08/03/2021 at 3:30 PM #28318Anonymous UserParticipant
The paradox of the superconducting / insulating Josephson junction solved
In 1983, it was theoretically predicted by A. Schmid that any Josephson junction in its ground state, shunted by a high value resistance R should not be superconducting but insulating.
Although several experiments had claimed to confirm this , physicists from SPEC and German universities have just demonstrated that the state expected does not exist. This result thus resolves the created by the prediction, according to which at the limit R , no Josephson junction should be superconducting!
In 1962 Josephson predicted that a can circulate without applied through a thin insulating layer separating two superconductors. Following this discovery of the “Josephson junction” (JJ), a quantum electronic devices with unique properties have flourished: SQUID magnetometers, parametric amplifiers, RSFQ (Rapid Single Quantum), superconducting qubits … Among these devices, the Josephson voltage standard notably made it possible to base the international system of units on quantum effects in 2019.
In recent years, quantum electronic devices based on a JJ have also been widely cited, due to the intense research and enormous progress made in the manufacture of “quantum processors” capable of performing certain calculations beyond the reach of supercomputers. . All these success stories testify to the fact that the behavior of the JJs seems to be well understood.
Yet a team of SPEC researchers and German theorists  shows, in a recent publication which may seem surprising, that the electromagnetic coupling of a JJ with a simple resistance was until now poorly understood and inconsistent.
Specifically, it was believed that any Josephson junction in his , shunted by a resistor R of great value should not be superconducting but insulating, due to a dissipative quantum occurring at R = RQ = h / 4e² ≈ 6.5 kΩ. This original prediction, made by A. Schmid in 1983, has since been amply confirmed theoretically and some experiments carried out in the 90s have claimed to have verified this prediction. The original article by A. Schmid was thus about 400 times and his prediction was widely regarded as unchallengeable.
Despite this unanimity, the initial prediction of an insulating state by dissipation in a JJ remained troubling, because it leads to a paradox in the limit R → ∞: for this limit, the resistance can in fact simply be eliminated from the circuit and we must find the initial result of Josephson: the JJ must be superconducting, in with the prediction of A. Schmid. Oddly enough, few scientists seemed to care about this rather trivial, albeit old, theoretical inconsistency.
By performing well-controlled measurements of JJs connected to resistors greater than RQ, SPEC physicists and their German colleagues show that the linear response of JJs saturates at low , while preserving unambiguously a superconducting character (see Fig. 1). These experimental results exclude the existence of the transition from predicted by A. Schmid in the JJ.
Beyond the , the authors explain that A. Schmid’s prediction is in fact only valid for systems in the “normal” state, but not for superconducting systems (and therefore in particular not for a JJ). Indeed, although these two types of systems (normal conductor or superconductor) are described by similar effective 1D equations, a subtle difference in the (linear vs circular) of the coordinate that describes their coupling to resistor R (as dissipative) radically changes their .
The article therefore resolves this theoretical inconsistency and provides a simple and unified understanding of the of JJs with their electrical environment. This work may pave the way for new superconducting devices with high based on JJs, which were previously considered impractical due to the expected phase transition.
 Absence of a dissipative phase transition in Josephson junctions,
, Phys. Rev. X 10, 021003 (2020).
Philippe Joyez, Quantronics Group (SPEC/GQ)
– Department of Condensed State Physics (SPEC), UMR 3680 CEA-CNRS
– physical , University of Freiburg
– Institute for Complex Quantum Systems and IQST, University of Ulm.
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