Abstract
The development of scalable, high-fidelity qubits is a key challenge in quantum information science. Neutral atom qubits have progressed rapidly in recent years, demonstrating programmable processors1,2 and quantum simulators with scaling to hundreds of atoms3,4. Exploring new atomic species, such as alkaline earth atoms5,6,7, or combining multiple species8 can provide new paths to improving coherence, control and scalability. For example, for eventual application in quantum error correction, it is advantageous to realize qubits with structured error models, such as biased Pauli errors9 or conversion of errors into detectable erasures10. Here we demonstrate a new neutral atom qubit using the nuclear spin of a long-lived metastable state in 171Yb. The long coherence time and fast excitation to the Rydberg state allow one- and two-qubit gates with fidelities of 0.9990(1) and 0.980(1), respectively. Importantly, a large fraction of all gate errors result in decays out of the qubit subspace to the ground state. By performing fast, mid-circuit detection of these errors, we convert them into erasure errors; during detection, the induced error probability on qubits remaining in the computational space is less than 10−5. This work establishes metastable 171Yb as a promising platform for realizing fault-tolerant quantum computing.
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Data availability
The data reported in this manuscript are available in the Harvard Dataverse online repository at https://doi.org/10.7910/DVN/TJ6OIF.
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Acknowledgements
We acknowledge helpful conversations with S. Kolkowitz and M. Gullans. This work was supported by the Army Research Office (W911NF-1810215), the Office of Naval Research (N00014-20-1-2426), DARPA ONISQ (W911NF-20-10021), the National Science Foundation (QLCI grant OMA-2120757) and the Sloan Foundation. This research also received funding from the European Union’s Horizon 2020 programme under the Marie Sklodowska-Curie project 955479 (MOQS), the Horizon Europe programme HORIZON-CL4-2021-DIGITAL-EMERGING-01-30 via the project 101070144 (EuRyQa) and from the French National Research Agency under the Investments of the Future Program project ANR-21-ESRE-0032 (aQCess).
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S.M., G.L., P.P. and B.Z. performed the experiments described in the main text and analysed the data. A.P.B. contributed to experiments developing the initialization and readout procedures for the metastable qubit. S.J. and G.P. performed theoretical modelling of optimal control gate sequences and collaborated with the experimental team on gate optimization. J.C., S.P., P.P. and J.D.T. developed approaches to benchmark gates in the presence of erasure errors. All authors discussed the results. S.M., G.L., P.P., B.Z. and J.D.T. wrote the manuscript with input from all authors.
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G.P. is co-founder and shareholder of QPerfect.
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Extended data figures and tables
Extended Data Fig. 1 Level diagrams and laser beam geometry.
a, Partial level diagram showing the transitions used to optically pump into the state \(\left|1\right\rangle \) for initialization. b, Partial level diagram indicating the transitions used to measure the spin state in 3P0. First, atoms in \(\left|1\right\rangle \) are removed from the trap using Rydberg excitation and subsequent autoionization. Then, all population in 3P0 is pumped back to 1S0 and imaged. c, Propagation directions and polarizations of the lasers addressing the atoms. The 556 nm and 399 nm imaging beams are not shown, but are co-propagating with the 770 nm beam and retro-reflected. The microscope objective used to project the tweezers and image the atoms (numerical aperture NA=0.6) is positioned above the glass cell. d, Partial level diagram showing the transitions between 3P0 and the Rydberg manifold used in this work. The detuning between the Rydberg states is 5.8 times larger than ΩUV. The 302 nm beam is linearly polarized perpendicular to the magnetic field, which is constrained by the geometry of our apparatus. In the future, using a pure σ+-polarized 302 nm beam would increase the gate speed by a factor of \(\sqrt{2}\) for the same laser power.
Extended Data Fig. 2 Rydberg laser system.
a, The 302 nm light is generated by a resonant cavity. The output beam is power-stabilized by a servo formed by AOM1 and PD1, and pulses are generated by AOM2. The pulsed light is coupled into a fibre and delivered to a monolithic breadboard next to the glass cell. PD2 monitors the pulse power on the breadboard. To monitor the pulse phase, a small fraction of the light is sent back to the optical table with a second fibre, and interfered with the un-modulated laser to form a beatnote on PD3 (at the frequency of AOM2). The beatnote is digitized and digitally demodulated to extract the amplitude and phase profiles shown in b and c, together with the target pulse shapes (solid lines). Programming AOM2 with a naive waveform results in phase distortion; the waveform shown in c is obtained after closed-loop correction.
Extended Data Fig. 3 Randomized circuit benchmarking experiment.
a, Sequence of operations for the entangling gate randomized circuit benchmarking experiment (the horizontal axis is not to scale). b, Comparison of the simulated gate fidelity and simulated randomized circuit benchmarking fidelity for the error model discussed in the Methods. The strength of each noise term is varied randomly around the nominal value expected for the experiment. The randomized circuit benchmarking infidelity is typically within 10% of the true gate error (shaded region), indicating that it is a good estimator of the true gate fidelity.
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Ma, S., Liu, G., Peng, P. et al. High-fidelity gates and mid-circuit erasure conversion in an atomic qubit. Nature 622, 279–284 (2023). https://doi.org/10.1038/s41586-023-06438-1
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DOI: https://doi.org/10.1038/s41586-023-06438-1
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