Hot Qubits, Cool Logic
28 March 2024 by Diraq

Quantum computing, poised to revolutionise various industries such as materials science, medicine and pharmaceuticals, and banking and finance, has faced formidable obstacles in its development, with heat being a significant hindrance. Diraq’s recent technological breakthrough, published in the prestigious Nature journal, features the capability of their spin-based quantum processors to operate at temperatures more than ten times hotter than previously possible whilst maintaining stability and high accuracy. This innovation allows Diraq quantum computers to be faster, more cost effective, and more environmentally sustainable.

Semiconductor spins are widely recognised as one of the most scalable technologies for quantum processors, due to their compact size and compatibility with existing silicon chip manufacturing [Zwerver et al., Nat Electron 5, 184–190 (2022); Gonzalez-Zalba et al., Nat Electron 4, 872–884 (2021)]. Crucially, they can operate above one kelvin (1 K), as discovered in two inaugural studies in 2020 – one by led by Dr Henry Yang, Head of Quantum Control at Diraq, and another at TU Delft [Yang et al., Nature 580, 350–354 (2020); Petit et al., Nature 580, 355–359 (2020)]. 

“Compared to millikelvin temperatures, temperatures above one Kelvin relax the constraints imposed by the tiny cooling power of dilution refrigerators by many orders of magnitude, allowing a practical quantum computer with integrated classical control electronics to be operated in a simple cryo-platform,” states Professor Andrew Dzurak, CEO and Founder of Diraq.

While showing promise, the earlier studies noted above showed only limited fidelities for qubit control, initialisation, and readout, representing a set of fundamental challenges posed at elevated temperatures. Firstly, there was no natural means to set the qubits to a certain initial state because the spins are easily excited by the much greater thermal energy above one kelvin. As a result, the spin coherence times are shorter, being more prone to erroneous changes in their polarisation and phase, which increases qubit error rates. Furthermore, qubit readout fidelity was degraded due to the charge sensors becoming less sensitive at elevated temperatures. These obstacles appeared to make the prospect of fault-tolerance at elevated temperatures unlikely, or highly challenging at best.

In our new work led by Jonathan Huang, Research Associate at Diraq and PhD student at UNSW, Sydney (refer to article: High-fidelity spin qubit operation and algorithmic initialization above 1K) we made several important advances in addressing these formidable challenges. Firstly, we demonstrated deterministic initialisation of a pair of qubits from a fully thermalised state, fulfilling an important prerequisite for all quantum applications at elevated temperatures. Secondly, backed by state-of-the-art readout techniques, we achieved the highest-ever qubit initialisation and readout fidelities above one kelvin. Thirdly, we benchmarked the qubits and measured the most accurate universal quantum logic in semiconductor spin qubits at this temperature, and additionally presented a full analysis of the errors in the logic gates.

This campaign dated back to shortly after Dr Yang's work in 2020, when an idea to initialise a pure two-qubit state was formulated by Mr Huang, Dr Yang and Dr Andre Saraiva, Head of Solid-State Theory at Diraq. In this new concept, we employ an entropy transfer engine, once described as “a being who can play a game of skill with the molecules” by Maxwell, to cool the qubits to their lowest-energy state. Specifically, we devised an algorithm that manipulates and filters the two-qubit state, allowing only the target state to pass. On the experimental front, this was realised using two-qubit logic gates and high-fidelity radio-frequency readout, together with hardware with fast real-time logic. As a result, in less than 200 microseconds, we were able to initialise and read out the qubits with fidelities up to 99.34 per cent above one kelvin, reducing the errors by an order of magnitude compared with previous results. Moreover, this method works at ultra-low external magnetic fields, where the thermal energy is more than 20 times larger than the qubit energy!

With the qubits well-initialised, we mapped out the relaxation, excitation, and decoherence of qubits at different temperatures and magnetic fields. We tailored the qubit control strategies accordingly to achieve the highest to-date single-qubit gate fidelity of 99.85 percent and the highest to-date two-qubit gate fidelity of 98.92 per cent at the technologically significant temperature of one kelvin. These advances now bring semiconductor spin qubits operating above one kelvin into the realm of fault tolerance.

To gain insight into the remaining qubit errors, we employed machine learning and Bayesian tomography to obtain a full diagnosis of qubit initialisation, readout, and control at different temperatures, which will guide the development of future quantum error correction protocols.

This engineering and scientific feat by Diraq reports not only a significant milestone in spin-based quantum computing, but also comprehensive information for future studies covering spin physics, quantum information, computer science, quantum thermodynamics, and nano-device engineering. “These outcomes substantiate the prospect of operating a large-scale quantum computer, built using CMOS chip manufacturing methods, without the restrictions on thermal budget imposed by millikelvin operation.” commented co-author Professor Andrea Morello, an Academic Partner of Diraq’s, and Leader in Spin Dynamics and Education at UNSW Sydney.

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