Quantum memories are entangled using interference of their emitted photons. A fast laser pulse excites an atom, and the subsequent spontaneously emitted photon is entangled with the state of the atom. These entangled photons interfere on a beam splitter, projecting the atoms in to an entangled state. This photon interference scheme relies on the interference of identical photons. If the photons are not identical, the resulting entanglement fidelity between the quantum memories suffers. Researchers in the past have boosted the fidelity by only accepting photons in a short time interval, thereby “erasing” the frequency difference of the photons at the cost of entanglement speed.

However, photons are waves and waves of differing frequency always interfere. By keeping track of the time-dependent ripples of the resulting photon interference, we recover the fidelity of the quantum memories without sacrificing entanglement rate. The techniques we show in this paper can be extended to entangle distinguishable quantum memories using photons with arbitrary frequency difference.

- “Entanglement of Distinguishable Quantum Memories,” G. Vittorini, D. Hucul, I.V. Inlek, C. Crocker, and C. Monroe. arXiv:1409.0907 (2014).