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翻訳と辞書　辞書検索 [ 開発暫定版 ] スポンサード リンク Josephson voltage standard ： ウィキペディア英語版 Josephson voltage standard A Josephson voltage standard is a complex system that uses a superconductive integrated circuit chip operating at 4 K to generate stable voltages that depend only on an applied frequency and fundamental constants. It is an intrinsic standard in the sense that it does not depend on any physical artifact. It is the most accurate method to generate or measure voltage and, by international agreement, is the basis for voltage standards around the World. == The Josephson effect== In 1962, Brian Josephson, a graduate student at Cambridge University, derived equations for the current and voltage across a junction consisting of a thin insulating barrier separating two superconductors - now generally known as a Josephson junction.〔B.D. Josephson, Phys. Lett., 1, 251 (1962)〕 His equations predicted that if a junction is driven at frequency $f$, then its current-voltage (I-V) curve will develop regions of constant voltage at the values $nhf/2e$ , where $n$ is an integer and $h/e$ is the ratio of the Planck constant $h$ to the elementary charge $e$. This prediction was verified experimentally by Shapiro 〔S. Shapiro, Phys. Rev. Lett. 11 80 (1963)〕 in 1963 and has become known as the ac Josephson effect. This effect found immediate application in metrology because it relates the volt to the second through a proportionality involving only fundamental constants. Initially, this led to an improved value of the ratio $h/e$. Today it is the basis for all primary voltage standards. Josephson's equation for the supercurrent through a superconductive tunnel junction is given by $I\; =\; I\_c\; sin\; ((4\; \backslash pi\; e/h)\; \backslash int\; V\; dt\; )$ where $I$ is the junction current, $I\_c$ is the critical current, $V$ is the junction voltage. $I\_c$ is a function of the junction geometry, the temperature, and any residual magnetic field inside the magnetic shields that are used with voltage standard devices. When a dc voltage is applied across the junction, Eq. (1) shows that the current will oscillate at a frequency $f\_J=2eV/h$ , where $2e/h$ is approximately equal to 484 GHz/mV. The very high frequency and low level of this oscillation make it difficult to observe directly. However, if an ac current at frequency $f$ is applied to the junction, the junction oscillation $f\_J$ tends to phase lock to the applied frequency. Under this phase lock, the average voltage across the junction equals $hf/2e$. This effect, known as the ac Josephson effect, is observed as a constant voltage step at $V\; =\; hf/2e$ in the voltage-current (I-V) curve of the junction. It is also possible for the junction to phase lock to harmonics of $f$. This results in a series of steps at voltages $V\; =\; nhf/2e$, where $n$ is an integer, as shown in Fig. 1a. The Josephson effect was initially used to improve the measurement of the constant $2e/h$ based on voltage values derived from the SI volt realization as maintained by Weston cells. The uncertainty of these measurements was limited by the uncertainty of the SI volt realization and the stability of the Weston cells.〔W.H. Parker, D.N. Langenberg, A. Denenstein and B.N. Taylor, Phys. Rev. 177 639 (1969)〕〔T.F. Finnegan, A. Denenstein and D.N. Langenberg, Phys. Rev. B 4 1487 (1971)〕 The stability of the Josephson volt depends only on the stability of $f$ (which can easily be a part in 1012), and is at least four orders of magnitude better than the stability of Weston cells. Thus, in the early 1970s, many national standards laboratories adopted a value for the Josephson constant $K\_J\; =\; 2e/h$ and began using the ac Josephson effect as the practical standard of voltage.