NSF recipient Vatsal Bandaru, a first-year doctoral student in Dr. Anthony Sigillito’s lab, co-invented a theoretical architecture at the University of Wisconsin, Madison that enables robust, shuttling-based semiconductor qubit processors.
The work in Dr. Sigillito’s lab aligns with Bandaru’s overall research goals: to scale-up semiconductor qubit systems.
Qubits or quantum bits represent the building blocks for quantum computing as they leverage quantum mechanical phenomena. And semiconductor materials can host qubits in carefully isolated quantum dots. In Dr. Sigillito’s lab, researchers make qubits using the spins of individual electrons trapped in silicon. Bandaru’s work focuses on two interlinked challenges: scalability and coupling distant qubits.
According to Bandaru, scalability is a major hurdle. Semiconductor heterostructures require gate electrodes to confine quantum dots, “which host spin qubits.” A fan-out problem arises because as the number of qubits in an array increases, the number of necessary gates becomes too large to fit on the chip. Moreover, error-stabilizing quantum gate operations are currently performed serially, leading to timescales that conflict with qubit relaxation times. “It’s not feasible,” Vatsal said, referring to scaling-up and controlling spin-qubit arrays with current methods.
Coupling distant qubits also poses problems. For quantum algorithms, distant qubits may need to be entangled, Bandaru said. Problems arise because within current spin qubits, entanglements require the qubits to remain very close. “It’s something known as exchange,” Bandaru said, explaining that the need to keep them close is inconvenient for scalability. “With the spin qubit arrays, you can’t necessarily have qubits that are this far apart interact very easily unless you conduct SWAP operations, which brings them closer.”
While SWAP operations bring qubit states closer, they have their limitations as well. For example, if one conducts 100 SWAP gate operations, this can quickly lead to a loss of fidelity due to relevant qubit timescales. The large numbers, “can lead to errors somewhere,” Bandaru explained.
Dr. Sigillito “has ideas to address both the [scalability] and distant coupling problems,” Bandaru explained, adding that the “hope is that they will enable scaling-up to utility-scale quantum computing.”
As for the timeline? “That’s an ongoing process,” Bandaru said. “New problems consistently arise in experimental work. We just need to keep plugging away at it.”
When Bandaru’s not working, he enjoys cooking, playing the flute, video games, and reading.