When a mirror reflects light, it experiences a slight push but it is negligible in our everyday lives. Our furniture is not moving because due to radiation pressure of light, a 100 Watt light-bulb causes a radiation pressure that is only a trillionth (one part to 1000000000000) of the normal atmospheric pressure.

Radiation certainly pressure matters in space, the tails of comets typically point away from the Sun due to radiation pressure, and it has been proposed as the propulsion for solar sails, should we ever venture more than 400 miles from Earth again. On terra firma, radiation has been harnessed in the field of laser physics, it can be used to couple the electromagnetic laser field to the movement of the small mechanical oscillators that can be found inside ordinary watches. Due to the weakness of the interaction, one typically needs substantially strong laser fields.

A new study shows this radiation pressure can be increased considerably - with the help of a small superconducting island.

Jani Tuorila from the University of Oulu explains that radiation pressure physics in systems are measurable only when the oscillator is hit by millions of photons but by placing a superconducting island in between the electromagnetic field and the oscillator to mediate the interaction, the strength of the radiation pressure coupling can be considerably increased. 

"In the measurements, we exploited the Josephson coupling of the superconducting junctions, especially its nonlinear character, says co-author Juha Pirkkalainen from Aalto University, the post-doctoral researcher who conducted the measurements. The researchers were able to alter the radiation pressure coupling significantly. With the superconducting island, the radiation pressure increased a millionfold the value we had previously achieved.

Because of the increased radiation pressure coupling, the oscillator observes the electromagnetic field with the precision of a single photon. Correspondingly, the oscillators reveal themselves to the field with the resolution of a single quantum of oscillations, a phonon.

The research enables the observation of quantum phenomena in larger structures than before, so it allows studying the validity of the quantum mechanical laws in large structures. Does this hold up only with very small particles? The existence of an upper limit for the validity region has not been found yet.

Citation: J.-M. Pirkkalainen, S.U. Cho, F. Massel, J. Tuorila, T.T. Heikkilä, P.J. Hakonen, and M.A. Sillanpää, Cavity optomechanics mediated by a quantum two-level system, Nature Communications 6, 27 April 2015