Alkaline-earth atom qubits in nuclear spin states

Short answer: Alkaline-earth atoms such as strontium-87 and ytterbium-171 can store a qubit in the spin of the nucleus, which is shielded from many disturbances. Three 2022 papers report nuclear spin qubits in optical tweezer arrays, with coherence times from several seconds to tens of seconds and single-qubit error rates near 0.1 percent in one case. They also report entangling gates using Rydberg blockade. These are laboratory experiments with small arrays.

Why alkaline-earth atoms

The 2022 Nature Communications paper explains that a register of highly coherent but independent qubits is a prerequisite for universal quantum computation. Individually trapped neutral atoms in optical tweezers are a promising platform, combining control over the quantum state of single atoms with efficient scaling to larger numbers of atoms. Until recently these systems mostly used alkali metal atoms, with favourable level structures for loading and cooling and ground-state hyperfine structure used with microwaves. The Physical Review X paper on ytterbium notes that alkaline-earth-like atoms have a more complex electronic structure that offers several advantages, with experiments showing very efficient cooling and detection, higher-fidelity entangling operations and uses in optical atomic clocks, though universal gates on a ground-state spin had not yet been realised.

Strontium-87 qubits

The Nature Communications authors encode a qubit in two nuclear spin states of a single strontium-87 atom and demonstrate coherence approaching the minute scale within an assembled register of individually controlled qubits. They say other systems have achieved long coherence through shielding, careful trapping, global operations and dynamical decoupling, while they reach comparable coherence times while driving multiple qubits individually and in parallel. With several qubits manipulated at once, they observe coherence more than 10^5 times the length of the current operations, with an echo coherence time of 40 plus or minus 7 seconds. They expect nuclear spin qubits to combine with larger arrays of individually trapped atoms and high-fidelity entangling operations.

Ytterbium-171: a universal gate set

The first Physical Review X paper presents what it calls the first demonstration of a universal set of gate operations on a nuclear spin qubit in an alkaline-earth atom, using ytterbium-171. It uses narrow-line cooling and imaging at a newly found magic trapping wavelength of 486.78 nm, and demonstrates nuclear spin initialisation, readout and single-qubit gates. Reported coherence times are T1 of about 20 s and T2* of 1.24 s, with a single-qubit fidelity of 0.99959. It also shows two-qubit entangling gates through Rydberg blockade, with coherent control of the gates using light shifts on the Yb+ ion core transition at 369 nm, and calls the results a step toward highly coherent gates in alkaline-earth tweezer arrays.

Ytterbium-171: loading and rotations

The second Physical Review X paper reports a fast, scalable, high-fidelity architecture based on ytterbium-171 in a tweezer array, where the nuclear spin of 1/2 serves as a long-lived two-level system and the electronic structure allows low-entropy preparation, fast control and high-fidelity readout. A near-deterministic loading protocol fills a 10 by 10 array with 92.73 percent efficiency and a single tweezer with 96.0 percent. Submicrosecond rotations show an error of 5.2 times 10^-3 per Clifford gate in randomised benchmarking. Measured coherence is T2* of 3.7 s and T2 of 7.9 s, many orders of magnitude longer than the pulses. Spin depolarisation times are tens of seconds, extendable to the 100 s scale with a magnetic field of a few gauss. The authors also use Raman-sideband cooling to bring atoms near their motional ground state, for future two-qubit gates. A Physics commentary quoted on the paper pages says two independent teams showed neutral ytterbium-171 atoms can be used for quantum information processing.

What this does not tell you

The papers use different atoms, arrays and benchmarks, so their numbers are not directly comparable. They show single qubits, small registers and early two-qubit gates, not a large processor.

FAQ

What is a nuclear spin qubit?

A qubit stored in the spin states of an atom's nucleus rather than its electrons.

Which atoms were used?

Strontium-87 in one paper and ytterbium-171 in two others, all in optical tweezers.

How long do they stay coherent?

The papers report echo coherence of 40 plus or minus 7 s for strontium-87 and T2 of 7.9 s for ytterbium-171.

Have gates been shown?

Yes. The ytterbium work reports single-qubit gates and Rydberg-blockade entangling gates.

Is this a finished computer?

No. These are small-array laboratory experiments.