Sudden transition in superconductors could shift quantum technology into high gear

Insider’s memory

• Researchers have discovered that highly disordered superconductors, such as indium oxide films, undergo abrupt first-order quantum phase transitions, challenging traditional theories that assume gradual transitions.
• The results, which reveal a sharp drop in superfluid stiffness at a critical disorder level, have significant implications for quantum computing, particularly in the design of superconducting circuits and stable superinductors.
• The study highlights the need to revisit existing models of quantum phase transitions and explore how disorder influences superconductivity in other materials.

Researchers have discovered that some disordered superconductors exhibit abrupt phase transitions, a finding that challenges established theories and could have implications for quantum computing.

A study published in Nature by researchers studying films of indium oxide – a highly disordered superconductor – shows that their transition from a superconducting to an insulating state is not gradual, as traditionally assumed, but sudden . This abrupt change, known as a first-order quantum phase transition, contrasts with the continuous second-order transitions commonly observed in superconductors.

Key measurements revealed a sharp drop in the rigidity of the superfluid, a property that reflects the ability of the superconducting state to resist phase distortions, at a critical level of disorder. Interestingly, the critical temperature of these films, where superconductivity collapses, no longer depends on the strength of the electronic pairing but rather on the rigidity of the superfluid. This behavior aligns with a pseudogap regime, in which electron pairs exist but lack the coherence necessary for superconductivity.

“This discontinuous transition highlights the role of repulsive interactions between Cooper pairs and the subsequent competition between superconductivity and insulating glass Cooper pairs,” the researchers write in the study. “Moreover, we show that the critical temperature of the films is no longer linked to the matching amplitude but aligns with the superfluid stiffness, which is consistent with the pseudogap regime of the preforms. Our results raise new fundamental questions about the role of disorder in quantum phase transitions and have implications for superinductances in quantum circuits.

Integral part of quantum computing

The study has direct implications for quantum computing hardware, particularly in the design of superconducting circuits. Superconductors are an integral part of quantum computers and form the basis of qubits and components such as superinductors. The observed sudden phase transitions could shed light on how materials are designed to improve the stability and performance of quantum systems.

Materials with low superfluid stiffness but retaining Cooper pair formation, like those in the study, could also help create more efficient superinductors. These are necessary to protect qubits from external disturbances and improve coherence times, which are essential for practical applications of quantum computing.

Control manufacturing conditions

The researchers studied thin films of amorphous indium oxide, adjusting their level of disorder by controlling their manufacturing conditions. The team used advanced microwave spectroscopy to measure the plasmonic spectrum of superconducting resonators made from these films. These measurements allowed a precise extraction of the superfluid rigidity and its behavior under increasing disorder.

They observed that as disorder increased, superfluid stiffness exhibited an unexpected sharp drop instead of the gradual decline predicted by existing theories. This jump signaled a break in macroscopic coherence, marking the transition from a superconducting state to an insulating state.

Boundaries

Although the study provides compelling evidence for a first-order transition, it raises new questions about the underlying mechanisms. The role of repulsive interactions between Cooper pairs and the emergence of a localized Cooper pair glass – a state in which electron pairs are immobilized – are not fully understood. Further research into the microscopic details of these interactions is needed to develop comprehensive theories.

Research has also been limited to indium oxide, a specific type of disordered superconductor. The question of whether similar transitions occur in other materials remains open.

Future Directions

The results highlight the need to revisit established models of quantum phase transitions, particularly in disordered systems. Future work could explore the applicability of these results to other superconductors and study the interplay between material properties, disorder and quantum phenomena.

Additionally, the study highlights the potential for developing new quantum circuit components. Superinductors, for example, could benefit from a deeper understanding of how disorder influences superfluid stiffness and phase transitions.

Researchers and Institutions

This study was carried out by Thibault Charpentier, David Perconte, Sébastien Léger, Kazi Rafsandjani Amin, Florent Blondelle, Frédéric Gay, Olivier Buisson, Nicolas Roch and Benjamin Sacépé of the University of Grenoble Alpes, CNRS, Grenoble INP, Institut Néel; Lev Ioffe of Google Research, USA; Anton Khvalyuk and Mikhail Feigel’man from LPMMC, Université Grenoble Alpes; Igor Poboiko from the Karlsruhe Institute of Technology; and Mikhail Feigel’man also affiliated with the CENN Nanocenter and the Jožef Stefan Institute.