Short answer: The negatively charged tin-vacancy (SnV) center in diamond is a spin qubit that keeps its coherence at temperatures far above a dilution refrigerator, around 1.7 to 2.9 kelvin. A 2020 Physical Review Letters paper reported spin coherence reaching the nuclear spin-bath limit at 2.9 K, a 2021 Physical Review X paper reported all-optical control with a dynamical-decoupling coherence time of 0.30 ms, and a 2023 paper reported 99.51% microwave pi-pulse fidelity and a Hahn-echo time of 170 microseconds. All are single-center laboratory results.
Why tin-vacancy
The 2021 abstract explains that the SnV has a large spin-orbit coupling, which offers strong protection against phonon dephasing and robust cyclicity of its optical transitions toward spin-photon entanglement schemes. The 2020 abstract says that coherent optical transitions combined with long spin coherence without dilution refrigeration make the SnV a promising candidate for scalable quantum networking. The authors compare it with other inversion-symmetric color centers, for which similar values require millikelvin temperatures.
2020: spin and optical properties
The Physical Review Letters paper reports spin and optical properties of individual SnV centers in diamond nanostructures. Using cryogenic magneto-optical and spin spectroscopy, the authors verify the inversion-symmetric electronic structure, identify spin-conserving and spin-flipping transitions, characterize linewidths, measure electron spin lifetimes and evaluate the dephasing time. They find the optical transitions consistent with the radiative lifetime limit even in nanofabricated structures. The spin lifetime is phonon limited with an exponential temperature scaling, and the coherence time reaches the nuclear spin-bath limit on cooling to 2.9 K. The numeric values for lifetime and coherence did not render in the retrieved text and are not quoted.
2021: all-optical control
The Physical Review X paper demonstrates multiaxis coherent control of the SnV spin qubit using an all-optical stimulated Raman drive between ground and excited states. The authors use coherent population trapping and optically driven electron spin resonance to confirm coherent access at 1.7 K, and obtain spin Rabi oscillations at 19.0 MHz. All-optical Ramsey interferometry reveals a dephasing time T2* of 1.3 microseconds, and four-pulse dynamical decoupling extends the spin coherence time to 0.30 ms.
2023: microwave control
The 2023 Physical Review X abstract notes that the large spin-orbit coupling that gives long spin lifetimes also suppresses the magnetic dipole transitions needed for control. By using a naturally strained center, the authors achieve high-fidelity microwave spin control, with a pi-pulse fidelity of up to 99.51% (uncertainty 0.03) and a Hahn-echo coherence time of 170.0 microseconds (uncertainty 2.8), which they say are the highest reported for the SnV platform. This is at 1.7 K, where they say ample cooling power is available to mitigate drive-induced heating, and without compromise to optical stability.
Limits
The experiments use individual centers under cryogenic conditions, and each paper uses a different control method, so the numbers are not directly comparable. None of the abstracts reports a working network node.
FAQ
How cold does the SnV need to be?
The papers report operation around 1.7 K and 2.9 K, as opposed to millikelvin.
What coherence times were reported?
0.30 ms with dynamical decoupling in 2021 and 170 microseconds Hahn-echo in 2023.
Why is strain used?
It overcomes the suppressed magnetic dipole transitions, per the 2023 authors.
Is this a quantum network?
No. These are single-center control results.
How does it compare to other centers?
The 2020 authors say similar values for other inversion-symmetric centers need millikelvin temperatures.
All field notes