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Researching Abroad for Applications to be used in Quantum Computing

NDEE Grad. Student Thomas Zirkle and the active-interferometry technique

Researching Abroad for Applications to be used in Quantum Computing

Written by: Leslie Lestinsky

NDEE graduate student, Thomas Zirkle was invited to take his cutting-edge research abroad. Zirkle’s primary research focus is the measurement of cryogenic electronic devices, under the supervision of professors Gregory Snider and Alexi Orlov. In June 2017, he received the Châteaubriand Fellowship, awarded by the French Embassy. The fellowship afforded him the opportunity to work at the French Alternative Energies and Atomic Energies Commission, at the Grenoble, France center. Zirkle worked with a team of scientists, doctoral students, and post-doctoral associates under the advising of professors Xavier Jehl and Silvano De Franceschi.  Thomas Zirkle in Toulouse

During his time there, he was able to use an active-interferometry technique to take cryogenic measurements, which until now, has not been used on single electron devices. He used this technique to take and analyze measurements of silicon-based quantum dots. These dots have the potential to be used as qubits in quantum computing. “Quantum computing is still in its infancy, creating a true quantum computer is difficult,”, said Zirkle. “One of the challenges is trying to measure qubits without having an extra measuring apparatus associated with it.”

The active-interferometry technique begins by reflecting radio frequency waves off the gate, which controls the state of the quantum dot. Similar to that of a valve on a hose, it controls the flow of electrons through the dot. When the radio frequency waves reflect off the gate, they are coupled to the dot and as the dot changes states, the changes can be sensed through variations in the reflected radio frequency wave. “In the past, this has been done with inductors and capacitors,” explained Zirkle. Interferometry is the process by which a secondary wave is injected to null the signal, which results in greater sensitivity to changes in reflective wave. “With inductors and capacitors, you can’t easily predict what will happen as the temperature changes. The active-interferometry technique is tunable, so that we can tune the measurement as the temperature changes,” explained Zirkle.  

Large-scale applications of his research are potentially in quantum computing, and other future computing technologies. One of the major challenges for quantum computing to work, is the need for many qubits. Currently, to make just a few qubits you need a whole cryogenic system which needs to be thermally isolated and requires several costly, heavy cables. “It becomes very complicated, very fast,” explained Zirkle. Scalability is also an issue. “You can make one qubit, that’s great but then you need to take all the pieces and multiply that by thousands,” explained Zirkle. “Cryostats are only so big. The technology is moving forward and becoming better all the time but at what point are they too expensive and too big?”

Zirkle’s work with gate-coupled reflectometry will hopefully reduce the amount of cabling required for quantum computing. “If you’re doing reflectometry off the gate, your signal goes through the same cable essentially that you’ll use to control the gate voltage. You’ll need that cable anyhow,” said Zirkle. For these silicon qubits, using the gate-based reflectometry is a simpler practice.  In the past, to measure a qubit, a single electron transistor (SET), off to the side, was essential. It would couple capacitively to the qubit, or quantum dot, and changes in the qubit would affect the SET, producing a measurable signal. Now, by measuring through the gate, the need to have an SET, or quantum point contact is removed. “This simplifies things and makes it easier to scale, so that someday if we want to make large systems, we’ll need fewer wires by using this gate-coupled reflectometry technique,” explained Zirkle

What they thought would take two weeks, to test the active-interferometry technique, took four months. However, Zirkle is satisfied with that work, because at the end of the day, he got the technique to work and was able to gather results. “It gives a graduate student a little more life when you get something to work on your own,” said Zirkle. He submitted a paper explaining the results to the Silicon Nanoelectronics Workshop and gave a poster presentation at their June 2018 conference.

Zirkle in France

As Zirkle was exposed to new people and cultures, he realized and appreciated that he could interact professionally in a foreign country. “Notre Dame gave me the background and education to be able to collaborate and communicate well with international peers in my field,” said Zirkle. “Being exposed to different ideas and people, seeing another country’s take on the world was a great experience."