Speediest quantum operation yet

Australian scientists have linked together two electrons spins embedded in a silicon chip to form a two-qubit gate, the fundamental building block of a quantum computer.

The gate is 200 times faster than any other of its type, taking a mere 0.8 nanoseconds to complete an operation, and uses atom-based qubits that are known for their high accuracy and extremely low noise.

The team from UNSW Sydney reports in the journal Nature that the device is of a type known as a SWAP gate, in which quantum information is exchanged between the two qubits: electron spins attached to phosphorus atoms embedded in the silicon crystal.

The speed of the gate comes from the close proximity of the two atoms, just 13 nanometres apart. At this small separation their interaction can be strong yet still tightly controlled. Although isolated, they were close enough to be pushed together by a voltage to enable information swapping.

Team leader, and 2018 Australian of the Year, Michelle Simmons says the precision required to make the device was at the limit of what was humanly possible.

“A lot of people thought this would not be possible. To be able to control nature at its very smallest level so that we can create interactions between two qubits but also individually talk to each qubit without disturbing the other is incredible,” she says.

“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.”

Now the team has created a two-qubit gate, it plans to combine them to start building a fully-fledged quantum computer. Its goal is a 10-qubit integrated circuit, which it aims to create in the next three to four years.

A two-qubit gate is an important step towards that goal. It is the equivalent of a logic gate in a conventional computer; and, when combined with single qubit gate, can be used to run any quantum algorithm.

The team at UNSW uses a scanning tunneling microscope to profile the exact shape of each electron orbital, known as its wavefunction, to work out the best angles and distances required for buiding a scalable processor.

“We can actually image the wavefunction directly with the scanning probe tip, and feed that into the mathematical model to find the best way to design it,” Simmons says.

One of the surprises was that the gate runs better using asymmetric qubits.  One qubit is an electron sitting in a group of three phosphorus atoms, while the second qubit contains only two atoms.

The team also found a way to increase the speed of the initialisation and read-out of the qubits by attaching a type of circuit called a tank circuit, which uses AC rather than DC measurements.

The power of a quantum computer grows exponentially with the number of qubits, but Simmons points out that before that power can be realised, quantum error correction is needed requiring additional qubits to correct for errors.

“To make a universal quantum computer you want to make sure the errors are not accumulating,” she says.

“So you surround the data qubits with ancillary qubits that you can entangle with them to probe what’s going on without disturbing the data qubit.

“Error correction is a complex process, necessary for large-scale computers, and increases the numbers of physical qubits ultimately needed.”