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Physical Review Applied

Magnetic Force Microscopy for Diagnosis of Complex Superconducting Circuits

Sergey Grebenchuk, Razmik A. Hovhannisyan, A.G. Shishkin, Viacheslav Dremov, Vasily S. Stolyarov

Published Nov 14, 2022• 9 citations
Publisher

Abstract

The possibility of controlling Josephson vortices and fluxon states has always been an attractive area of research that can influence the development of superconducting devices and quantum computing. Here we demonstrate the capability of magnetic force microscopy to detect and manipulate Josephson vortices (JVs) and fluxons below the critical current in complex superconducting systems on the example of a dc superconducting quantum interference device. The simultaneous magnetic force microscopy and transport investigation method make it possible to generate JVs by direct current in Josephson junctions and simultaneously analyze their dynamics under various external conditions. Our results show a dynamic process of transition of $2\ensuremath{\pi}$-phase singularities of JVs and fluxons from one to another. Finally, we demonstrated that the magnetic force microscopy method could distinguish the difference in critical currents between several Josephson junctions in complex superconducting circuits without the need of direct electron transport measurements.

DOI
10.1103/physrevapplied.18.054035

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The possibility of controlling Josephson vortices and fluxon states has always been an attractive area of research that can influence the development of superconducting devices and quantum computing. Here we demonstrate the capability of magnetic force microscopy to detect and manipulate Josephson vortices (JVs) and fluxons below the critical current in complex superconducting systems on the example of a dc superconducting quantum interference device. The simultaneous magnetic force microscopy and transport investigation method make it possible to generate JVs by direct current in Josephson junctions and simultaneously analyze their dynamics under various external conditions. Our results show a dynamic process of transition of 2\ensuremathπ2\ensuremath{\pi}2\ensuremathπ-phase singularities of JVs and fluxons from one to another. Finally, we demonstrated that the magnetic force microscopy method could distinguish the difference in critical currents between several Josephson junctions in complex superconducting circuits without the need of direct electron transport measurements.
Josephson vortices play an essential role in superconducting quantum electronics devices. Often seen as purely conceptual topological objects, 2π-phase singularities, their observation and manipulation are challenging. Here we show that in Superconductor-Normal metal-Superconductor lateral junctions Josephson vortices have a peculiar magnetic fingerprint that we reveal in Magnetic Force Microscopy (MFM) experiments. Based on this discovery, we demonstrate the possibility of the Josephson vortex generation and manipulation by the magnetic tip of a MFM, thus paving a way for the remote inspection and control of individual nano-components of superconducting quantum circuits.
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We have fabricated and studied a promising kind of direct current superconducting quantum interference device (dc-SQUID) in which the magnitude and sign of the critical current of the individual Josephson junctions can be controlled by additional voltage probes connected to the junctions. We show that the amplitude of the voltage oscillations of the SQUID as a function of the applied magnetic field can be tuned and that the phase of the oscillations can be switched between 0 and π in the temperature range of 0.1–4.2 K using a suitable control voltage. This is equivalent to the external application of (n+1/2) flux quantum.
A neutral quantum particle with magnetic moment encircling a static electric charge acquires a quantum-mechanical phase (Aharonov-Casher effect). In superconducting electronics, the neutral particle becomes a fluxon that moves around superconducting islands connected by Josephson junctions. The full understanding of this effect in systems of many junctions is crucial for the design of novel quantum circuits. Here, we present measurements and quantitative analysis of fluxon interference patterns in a six Josephson junction chain. In this multijunction circuit, the fluxon can encircle any combination of charges on five superconducting islands, resulting in a complex pattern. We compare the experimental results with predictions of a simplified model that treats fluxons as independent excitations and with the results of the full diagonalization of the quantum problem. Our results demonstrate the accuracy of the fluxon interference description and the quantum coherence of these arrays.
A dc superconducting quantum interference device (SQUID) consisting of a symmetric ring circuit with four Josephson junctions and two independent external currents I1{I}_{1}I1​ and I2{I}_{2}I2​, which enter the circuit at different points, has a phase boundary between the fully superconducting and the normal states that is different from that of a conventional SQUID. The control current I2{I}_{2}I2​ modifies the magnetic flux (\ensuremath{\varphi}) periodicity and the value of the maximum measuring current I1m{I}_{1m}I1m​(\ensuremath{\varphi}). For values of I2{I}_{2}I2​>Ic{I}_{c}Ic​ (critical current of Josephson junction) the fully resistive state exists over flux intervals centered around (n+(1/2)${\ensuremath{\varphi}}_{0}$, where n is an integer and ${\ensuremath{\varphi}}_{0}$ is the fluxoid quantum. A superconducting circuit without Josephson junctions is suggested which should have a phase boundary similar to that found here. Minimization procedures which are usually employed in analyzing SQUID's are discussed.
Experiments towards realizing a readout of superconducting qubits by using ballistic Josephson vortices are reported. We measured the microwave radiation induced by a fluxon moving in an annular Josephson junction. By coupling a flux qubit as a current dipole to the annular junction, we detect periodic variations of the fluxon's oscillation frequency versus magnetic flux through the qubit. We found that the scattering of a fluxon on a current dipole can lead to the acceleration of a fluxon regardless of a dipole polarity. We use the perturbation theory and numerical simulations of the perturbed sine-Gordon equation to analyze our results.