Ytterbium-171 nuclear spin qubits in tweezer arrays

Short answer: Ytterbium-171 is a neutral atom whose nuclear spin of 1/2 can serve as a long-lived qubit in optical tweezer arrays. Two Physical Review X papers from 2022 report high-fidelity single-qubit operations, long coherence times and, in one, entangling gates using the Rydberg blockade. A 2024 npj Quantum Information paper proposes encoding two qubits in one atom. These are laboratory experiments and a theory proposal.

Why ytterbium-171

The first 2022 Physical Review X paper (volume 12, article 021027) reports a qubit architecture based on ytterbium-171 atoms in an optical tweezer array. The authors say the nuclear spin of 1/2 serves as a long-lived and coherent two-level system, while the alkaline-earth-like electronic structure allows low-entropy preparation, fast qubit control and high-fidelity readout.

Loading, coherence and gates in the first 2022 paper

The authors present a near-deterministic loading protocol that fills a 10 by 10 tweezer array with 92.73(8)% efficiency and a single tweezer with 96.0(1.4)% efficiency, which they say will enable efficient and uniform loading of target arrays in the future. They perform submicrosecond qubit rotations with an optical approach, and randomized benchmarking gives an error of 5.2(5) times 10 to the minus 3 per Clifford gate. They report coherence of T2* = 3.7(4) s and T2 = 7.9(4) s, many orders of magnitude longer than the rotation pulses, and spin depolarization times of tens of seconds that can reach the 100 s scale with a several-gauss magnetic field. They use 3D Raman-sideband cooling to bring atoms near the motional ground state, which they say will be central to future two-qubit gates.

Universal gates in the second 2022 paper

The second 2022 Physical Review X paper (article 021028) says alkaline earth atoms have attracted interest because their level structure offers opportunities for improved performance, and presents what it calls the first demonstration of a universal set of quantum gate operations on a nuclear spin qubit in an alkaline earth atom, using ytterbium-171. The authors use narrow-line cooling and imaging at a newly discovered magic trapping wavelength of 486.78 nm. They demonstrate nuclear spin initialization, readout and single-qubit gates, with coherence times of T1 of about 20 s and T2* = 1.24(5) s and a single-qubit fidelity of 0.99959(6). They also demonstrate two-qubit entangling gates using the Rydberg blockade, and coherent control of these gates using light shifts on the ytterbium ion core transition at 369 nm. They describe the results as a significant step toward highly coherent gates in these tweezer arrays.

Two qubits in one atom

The 2024 npj Quantum Information paper presents an architecture for encoding two qubits within the optical clock transition and the nuclear spin-1/2 of a neutral ytterbium-171 atom. It describes a toolbox of single-atom one- and two-qubit gates, Rydberg-based two- and four-qubit gates between atoms, and quantum nondemolition readout, and uses it to show advantages for entanglement distillation and quantum error correction, which it says exhibit superior hardware efficiency because fewer two-atom operations are needed. It also presents an approach to interactive circuits and to realizing a symmetry-protected topological phase of a spin-1 chain with a shallow, constant-depth circuit. This is a proposal built on the authors' reading of recent high-fidelity control of the four-dimensional space.

Limits

The two 2022 coherence figures come from different experiments and are not directly comparable. The 2022 gate paper's entangling gates are demonstrations, and its authors frame the work as a step toward highly coherent gates, not a finished processor. The 2024 paper is a proposal.

FAQ

What makes ytterbium-171 useful as a qubit?

The first 2022 paper says its nuclear spin of 1/2 is a long-lived, coherent two-level system and its electronic structure allows low-entropy preparation, fast control and high-fidelity readout.

How efficiently were atoms loaded into the array?

The first 2022 paper reports 92.73(8)% for a 10 by 10 tweezer array and 96.0(1.4)% for a single tweezer.

How long do the qubits stay coherent?

The first 2022 paper reports T2 = 3.7(4) s and T2 = 7.9(4) s. The second reports T1 of about 20 s and T2 = 1.24(5) s.

How were two-qubit gates done?

The second 2022 paper reports entangling gates using the Rydberg blockade, with coherent control through light shifts on the ytterbium ion core transition at 369 nm.

Can one atom hold two qubits?

The 2024 paper proposes it, using the optical clock transition and the nuclear spin-1/2, and describes the gates and readout that would be needed.