According to recent research, the previously puzzling thermal energy loss in qubits can now be clarified using a surprisingly straightforward experimental arrangement.
Researchers from Aalto University in Finland, in collaboration with an international team, have demonstrated both theoretically and experimentally that the loss of coherence in superconducting qubits can be measured as thermal dissipation in the electrical circuit that supports the qubit.
Superconducting Josephson junctions, integral components of qubits—also referred to as quantum bits—are essential for the most sophisticated quantum computers and highly sensitive detectors. These qubits and their associated circuitry are known for their exceptional ability to conduct electricity.
‘Despite rapid advancements in developing high-quality qubits, an important question has remained: how and where does thermal dissipation happen?’ states Bayan Karimi, a postdoctoral researcher in the Pico research group at Aalto University and the paper’s lead author.
‘We have spent significant time perfecting measurement techniques for this loss based on our group’s expertise in quantum thermodynamics,’ comments Jukka Pekola, the head of the Pico research group at Aalto University.
As physicists strive to create ever more efficient qubits in the development of quantum technology, this new information helps researchers gain insights into the decay of their qubits. Longer coherence times in qubits enable more operations, allowing for complex calculations that cannot be accomplished using classical computers.
Warmth in the air
Supercurrents are transmitted thanks to the Josephson effect, where two closely spaced superconducting materials can carry current without any voltage applied. This study has identified previously unexplained energy loss as thermal radiation originating from the qubits and traveling down the leads.
Consider a campfire warming a person at the beach—the surrounding air remains cool, yet the individual feels warmth radiating from the fire. Karimi explains that a similar type of radiation causes dissipation in the qubit.
This loss has been previously observed by physicists studying extensive arrays of hundreds of Josephson junctions arranged in a circuit. Like a game of telephone, when one junction destabilizes, it can affect those further down the line.
Starting with experiments involving these numerous junctions, Karimi, Pekola, and their team simplified their approach step by step. Their ultimate experimental setup involved examining the effects of adjusting the voltage at a single Josephson junction. They positioned an ultrasensitive thermal absorber next to this junction, allowing them to passively detect the very faint radiation emitted during each phase transition across a wide range of frequencies, up to 100 gigahertz.
Their theoretical analysis was conducted in collaboration with colleagues from the University of Madrid. This research was published in Nature Nanotechnology on August 22nd.
The project was conducted in partnership with Professor Charles Marcus of the InstituteQ Chair of Excellence at the University of Washington in the USA, and at the Niels Bohr Institute in Copenhagen, Denmark. The devices utilized in these experiments were made in OtaNano, Finland’s national research facility for micro- and nanotechnologies. Additionally, support from the Research Council of Finland through the Quantum Technology Finland (QTF) Centre of Excellence and THEPOW consortium made this work possible.
