  A qubit memory is the building block for quantum information. Cavity-enhanced
spin-wave-photon entanglement has been achieved by applying dual-control modes.
However, owing to cross readouts between the modes, the qubit retrieval
efficiency is about one quarter lower than that for a single spin-wave mode at
all storage times. Here, we overcome cross readouts using a multiplexed ring
cavity. The cavity is embedded with a polarization interferometer, and we
create a write-out photonic qubit entangled with a magnetic-field-insensitive
spin-wave qubit by applying a single-mode write-laser beam to cold atoms. The
spin-wave qubit is retrieved with a single-mode read-laser beam, and the
quarter retrieval-efficiency loss is avoided at all storage times. Our
experiment demonstrates 50% intrinsic retrieval efficiency for 540 microsecond
storage time, which is 13.5 times longer than the best reported result.
Importantly, our multiplexed-cavity scheme paves one road to generate
perfect-cavity-enhanced and large-scale multiplexed spin-wave-photon
entanglement with a long lifetime.
