Short answer: The nuclear spin of a phosphorus-31 donor in silicon is a long-lived qubit, and two papers report how well it can be read and held. A 2013 Nature paper reports electrical single-shot readout of a single nuclear spin with a fidelity above 99.8%, a coherence time of 60 milliseconds and a one-qubit gate fidelity above 98%. A 2013 Science paper reports a room-temperature coherence time of over 39 minutes for an ensemble in silicon-28. These are laboratory results on one donor type.
Why the nuclear spin
The Nature abstract notes that nuclear spins are extremely isolated from their environment, which makes them desirable for quantum information processing. It says the nuclear spin of a phosphorus-31 donor in silicon is very promising as a qubit, since bulk measurements indicate excellent coherence times and silicon is the dominant material of the microelectronics industry.
2013, Nature: one nuclear spin, read electrically
The authors demonstrate electrical detection and coherent manipulation of a single phosphorus-31 nuclear spin qubit with fidelities they describe as high enough for fault-tolerant quantum computing. They combine single-shot readout of the electron spin with on-chip electron spin resonance to read the nuclear spin in a quantum non-demolition way. The readout fidelity is reported as higher than 99.8%, which they call the highest so far reported for any solid-state qubit. They operate the nuclear spin as a qubit with coherent radio-frequency pulses. For an ionized donor, they find a nuclear spin coherence time of 60 milliseconds and a one-qubit gate fidelity above 98%. The sentence that follows these figures in the abstract was cut off here and is not described.
2013, Science: room-temperature coherence
The Science abstract says quantum memories that can store coherent information for long times at room temperature would enable many technologies. Shallow neutral donors have been studied extensively but work only at low temperatures, about 10 kelvin or below, while the nuclear spins of ionized donors could work at high temperature. The authors use optical methods and dynamical decoupling on an ensemble of phosphorus-31 donors in isotopically purified silicon-28 and observe a room-temperature coherence time of over 39 minutes. They show that a coherent spin superposition can be cycled from 4.2 kelvin to room temperature and back, and report a cryogenic coherence time of 3 hours in the same system.
A third source
An arXiv preprint, 1109.3864, was listed as a source, but its page returned no content, so it is not described.
Limits
The two results use different systems. The Nature work is a single electrically read nuclear spin, and the Science work is an ensemble measured optically, so the 39-minute coherence time is not a single-qubit figure. Neither abstract reports a two-qubit gate between nuclear spins, and the 98% gate fidelity is a single-qubit result.
FAQ
What is the qubit?
The nuclear spin of a phosphorus-31 donor atom in silicon.
How good was the readout?
Above 99.8% single-shot fidelity, per the 2013 Nature authors.
How long did coherence last?
60 milliseconds for a single ionized donor spin, and over 39 minutes at room temperature for an ensemble in silicon-28.
Why silicon-28?
The Science paper uses isotopically purified silicon-28, which removes nuclear spins from the host.
Is this a silicon quantum computer?
No. These are single-qubit and ensemble memory results.
All field notes