Optically trapped nanoparticles experience at least two types of interaction: collisions with gas particles in the nearby environment, and collisions with the photons of the trapping field. Both of these interactions contribute to noise in the measurements of these systems. While collisions with other particles can be minimized under ultrahigh-vacuum conditions, the individual kicks from the photons represent a fundamental limit to the degree of precision of such measurements: low laser energies decrease the noise but also increase the uncertainty of the particle's position, and at high laser energies the reverse is observed. Now, Lukas Novotny and collaborators from ETH Zürich, the University of Vienna and ICFO in Spain describe an experiment in which the contribution of photon shot noise can be measured directly.
A nanoparticle of ∼50 nm diameter was confined in an optical trap and an ultrahigh vacuum applied. The particle was then cooled to mK temperatures and kept there under steady-state conditions. Under these conditions, the interaction with the laser field was the only source of heating. The researchers could measure this interaction directly by watching the trajectory of the levitated particle (an indication of how much it has heated up) as soon as they turned off the cooling mechanism. They found that the photon shot noise heats the nanoparticle at a rate of 10,000 oscillator quanta per second. This value sets a lower boundary for the sensitivity of levitated nanoparticle measurements.