  Broadband quantum memories hold great promise as multiplexing elements in
future photonic quantum information protocols. Alkali vapour Raman memories
combine high-bandwidth storage, on-demand read-out, and operation at room
temperature without collisional fluorescence noise. However, previous
implementations have required large control pulse energies and suffered from
four-wave mixing noise. Here we present a Raman memory where the storage
interaction is enhanced by a low-finesse birefringent cavity tuned into
simultaneous resonance with the signal and control fields, dramatically
reducing the energy required to drive the memory. By engineering anti-resonance
for the anti-Stokes field, we also suppress the four-wave mixing noise and
report the lowest unconditional noise floor yet achieved in a Raman-type warm
vapour memory, $(15\pm2)\times10^{-3}$ photons per pulse, with a total
efficiency of $(9.5\pm0.5)$%.
