Andrew Houck: New materials that extend performance of quantum computers

Nov. 2, 2021

A significant boost in the stability of qubits — the key components at the heart of quantum computers — could lead to significant improvements in performance.

Quantum computers work by manipulating quantum bits, or quibits — units of computing hardware that obey the laws of quantum mechanics — to tackle problems that ordinary computers cannot. The most advanced quantum computers rely on superconducting bits known as transmon qubits, but these can easily be destroyed by heat, contaminants or defects in its materials.

Now a team of researchers at Princeton has dramatically improved the performance of transmon qubits by replacing their superconducting innards with another, previously untried superconducting metal. The team used tantalum rather than the usual niobium to craft two key parts of the qubit, the capacitor and the microwave resonator.

Quantum computer

Transmon qubits consist of nano-sized circuits patterned from superconducting materials onto the surface of a crystal such as sapphire. The depositing of tantalum in places that would normally contain niobium required substantial reconfiguring of equipment and procedures.

The result is a qubit with a lifetime that is three times longer previous qubits, a significant jump in viability. The resulting qubits have remarkably consistent performance. Replacing niobium with tantalum improved transmon qubit lifetimes across various designs, shapes and fabrication processes.

 Tantalum transmon

The staying power of the qubits at the heart of quantum computers can be dramatically improved by crafting components from tantalum (blue) on sapphire (gray).

The researchers theorize that tantalum’s performance can be explained by its reaction with oxygen. Tantalum oxides are insulating and can reduce loss in the device, whereas the oxides at the niobium surface can include noninsulating materials that lead to microwave loss.

"These qubits, which we engineered by systematically trying different superconducting materials, are delivering a whole new level of performance." – Andrew Houck

Andrew Houck, Professor of Electrical and Computer Engineering

Nathalie de Leon, Assistant Professor of Electrical and Computer Engineering

Robert Cava, Russell Wellman Moore Professor of Chemistry

Mattias Fitzpatrick, Princeton Ph.D. 2019, now at IBM Research

Alex Place, Lila Rodgers and Basil Smitham, Graduate Students in Electrical and Computer Engineering

Berthold Jäck, Hong Kong University of Science and Technology;

András Gyenis, Princeton Ph.D. 2016, University of Colorado-Boulder;

Nan Yao, Senior Research Scholar, Princeton Institute for the Science and Technology of Materials and Director, Imaging and Analysis Center

Team members:
Pranav Mundada, Zhaoqi Leng and Andrei Vrajitoarea, Princeton Ph.D.s 2021; Anjali Premkumar and Jacob Bryon, Graduate Students in Electrical and Computer Engineering; Trisha Madhavan and Harshvardhan Babla, Class of 2021; Xuan Hoang Le and Youqi Gang, Class of 2022.

Development status:
Patent protection is pending. Princeton is seeking outside interest for the development of this technology.

National Science Foundation, U.S. Army Research Office

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