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Quantum interference effects in statistical systems of Josephson junctions in the presence of fluctuations

I. I. Yurchenko, V. S. Kartavtsev, V. I. Matveev, I. M. Dmitrenko

Published Jul 1, 1973• 0 citations
OpenAlex

Abstract

An experimental study is reported of the interference effects in systems consisting of a large number of superconducting particles coupled by the Josephson forces, the coupling energy beirg comparable with the energy of fluctuations, as in the case of a single junction. The reduction in the coupling energy to the fluctuation level should increase the responsivity of the system to external microwave fields, by analogy with the case of a single junction. Therefore, it would be interesting to consider the possibility of using such systems as sensitive microwave detectors. The experiments were carried out on a niobium powder consistirg of particles about 10 mu in diameter. (auth)

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An experimental study is reported of the interference effects in systems consisting of a large number of superconducting particles coupled by the Josephson forces, the coupling energy beirg comparable with the energy of fluctuations, as in the case of a single junction. The reduction in the coupling energy to the fluctuation level should increase the responsivity of the system to external microwave fields, by analogy with the case of a single junction. Therefore, it would be interesting to consider the possibility of using such systems as sensitive microwave detectors. The experiments were carried out on a niobium powder consistirg of particles about 10 mu in diameter. (auth)
Josephson junction devices are good candidates for quantum computation. A large energy splitting was observed in the spectroscopy of a superconducting Josephson junction. The presence of the critical current fluctuation near the energy splitting indicated coupling between the junction and a two-level system. Furthermore, we find that this fluctuation is microwave dependent. It only appears at certain microwave frequency. This relation suggested that the decoherence of qubits is influenced by the necessary computing operations.
Thermodynamic fluctuations limit the ultimate performance of superconducting devices of practical interest, devices such as the quantum interference magnetometer and the Josephson junction radiation detector. We report direct observations of thermodynamically driven quantum phase fluctuations near the superconducting critical temperature. The experiments used point-contact quantum interference techniques with niobium points on a single-crystal tin substrate. Near the superconducting transition of tin, TC=3.72°K, the niobium points served as probes to measure fluctuations in the phase of the superconducting order parameter within the tin crystal. The temperature was slowly increased, beginning well below TC for tin, where the quantum phase was stable. Quantum phase fluctuations were first observed approximately 5 mdeg below TC. As the temperature continued to increase through this 5 mdeg range near TC, the characteristic frequency of the phase fluctuations increased 6 orders of magnitude. At TC a discontinuous transition to a resistive state gave rise to a finite voltage difference in the tin with the associated driven quantum phase precession. Thermodynamic Johnson noise voltages developed across this resistance of order 10−10Ω observably broadened the precession line. We will present the detailed temperature dependence of the frequency power spectrum of quantum phase fluctuations below and above TC.
Microwave detector response ( eta ) for three types of superconducting structures-single edge-type Josephson junctions, arrays of series-connected junctions, and arrays of RF-series and DC parallel-connected junctions-were investigated in the frequency range 35-120 GHz. Experimental dependence of eta versus biasing voltage is mainly determined by the interaction of junctions in the array. The selective response, which means a narrow linewidth of Josephson radiation, is observed for the third type of superconducting structure in a small magnetic field changing the phase shift between closely-spaced junctions. These arrays have the advantage in coupling with the external microwave system.
We investigate the possibility to employ magnetic Josephson junctions as magnetic noise detectors. To illustrate our idea, we consider a system consisting of a quantum dot coupled to superconducting leads in the presence of an external magnetic field. Under appropriate assumptions, we relate the noise in the Josephson current to magnetization noise. At the magnetic field driven 0-$\ensuremath{\pi}$ transition, the junction sensitivity as magnetic noise detector is strongly enhanced and it diverges in the zero temperature limit. Moreover, we demonstrate that, if also dot energy is affected by fluctuations, only the magnetic noise channel contributes to Josephson current noise response when the quantum dot is tuned in resonance with superconducting leads.
A single-junction superconducting quantum interferometer with external high-frequency excitation is considered, in which the weak coupling is described within the framework of the resistive model. An analysis for the hysteresis-free state serves to define the parameters characterizing the Josephson junction, including the current as a function of the phase difference at the junction, as well as the interference term in the dissipative component of the current. The theoretical results are compared with experiment to define certain parameters of the Josephson junction and the sign of the interference term.
Two-level system fluctuators (TLS's) in the tunnel barrier of a Josephson junction have recently been demonstrated to cause novel energy splittings in spectroscopic measurements of superconducting phase qubits. With their strong coupling to the Josephson junction and relatively long decoherence times, TLS's can be considered as potential qubits and demonstrate coherent quantum effects. Here, we study the effective interaction between the TLS qubits that is mediated by a Josephson junction resonator driven by an external microwave source. This effective interaction can enable controlled quantum logic gates between the TLS's. Our study can be extended to other superconducting resonators coupling with TLS's.