〔B.N. Taylor, W.H. Parker, D.N. Langenberg, and A. Denenstein, Metrologia 3 89 (1967)〕〔B.F. Field, T.F. Finnegan and J. Toots, Metrologia 9 155 (1973)〕 Owing to small differences in existing national standards, different values of $K\_J$ were adopted by various countries. This inconsistency was corrected in 1990 when, by international agreement, the constant $K\_\backslash text$ was assigned the value 483597.9 GHz/V and adopted by all standards laboratories.〔T.J. Quinn, Metrologia 26 69 (1989)〕 The assigned value is based on a weighted average of volt realization measurements made prior to 1990 at many national measurement institutions. The uncertainty in $K\_\backslash text$ is 0.4 ppm. Standards such as the Josephson volt that depend on fundamental constants rather than physical artifacts are known as intrinsic standards. Although the Josephson voltage standard (JVS) does not realize the SI definition of the volt, it provides a very stable reference voltage that can be reproduced anywhere without the need to transfer artifacts such as Weston cells. The accuracy of the Josephson voltage-frequency relation $V\; =\; nf/K\_J$, and its independence from experimental conditions, such as bias current, temperature, and junction materials, have been subjected to many tests.〔J. Clarke, Phys. Rev. Lett. 21 1566 (1968)〕〔T. D. Bracken, and W.O. Hamilton, Phys. Rev. B 6 2603 (1972)〕〔I. K. Harvey, J.C. MacFarlane, and R.B. Frenkel, Metrologia 8 114 (1972)〕〔F. Bloch, Phys. Rev. Lett. 21 1241 (1968), Phys. Rev. B, 2 109 (1970)〕〔T. R. Rieger, D. J. Scalapino, and J. E. Mercereau, Phys. Rev. Lett. 27 1787 (1971)〕〔T. A. Fulton, Phys. Rev. B, 7 981 (1973)〕〔J. Niemeyer, L. Grimm, C.A. Hamilton, and R. L. Steiner IEEE Electron Dev. Lett. 7 44 (1986)〕〔V. Kose, and J. Niemeyer: in The Art of Measurement, ed. B. Kramer (Weinheim: VCH) 249 (1988)〕 No significant deviation from this relation has ever been found. In the most precise of these experiments, two Josephson devices are driven by the same frequency source, biased on the same step, and connected in a series opposition loop across a small inductor. Since this loop is entirely superconductive, any voltage difference leads to a changing magnetic field in the inductor. This field is detected with a SQUID magnetometer and its constancy has set an upper limit on the voltage difference of less than 3 parts in 1019.〔J-S Tsai, A. K. Jain, and J. E. Lukens, Phys. Rev. Lett. 51 316 (1983)〕〔A.K. Jain, J.E. Lukens, and J.S. Tsai, Phys. Rev. Lett. 58 1165 (1987)〕 Figure 2 is a semilog plot that illustrates how typical differences in dc voltage measurements among National Measurement Institutes (NMIs) have decreased over the last 70 years.〔B. W. Petley: in Quantum Metrology and Fundamental Physical Constants, edited by P.H. Cutler and A. A. Lucas (Plenum, New York) 293 (1983)〕〔F. Melchert, Tech. Messen 2 59 (1979).〕〔D. Reymann and T. J. Witt, IEEE Trans. Instrum. Meas. 42 596 (1993).〕〔J. P. Lo-Hive, D. Reymann and G. Genevès, IEEE Trans. Instrum. Meas. 44 230 (1995)〕〔D. Reymann, T.J. Witt, G. Eklund, H. Pajander, and H. Nilsson, IEEE Trans. Instrum. Meas. 46 220 (1997)〕〔D. Reymann, T. J. Witt, G. Eklund, H. Pajander, H. Nilsson, R. Behr, T. Funck and F.Müller, IEEE Trans. Instrum. Meas. 48 257 (1999).〕〔J. Niemeyer in Handbook of Applied Superconductivity, ed. Bernd Seeber, (Inst. Of Physics Publishing, Philadelphia), 2 p. 1813 (1998)〕 The two major improvements coincide with the introduction of single-junction Josephson standards in the early 1970s and the introduction of series-array Josephson standards beginning in 1984. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Josephson voltage standard」の詳細全文を読む スポンサード リンク 翻訳と辞書 : 翻訳のためのインターネットリソース Copyright(C) kotoba.ne.jp 1997-2016. All Rights Reserved.