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Because it avoids the need for per-qubit lasers, it is more easily scaled to many qubits, according to Oxford Ionics, the company behind it, which is working with the Universities of Oxford and Oregon.
Oxford Ionics is also claiming a record for its IC among trapped-ion quantum logic: 99.97% fidelity with two-qubit gates, and also 99.9992% fidelity for single-qubit operations. “Previous world records have been achieved with the use of error correction to reduce errors in hardware. Oxford Ionics chips provide over twice the performance, without needing error correction, using 10x fewer qubits,” it said.
In its demonstrator, which has sites (~350μm across) for seven calcium ions in a row held 40μm above the IC surface, a pair of linear conductor carrying RF alternating current passes under all of the sites and impose a common alternating magnetic field on all of them. At the same time, a static ~8.5mT magnetic bias field is arranged across the surface of the whole IC.
A third linear conductor, also passing under all of the sites, is pulsed with AC to initiate quantum interactions once pre-interaction conditions have been set.
Key to the scheme is a way to individually fine-tune each ion by creating a locally-shaped electric field – as an alternative to laser fine-tuning.
This electric field comes from multiple conductive surface patches around each ion site, where each of the patches can be individually set to arbitrary dc voltages – an approach well suited to integrated circuit techniques, the company points out.
By adjusting the shared AC magnetic drive and local DC electrostatic drives, all seven qubits can be prepared for interaction, or some can be temporarily hidden while two or any other remaining sub-set are prepared for interaction, or, say, a pair at one end of the seven and a pair at the other end can be separately prepared.
The same DC electrodes are also used to create the quantum wells which hold the ions in place, and are coordinated with neighbouring electrodes to shunt ions across the IC.
“This unique, embedded approach takes the highest performing qubit technology – trapped ions – and integrates everything needed to control them into a silicon chip that can be mass-produced using standard semiconductor manufacturing facilities and processes,” said Oxford Ionics.
Although qubit interactions in the demonstrator are laser-free, a set of lasers above the middle of the seven sites are used for support functions including ion loading, ion cooling, qubit initialisation and qubit measurement.
The company’s aim is to develop the concept into integrated 2D arrays of qubits, still sharing a three AC magnetic drive and trigger electrodes, and having individually-sculptured local electrostatic fields, to “build a 256 qubit chip that can be manufactured on existing semiconductor production lines” it said.
The company has funding to produce ‘Quartet’, one of several prototype quantum computers selected for the UK’s National Quantum Computing Centre (NQCC).
“The new results mark a pivotal step forwards in ion trap quantum computing and validate the scalability of the technology,” said NQCC director Michael Cuthbert. “The reported one and two qubit gate results out-perform other players’ achievements to date, meaning error correction becomes achievable with minimal overheads.”
Oxford Ionics was founded in 2019 by Tom Harty (now CTO) and Chris Ballance (now CEO). Its team, according to the company, includes “55 global experts across physics, quantum architecture, engineering and software”, which is expect to grow to ~150 in three years. So far it has £37m from investors including Braavos, OSE, Lansdowne Partners, Prosus Ventures, 2xN and Hermann Hauser. It also has an R&D partnership with Infineon Technologies.
The work is described in the paper ‘Scalable, high-fidelity all-electronic control of trapped-ion qubits‘, submitted to the open-access archive Arxiv.
