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Qubit Reader Brings Quantum Computing Closer

The development of a compact sensor that can access information stored in the electrons of individual atoms brings scalable quantum computing in a silicon chip one step closer. The development, conducted within Prof Michelle Simmons’ group at the Centre of Excellence for Quantum Computation and Communication Technology at The University of NSW, was published in Physical Review X (

Quantum bits (or qubits) made from electrons hosted on single atoms in semiconductors are a promising platform for large-scale quantum computers due to their stability. Creating qubits by precisely positioning and encapsulating individual phosphorus atoms within a silicon chip is a unique Australian approach that Simmons’ team has been pursuing.

However, the number of connections and gates required to scale up the phosphorus atom architecture has been a challenge. “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 says. “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.”

Compared with other approaches for making a quantum computer, Simmons’ system already had a relatively low gate density. However, conventional measurement still required at least four gates per qubit: one to control it and three to read it. By integrating the read-out sensor into one of the control gates, the team has been able to drop this to just two gates: 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,” says lead author Prasanna Pakkiam. “And we’ve shown that we can do this in real-time with just one measurement – single shot – without the need to repeat the experiment and average the outcomes.”

Simmons says that “this represents a major advance in how we read information embedded in our qubits. 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.”