Josephson whirlwind

2019: Possibility of local management of Josephson whirlwinds

On October 7, 2019 MIPT reported to TAdviser that his physicists showed a possibility of local management of Josephson whirlwinds. Opening can be demanded in superconducting devices of quantum electronics, in future quantum processors.

Illustration. The designer is Elena Havina, the press service of MIPT

The Josephson whirlwind is the whirlwind of currents arising in a system from two superconductors separated by loose coupling (dielectric, normal metal, etc.) in the presence of an external magnetic field. In 1962 Josephson predicted effect of course of superconducting current through the thin layer of the insulator separating two superconductors. Such current was called a Josephson current, and such compound of superconductors — Josephson contact. Between two superconductors through the dielectric or metal which is not superconductor the communication called weak is formed and the macroscopic quantum coherence is established. When this system is placed in magnetic field, superconductors push out magnetic field. Than bigger magnetic field is put, that more superconductivity resists penetration of magnetic field into a Josephson system. However loose coupling is a place into which the field can get in the form of the separate Josephson whirlwinds bearing quantum of a magnetic flux. Josephson's whirlwinds often are considered as these topological objects, 2π-фазовые singularities, observation and manipulation of which is rather difficult.

Scientists from laboratory of the topological quantum phenomena in the superconducting systems of MIPT decided to apply a magnetic and power microscope (MCM) to studying of Josephson whirlwinds in a system from two superconducting contacts from niobium and a layer from copper (Nb/Cu/Nb) playing a role of loose coupling.

We showed that in planar (flat) contacts 'superconductor — normal metal — superconductor' Josephson whirlwinds have a peculiar print. It was detected when carrying out magnetic and power microscopy of such structures. Based on this opening, we showed a possibility of local generation of a Josephson whirlwind and manipulation to them a magnetic kantilever of a microscope. Our research is another step to creation of future superconducting quantum calculators,

A variety of supersensitive superconducting devices, qubits and architecture for quantum computings quickly grows. It is expected that devices of superconducting quantum electronics will throw down a challenge to normal semiconductor devices in the near future. Josephson contacts are construction blocks of similar devices.

Рисунок 1. Экспериментальная установка: ниобий Nb (синий), медь Cu (оранжевый). Эллипс отмечает область Джозефсоновского перехода. Игла магнитно-силового микроскопа с магнитным покрытием из Co/Cr колеблется пьезоэлементом (dither); optical fiber it is used for reading of fluctuations.

Visualize Josephson whirlwinds rather difficult as they are badly localized. We found a method to measure the dissipation arising at birth / destruction of such whirlwind in the field of loose coupling. Dissipation is a small energy liberation. In our case energy liberation happens at the movement of a whirlwind in planar Josephson contact. Thus, by means of our magnetic and power microscope we well detect not only a static magnetic portrait of superconducting structure, but also dynamic processes in it, adds Vasily Stolyarov

Figure 2. Detecting of Josephson whirlwinds. (a) — the topographical image of structure received using atomic and power microscopy. (b-c) — magnetic and power images of phase contrast (phase change of fluctuations of a kantilever was measured). (b) — the sample was cooled in an external magnetic field 90 E. Small separate white circles are Abrikosov's whirlwinds. (c) — magnetic field 90 E is attached after the sample was cooled in a zero external magnetic field. In the neighborhood of transition appeared several black rings which are sharp falling of a phase of fluctuation of a needle when it is in certain places. (d) — in a zero external magnetic field. Several rings showing influence of own magnetic field of a needle on structure are visible. (e) — distribution of a phase signal along the line shown by a red arrow on (d). Each falling of a phase defines borders between different configurations with different number of Josephson whirlwinds of n=0, 1, 2. (f) — dependence of oscillation phase of a needle on its height over a surface when the needle is over the center of structure.

Authors of work showed a method of remote generation, detecting and manipulation of Josephson whirlwinds in planar Josephson junctions using a low-temperature magnetic and power microscope.

At certain parameters (location of the probe, temperature, an external magnetic field, electric current through a sample) scientists observed a special response of a kantilever of a microscope. It was followed by emergence of sharp rings / arcs on images.

Researchers identified these features as the bifurcation points between the next Josephson statuses which are characterized by different number or provision of Josephson whirlwinds in transition. Process is followed by exchange of energy of a kantilever with a sample in points of bifurcation and shows that the magnetic and power microscope can provide unique information on a status of a whirlwind of Josephson.

It is expected that results of work will be an incitement for development of the authors of methods of local contactless diagnostics and control of modern superconducting devices and superconductor quantum electronics based on opening.

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