Absorption–emission quantum repeater using diamond quantum memories

Long-distance quantum communication is fundamentally limited by exponential photon loss over fiber transmission links. Quantum repeaters are essential for overcoming this barrier and building scalable quantum networks. While many repeater designs rely on complex multi-photon interference between remote nodes or strong cavity coupling, this paper investigates an alternative: an absorption-emission-based quantum repeater architecture using a single nitrogen-vacancy (NV) center in diamond. This approach relies on local light-matter interactions rather than remote photon interference, aiming to relax strict optical synchronization requirements while retaining key repeater functionality.

The researchers implement a complete single-node quantum repeater using a single diamond NV center at cryogenic temperatures. The operation integrates three main components:

1. Heralded Absorption: A polarization-encoded input photon is resonantly absorbed by an NV center initialized in an electron-nuclear spin entangled state, transferring the photonic state to the nitrogen nuclear spin memory via quantum teleportation.
2. Repeat-Until-Success (RUS) Emission: The nuclear-spin-stored state is transferred to an emitted zero-phonon-line (ZPL) photon via an optical excitation protocol that can be repeated multiple times until success, mitigating photon loss while protecting the memory.
3. Bell-State Measurement (BSM): A local projection between the electron and nuclear spins completes the end-to-end polarization state transfer.

The complete repeater channel from the absorbed photon to the emitted photon is characterized using quantum process tomography.

The study successfully demonstrates end-to-end quantum repeater-node operation. Photon-to-nuclear-spin quantum state transfer achieves an average state fidelity of 93%. By optimizing the synchronization of the excitation repetition period to match the hyperfine interaction, the repeat-until-success protocol preserves arbitrary nuclear-spin states over ten excitation attempts with an average fidelity of 87%, successfully balancing state preservation against photon collection efficiency. Finally, quantum process tomography on the complete absorption-emission channel reveals a process fidelity of 78% with respect to the identity operation, confirming genuine quantum repeater-node functionality.