Australian scientists get a compact and sensitive reading of the qubit

Michelle Simmons, led by the Australian of the year, overcame another technical hurdle essential to building a silicon-based quantum computer.

Simmons' team at UNSW in Sydney has presented a compact sensor to access information stored in the electrons of individual atoms – a breakthrough that brings us closer to evolving quantum computing in silicon.

The research, conducted within the Simmons group of the Center for Excellence in Quantum Computing and Communication Technologies (CQC2T) with PhD student Prasanna Pakkiam as lead author, was published on November 27 in the Physical Review X.

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

READ: International scientists discuss silicon quantum computing in Australia

The creation of qubits by precisely positioning and encapsulating individual phosphorus atoms in a silicon chip is a unique Australian approach that the Simmons team leads globally.

But adding all the connections and all the access points needed to develop the architecture of the phosphorus atom was a challenge – until now.

"To monitor even a single bit, you have to create multiple connections and gates around individual atoms, where there is not much room," Simmons said.

"Plus, you need high quality qubits close to each other so they can talk to each other – which is only possible if you have as little door infrastructure as possible."

Compared to other approaches for the fabrication of a quantum computer, the Simmons system already had a relatively low gate density. However, the conventional measurement still required at least 4 gates per qubit: 1 to control it and 3 to read it.

By integrating the reading sensor into one of the control gates, the UNSW team was able to reduce it to just two doors: one for control and one for reading.

Lead author Pakkiam stated that not only was the system more compact, but that by integrating a superconducting circuit attached to the gate, the team now had the sensitivity needed to determine the quantum state of the qubit by measuring whether an electron was moving between two neighboring atoms.

"And we showed that we could do it in real time with just one measurement – just one shot – without it being necessary to repeat the experiment and average the results," Pakkiam said.

Simmons said this represents a major breakthrough in reading the information embedded in our qubits.

"The result confirms that single-gate qubit playback now has the sensitivity needed to perform the quantum error correction required for an evolving quantum computer," Simmons said.