Researchers Achieve First Two-Qubit Gate Between Silicon Atom Qubits

A team of physicists led by UNSW Sydney has developed a super-fast version of the major building block of a quantum computer. The study is the landmark outcome of a vision first proposed by scientists two decades earlier.

From left to right: Professor Michelle Simmons; Dr Sam Gorman, Postdoc Research Associate; Dr Yu He, Postdoc Research Associate; Ludwik Kranz, PhD student; Dr Joris Keizer, Senior Research Fellow; Daniel Keith, PhD student. (Image credit: UNSW Sydney)

Led by Professor Michelle Simmons, 2018 Australian of the Year, a research team has realized the first-ever two-qubit gate between atomic qubits in silicon—a significant breakthrough in the team’s mission to develop an atom-scale quantum computer. The crucial study was reported in Nature, a world-renowned journal, on July 18th, 2019.

A two-qubit gate is the backbone of any quantum computer. The UNSW team’s version of it is the fastest ever to demonstrated in silicon, finishing an operation in 0.8 ns—about 200 times faster than other current spin-based two-qubit gates.

In the approach of the Simmons’ team, a two-qubit gate is a function between two electron spins—similar to the role played by classical logic gates in traditional electronics. For the first time, the researchers could develop a two-qubit gate by positioning two atom qubits closer to each other than ever before, and then controllably detecting and measuring their spin states in real time.

The unique technique used by the team for quantum computing necessitates not just the placement of individual atom qubits in silicon but all the related circuitry to initialize, control, and read-out the qubits at the nanoscale—an idea that mandates exceptional precision that it was long considered to be impossible. However, this major breakthrough will now enable the researchers to translate their technology into scalable processors.

According to Professor Simmons, Director of the Centre of Excellence for Quantum Computation and Communication Technology (CQC2T) and founder of Silicon Quantum Computing Pty Ltd, previous results from the last decade perfectly set the team up to transfer the boundaries of what is considered to be “humanly possible.”

Atom qubits hold the world record for the longest coherence times of a qubit in silicon with the highest fidelities. Using our unique fabrication technologies, we have already demonstrated the ability to read and initialise single electron spins on atom qubits in silicon with very high accuracy. We’ve also demonstrated that our atomic-scale circuitry has the lowest electrical noise of any system yet devised to connect to a semiconductor qubit.

Professor Michelle Simmons, Director, Centre of Excellence for Quantum Computation and Communication Technology

Simmons continued, “Optimising every aspect of the device design with atomic precision has now allowed us to build a really fast, highly accurate two-qubit gate, which is the fundamental building block of a scalable, silicon-based quantum computer.”

We’ve really shown that it is possible to control the world at the atomic scale—and that the benefits of the approach are transformational, including the remarkable speed at which our system operates.”

Professor Emma Johnston AO, UNSW Science Dean, stated that this important study further demonstrates just how pathbreaking Professor Simmons’ research is.

Johnston added, “This was one of Michelle’s team’s final milestones to demonstrate that they can actually make a quantum computer using atom qubits. Their next major goal is building a 10-qubit quantum integrated circuit—and we hope they reach that within 3-4 years.”

Getting up and Close with Qubits—Engineering with a Precision of Just Thousand-Millionths of a Meter

The researchers had to first calculate the optimal distance between two qubits to facilitate the crucial operation, with the help of a scanning tunnelling microscope to precision-place and encapsulate phosphorus atoms in silicon.

Our fabrication technique allows us to place the qubits exactly where we want them. This allows us to engineer our two-qubit gate to be as fast as possible. Not only have we brought the qubits closer together since our last breakthrough, but we have learnt to control every aspect of the device design with sub-nanometer precision to maintain the high fidelities.

Sam Gorman, Study Lead Co-Author, Centre of Excellence for Quantum Computation and Communication Technology

Observing and Controlling Qubit Interactions in Real Time

Then, the researchers were able to measure the evolution of the qubits states in real time. Moreover, most fascinatingly, the scientists demonstrated ways to control the interaction strength between two electrons on the nanosecond timescale.

Importantly, we were able to bring the qubit’s electrons closer or further apart, effectively turning on and off the interaction between them, a prerequisite for a quantum gate. The tight confinement of the qubit’s electrons, unique to our approach, and the inherently low noise in our system enabled us to demonstrate the fastest two qubit gate in silicon to date.

Yu He, Study Lead Co-Author and Postdoc Research Associate, UNSW Sydney

Yu He added, “The quantum gate we demonstrated, the so-called SWAP gate, is also ideally suited to shuttle quantum information between qubits—and, when combined with a single qubit gate, allows you to run any quantum algorithm.”

A Thing of Physical Impossibility? Not Anymore

According to Professor Simmons, this is the pinnacle of two decades’ worth of studies.

Simmons further stated that “This is a massive advance: to be able to control nature at its very smallest level so that we can create interactions between two atoms but also individually talk to each one without disturbing the other is incredible. A lot of people thought this would not be possible.”

The promise has always been that if we could control the qubit world at this scale, they would be fast, and they sure are!”


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