rotations by using interfering atomic waves. As the sensitivity of such sensors increases with increasing mass of the quantum mechanically interfering object, the sensors could be immensely improved with nanospheres.