Exploring quantum building blocks
Superconducting qubits, typically composed of thin layers of superconducting, semiconducting and insulating materials, serve as a quantum computer’s basic units of information. Adopting a similar methodology pioneered by the Fermilab SRF (superconducting radio frequency) research group, the SQMS Center is pushing the performance of quantum devices, starting with a fundamental understanding of the nanostructural and superconducting properties of the materials employed. Cavities and qubits of varying performance levels are being dissected and analyzed with a broad array of experimental techniques. The investigations provide researchers with insights into the nanoscale and atomic scale mechanisms limiting quantum coherence.
Qubits require near-perfect conditions to exist and certain material characteristics can decrease the qubit lifespan. This phenomenon, called quantum decoherence, is a critical obstacle to overcome to be able to solve impactful computational problems with quantum processors.
The first step to reduce or eliminate quantum decoherence is to develop a solid understanding of its root causes in quantum devices. SQMS scientists are studying in great depth a broadly employed qubit device called the transmon qubit. It is made of several layers or films of semiconducting, dielectric and superconducting materials. Each layer and each interface can play an important role in quantum decoherence. They offer “traps” where microwave photons, key in storing and processing quantum information, can be absorbed and disappear.
When it comes to developing a state-of-the-art quantum computer, you must start at the materials level.
Carrying measurements of the qubit in its entirety does not allow researchers to easily distinguish where those traps are located, nor specifically which of the various materials or interfaces are the primary drivers of decoherence. Using very sensitive tools, scientists at the SQMS Center are taking the systematic approach of studying the effect of each of the materials making up the transmon qubits.
SQMS has launched the largest coordinated study of qubit performance: hundreds of qubits of different performance levels are cut into fragments and studied with the most advanced surface science and superconducting characterization techniques. Experimentalists and theorists interpret these results and give feedback to the device fabrication experts on what kinds of defects or material properties may be responsible for the different levels of performance.
SQMS has also launched a nanofabrication task force, a coordinated study across many national qubit foundries producing transmon qubits, including the PNF facility at University of Chicago, NUANCE at Northwestern University, NIST Colorado nanofab facilities and our industrial partner, Rigetti Computing.
Moreover, SQMS researchers are using the uniquely sensitive high Q SRF resonators as sample host devices to directly measure the impact of various materials making up the qubits on losses in the tens-of-millikelvin temperature range and with parts-per-billion precision.
Access to the unique array of SQMS facilities at Fermilab and the Center’s partner institutions enables researchers to conduct in-depth materials analysis of their devices, from microscopic scales down to the atomic scale. Through these findings, researchers are working to engineer materials with the ideal chemical and physical structures to achieve targeted performance metrics for a quantum computer housed at SQMS.