The compact sensor, say the researchers, represents a breakthrough toward the goal of scalable quantum computing in silicon - a promising platform for large-scale quantum computers. The researchers have been working on an approach for creating qubits - the basic unit of quantum information - by precisely positioning and encapsulating individual phosphorus atoms within a silicon chip.
But adding in all the connections and gates required for scale-up of the phosphorus atom architecture presented a significant 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," says Professor Michelle Simmons, who led the effort. "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."
Conventional measurement, say the researchers, requires at least four gates per qubit: one to control it and three to read it. However, by integrating a read-out sensor into one of the control gates, the researchers have been able to reduce this to just two gates: one for control and one for reading.
"Not only is our system more compact," says PhD student Prasanna Pakkiam and lead author of a paper on the research, "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 neighboring atoms. 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."
"This represents a major advance in how we read information embedded in our qubits," says Simmons. "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."