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The cascaded biphoton state generated from a cold atomic ensemble presentsone of the strongly correlated resources that can preserve and relay quantuminformation. Under the four-wave mixing condition, the emitted signal and idlerphotons from the upper and lower excited states become highly correlated intheir traveling directions and entangled in continuous frequency spaces. Inthis system, we theoretically study the spectral compression of the biphotonsource using an external cavity and show the reduction in its frequencyentanglement entropy. This indicates, respectively, an improved lightabsorption efficiency for the idler photon as well as an almost pure biphotonsource which is useful in optical quantum networks. We further investigate thelimit of the spectral compression that can be achieved by using multiplecavities. Our results show the capability and potential of the biphoton sourcewith external cavities, where the performance of atom-based quantum memory canbe enhanced and the entanglement property can be manipulated by tailoring thespectral compression.

Spectral compression and entanglement reduction in the cascaded biphoton state with cavities