Single-shot readout of nuclear spins near NV centers

Short answer: The nuclear spins near a nitrogen-vacancy (NV) center in diamond can act as quantum memories, but reading one out in a single shot is hard. A 2013 Physical Review Letters paper reported single-shot readout of a carbon-13 nuclear spin at room temperature, a 2017 paper reported single-shot readout of a weakly coupled carbon-13 spin with 95.5% fidelity in 200 ms, and a UCL thesis abstract describes repetitive readout with a scanning probe. These are single-defect experiments.

2013: watching a nuclear spin

The Physical Review Letters abstract (Dréau and coauthors) uses the electronic spin of a single NV defect to observe the real-time evolution of neighboring single nuclear spins under ambient conditions. In a diamond with natural carbon-13 abundance, the authors first demonstrate high-fidelity initialization and single-shot readout of an individual carbon-13 nuclear spin. Then, by including the NV center's intrinsic nitrogen-14 nuclear spin in the quantum register, they report simultaneous observation of quantum jumps linked to both nuclear spin species, which gives an efficient initialization of the two qubits. They say these results open avenues for diamond-based quantum information processing, including active feedback in quantum error correction and tests of quantum correlations with solid-state spins at room temperature. Those are expectations.

2017: a weakly coupled spin

The 2017 abstract says single-shot readout of qubits is required for scalable quantum computing, and that nuclear spins are superb memories because of long coherence times but hard to read in one shot because of weak coupling to probes. The authors demonstrate single-shot readout of a weakly coupled carbon-13 nuclear spin at room temperature, one that traditional protocols cannot resolve. They trap and read out its states projectively through sequential weak measurements, implemented with dynamical decoupling pulses. A nuclear spin coupled to the NV center at 330 kHz is read out in 200 ms with a fidelity of 95.5%. They present this as a general protocol for weakly coupled qubits at room temperature.

A thesis on repetitive readout

A University College London thesis abstract (the listing is a thesis, not a journal article) says NV centers display spin coherence and optical addressability from cryogenic to room temperatures, and are suited to sensing at scales from nano to macroscopic. It reports temperature-dependent studies of near-surface NV ensembles, finding consistency with previously reported non-monotonic behavior of deep isolated NVs in the 20 to 100 kelvin range. It also describes nuclear-ancilla-assisted repetitive readout for a single NV in an atomic force microscope probe, with an enhanced signal-to-noise ratio for relaxometry under ambient conditions, and a study of the trade-off in single-shot end-to-end fidelity against readout number and re-initialization time. The thesis abstract does not give a fidelity figure in the text read here.

Limits

The two journal papers read out nuclear spins coupled to a single NV center, and they report no gate or entanglement between distant qubits. The thesis is not peer reviewed in the usual sense. Readout fidelity in these abstracts is tied to long measurement times, such as 200 ms in 2017.

FAQ

Why read out nuclear spins?

They are long-lived memories, but weakly coupled to probes, per the 2017 abstract.

How good was the readout?

95.5% in 200 ms for a spin coupled at 330 kHz.

Does it work at room temperature?

Yes. Both journal papers report ambient conditions.

What is repetitive readout?

Measuring a nuclear ancilla many times to improve fidelity, as the thesis describes.

Is this a quantum computer?

No. These are single-defect readout demonstrations.