Two-qubit gate achieves superfast quantum operations

A group of physicists at UNSW Sydney, led by 2018 Australian of the Year Professor Michelle Simmons, have achieved what they claim to be the first two-qubit gate between atom qubits in silicon. Described in the journal Nature, the breakthrough marks a major milestone in the team’s quest to build an atom-scale quantum computer.

A two-qubit gate is the central building block of any quantum computer. The UNSW team’s version of the gate is said to be the fastest that’s ever been demonstrated in silicon, completing an operation in 0.8 ns — around 200 times faster than other existing spin-based two-qubit gates.

The group approached the two-qubit gate as an operation between two electron spins, comparable to the role that classical logic gates play in conventional electronics. The team was able to build the gate by placing two atom qubits closer together than ever before, and then — in real time — controllably observing and measuring their spin states.

The team’s approach to quantum computing requires not only the placement of individual atom qubits in silicon but all the associated circuitry to initialise, control and read out the qubits at the nanoscale — a concept that requires such precision it was long thought to be impossible. But with this major milestone, the team is now positioned to translate their technology into scalable processors.

“Atom qubits hold the world record for the longest coherence times of a qubit in silicon with the highest fidelities,” said Prof Simmons, Director of the Centre of Excellence for Quantum Computation and Communication Technology (CQC²T) and founder of Silicon Quantum Computing.

“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.

“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.”

The atomically engineered two-qubit gate. The researchers not only brought the qubits closer together since their last breakthrough — just 13 nm apart — they also engineered all the control circuitry with sub-nanometre precision.

Using a scanning tunnelling microscope to precision-place and encapsulate phosphorus atoms in silicon, the team first had to work out the optimal distance between two qubits to enable the crucial operation. As explained by CQC²T’s Dr Sam Gorman, lead co-author on the study, “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-nanometre precision to maintain the high fidelities.”

The team was then able to measure how the qubits’ states evolved in real time. Most excitingly, the researchers showed how to control the interaction strength between two electrons on the nanosecond timescale.

“Importantly, we were able to bring the qubits’ electrons closer or further apart, effectively turning on and off the interaction between them — a prerequisite for a quantum gate,” said lead co-author Dr Yu He.

“The tight confinement of the qubits’ 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.

“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.”

Prof Simmons said the breakthrough is a “massive advance” and the culmination of two decades’ worth of work.

“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,” she said. “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!”

UNSW Science Dean Professor Emma Johnston AO added that the paper shows just how groundbreaking Prof Simmons’ research is.

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

Top image caption: Dr Joris Keizer, Professor Michelle Simmons and Dr Yu He at CQC²T.

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