• Atomic spontaneous emission is proportional to the vacuum energy density, which is inversely proportional to the mode volume
• The spontaneous emission can therefore be enhanced in a small cavity (small V) by the Purcell factor F_p given by

where

	the vacuum Rabi frequency
	the cavity quality factor

• As seen from its relation with the vacuum Rabi frequency, the Purcell factor comes from the cavity-QED principle.