Short answer: Silicon-vacancy (SiV) color centers in diamond can be coupled to nanophotonic cavities to link a spin qubit to single photons. A 2019 Physical Review Letters paper reported cavity cooperativity above 30 with spin coherence times above 1 ms, and a second 2019 paper in Science reported interaction between two SiV centers in one cavity. A 2023 paper in Communications Physics used SiV centers in a nanodiamond with a microcavity, with a spin initialization fidelity of 80%. These are laboratory steps toward quantum repeaters.
The network-node idea
The Communications Physics abstract explains the need: large quantum networks require quantum repeaters because of signal loss in optical fibers, and an ideal repeater node efficiently links a quantum memory with photons. Color centers in diamond, particularly the negatively charged SiV, are promising candidates. A major obstacle is the inefficient connection between the spin and the fiber network, which optical resonators could address.
2019: an efficient interface and a two-qubit register
The Physical Review Letters abstract reports a diamond nanocavity with an embedded SiV and a proximal nuclear spin. The authors show that efficient SiV-cavity coupling, with cooperativity above 30, provides a nearly deterministic interface between photons and the electron spin memory, with coherence times above 1 ms. Using coherent microwave control, they demonstrate heralded single-photon storage in the long-lived spin memory, and universal control over a cavity-coupled two-qubit register made of an SiV and a nearby carbon-13 nuclear spin with nearly second-long coherence time. They say this lays groundwork for implementing quantum repeaters, which is their outlook.
2019: two centers sharing a cavity
The Science abstract says photon-mediated interactions between quantum systems are essential for networks and scalable quantum information processing. The authors demonstrate such interactions between pairs of SiV centers coupled to a diamond nanophotonic cavity. When the optical transitions of the two centers are tuned into resonance, coupling to the common cavity mode gives a coherent interaction, resulting in spectrally resolved superradiant and subradiant states. They use the electronic spin of the centers to control these interactions, and say such control will be crucial for cavity-mediated gates between spin qubits and scalable network nodes.
2023: nanodiamonds in a microcavity
The Communications Physics paper couples individual SiV centers in a nanodiamond to a hemispherical, stable Fabry-Perot microcavity, achieving Purcell factors larger than 1. The authors demonstrate coherent optical driving between ground and excited states with a Rabi frequency of 330 MHz, and all-optical initialization and readout of the electron spin in magnetic fields up to 3.2 tesla. Spin initialization takes 67 ns with 80% fidelity and a lifetime of 350 ns. They say this opens the way toward quantum repeater applications.
Limits
The three experiments use different cavity designs and do not report a working repeater. A Purcell factor above 1 and 80% initialization fidelity are modest next to the 2019 nanocavity numbers, and the nanodiamond route trades performance for a simpler geometry.
FAQ
What is cooperativity?
A measure of how strongly an emitter couples to a cavity, reported as above 30 in the 2019 paper.
How long did the spin stay coherent?
Above 1 ms for the electron spin, and nearly a second for the nearby nuclear spin, per the 2019 authors.
What happens with two centers in one cavity?
They interact coherently, producing superradiant and subradiant states.
Why nanodiamonds?
The 2023 authors use them with a microcavity as a different route to a spin-photon interface.
Is a quantum repeater built?
Not in these abstracts.
All field notes