Lower errors, longer lifetimes: Building better qubits at Rigetti

By Damon Russell, CTO

Building ever more powerful quantum computers requires more than increasing the number of qubits — the quality of individual qubits is equally important. Last summer we shared our plans to build and deploy a 128-qubit system, and today we’re sharing new results that pave a clear path for producing the high-performance qubits necessary to operate at that scale.

Qubits need two properties for maximized performance: lower error rates and longer lifetimes. These are enabled through the use of low-noise control electronics to perform operations on qubits, and by preventing unwanted interactions between qubits and their environment. Carefully balancing these dual goals of control and isolation increases the likelihood of achieving high-quality results.

Fine-tuned control

Unlocking the power of larger chips relies on how well we can perform operations called two-qubit gates, which are a critical ingredient for any quantum computation. Off-the-shelf control electronics typically used to create these gates also introduce noise into the system, which can lead to higher error rates.

Last year we discovered a “sweet spot” where qubits become much less sensitive to this noise. Enhanced by our custom-built control electronics that deliver cleaner signals, we were able to operate two-qubit gates at this sweet spot with fidelities as high as 99.2%. What’s more, these improvements are scalable to larger lattices, and will be incorporated in devices deployed to our Quantum Cloud Services platform later this year.

High-performance chips

Qubit lifetimes are a driving factor in the ability to scale quantum programs with our chips. Lifetimes are generally defined by how long it takes energy to drain from a qubit into its environment. For superconducting qubits, our goal is to prevent the silicon and metal interfaces on the chip from absorbing this energy for as long as possible.

Since opening Fab-1 in 2017, we have marched steadily toward creating qubits with reproducible lifetimes across devices. Our latest work describes, for the first time, an end-to-end series of optimizations at various device interfaces that together achieve functional qubit lifetimes as high as 110 microseconds.

Both of these results rival state-of-the-art performance in superconducting qubits. We are actively working to combine insights from both bodies of work to yield even greater performance gains in the future, and are excited to continue to deliver steady performance improvements to QCS as we pursue quantum advantage.

1] S. Hong, A. Papageorge, and P. Sivarajah, et al (2019). “Demonstration of a Parametrically-Activated Entangling Gate Protected from Flux Noise”. https://arxiv.org/abs/1901.08035

2] A. Nersisyan, S. Poletto, N. Alidoust, R. Manenti, et al (2019). “Manufacturing low dissipation superconducting quantum processors”. https://arxiv.org/abs/1901.08042