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The European Metrology Research Program EMRP addresses a number of “Grand Challenges” in fundamental and applied metrology. In its sub-program “SI and Fundamental Metrology” a Joint Re-search Project “Foundations for a Redefinition of the SI base unit ampere, REUNIAM” has been launched to specifically address tasks related to a new definition of the ampere which, among other redefinitions, is scheduled for 2011 by the CIPM Recommendation CI-2005. Six European Metrology Institutes and one from academia participate in REUNIAM: PTB (Germany), NPL (UK), LNE (France), MIKES (Finland), NMi-VSL (The Netherlands), METAS (Switzerland), and TKK (Helsinki Technical University, Finland). The current practical system of electrical units relies on the Josephson effect (JE) and the quantum Hall effect (QHE) to relate the units of voltage and resistance with extreme precision to combinations of h (Planck’s constant) and e (elementary charge), fundamental constants invariable in space and time. Strictly speaking, it is a system outside the SI. For a re-unified SI the favored scenario would define the ampere through e, and the kilogram through h. This redefinition requires the results of two experiments; the Watt balance (subject of a separate EMRP project), and the quantum metrological triangle (QMT). The QMT, addressed in the project REUNIAM, will combine JE and QHE standards with a quantum current standard based on single charge transport (SCT), and reveal any inconsistency in the orthodox understanding of the three effects. Two variants of the QMT are pursued in the project. In the first a (precisely amplified) SCT current is fed through a QHE-resistance to compare its Hall voltage with a JE-voltage. Being limited in precision with the available low current SCT devices, this direct variant will allow a more precise QMT experiment when new high current devices become available. An indirect variant for closing the QMT relies on collecting a known number of charges on a capacitor C, and to compare C with an impedance derived from the QHE. The experiment was performed with an uncertainty of around one part in 10^6 by NIST in 1999. For a significant scientific impact an uncertainty of 1 part in 10^7 or lower is required, which is the target uncertainty of REUNIAM. Both challenging QMT experiments are being set up at LNE and PTB, respectively, and the combination of effort and experience of these and other groups within REUNIAM will warrant their success. In a second and equally important project thread the REUNIAM partners will study and develop technologies for creating novel SCT devices to deliver significantly larger currents than the existing devices which are limited to the pA range. For a future practical laboratory current standard, when a ‘new’ ampere will be based on the value of e, such high current devices are needed. The high current SCT thread will follow parallel routes, two of them based on technologies derived from the existing aluminum devices and from gated semiconductor devices. A third route exploits the dual analog of a Josephson junction, the quantum phase-slip (QPS) effect occurring in nanometer-size superconducting wires with high-impedance environment. This novel and most challenging route holds the highest risk as well as the highest promise since fundamental constant based currents as high as 10 nA have been predicted for QPS devices. |
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