A passive, heralded, quantum memory with crossed fiber cavities

Quantum memories have been implemented in various physical systems ranging from atoms to solids, and from ensembles to single emitters. Despite progress, a large challenge concerns the always present photon loss and the always finite efficiency. Both limitations can be remedied with a herald that signals successful operation of the quantum memory. We have set up a new experiment with single neutral atoms trapped at the center of two crossed Fabry-Perot fiber cavities. Exploiting the possibilities given by the new system, we have realized a quantum memory for photonic polarization qubits which provides a herald that signals successful storage without destruction of the employed qubit \footnote{Brekenfeld et al, accepted in \textbf{Nat. Phys.} (2020)}. In addition, the memory couples to two spatially and spectrally distinct cavity modes. One mode is used for sending in and reading out the photonic qubit. The second mode replaces amplitude- and phase-critical control fields including no need for feedback-loops, rendering this memory fully passive. Our memory is robust and fits naturally into a fiber-based network. Therefore it is an important step towards the goal of realizing a practical quantum repeater \footnote{Uphoff et al, \textbf{Appl. Phys. B} 122, 46 (2016)}.