Masakazu Nakanishi

Integrated DC–SQUID Magnetometer

A magnetometer using a refractory material integrated with a pickup coil and a DC superconducting quantum interference device (DC–SQUID) has been developed for biomagnetic applications. It has an overall size of 8 ×9 mm2 and a field resolution of less than 40 fT/ √Hz which is limited by the electronics at frequencies above 20 Hz.

Measurement of frequency dependence of standard capacitors based on the QHR in the range between 1 kHz and 1.592 kHz

A quantized Hall resistance (QHR)-based capacitance measurement chain has been developed. The chain comprises an AC resistance bridge together with an AC-DC calculable resistor, a multifrequency quadrature bridge, and a capacitance bridge. The use of the multifrequency quadrature bridge enables the measurement of the frequency dependence of capacitors in the range between 1-1.592 kHz. The results demonstrate that the frequency dependence of 10-pF standard capacitors can be measured based on the QHR with a relative standard uncertainty of 3/spl times/10/sup -8/.

Ten-volt Josephson junction array

An array of 20144 Josephson junctions divided into four strip line branches has been developed and tested. The array generates quantized voltages up to about 14 V under millimeter-wave irradiation of 90 mW at 92.5 GHz, and will be used in a 10-V voltage standard. >

Improved Quantum Hall Standard of Resistance at Electrotechnical Laboratory

We improved the quantum Hall standard of resistance at the Electrotechnical Laboratory to measure 1 Ω, 100 Ω and 10 kΩ resistors. It is composed of an 11 T superconducting magnet system with a 3He insert, a resistance ratio bridge using a cryogenic current comparator (CCC bridge) and a 100 Ω transfer resistor. The 100 Ω transfer resistor is directly compared to the second or fourth plateau of the quantized Hall resistance using the CCC bridge, whose combined uncertainty is estimated to be about 4 ×10-9. The 1 Ω, 100 Ω and 10 kΩ resistors are compared to the 100 Ω transfer resistor using the same CCC bridge with different settings, whose combined uncertainty is estimated to be about 3 ×10-9 or less. Total uncertainty of measuring the resistor is given as the root sum squares of these combined uncertainties and estimated to be about 5 ×10-9, because these two comparisons are independent of each other. This is the first report of the procedure for resistance measurement and its uncertainty.

Current dependence measurement of 1 /spl Omega/ standard resistors using a cryogenic current source with linear output

We have developed a cryogenic current source with linear output up to 1 A, by combining a 1 mA current source with a cryogenic current comparator operating as a current amplifier. This source was used, along with a 1 V Josephson potentiometer, to determine the current dependence of two types of 1 /spl Omega/ resistors.

Cryogenic Current Comparator Bridge Supplying Current up to 1 A and Current Dependence Measurement of 1 Ω Resistor

We developed a resistance ratio bridge using a cryogenic current comparator (CCC bridge), which supplied current of up to 1 A. We used it to measure the current dependence of 1 Ω resistors from 50 mA to 1 A by comparing them to a 100 Ω resistor with a small power coefficient. The typical value of the combined standard uncertainty of the measurements was estimated to be approximately 0.03 ppm.

Resistance Ratio Bridge Using Cryogenic Current Comparator with DC-Superconducting Quantum Interference Device Magnetometer

A resistance ratio bridge using a cryogenic current comparator (CCC) with a dc superconducting quantum interference device (dc-SQUID) was developed. Its structure, driving circuit and principle of operation were based on those of a resistance ratio bridge using a CCC with a commercially available rf-SQUID. Because of the small intrinsic noise of the dc-SQUID, its resolution was determined not by the dc-SQUID noise but by the thermal noise of the resistor being tested. The ratio of a resistor having a nominal resistance of 100 Ω and a resistor with that of 1 Ω was measured with an uncertainty of 1.1×10-8.

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.

Low-Noise Voltage Amplifier Using RF-Superconducting Quantum Interference Device

We developed a low-noise voltage amplifier using a commercially available rf-superconducting quantum interference device (rf-SQUID) operated with a feedback circuit, which increased the input impedance to approximately 200 MΩ. Its voltage resolution in the frequency range less than 1 Hz was measured to be approximately 0.4 nV. It was designed for a voltage detector of a resistance ratio bridge using a cryogenic current comparator (CCC-bridge).

Current dependence measurement of resistance using a cryogenic constant current source with linear output

We developed a cryogenic constant current source with linear output by using a cryogenic current comparator. When changing the current supplied to a resistor, nonlinear change of its voltage drop corresponds to current dependence of its resistance. We used a Josephson potentiometer with high linearity to measure the voltage drop.