Ko Hara

Absolute determination of the magnetic flux quantum using superconducting magnetic levitation

The current status of our superconducting magnetic levitation experiment for determining the magnetic flux quantum is described. The flux-up system has been improved significantly by using a Josephson voltage standard. Studies are also in progress to improve the mechanical measurement relevant to the floating body with the goal of reducing the uncertainty to less than 1 ppm. >

A Study of an Absolute Determination of the Magnetic Flux Quantum Using a Superconducting Levitation System

A study on a new method of determining the magnetic flux quantum 0 possibly at subppm level using a superconducting levitation system is described with some data from a preliminary experiment. The method employs a superconducting levitation system to substitute the gravitational potential energy of the floating body for the magnetic energy. The estimation of the expected accuracy and methods of improving the experiment are discussed.

A buoyancy-free mass comparator with precision better than 10/sup -6/ for use in a superconducting magnetic levitation system

A system for measuring mass without the necessity of correcting for the buoyancy of air with a relative standard uncertainty better than 10/sup -6/ has been developed. The system is intended for use in determining the mass of the floating body in the superconducting magnetic levitation system.

Improvement of the Superconducting Magnetic Levitation System for an Absolute Determination of the Magnetic Flux Quantum

The superconducting magnetic levitation system for the absolute determination of the magnetic flux quantum 0 is improved. Resolution of the vertical position of the floating body with laser interferometer becomes 1/100 µm order, which corresponds to 1 ppm of the gravitational potential energy of the floating body. An outline of this system and some temporary results are described.

Present status of the NRLM superconducting magnetic levitation system for absolute determination of the magnetic flux quantum

The NRLM superconducting magnetic levitation is based on the concept of quasi-static energy injection process, where the energy injected from external source to the coil system is equal to the sum of the increase in magnetic energy and gravitational potential energy. The levitation system uses a niobium coil and floating body with mass of 25 g. The measurement consists of two processes: (1) to increase the magnetic flux through the coil in terms of the Josephson voltage which we call flux-up, (2) to measure the coil current and the vertical displacement of the center of gravity of a floating body in equilibrium. By the repetition of these processes, the relation which we call equilibrium trajectory is obtained. The absolute value of the magnetic flux quantum is calculated in terms of the energy equation using the trajectory data. Hence, a quasi-static process is not vital. The levitation system has been developed at NRLM and the reproducibility of magnetic flux and vertical displacement in equilibrium at 0.1 ppm level was confirmed including Meissner effect of the superconductive material. In this paper, we report a recent improvements concerning mechanical energy determination as well as the changes to the electrical system.

Electrical Analog of a Single-Flux Quantum Device for Logic Design

Some of the behaviour of a quantron (one-junction interferometer) with a Josephson gate is simulated. This quantron was proposed by Likharev in 1977 and is called the Parametric Quantron. By proper control of its critical current with time, alternative stable states of the quantron can be selected by a switching signal whose product of the energy dissipation Ec and the switching time τ is as low as 102h (where h is Plancks constant). In this experiment, a simulator based on the analogy between a Josephson junction and a phase-locked loop is used. Analog systems are flexible and easier to assemble than the mechanical analog.

Improved Fabrication Method of the Microbridge Utilizing Step Edge

A microbridge is regarded as an ideal application of a Josephson junction. A simple fabrication technology for a microbridge having a very short link was previously proposed. Such a link is fabricated by utilizing a step edge; such a microbridge is named the MUSE. This stands for a Microbridge Utilizing the Step Edge. Another fabrication method for improving the stability and reliability of a microbridge is discussed in this paper.

Experimental Test of the Composite Josephson Device

A circuit composed of superconductive loops of 4 Josephson junctions (abbreviated as JJ) or more with infinitesimal loop area is named as a Composite Josephson device (abbreviated as CJD). Its numerical analysis has been reported before. One of the CJD, a 3 terminal 6 JJ loop device, is fabricated by the microbridge utilizing step edge technology and its experimental tests are reported in this paper. Its numerical analysis is proved by the experimental tests.

Composite Josephson Device

A superconductive loop circuit including more than four Josephson junctions (JJ), which is called a Composite Josephson Device (CJD), can support a fluxoid stably without inductance and has multiple stable states depending on the number of fluxoids. One of the simplest cases of CJD with a three-terminal six JJ loop device is analyzed numerically. The result shows that it has five stable areas and is able to operate as a current amplifier within a certain parameter range.

I–V Characteristics of NbN–NbN Josephson Point Contacts

Current-voltage characteristics of NbN-NbN Josephson point contacts have been examined. The point contacts were formed by NbN thin films deposited on Nb or W whiskers and Nb posts. The current-voltage characteristics of those contacts were investigated with and without irradiation of microwaves and far-infrared light. High values of the IcRn product and the energy gap structure were observed, which corresponded to the high-energy gap of NbN. Shapiro steps for far-infrared light up to 170 µm were observed and the possibility of replacing Nb Josephson point contacts with NbN ones is indicated.