Electrical Metrology and the Revised SI
Part 1: Murray Early, Electrical Metrology, MSL, New Zealand Part 2: Ilya Budovsky, Electricity Section, NMI, Australia
14 September 2017 The 2019 SI Revision of Electrical Metrology…Overview Part 1: Murray The need for change to the SI electrical units The challenge of electrical units Part 2: Ilya Preparations for change Support for stakeholders Questions
Metrology: the science of measurement
2 Proposed Changes to the SI A revised SI, not new – same units, different definitions Based on 7 fixed constants (values to be finalised)
To be fixed in 2019 Already fixed 1 h = 6.626 070 04 ×10–34J s 5 c = 299 792 548 m s-1
–19 2 e = 1.602 176 621 ×10 C 6 ΔνCs = 9 192 631 770 Hz
–23 –1 -1 3 k = 1.380 648 5 ×10 J K 7 Kcd= 683 lm W
23 –1 4 NA= 6.022 140 857 ×10 mol
3 Why Change? 1. Concern over stability of the IPK (international prototype kilogram) 2. Link to the IPK limits accuracy of electrical units 3. Present base units are a mix of definition and realisation 4. Base the system on the most stable quantities available 5. Opportunity to base mole on Avogadro’s constant 6. Opportunity to base kelvin on Boltzmann’s constant
NB: second, metre and candela will not change in 2019
4 Electrical Units – quantum effects
Electrical measurements revolutionised by quantum effects:
Year Equation Constant ℎ Josephson 2푒 1962 Josephson effect 푉=푛 푓 퐾 = 2푒 constant ℎ 1 ℎ von Klitzing ℎ 휇 푐 1980 Quantum Hall effect 푅 = 푅 = = 푖 푒 constant 푒 2훼
1987 Single electron counting 퐼 = 푛푒푓 Electronic charge 푒
NB: α≅ 1/137, is the fine structure constant 1990 - Conventional values defined:
• KJ–90 = 483 597.9 GHz/V (ur = 0.4 ppm)
• RK–90 = 25 812.807 Ω (ur = 0.1 ppm) • uncertainties dominated by link to mechanical units (force, energy, power) reproducibility > 1000 times better! • best calibrations not strictly traceable to the SI? h The Josephson Effect f 0.14 mV 2e Josephson SIS junction: (for f = 70 GHz, step n = 1) • Superconductor-Insulator-Superconductor sandwich • radiated by microwaves, frequency f • small dc voltage generated 1990s key advance: put 104 to > 105 in a series array – 10 V output! Mature technology: > 20(?) arrays in use in North America (industry and the military) Various kinds of junctions (SNS, SINIS,…) Various array designs (conventional, programmable, pulse) Programmable AC voltages – applied to impedance and power standards 10 V SIS array 6 h The Quantum Hall Effect 25.8 k Klaus von Klitzing (1985) e2 Quantum Hall Resistor: “for the discovery of the quantized Hall effect” • 2D electron layer • Large magnetic field (~10 T) • Low temperatures (< 2.3 K)
Laughlin, Stormer and Tsui (1998) “for their discovery of a new form of quantum fluid with fractionally charged excitations”
Directly compare Graphene and GaAs devices: (Janssen et al, NPL) Thouless, Haldane, Kosterlitz (2016) ”for theoretical discoveries of topological phase 2 -11 7 (RGaAs/AlGaAs-RGraphene) / (h/2e ) = (-4.7 ± 8.6) x 10 transitions and topological phases of matter” Quantum Current Source ef 1.6 pA SOLID STATE ENTANGLER Clocking electrons one by one (for f = 10 MHz) • Currents still too small for high accuracy - ongoing research challenges Various device technologies • SET (single electron tunneling) turnstile
V/2 -V/2 • SAW (surface acoustic wave) U1 U2 • Tunable barrier pumps, carbon nanotubes etc
3-e 1-e 2-e These technologies have strong relevance to: Tunnelling rate vs gate voltage • future electronic devices (impedance (e.g. memory) suppresses unwanted • new physics (e.g. qubits for processes quantum computing) • manipulation of entanglement –fully quantum Hybrid superconductor-normal-metal turnstile 8 Pekola et al, Nature Physics 4 (2008) 120 Linking Electricity to Mass - The Kibble Balance Mechanical versus electrical force (power) • Two modes: weighing and calibration (static & dynamic) • Weighing current I, induced voltage U, coil velocity • Geometric factor , magnetic field B – eliminated by combining the 2 modes
Weighing mode Calibration mode
F I dl B I m g U B dl dl B v v mg U UI so m (these all known to < 10-8) I v gv 9 UI The Kibble Balance m gv
Equivalence of Electrical and Mechanical Power: UI Pelec Pmech mg U 2 U Virtual electrical power:P ~ since: I s elec R R h Measure resistance in terms of quantum Hall effect: R ~ e2 h Measure voltages in terms of Josephson effect: U ~ f 2e MSL Kibble Balance Project h 2 • Designed by Chris Sutton Then the electrical power is related to Planck’s constant: 2e • Based on twin pressure-balance (actually a measure of the Planck constant h) P ~ ~ h concept elec h 2 • May be used in an oscillatory mode Redefinition: define h and derive mass scale e
10 Current Status of NA (h) Measurement Results