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arXiv:0907.2319 · Superconductor Science and Technology

Quantum jump approach to the switching process of a Josephson junction coupled to a microscopic two-level system

Xueda Wen, Yiwen Wang, Ning Dong, Yang Yu, Guozhu Sun, Jian Chen, Lin Kang, Weiwei Xu, Peiheng Wu

Published Oct 19, 2009• 0 citations
arXivHTMLPDF

Abstract

With microwave irradiation, the switching current of a Josephson junction coupled to a microscopic two-level system jumps randomly between two discrete states. We modeled the switching process of the coupled system with quantum jump approach that was generally used in quantum optics. The parameters that affect the character of the quantum jumps between macroscopic quantum states are discussed. The results obtained from our theoretical analysis agree well with those of the experiments and provide a clear physical picture for the macroscopic quantum jumps in Josephson junctions coupled with two-level systems. In addition, quantum jumps may serve as a useful tool to investigate the microscopic two-level structures in solid-state systems.

DOI
10.1088/0953-2048/22/11/114016

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With microwave irradiation, the switching current of a Josephson junction coupled to a microscopic two-level system jumps randomly between two discrete states. We modeled the switching process of the coupled system with quantum jump approach that was generally used in quantum optics. The parameters that affect the character of the quantum jumps between macroscopic quantum states are discussed. The results obtained from our theoretical analysis agree well with those of the experiments and provide a clear physical picture for the macroscopic quantum jumps in Josephson junctions coupled with two-level systems. In addition, quantum jumps may serve as a useful tool to investigate the microscopic two-level structures in solid-state systems.
We propose a scheme to demonstrate macroscopic quantum jumps in a superconducting phase qubit coupled to a microscopic two-level system in the Josephson tunnel junction. Irradiated with suitable microwaves, the Rabi oscillations of the qubit exhibit signatures of quantum jumps: a random telegraph signal with long intervals of intense macroscopic quantum tunneling events (bright periods) interrupted by the complete absence of tunneling events (dark periods). An analytical model is developed to describe the width of the dark periods quantitatively. The numerical simulations indicate that our analytical model can capture underlying physics of the system. Besides calibrating the quality of the microscopic two-level system, our results have significance in quantum information process since dark periods in Rabi oscillations are also responsible for errors in quantum computing with superconducting qubits.
Quantum mechanics had been considered as a theory of ensembles for a long time since its foundation.However,quantum jumps demonstrated in atomic system after 1980s leads to new approaches,such as quantum jumps approach,that can be used to describe the trajectory of single quantum system.Here we summarized our recent efforts to develop quantum jump approach to the Josephson-tunnel-junction based superconducting qubits.We experimentally demonstrated quantum jumps in a macroscopic quantum system.In addition,quantum jumps can serve as a useful tool to investigate the properties of the two-level systems that are ubiquitous in solid-state systems.
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 Tcr{T}_{cr}Tcr​ the thermal fluctuations of the Josephson phase induce such a switching, and as T<TcrT<{T}_{cr}T<Tcr​ 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 ${T}_{cr}(\mathrm{\ensuremath{\Phi}})$, where $\mathrm{\ensuremath{\Phi}}$ 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 ${T}_{cr}(\mathrm{\ensuremath{\Phi}})$ occurring in the intermediate range of $\mathrm{\ensuremath{\Phi}}$, i.e., $0<\mathrm{\ensuremath{\Phi}}<{\ensuremath{\phi}}_{0}$ (${\ensuremath{\phi}}_{0}$ 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.
A nanometer-thick small mesa consiting of only two or three Bi 2 Sr 2 CaCu 2 O y intrinsic Josephson junctions (IJJs) is studied through the switching current distribution measurements down to 0.4 K. Experimental results clearly show that the first switching events from the zero-voltage state for 1 K < T < 4 K are successfully described by a conventional thermal activation (TA) theory for a single Josephson junction, and that they become independent of temperature below T* ∼ 0.7 K. We observe the microwave-induced peak in the switching distribution at 0.4 K, which is induced by the microwave irradiation at 55 GHz. These results strongly suggest that the system crossovers to macroscopic quantum tunneling (MQT) regime below T* , which is as high as the previously reported value for a stacked IJJs with several tens of junctions, in contrast to the recent result on a similar mesa-structured surface IJJ.
The discreteness of permitted state spectrum postulated on atomic level can be macroscopic in nanostructures and larger structures because of macroscopic quantum phenomena such as superconductivity. The change by jump of measured parameters because of the macroscopic discreteness may be used for some applications. A device is proposed measured parameters of which can change by jump at a weak change of external parameters. The devise consists of two superconductor loop connected with two Josephson junctions. The macroscopic parameter - maximum value of the super-current through the two Josephson junctions - can change with the quantum number determining macroscopic angular momentum of superconducting pairs in one of the two loops.