'This result has been more than a decade in the making': Millions of qubits on a single quantum processor now possible after cryogenic breakthrough

Scientists have developed a new type of computer chip that removes a major obstacle to practical quantum computers, making it possible for the first time to place millions of qubits and their control systems on the same device.

The new control chip operates at cryogenic temperatures close to absolute zero (about minus 459.67 degrees Fahrenheit, or minus 273.15 degrees Celsius) and, crucially, can be placed close to qubits without disrupting their quantum state.

"This result has been more than a decade in the making, building up the know-how to design electronic systems that dissipate tiny amounts of power and operate near absolute zero," lead researcher David Reilly, professor at the University of Sydney Nano Institute and School of Physics, said in a statement.

The scientists described the result as a "vital proof of principle" for integrating quantum and classical components in the same chip — a major step toward the kind of practical, scalable processors needed to make quantum computing a reality. The researchers published their findings June 25 in the journal Nature.

Qubits are the quantum equivalent of binary bits found in today's classical computers. However, where a classical bit can represent either 0 or 1, a qubit can exist in a "superposition" of both states. This enables quantum computers to perform multiple calculations in parallel, making them capable of solving problems far beyond the reach of today's computers.

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Spin qubits, a type of qubit that encodes information in the spin state of an electron, have piqued the interest of scientists because they can be built using complementary metal-oxide-semiconductor (CMOS) technology.

This is the same process used to fabricate the chips found inside modern smartphones and PCs. In theory, this makes spin qubits much easier to produce at scale as it slips into normal manufacturing methods.

Other quantum computers use different types of qubits, including superconducting, photonic or trapped-ion qubits. But unlike these other types, spin qubits can be made on a massive scale using existing equipment.

However, spin qubits need to be kept at temperatures below 1 kelvin (just above absolute zero) to preserve "coherence." This is a qubit’s ability to maintain superposition and entanglement over time, and what is needed to unlock the parallel processing power that makes quantum computing so promising. Spin qubits also need electronic equipment to measure and control their activity.

"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," Reilly added.

The road to a single million-qubit chip

Integrating the electronics required to control and measure spin qubits has long posed a challenge, as even small amounts of heat or electrical interference can disrupt the qubits' fragile quantum state.

But this new, custom CMOS chip is designed to operate in cryogenic environments and at ultra-low power levels, meaning it can be integrated onto a chip alongside qubits without introducing thermal or electrical noise that would otherwise interrupt coherence.

In tests, the researchers ran single-gate and two-qubit gate operations with the control chip positioned less than 1 millimeter (0.04 inches) from the qubits. The control chip introduced no measurable electrical noise and caused no drop in accuracy, stability or coherence, the researchers said.

Additionally, the control chip consumed just 10 microwatts (0.00001 watts) of power in total, with the analogue components — used to control the qubits with electrical pulses — using 20 nanowatts (0.00000002 watts) per megahertz.

"This validates the hope that indeed qubits can be controlled at scale by integrating complex electronics at cryogenic temperatures," Reilly 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," he added.

"We see many further diverse uses for this technology, spanning near-term sensing systems to the data centres of the future."

The findings could prompt more researchers to explore the power of spin qubits.

"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," study co-author Kushal Das, senior hardware engineer at Emergence Quantum and a researcher at the University of Sydney who designed the chip, said in the statement.

"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."