Benchmarking memory and logic gates for trapped-ion quantum computing
<p>Trapped ions are a promising platform for experimental quantum computing, possessing the longest decoherence times and the highest fidelity logic gates of any candidate technology. The challenge remains to scale up ion-trap systems to larger numbers of qubits. This thesis benchmarks two fun...
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| Other Authors: | |
| Format: | Thesis |
| Language: | English |
| Published: |
2021
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| _version_ | 1817932881915281408 |
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| author | Hughes, AC |
| author2 | Lucas, D |
| author_facet | Lucas, D Hughes, AC |
| author_sort | Hughes, AC |
| collection | OXFORD |
| description | <p>Trapped ions are a promising platform for experimental quantum computing, possessing the longest decoherence times and the highest fidelity logic gates of any candidate technology. The challenge remains to scale up ion-trap systems to larger numbers of qubits. This thesis benchmarks two fundamental operations for scalable trapped-ion quantum computing: the memory performance of a <sup>43</sup>Ca<sup>+</sup> hyperfine qubit, and mixed-element entangling gates between <sup>43</sup>Ca<sup>+</sup> and <sup>88</sup>Sr<sup>+</sup>.</p>
<p>Decoherence is usually measured over long timescales, where the memory errors ε are large compared to state-preparation and measurement errors. Information about the small-error regime relevant for quantum computing is inferred by extrapolation. In this work we use randomised benchmarking to directly measure errors as small as 1.2(7) × 10<sup>−6</sup> after a storage time of 1 ms, which is around an order of magnitude smaller than
would be expected based on the usual model of exponential decay. We find ε<sub>m</sub> < 10<sup>−4</sup> for up to 50 ms with no additional dynamical decoupling, and ε<sub>m</sub> < 10<sup>−3</sup> for up to 2 seconds using a simple CPMG sequence. These
timescales exceed previously-demonstrated gate or measurement times in trapped-ion systems by several orders of magnitude — a requirement for quantum error correction, and a highly desirable feature for near-term
processors. The qubit is robust to offsets of the external magnetic-field strength, with ε<sub>m</sub> < 10<sup>-4</sup> for 1 ms even at a 50 mG offset, and we identify phase noise on the reference oscillator as the limiting factor on the
memory performance.</p>
<p>We provide a comparison of different implementations of mixed-element geometric phase gates in the same experimental system. This includes a light-shift gate which can be implemented on both ion species using a single laser, with a fidelity of 99.8(1)% or 99.7(1)%, measured using partial state tomography or interleaved randomised benchmarking respectively. We also demonstrate several Mølmer–Sørensen gates with measured fidelities of up to 99.6(2)% (by partial state tomography); this mechanism is more susceptible to errors arising from instability of the external magnetic field. For the first time, this pushes mixed-element entangling gate fidelities over the fault-tolerant threshold level (above which error correction is possible), and puts them on par with state-of-the-art single-species gates.</p> |
| first_indexed | 2024-03-06T21:16:10Z |
| format | Thesis |
| id | oxford-uuid:3fdb0b98-3c50-483c-b66e-132495ce71af |
| institution | University of Oxford |
| language | English |
| last_indexed | 2024-12-09T03:44:58Z |
| publishDate | 2021 |
| record_format | dspace |
| spelling | oxford-uuid:3fdb0b98-3c50-483c-b66e-132495ce71af2024-12-07T16:37:17ZBenchmarking memory and logic gates for trapped-ion quantum computingThesishttp://purl.org/coar/resource_type/c_db06uuid:3fdb0b98-3c50-483c-b66e-132495ce71afEnglishHyrax Deposit2021Hughes, ACLucas, D<p>Trapped ions are a promising platform for experimental quantum computing, possessing the longest decoherence times and the highest fidelity logic gates of any candidate technology. The challenge remains to scale up ion-trap systems to larger numbers of qubits. This thesis benchmarks two fundamental operations for scalable trapped-ion quantum computing: the memory performance of a <sup>43</sup>Ca<sup>+</sup> hyperfine qubit, and mixed-element entangling gates between <sup>43</sup>Ca<sup>+</sup> and <sup>88</sup>Sr<sup>+</sup>.</p> <p>Decoherence is usually measured over long timescales, where the memory errors ε are large compared to state-preparation and measurement errors. Information about the small-error regime relevant for quantum computing is inferred by extrapolation. In this work we use randomised benchmarking to directly measure errors as small as 1.2(7) × 10<sup>−6</sup> after a storage time of 1 ms, which is around an order of magnitude smaller than would be expected based on the usual model of exponential decay. We find ε<sub>m</sub> < 10<sup>−4</sup> for up to 50 ms with no additional dynamical decoupling, and ε<sub>m</sub> < 10<sup>−3</sup> for up to 2 seconds using a simple CPMG sequence. These timescales exceed previously-demonstrated gate or measurement times in trapped-ion systems by several orders of magnitude — a requirement for quantum error correction, and a highly desirable feature for near-term processors. The qubit is robust to offsets of the external magnetic-field strength, with ε<sub>m</sub> < 10<sup>-4</sup> for 1 ms even at a 50 mG offset, and we identify phase noise on the reference oscillator as the limiting factor on the memory performance.</p> <p>We provide a comparison of different implementations of mixed-element geometric phase gates in the same experimental system. This includes a light-shift gate which can be implemented on both ion species using a single laser, with a fidelity of 99.8(1)% or 99.7(1)%, measured using partial state tomography or interleaved randomised benchmarking respectively. We also demonstrate several Mølmer–Sørensen gates with measured fidelities of up to 99.6(2)% (by partial state tomography); this mechanism is more susceptible to errors arising from instability of the external magnetic field. For the first time, this pushes mixed-element entangling gate fidelities over the fault-tolerant threshold level (above which error correction is possible), and puts them on par with state-of-the-art single-species gates.</p> |
| spellingShingle | Hughes, AC Benchmarking memory and logic gates for trapped-ion quantum computing |
| title | Benchmarking memory and logic gates for trapped-ion quantum computing |
| title_full | Benchmarking memory and logic gates for trapped-ion quantum computing |
| title_fullStr | Benchmarking memory and logic gates for trapped-ion quantum computing |
| title_full_unstemmed | Benchmarking memory and logic gates for trapped-ion quantum computing |
| title_short | Benchmarking memory and logic gates for trapped-ion quantum computing |
| title_sort | benchmarking memory and logic gates for trapped ion quantum computing |
| work_keys_str_mv | AT hughesac benchmarkingmemoryandlogicgatesfortrappedionquantumcomputing |