Junctions between two superconductors, so-called Josephson junctions, can carry a dissipationless supercurrent. They play a crucial role in modern quantum technologies. Indeed, they are the key element to realize superconducting qubits, quantum systems with two different energy levels that represent the logical 0 and 1 necessary to encode “quantum information”. In the simplest case, the amplitude of the supercurrent in a Josephson junction is controlled by a single variable, the superconducting phase difference across the junction. If the junction is embedded in an electric circuit, this variable follows the rules of quantum mechanics, giving rise to the discrete energy levels needed for the realization of a qubit.
On a microscopic level, the energy of the junction is related to states localized in the junction. These bound states may be empty or filled. A description of the junction in terms of the superconducting phase difference only is possible when the bound states are empty. As observed in recent experiments, this is not always the case. Notably, a single quasiparticle may get trapped in a bound state and stay there for a long time. When the bound state is filled, the junction is said to be “poisoned” and the supercurrent was so far expected to vanish.
Researchers in the theory group of Pheliqs in collaboration with TU Delft showed that the quantum mechanics of a poisoned junction is fundamentally different from the conventional case. Strikingly, while the supercurrent is always suppressed by the electric circuit in the conventional case, it is restored in a poisoned junction and may even surpass the supercurrent in the conventional case.
Hybrid superconductor/semiconductor/superconductor junctions are a promising platform to study these effects. These systems are extensively studied in the context of topological qubits, a type of qubits that is expected to allow for intrinsically robust encoding of quantum information. The present work opens up new perspectives in the understanding of their intriguing properties.
Fundings
Collaboration
Yuli Nazarov
TU Delft, The Netherlands