Control of spin qubits at near absolute zero provides path forward for scalable quantum computing

Developing technology that allows quantum information to be both stable and accessible is a critical challenge in the development of useful quantum computers that operate at scale. Research published in the journal Nature provides a pathway for scaling the number of quantum transistors (known as qubits) on a chip from current numbers under 100 to the millions needed to make quantum computation a practical reality. The result is enabled by new cryogenic control electronics that operate at close to absolute zero, developed at the University of Sydney.

Lead researcher Professor David Reilly from the University of Sydney Nano Institute and School of Physics said, “This will take us from the realm of quantum computers being fascinating laboratory machines to the stage where we can start discovering the real-world problems that these devices can solve for humanity.”

The paper is the result of industry cooperation between the University of Sydney and the University of New South Wales through the respective quantum tech spin-out companies Emergence Quantum and Diraq. Professor Reilly’s company, Emergence Quantum, was established this year to commercialize quantum control technologies and other advanced electronics like the chip presented in this Nature paper.

For this research, his team developed a silicon chip that can control spin qubits at milli-kelvin temperatures. That’s just slightly above absolute zero (-273.15 degrees Celsius), the temperature at which—theoretically—matter ceases moving.

Of the many emerging qubit technologies, experts think that spin qubits (where information is encoded onto the magnetic direction of single electrons) could more easily scale up, as they are based on common CMOS (complementary metal-oxide-semiconductor) technology that underpins modern conventional computing and is already used to print billions of transistors.

World-first semiconductor control system integrated with qubits

However, spin qubits must be kept at temperatures below 1 kelvin to preserve their information. To scale up, they must also be controlled and measured using complex integrated electronics. This created a serious concern that even if the control system could work at that temperature, the heat and electrical interference generated by placing the control so close to the qubits would degrade their performance.

Professor Reilly’s team has, for the first time, shown that with careful design, this need not be the case—a vital proof-of-principle demonstration that spin qubits in CMOS could be scaled up to the millions of qubits to make a useful machine.

Professor Reilly said, “This result has been more than a decade in the making, building up the know-how to be able to design electronic systems that dissipate tiny amounts of power and operate near absolute zero. We have now demonstrated a scalable control platform that can be integrated with qubits without destroying the fragile quantum states.

“This validates the hope that indeed qubits can be controlled at scale by integrating complex electronics at cryogenic temperatures. Our paper shows that with careful design of the control system, fragile qubits hardly notice the switching of transistors in a chip less than a millimeter away.”

Good science, good commercialization

The qubits were supplied by Diraq, a UNSW spin-out established by Professor Andrew Dzurak, and the knowledge that enabled the University of Sydney-designed control chip will now carry over to underpin much of the work of the new company, Emergence Quantum, co-founded by Professor Reilly and Dr. Thomas Ohki.

Professor Reilly said, “As well as good science, this is a good commercialization story, too, and is more evidence of why Sydney is a vital cog in the global quantum industry.”

Lead author Dr. Sam Bartee, who undertook experiments as a Ph.D. student with Professor Reilly at the University of Sydney, is now working at Diraq.

He said, “It’s extremely exciting to be part of this work, to be involved in the development of such powerful technologies and to sit in this hotspot of quantum computing research. Sydney really is a remarkable place to be a quantum engineer at the moment.”

Dr. Kushal Das was the lead designer of the control chip. He holds a joint position with the University of Sydney and Emergence Quantum.

Dr. Das said, “Now that we have shown that milli-kelvin control does not degrade the performance of single- and two-qubit quantum gates, we expect many will follow our lead. Fortunately for us, this is not so easy, but requires years to build up the know-how and expertise to design low-noise cryogenic electronics that need only tiny amounts of power.”

Professor Reilly added, “Here we are showing the impact that cryogenic electronics can have on scaling up qubits, but we see many further diverse applications for this technology, spanning near-term sensing systems to the data centers of the future.”

Diraq CEO Professor Andrew Dzurak said, “This advance underpins Diraq’s goal of integrating our silicon qubits with classical control electronics in one compact package, opening the path to affordable quantum computers that consume much less energy.”

Dr. Bartee and his co-authors measured the performance characteristics of one- and two-qubit operations controlled by the cryo-CMOS chiplet. They compared its performance against that of a standard cable-connected room-temperature control system.

Their findings include:

• Negligible fidelity loss for single-qubit operations;
• No measurable reduction of the coherence time for one- and two-qubit operations;
• Comparable behavior of qubit interactions, indicating negligible interference from electrical noise.

Remarkably, these feats were achieved within a power envelope of just 10 microwatts, the vast majority of which was expended on the digital systems. The analog components dissipate only around 20 nanowatts per megahertz, which means that the system can be scaled up to millions of qubits without a significant increase in power usage.