Achievability of Quantum Regime in Optomechanics of Electromagnetically   Levitated Nanoparticles: Inclusion of Gradient Force Fluctuations
  Counterintuitively, experiments show that an electromagnetically levitated particle escapes from its trap when the ambient pressure is reduced below a certain level even if the particle's motion is cooled by a resonator-based or feedback-based mechanism. Here, we theoretically show that the ambient pressure must be kept well above a critical value arising from gradient force fluctuations (viz., fluctuations in part of the EM force whose Hamiltonian is quadratic in the position of the particle). Also, we consider other force fluctuations, and determine whether different realizations of feedback cooling are able to reach the ground state. In some realizations, the cooling rate must be kept well below a critical value arising from feedback-induced gradient force fluctuations. 
Amir M. Jazayeri and Khashayar Mehrany
quant-ph physics.optics