Mechanical vibrations cause errors in superconducting qubits

Quantum computers, which use quantum bits or "qubits" to process information, hold the potential to solve problems much faster than traditional computers. However, keeping qubits stable and minimizing errors is a significant challenge that researchers are striving to overcome.

Superconducting qubits, based on Josephson junctions, are the leading platform for quantum computing due to their fast, high-fidelity control and scalability, pursued by major companies like Google and IBM. Despite their potential, their error rates are primarily limited by their quantum lifetimes. While there have been considerable studies and significant improvements in the qubit lifetimes over the decades, the exact causes of errors in superconducting qubits are not fully understood.

Errors in qubits can come from many sources, including dielectric loss, defects in materials, and spontaneous emissions. Until now, the role of mechanical vibrations in causing errors has not been well understood in superconducting devices, even though these vibrations could be affecting the performance of quantum computers.

Now, researchers led by Tobias Kippenberg, have discovered that mechanical vibrations from the cooling systems used in superconducting quantum computers can cause errors in qubits. These vibrations, originating from the pulse tube cooler in a dilution refrigerator, induce changes in the qubits, causing them to make mistakes simultaneously.

“Coherent and stable superconducting qubits are essential to realize quantum computing. However, as qubits become more coherent, they also become more sensitive to small changes in their environments,” says Shingo Kono, who led the project. “We capitalized on this sensitivity to investigate the error mechanisms affecting state-of-the-art superconducting devices.”

The researchers created highly coherent qubits using advanced nanofabrication processes and characterized their performance over time. The superconducting qubits featured long “relaxation times” (the time it takes for a system to return to its original state), exceeding 0.4 milliseconds, which is significant in itself, as longer relaxation times are useful for quantum computing.

The team synchronized their measurements with the operation of the pulse tube cooler to see how mechanical vibrations affected the qubits. This innovative approach allowed them to link specific mechanical vibrations to errors in the qubits.

The study revealed that mechanical vibrations from the cooler affected the entire chip, causing simultaneous errors in multiple qubits, known as “correlated errors.” This error mechanism would pose a significant challenge for achieving fault tolerance toward large-scale quantum computing, as error correction protocols strictly assume that errors occur independently.

“Although the physical origin could not be determined unambiguously in this work, our findings deepen our understanding of qubit error mechanisms,” says Kono. “More importantly, our observations provide valuable insights into potential error-mitigation strategies for achieving fault tolerance by decoupling superconducting qubits from their mechanical environments.”