Trapping a single atom with a fraction of a photon using a photonic crystal nanocavity

We consider the interaction between a single rubidium atom and a photonic crystal nanocavity. Because of the ultrasmall mode volume of the nanocavity, an extremely strong coupling regime can be achieved in which the atom can shift the cavity resonance by many cavity linewidths. We show that this shift can be exploited to trap a single atom above the cavity. The atom is trapped by light that is only inside the cavity as a result of the shift
of the cavity resonance caused by the proximity of the atom itself. This atom trap requires only a fraction of a photon inside the cavity. Surprisingly, the damping of the cavity plays a pivotal role in this trapping mechanism.