Short answer: The electron spin of a nitrogen-vacancy (NV) centre in diamond lives longer when the surrounding crystal has few nuclear spins, which means few carbon-13 atoms and few other paramagnetic impurities. A 2009 paper reported a room-temperature dephasing time of 1.8 ms in isotopically controlled diamond. A 2023 paper reports long coherence in isotopically purified nanodiamonds, and a 2026 preprint reports a Hahn-echo T2 of 6.8 ms and 11.2 s under dynamical decoupling. All are laboratory measurements.
Why the host crystal matters
A 2023 Physical Review Applied paper says the long spin coherence and relaxation times of the NV centre at room temperature are crucial to its many sensing uses, since they often limit sensitivity. It adds that the coherence time depends heavily on the host material.
Ultrapure diamond, 2009
A 2009 Nature Materials paper reports synthesising ultrapure, isotopically controlled single-crystal chemical vapour deposition diamond with a very low concentration of paramagnetic impurities. The content of carbon-13 nuclear spins was depleted to 0.3% and other paramagnetic defects were measured below 10^13 per cubic centimetre. In this nearly spin-free lattice, single electron spins show what the authors describe as the longest room-temperature spin dephasing times observed in solid-state systems at the time, T2 of 1.8 ms. They suggest it could allow coherent coupling between spins separated by a few tens of nanometres.
Isotopically purified nanodiamonds, 2023
The 2023 paper addresses nanodiamonds, which can operate in environments bulk diamond cannot, such as inside cells. It reports single NV centres in isotopically purified, polycrystalline, ball-milled nanodiamonds. With a spin-locking sequence the authors observe T2 up to 786 +/- 200 microseconds, T2 up to 2.06 +/- 0.24 microseconds and T1 up to 4.32 +/- 0.60 ms. They say these improve on the leading literature values for T2 and T1 in nano- and microdiamond and for T2 in nanodiamond. They report that the diamond hosting the longest T1 is smaller than 100 nm, and give a substitutional nitrogen concentration of 0.15 +/- 0.02 parts per million.
Networks and spin-photon entanglement, 2026
A 2026 arXiv preprint studies single NV centres in (111)-grown isotopically engineered diamond, with controlled carbon-13 concentration and low parts-per-billion nitrogen. With a real-time feedforward scheme to mitigate 50 Hz noise and tailored decoupling sequences, the authors report an electron-spin T2 of 6.8(1) ms for a Hahn echo and 11.2(8) s under dynamical decoupling. They also report coherent optical transitions with a near-lifetime-limited homogeneous linewidth of 16.9(4) MHz. They argue this is relevant for quantum networks, which need long spin coherence together with a coherent optical transition.
Reading the numbers
The measurements use different diamonds and different pulse sequences. A Hahn-echo T2, a dephasing time and a decoupled coherence time are different quantities, so the 2009, 2023 and 2026 figures are not a simple progression. The 2026 result is a preprint.
FAQ
Why does carbon-13 shorten coherence?
It carries a nuclear spin that couples to the NV electron spin. The 2009 paper lowers it to 0.3% to reduce that effect.
How long did the coherence reach?
1.8 ms dephasing in 2009, and in 2026 a Hahn-echo T2 of 6.8 ms and 11.2 s under dynamical decoupling.
Does it work for nanodiamonds?
The 2023 paper reports T2 up to 786 microseconds in isotopically purified nanodiamonds.
Why does it matter for networks?
Networks need long spin coherence and a coherent optical transition together, per the 2026 preprint.
Are these devices available commercially?
These papers report laboratory measurements and make no such claim.
All field notes