Qubit coherence loss linked to thermal dissipation in superconducting circuits

Qubit coherence loss linked to thermal dissipation in superconducting circuits
by Robert Schreiber
Berlin, Germany (SPX) Aug 26, 2024
Physicists at Aalto University in Finland, in collaboration with an international team, have both theoretically and experimentally demonstrated that the loss of coherence in superconducting qubits can be directly attributed to thermal dissipation within the electrical circuits housing the qubits.

Superconducting Josephson junctions are the fundamental components of qubits-the essential units of quantum information in advanced quantum computers and ultrasensitive detectors. These qubits and their associated circuits are known for their exceptional electrical conductivity.

"Despite the fast progress of making high-quality qubits, there has remained an important unresolved question: how and where does thermal dissipation occur?" stated Bayan Karimi, a postdoctoral researcher in the Pico research group at Aalto University and the first author of the study.

"We have developed for a long time the methods for measuring this loss based on our group's expertise in quantum thermodynamics," added Jukka Pekola, the professor leading the Pico research group at Aalto University.

As researchers continue their efforts to improve the efficiency of qubits, this new understanding of qubit decay provides valuable insights. In quantum computing, longer coherence times for qubits translate into the ability to perform more operations, enabling the execution of more complex calculations that are beyond the reach of classical computers.

Thermal Radiation Identified as the Culprit
The Josephson effect, which enables supercurrent transmission between two closely spaced superconducting materials without any applied voltage, plays a critical role in the function of qubits. The study reveals that previously unexplained energy loss in these systems can now be traced back to thermal radiation that originates at the qubits and travels through the connecting leads.

Karimi likened this thermal dissipation to the experience of feeling warmth from a campfire on a cold beach: the surrounding air remains cold, but the warmth radiates from the fire to the individual. This same type of radiation is responsible for energy loss in qubits.

This phenomenon has been observed in previous experiments involving large arrays of Josephson junctions. In these arrays, thermal dissipation at one junction seemed to destabilize others in the circuit, similar to a chain reaction.

Karimi, Pekola, and their colleagues initially explored this dissipation by experimenting with arrays of many Josephson junctions. However, they gradually refined their experiments to simpler setups, ultimately focusing on a single Josephson junction. By placing an ultrasensitive thermal absorber near this junction, the team could measure the weak radiation emitted during phase transitions across a wide frequency range, up to 100 gigahertz.

Research Report:Bolometric Detection of Josephson Radiation