Australian scientists reach compact and sensitive qubit reading



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The authors of the article, from left to right: doctoral student Mark R. Hogg; Professor Michelle Simmons; Post Doc Matthew G. House; PhD student Prasanna Pakkiam; Post Doc Andrey Timofeev

The authors of the article, from left to right: doctoral student Mark R. Hogg; Professor Michelle Simmons; Post Doc Matthew G. House; PhD student Prasanna Pakkiam; Post Doc Andrey Timofeev

An Australian-led group of the year, Michelle Simmons, overcame another critical technical hurdle for building a silicon-based quantum computer.

Simmons's team at UNSW Sydney demonstrated a compact sensor to access information stored in individual atom electrons – a breakthrough that brings us one step closer to silicon-scalable quantum computing.

The research, conducted within the Simmons group at the Center of Excellence for Computing and Quantum Communication Technology (CQC2T) with doctoral student Prasanna Pakkiam as lead author, was published Nov. 27 in the journal Physical Review X.

Quantum bits (or qubits) made from electrons housed in single atoms in semiconductors is a promising platform for large-scale quantum computers, thanks to their long-lasting stability.

Read: International scientists discuss quantum silicon computing in Australia

The creation of qubits precisely positioning and encapsulating individual phosphorus atoms within a silicon chip is a unique Australian approach that the Simmons team has been leading globally.

But adding all the connections and gates needed to scale up the architecture of phosphorus atoms would be a challenge – until now.

"To monitor up to a qubit, you need to build multiple connections and gates around individual atoms where there is not much space," Simmons said.

"In addition, you need high-quality qubits nearby so they can talk to each other, which is only feasible if you have the smallest gate infrastructure possible."

Compared to other approaches to making a quantum computer, the Simmons system already had a relatively low port density. However, conventional metering still required at least 4 ports per qubit: 1 to control it and 3 to read it.

By integrating the readout sensor into one of the control ports, the UNSW team managed to drop it in only two gates: 1 for control and 1 for reading.

Principal author Pakkiam said the system is not only more compact but by integrating a superconducting circuit connected to the gate, the team now has the sensitivity to determine the quantum state of the qubit by measuring whether an electron moves between two neighboring atoms.

"And we've shown that we can do this in real time with just one measurement – a single shot – without the need to repeat the experiment and average the results," said Pakkiam.

Simmons said that represents a breakthrough in how we read the information embedded in our qubits.

"The result confirms that reading single-port qubits is now reaching the sensitivity needed to perform the quantum error correction required for a scalable quantum computer," Simmons said.

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