Short answer: Alkaline earth atoms such as ytterbium-171 can store a qubit in the nuclear spin, which is useful because the ground state has little sensitivity to many noise sources. A 2022 Physical Review X paper reported the first universal set of gate operations on a nuclear spin qubit in this kind of atom, including Rydberg-blockade entangling gates. A separate theory paper proposes ways to excite such qubits to Rydberg states in weak magnetic fields. These are laboratory results in small arrays, and the theory is a proposal.
What the 2022 experiment reported
The Physical Review X abstract describes neutral atom arrays as a rapidly developing platform and says alkaline earth atoms (AEAs) have attracted interest because their level structure offers several opportunities for improved performance. The authors present what they call the first demonstration of a universal set of quantum gate operations on a nuclear spin qubit in an AEA, using ytterbium-171.
They report narrow-line cooling and imaging at a newly discovered magic trapping wavelength of 486.78 nm. They also demonstrate nuclear spin initialization, readout and single-qubit gates. The coherence times they report are T1 of about 20 seconds and T2* of 1.24 seconds, with an uncertainty of 0.05. The reported single-qubit operation fidelity is 0.99959, with an uncertainty of 0.00006.
For two-qubit operations, the authors demonstrate entangling gates using the Rydberg blockade. They also show coherent control of these gates using light shifts on the ytterbium ion core transition at 369 nm. The authors describe the results as a significant step toward highly coherent quantum gates in AEA tweezer arrays. That is their own characterization of progress, and the abstract does not report a large-scale computation.
A theory route to Rydberg excitation
A paper in Frontiers of Physics (the abstract was read on the journal site) addresses a specific obstacle. Alkaline-earth-like atoms with two valence electrons and nonzero nuclear spin can be excited to Rydberg states, but typical ground states have no hyperfine splitting, whereas a gigahertz-scale splitting seems necessary for Rydberg excitation. Strong magnetic fields can create such a splitting, but weak fields are preferable to avoid experimental noise.
The authors offer two theoretical solutions using realistic data for well-studied isotopes. In the first, the two nuclear spin qubit states are excited to Rydberg states with detunings of Delta and 0, where a megahertz-scale detuning comes from a weak field of order 1 gauss. In the second, appropriate intermediate states allow a two-photon Rydberg excitation to proceed with only one nuclear spin qubit state, and this works whatever the magnitude of the magnetic field. The authors say these theories combine the broad applicability of Rydberg blockade with the advantages of nuclear-spin quantum memory.
A third source that could not be read
A 2023 arXiv preprint titled "Fast nuclear-spin entangling gates compatible with large-scale atomic arrays" is a related reference. Its abstract could not be retrieved, so this post does not describe its content.
Limits
The experimental paper reports a universal gate set on small systems. It does not report an error-corrected or large-scale device. The theory paper proposes excitation schemes and reports no measurement. Fidelity and coherence values above are the authors' figures and were measured under their own lab conditions.
FAQ
Why use a nuclear spin as the qubit?
The 2022 authors report long coherence times for it, with T1 of about 20 seconds in ytterbium-171.
What gate fidelity was reported?
A single-qubit operation fidelity of 0.99959 in the 2022 paper, per the authors.
What is the Rydberg blockade used for here?
It enables the two-qubit entangling gates in the 2022 experiment.
What problem does the theory paper address?
Exciting nuclear spin qubits to Rydberg states without needing a strong magnetic field.
Is this a working quantum computer?
No. These are research demonstrations and proposals.
All field notes