The International System of Units (SI) is founded on seven base units, the metre, kilogram, second, ampere, kelvin, mole and candela corresponding to the seven base quantities of length, mass, time, electric current, thermodynamic temperature, amount of substance and luminous intensity. At its 94th meeting in October 2005, the International Committee for Weights and Measures (CIPM) adopted a recommendation on preparative steps towards redefining the kilogram, ampere, kelvin and mole so that these units are linked to exactly known values of fundamental constants. We propose here that these four base units should be given new definitions linking them to exactly defined values of the Planck constant h, elementary charge e, Boltzmann constant k and Avogadro constant N A , respectively. This would mean that six of the seven base units of the SI would be defined in terms of true invariants of nature. In addition, not only would these four fundamental constants have exactly defined values but also the uncertainties of many of the other fundamental constants of physics would be either eliminated or appreciably reduced. In this paper we present the background and discuss the merits of these proposed changes, and we also present possible wordings for the four new definitions. We also suggest a novel way to define the entire SI explicitly using such definitions without making any distinction between base units and derived units. We list a number of key points that should be addressed when the new definitions are adopted by the General Conference on Weights and Measures (CGPM), possibly by the 24th CGPM in 2011, and we discuss the implications of these changes for other aspects of metrology.

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The kilogram, the base unit of mass in the International System of Units (SI), is defined as the mass m(K) of the international prototype of the kilogram. Clearly, this definition has the effect of fixing the value of m(K) to be one kilogram exactly. In this paper, we review the benefits that would accrue if the kilogram were redefined so as to fix the value of either the Planck constant h or the Avogadro constant N A instead of m(K), without waiting for the experiments to determine h or N A currently underway to reach their desired relative standard uncertainty of about 10 −8 . A significant reduction in the uncertainties of the SI values of many other fundamental constants would result from either of these new definitions, at the expense of making the mass m(K) of the international prototype a quantity whose value would have to be determined by experiment. However, by assigning a conventional value to m(K), the present highly precise worldwide uniformity of mass standards could still be retained. The advantages of redefining the kilogram immediately outweigh any apparent disadvantages, and we review the alternative forms that a new definition might take.

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Tungsten thin films can form in one of two crystal structures: alpha (bcc), with a superconducting transition temperature (c) of 15 mK, and beta (A15), with a c between 1 and 4 K. Films with intermediate c s are composed of both alpha and beta phases. We have investigated how to tune the film c in order to obtain certain values (c 100 mK) suitable for the fabrication of photon number resolving transition-edge sensor (TES) and arrays of TES detectors for astronomical and quantum information applications. Variation of deposition conditions, and also the choice of the underlayer/coating for equal deposition conditions, affect the c s of tungsten films. We have used x-ray diffraction to determine the structure of tungsten thin films and film stress. The results indicates that the film stress state depends on the underlying substrate and coating. To understand the variation of c values and to allow precise tuning of these values, we have investigated substrates and coatings for tungsten film multilayer stacks and determined tungsten film stress by x-ray diffraction at both room temperature and 8 K.

Dynamic nonlinear behavior is reported at high currents in the quantum Hall regime of GaAs heterostructures, resulting from breakdown of the dissipationless current flow. It is demonstrated that this breakdown is spatially localized and transient switching is observed on microsecond time scales among a set of distinct dissipative states. A simple macroscopic picture is proposed to account for these novel phenomena.PACS numbers: 73.40.Lq, 72.20.Ht, 72.20.My, 72.70. + m The quantum Hall effect 1 ' 2 is of great import for both many-body physics and fundamental metrology. The extreme accuracy with which the Hall resistance is quantized, despite the presence of disorder in the inversion-layer devices, is now fairly well understood as being due to the nearly complete freedom from dissipation in the quantized Hall regime. However, the nature of the localized states in a high magnetic field, the role of finite electric fields, and the nature of various dissipative effects remain poorly understood. Ebert etal. 3 have recently discovered that there is a critical current density above which the dissipation suddenly rises by several orders of magnitude. We report in this Letter unexpected new phenomena associated with this breakdown. We show that the breakdown is spatially localized and exhibits a rich time-dependent structure. In addition to a strong background of broadband noise we observe transient switching on a microsecond time scale among a discrete set of distinct dissipative states. Our observations demonstrate the significance of this breakdown phenomenon and provide a deeper understanding of the novel transport properties associated with the quantum Hall effect.Two high-quality GaAs-Ga^Al^As (# = 0.29) heterojunction devices [hereafter referred to as GaAs(7) and GaAs (8)] were used in this study. Both devices have zero-magnetic-field mobilities in excess of 10 5 cm 2 /(V s) at 4.2 K, and at 1.1 K yield excellent 6453.2-£2 (i =4) Hall steps that are flat and reproducible to at least 0.02 ppm. Figure 1 gives the sample geometry and displays the current dependence of the Hall resistance R H = (V 3 -V 4 )// SD and the dissipative voltage V x = V 2 -V 4 (at its minimum) for GaAs (7). Table I shows that V x changes by 7 orders of magnitude between / SD =25 and 370 juA and becomes as large as one-tenth of the Hall voltage V H while (as shown in Fig. 1) the value of JR H decreases by only 0.1 ppm! Similarly the other Hall-probe resistance R H f = (^I-^ASD decreases by only 0.6 ppm. These changes in R u are -0.01% of what is expected from the mixing of V x into V H due to the known misalignment of the Hall probes (3 rel-1374

Using a moving coil watt balance, electric power measured in terms of the Josephson and quantum Hall effects is compared with mechanical power measured in terms of the meter, kilogram, and second. We find the Planck constant h 6.626 068 91͑58͒ 3 10 234 J s. The quoted standard uncertainty (1 standard deviation estimate) corresponds to ͑8.7 3 10 28 ͒h. Comparing this measurement to an earlier measurement places an upper limit of 2 3 10 28 ͞yr on the drift rate of the SI unit of mass, the kilogram. [S0031-9007(98)

The electronic kilogram project of NIST has improved the watt balance method to obtain a new determination of the Planck constant h by measuring the ratio of the SI unit of power W to the electrical realization unit W 90 , based on the conventional values for the Josephson constant K J-90 and von Klitzing constant R K-90 . The value h = 6.626 069 01(34) × 10 −34 J s verifies the NIST result from 1998 with a lower combined relative standard uncertainty of 52 nW/W. A value for the electron mass m e = 9.109 382 14(47) × 10 −31 kg can also be obtained from this result. With uncertainties approaching the limit of those commercially applicable to mass calibrations at the level of 1 kg, an electronically-derived standard for the mass unit kilogram is closer to fruition.