  We report 100% duty cycle generation of sub-MHz single photon pairs at the
Rubidium D$_1$ line using cavity-enhanced spontaneous parametric
downconversion. The temporal intensity crosscorrelation function exhibits a
bandwidth of $666 \pm 16$ kHz for the single photons, an order of magnitude
below the natural linewidth of the target transition. A half-wave plate inside
our cavity helps to achieve triple resonance between pump, signal and idler
photon, reducing the bandwidth and simplifying the locking scheme.
Additionally, stabilisation of the cavity to the pump frequency enables the
100% duty cycle. These photons are well-suited for storage in quantum memory
schemes with sub-natural linewidths, such as gradient echo memories.
