UNSW's new compact quantum sensor can read single atoms in real-time

A research team from the University of NSW has demonstrated a new compact sensor for accessing information stored in the electrons of individual atoms could be a valuable building block for scalable quantum computers.

The team led by professor Michelle Simmons at the Centre of Excellence for Quantum Computation and Communication Technology (CQC2T) has developed a gate array that can get a much closer read on the electrons of phosphorous atoms in silicon chips, increasing the speed and accuracy of the results.

“To monitor even one qubit, you have to build multiple connections and gates around individual atoms, where there is not a lot of room,” Simmons said.

“What’s more, you need high-quality qubits in close proximity so they can talk to each other – which is only achievable if you’ve got as little gate infrastructure around them as possible.”

An approach developed by Simmons of using phosphorous-based qubits already had a relatively low gate density, with four gates per qubit: one to control it, and three to read it.

However, the amount of space needed to add in all the extra gates reduced the sensitivity of the results, meaning the readings would have to be taken several times and the results averaged out to get a more accurate picture.

Now Simmons' team have demonstrated that, by integrating the read-out sensor into one of the control gates, it’s possible to achieve a compact array with only two gates per qubit: one for control, and one for reading.

“Not only is our system more compact, but by integrating a superconducting circuit attached to the gate we now have the sensitivity to determine the quantum state of the qubit by measuring whether an electron moves between two neighbouring atoms,” said lead author of the team’s research paper Prasanna Pakkiam.

“And we’ve shown that we can do this real-time with just one measurement – single shot – without the need to repeat the experiment and average the outcomes.”

Simmons said the results in the paper illustrate a major advance in how information embedded in qubits is read.

“The result confirms that single-gate reading of qubits is now reaching the sensitivity needed to perform the necessary quantum error correction for a scalable quantum computer.”

The new gate array will also inform Simmons’ work with the Silicon Quantum Computing company, a joint commercial venture by the Australian and NSW governments and the Commonwealth Bank of Australia which is collaborating with international researchers to produce a 10-qubit demonstration device by 2022.

However, there’s still a way to go before a functional quantum computer is developed, with other Australian researchers exploring the use of different particles, including photons, as the base for durable qubits.