Noise asymmetry and superconductor-insulator transition in Josephson junctions

Noise asymmetry

Phase coherence of qubits is a central issue in solid state quantum computation. This has given rise to the need of mesoscopic noise detectors that can track also the unsymmetric part of nonequilibrium noise. In addition, the non-Gaussian nature of shot noise (i.e the importance of odd moments) has enhanced the interest in high resolution noise measurements.

We have investigated the potential of a single mesoscopic JJ as a detector of shot noise focusing on the case of a strongly resistive environment, in which Coulomb blockade (CB) of Cooper pair current takes place owing to the delocalization of the phase variable. In this case the current enhances distinctly with added nonequilibrium noise.

We find that odd moments of shot noise can be observed using a Coulomb blockaded Josephson junction. This method is much more sensitive than other methods to probe asymmetry of the shot noise.

A scanning electron microscope picture of sample 1 and a schematic view of the circuit. The chrome resistor is denoted by Cr, the superconductor-normal junction by SIN, and the Josephson junction in a SQUID-loop configuration by JJ1 and JJ2.
The asymmetric response due to non-Gaussian shot noise.

Superconductor-insulator transition in Josephson junctions

A Josephson junction is the simplest system in which the dissipative phase transition (DPT), predicted for various systems, can be obserevd. The physical origin of this transition is the suppression of macroscopic quantum tunneling of the phase by the interaction with dissipative quantum-mechanical environment, described by the Caldeira-Leggett model. Suppression of tunneling in a Josephson junction changes the character of conductivity by enhancing it essentially. Hence, this transition is often called a superconductor-insulator transition (SIT).

VI-curves of resistively shunted single Josephson junctions with different capacitances and tunneling resistances are found to display a crossover at which a resistance bump (negative second derivative) appears at zero-bias. The crossover corresponds to the dissipative phase transition (superconductor-insulator transition) at which macroscopic quantum tunneling delocalizes the Josephson phase and destroys superconductivity.

Our measured phase diagram, displayed by the solid curve, confirms the concept of the dissipative phase transition, but it is essentially different from the original theoretical one (dashed line), being determined by the accuracy of voltage measurements and by thermal fluctuations.

Related publications

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Phys. Rev. Lett. 93, 197002 (2004)

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