Resonant enhancement of macroscopic quantum tunneling in Josephson junctions: Influence of coherent two-level systems

Abstract

We report a theoretical study of the macroscopic quantum tunneling (MQT) in small Josephson junctions containing randomly distributed two-level systems. We focus on a Josephson phase escape for switching from the superconducting (the zero-voltage) state to a resistive one. Above the crossover temperature T cr the thermal fluctuations of the Josephson phase induce such a switching, and as T < T cr the regime of the MQT occurs. In the absence of two-level systems (TLSs) a magnetic field applied parallel to the junction plane results in a smooth reduction of T cr ( ), where is an applied magnetic flux. As the TLSs are present in Josephson junctions we obtain a resonant enhancement of the MQT. This phenomenon manifests itself by a narrow peak in the dependence of T cr ( ) occurring in the intermediate range of , i.e., 0 < < φ 0 (φ 0 is the magnetic flux quantum). We explain this effect quantitatively by a strong resonant suppression of the potential barrier for the Josephson phase escape that is due to the coherent quantum Rabi oscillations in two-level systems present in the junction.

Atomic structure and oxygen deficiency of the ultrathin aluminium oxide barrier in Al/AlOx/Al Josephson junctions

“…Al/AlO x /Al Josephson junctions are the building blocks of a wide range of superconducting quantum devices that are key elements for quantum computers, extremely sensitive magnetometers and radiation detectors 1 2 3 . A fundamental limiting factor for the implementation of Josephson junction based superconducting quantum electronics is the coupling of the coherent quantum state with two-level systems (TLS) accommodated in the amorphous oxide material in the circuits, resulting in noise and decoherence 4 5 6 7 8 9 10 . The TLS in the insulating oxide barrier in Josephson junctions have been considered as a major source of energy dissipation and decoherence in superconducting electric circuits, especially in qubits 11 12 13 .…”

Macroscopic resonant tunneling in an rf-SQUID flux qubit under a single-cycle sinusoidal driving *

“…[8][9][10][11][12][13][14][15][16][17][18] It has also been shown that Landau-Zener transition can enhance the resonant tunneling rate. [19][20][21] MRT has been used to prove that the low-frequency flux noise in both Josephson junctions and rf-SQUID system is the crucial source of quantum noise. [22][23][24][25][26] In this article, we experimentally demonstrate the observation of MRT between macroscopic distinct states in an rf-SQUID system, which is driven by an external single-cycle sinusoidal driving signal.…”