We demonstrate that the superconducting critical temperature (T c ) of thin niobium films can be electrically modulated in a liquid-gated geometry device. T c can be suppressed and enhanced by applying positive and negative gate voltage, respectively, in a reversible manner within a range of about 0.1 K. At a fixed temperature below T c , we observed that the superconducting critical current can be modulated by gate voltage. This result suggests a possibility of an electrically-controlled switching device operating at or above liquid helium temperature, where superconductivity can be turned on or off solely by the applied gate voltage. a) These authors contributed equally to this work. b) Electronic address: yonuk@kriss.re.kr c) Electronic address: dhchae@kriss.re.kr

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The new International System of Units (SI) became effective on 20 May 2019. In the new SI, the complete system of units can be traced to seven fixed values of the fundamental constants, not to seven base units as in the old system. Electrical metrology has two important quantum mechanical foundations. Here, we introduce the basics and the metrological applications of the Josephson effect and the quantum Hall effect, which play key roles in linking electrical quantities to the fundamental constants, including the Planck constant h, the elementary charge e, and the transition frequency of cesium 133 ΔνCs. Finally, we discuss the redefinition of the kilogram as one of the important examples of electrical metrology based on quantum physics.