28th December 2017

Silicon quantum computer chip revealed

Researchers at the University of New South Wales (UNSW) have published a complete design for a quantum computer chip that can be manufactured using traditional silicon. Furthermore, their design can be scaled up to millions of qubits.



Artist's impression of a silicon CMOS architecture for a spin-based quantum computer. (Illustration: Tony Melov)

For years, researchers around the world have been exploring different ways to design a working computing chip able to integrate quantum interactions. Now, Australian and Dutch engineers believe they have cracked the problem – reimagining the traditional silicon microprocessors we know, to create a complete design for a quantum computer chip using mostly standard industry processes and components.

The new chip design, published in the journal Nature Communications, details a novel architecture that allows quantum calculations to be performed with existing semiconductor components, known as CMOS (complementary metal-oxide-semiconductor), the basis for all modern chips.

"We often think of landing on the Moon as humanity's greatest technological marvel," said Professor Andrew Dzurak, a Program Leader at Australia's famed Centre of Excellence for Quantum Computation and Communication Technology (CQC2T). "But 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.

"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," he added.

The power of this new design is such that, for the first time, it provides a conceivable engineering pathway to creating millions of quantum bits, or "qubits", according to study co-author, Menno Veldhorst.



Architecture of a silicon CMOS chip for a spin-based quantum computer; above is mostly standard CMOS components, and below the quantum bits (Illustration: Tony Melov)

"Remarkable as they are, today's computer chips cannot harness the quantum effects needed to solve the really important problems that quantum computers will," explains Veldhorst. "To solve problems that address major global challenges – like climate change, or 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," he adds. "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."

A quantum computer exponentially expands the vocabulary of binary code used in modern computers, by using two "spooky" principles of quantum physics – namely, 'entanglement' and 'superposition'. Qubits can store a 0, a 1, or an arbitrary combination of 0 and 1 at the same time. And just as a quantum computer can store multiple values at once, so it can process them simultaneously, doing multiple operations together. This would allow a universal quantum computer to be many orders of magnitude faster than any conventional computer.

To solve complex problems, however, 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," says Professor 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."

"We expect that there will still be modifications required to this design as we move towards manufacture, but all of the key components that are needed for quantum computing are here in one chip. And that's what will be needed if we are to make quantum computers a workhorse for calculations that are well beyond today's computers," Dzurak adds. "We've shown how to integrate the millions of qubits needed to realise the true promise of quantum computing."

There are currently at least five major approaches to quantum computing being explored worldwide:

• diamond vacancies

• ion traps

• silicon spin qubits

• superconducting loops

• topological qubits

UNSW's design is based on silicon spin 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 a huge system requiring bulky supporting equipment and costly infrastructure. That's why the UNSW design is so exciting: relying on its silicon spin qubit approach – which already mimics the solid-state devices in silicon that are the heart of the $400bn global semiconductor industry – it shows how to dovetail spin qubit error correcting code into existing chip designs, enabling true universal quantum computation.

"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.

"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 the work we see from this extraordinary team puts Australia in the driver's seat."

The UNSW team has struck a A$83m deal with Telstra, Commonwealth Bank and the Australian and New South Wales governments to develop a 10-qubit prototype silicon quantum integrated circuit by 2022 – a key step in building the world's first quantum computer in silicon.

---

• Follow us on Twitter

• Follow us on Facebook

• Subscribe to us on YouTube

Comments »