Qubit quality as measured by coherence times and gate fidelities is a very important factor in determining the overall performance of a quantum computer. In fact, many researchers will say that qubit quality is even more important than the raw number of qubits in a hardware design. There have been tremendous improvements in qubit quality metrics over the past several years and this trend is still continuing.
In a series of blog postings, Rigetti Computing disclosed significantly improved qubit quality metrics for their new Aspen architecture. The first improvement is in the T1 relaxation time. In a paper published on arXiv, Rigetti reported they have achieved an average T1 time of 76 microseconds across 24 bits with a T1 time greater than 110 microseconds on the best qubit. This compares to average T1 times of 13 to 20 microseconds in the previous architecture. And in another paper on arXiv, they reported a best case two-qubit gate fidelity of 99.2%, much better than their previous generation which had maximum gate fidelities in the 93-95% range. The Rigetti team achieved these results by implementing process fabrication improvements, optimizing the RF modulation of the control signals and moving to custom-built control electronics to provide cleaner signals with less noise.
Although we regard this data as preliminary, it does compare favorably with other superconducting qubit implementations that we show on our Qubit Quality page. However, we would point out that some key data is still not available including single qubit gate fidelities, readout fidelities and T2 phase coherence times. In addition, we are expecting improved qubit quality metrics from IBM’s new Q System One computer but this data is also not available yet. Google has also not yet publicly released qubit quality information on their Bristlecone chip, but we are aware that maximizing the qubit quality is an important goal for that program.