Design unveiled for a silicon quantum computer chip

As research teams worldwide explore ways to design a computer chip that can integrate quantum interactions, UNSW engineers believe they have found the solution.

The researchers have designed a novel architecture, described in the journal Nature Communications, that allows quantum calculations to be performed using existing semiconductor components, known as CMOS (complementary metal-oxide-semiconductor) — the basis for all modern chips. It is thus feasible that their hypothetical quantum computer chip could be manufactured using mostly standard industry processes and components.

“Creating a microprocessor chip with a billion operating devices integrated together to work like a symphony — that you can carry in your pocket! — is an astounding technical achievement, and one that’s revolutionised modern life,” said Andrew Dzurak, director of the Australian National Fabrication Facility at UNSW and program leader at the Centre of Excellence for Quantum Computation and Communication Technology (CQC²T).

“With quantum computing, we are on the verge of another technological leap that could be as deep and transformative. But a complete engineering design to realise this on a single chip has been elusive. I think what we have developed at UNSW now makes that possible. And, most importantly, it can be made in a modern semiconductor manufacturing plant.”

Menno Veldhorst, lead author on the paper, was a research fellow at UNSW when the conceptual work was done. Now a team leader in quantum technology at QuTech in the Netherlands, he said the new design charts a conceivable engineering pathway towards creating millions of quantum bits, or qubits.

“Remarkable as they are, today’s computer chips cannot harness the quantum effects needed to solve the really important problems that quantum computers will,” said Veldhorst. “To solve problems that address major global challenges — like climate change or complex diseases like cancer — it’s generally accepted we will need millions of qubits working in tandem. To do that, we will need to pack qubits together and integrate them, like we do with modern microprocessor chips. That’s what this new design aims to achieve.

“Our design incorporates conventional silicon transistor switches to ‘turn on’ operations between qubits in a vast two-dimensional array, using a grid-based ‘word’ and ‘bit’ select protocol similar to that used to select bits in a conventional computer memory chip. By selecting electrodes above a qubit, we can control a qubit’s spin, which stores the quantum binary code of a 0 or 1. And by selecting electrodes between the qubits, two-qubit logic interactions, or calculations, can be performed between qubits.”

Artist’s impression of the architecture of a silicon CMOS chip for a spin-based quantum computer; above is mostly standard CMOS components, and below the quantum bits in operation. Illustration: Tony Melov.

A quantum computer exponentially expands the vocabulary of binary code used in modern computers by using two principles of quantum physics: entanglement and superposition. Qubits can store a 0, a 1 or an arbitrary combination of 0 and 1 at the same time. Just as a quantum computer can store multiple values at once, so it can process them simultaneously, doing multiple operations at once. This would allow a universal quantum computer to be millions of times faster than any conventional computer when solving a range of important problems.

But to solve complex problems, a useful universal quantum computer will need a large number of qubits, possibly millions, because all types of qubits we know are fragile, and even tiny errors can be quickly amplified into wrong answers.

“So we need to use error-correcting codes which employ multiple qubits to store a single piece of data,” said Dzurak. “Our chip blueprint incorporates a new type of error-correcting code designed specifically for spin qubits, and involves a sophisticated protocol of operations across the millions of qubits. It’s the first attempt to integrate into a single chip all of the conventional silicon circuitry needed to control and read the millions of qubits needed for quantum computing.”

Video credit: UNSW TV. Animations: Tony Melov/UNSW.

There are at least five major quantum computing approaches being explored worldwide: silicon spin qubits (UNSW’s approach), ion traps, superconducting loops, diamond vacancies and topological qubits. The main problem with all of these approaches is that there is no clear pathway to scaling the number of quantum bits up to the millions needed without the computer becoming huge a system requiring bulky supporting equipment and costly infrastructure.

However, by relying on the silicon spin qubit approach — which already mimics much of the solid-state devices in silicon that are the heart of the US$380 billion global semiconductor industry — the UNSW design shows how to dovetail spin qubit error-correcting code into existing chip designs, enabling true universal quantum computation.

Unlike other groups, CQC²T’s quantum computing effort is focused on creating solid-state devices in silicon, from which all of the world’s computer chips are made. The centre is not just creating ornate designs to show off how many qubits can be packed together; rather, it is aiming to build qubits that could one day be easily fabricated — and scaled up.

“It’s kind of swept under the carpet a bit, but for large-scale quantum computing, we are going to need millions of qubits,” said Dzurak. “Here, we show a way that spin qubits can be scaled up massively. And that’s the key.”

The UNSW silicon quantum chip team. Photo: Grant Turner/UNSW.

Earlier this year, the UNSW team struck an $83 million deal between UNSW, Telstra, Commonwealth Bank and the Australian and NSW Governments to develop, by 2022, a 10-qubit prototype silicon quantum integrated circuit — the first step in building the world’s first quantum computer in silicon. This came almost two years after Dzurak and Veldhorst first showed how quantum logic calculations could be done in a real silicon device, with the creation of a two-qubit logic gate — the central building block of a quantum computer.

“Those were the first baby steps, the first demonstrations of how to turn this radical quantum computing concept into a practical device using components that underpin all modern computing,” said UNSW Dean of Engineering Mark Hoffman. “Our team now has a blueprint for scaling that up dramatically.

“We’ve been testing elements of this design in the lab, with very positive results. We just need to keep building on that — which is still a hell of a challenge, but the groundwork is there, and it’s very encouraging. It will still take great engineering to bring quantum computing to commercial reality, but clearly the work we see from this extraordinary team at CQC²T puts Australia in the driver’s seat.”

Top image caption: Artist’s impression of a silicon CMOS architecture for a spin-based quantum computer. Illustration: Tony Melov.