Precision Measurement Techniques

Precision Measurement Techniques Murray EarlyMeasurement Standards Laboratory of New Zealand, Industrial Research Ltd

Overview • A bit about MSL and IRL • MSL: Measurement Standards Laboratory of New Zealand • IRL: Industrial Research Limited • The International System of Units (the SI) • Describe the base units • Precision Measurement Techniques • Some of the methods used to realize the SI electrical quantities • Future Developments of the SI • Replacing the kg

0. Wellington, NZ • LatitudeNZ: 41 SJapan: 36 N • AreaNZ: 266 000 km2Japan: 378 000 km2 • PopulationNZ: 4.2 MJapan: 127 M

Progress • A bit about MSL and IRL • MSL: Measurement Standards Laboratory of New Zealand • IRL: Industrial Research Limited • The International System of Units (the SI) • describe the base units • Precision Measurement Techniques • some of the methods used to realize the SI electrical quantities • Future Developments of the SI • Replacing the kg

1. MSL, IRL, CRI, NMI ….!! • MSL • Measurement Standards Laboratory of New Zealand • About 30 staff, mainly scientists (15 PhD’s in Physics, 2 PhD’s in Chemistry) • Maintains the national physical measurement standards in New Zealand • Has a legislative role to define the values of physical unitsused in New Zealand • Is part of a bigger organization: IRL • IRL • Industrial Research Limited • One of 8 Crown Research Institutes(government owned companies) • About 300 staff (~ 200 scientists) • Major groups: Carbohydrate Chemistry (patents formaterials used in cancer therapy) High Temperature Superconductors (patent for the discovery of BSSCOor Bi-2223, one of the most usefulHTS materials)

1. National Metrology Institutes (NMI’s) • New Zealand’s NMI is MSL in Wellington (http://msl.irl.cri.nz/) • Part of IRL, one of 8 government research institutes • Japan’s NMI is NMIJ in Tsukuba (http://www.nmij.jp/) • Part of AIST, the main research institutes in Japan • Nearly all industrialised countries have an NMI • Good links via collaboration, long term relationships • Strong overlap of problems of interest

1. National Metrology Institutes (NMI’s) • More formal links through international arrangements • Global groupings called Regional Metrology Organisations • Japan and New Zealand are in APMP (Asia Pacific Metrology Program) • Global agreement about the recognition of measurements made in other countries • Based on a set of internationally accepted and published measurement capabilities (see BIPM website) • Rigorously verified by international measurement comparisons

1. NMI’s are Globally Connected Regional Metrology Organizations

1. National Metrology Institutes (NMI’s) • What do NMI’s do for their nation? • Provide traceability to the SI • research capability • maintain and develop primary standards • calibration services • training and advice • Provide the science behind the national quality infrastructure • standardisation, • metrology, • testing, • certification, • accreditation

1. National Quality Infrastructure

2. Why is this needed? • A measurement system becomes important when people exchange information: • Important for trade - stable society demands fair trade • Important for scientific communication • Measurement systems and units are not fundamental – chosen for convenience to humans. • e.g. theory h = c = 1

2. History • A long history • Egypt 2500BC – pyramids built to accuracy of 0.05% • “Do not use dishonest standards when measuring length, weight or quantity” Leviticus 19.35 (Bible)

2. A Confusing History • By 1800’s, there was a very large number of measurement systems in use around the world • Complicated, inefficient, not trustworthy • A need for a revolution…

2. A Revolution in Measurement • Around 1800 the French developed a decimal system based on the properties of the planet: • The meter: 10-7 of the length of the quadrant from the North Pole to the equator via Paris • The kilogram: the weight of 1 cubic decimeter of water • With the discovery of electrical phenomena, it was eventually realized that power and energy should be consistent between mechanical and electrical units (Maxwell, 1863) • Led to the units ampere, volt, coulomb, ohm and also joule and watt

2. A Revolution in Measurement • Advantages of the metric system: • uniform measures • across a nation and across the world • a scientific basis • time invariant and reproducible • a decimal scale • offers convenience of calculation • But note - there are many possible metric systems

2. The Metric System • Metric scale: ratios are meaningful (the scale has a natural zero) • time interval: 6 seconds/2 seconds = 3 • time of day: 6pm/11am =…..?? • time of day is only an interval scale: 6pm-11am = 7 hours • Key point: only 1 standard defines the entire scale • Need accurate scaling methods to build up the scale • Metrologist talk in ratios! • e.g. ppm, 10-6 , parts in 106 all mean the same • Metric property allows the measurement scales of different quantities to be combined (quantity calculus): • mass in kg  acceleration in m/s2 = force in newtons • Want a coherent system: no conversion factors • 1 watt = 1 volt  1 ampere • Conversion factors appear in formulae is 3.42 ppm

1875: The Metre Convention signed (now including the second) a diplomatic treaty originally 17 nations Japan signed 1881, New Zealand 1991! 2008: 51 member states, 27 associate states Other base units added: 1946: the ampere 1948: the kelvin and candela 1971: the mole 2. History of the Metric System

2. The beginning of the SI • 1960: standardised on the MKSA base units: • Meter, Kilogram, Second, Ampere + Kelvin, Candela (+ Mole in 1971) • Named in French: Le Système international d'unités (SI) or in English: The International System of Units • Base units are definitions – require practical methods to implement them

2. Structure of the Metric System • Formal structure from the top level body (CGPM) down to internationally representative technical committees (see http://www.bipm.org/) • For example both Japan and New Zealand have a representative on the Consulative Committee for Electricity and Magnetism (CCEM) • BIPM – laboratory where the primary artefacts were held (meter and kilogram)

Calibration Limit Region accessible to all Refinement Region Accuracy Limit 10-4 Present industrial ‘Best’ instrument / method 10-5 Next generation instrument / method 10-6 Relative Accuracy Accuracy Of National Standard Next generation standard 10-7 Present Time 10-8 1980 1990 2000 Region not yet accessible Year 2. Anticipating Needs • The SI sytem is not static • Accuracy improves on average about a factor of ten every 15 years (courtesy of Brian Petley of NPL, London)

2. The Measurement Arms Race • Continual scientific and technology advances lead to constant improvements in accuracy • Pressure on NMI’s to make continual improvements • some accuracy improvements actually simplify the standard

2. Measurement Uncertainty • A measurement without an uncertainty is meaningless • The physics of the measurement process is contained in the uncertainty • The 1993 publication of the ISO “Guide to the Expression for Uncertainty in Measurement” has led to much greater international consistency in the calculation of uncertainty • Quite a lot of research presently being done to ensure uncertainty calculation is based on rigorous statistics • Metrologists take uncertainty seriously!

2. SI definition of the Meter • “The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.” • Realized by counting wavelengths of an iodine stabilized He-Ne laser

2. The Meter • Initially made equal to one ten-millionth of the distance from the equator to the North Pole • Hence earth’s circumference is ~ 40,000 km • Speed of light was fixed in 1983 by the SI definition • Can resolve physical distances of 1 nm on macroscopic objects • Can achieve uncertainties of 10-12

2. SI definition of the kilogram • “The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.” • The international prototype kilogram made from Pt-Ir is held at the BIPM in Paris

2. The kilogram • The mass of a cubic decimeter of water at the ice point • Concern over undetectable drift • ~ 50 mg over 100 years (the mass of a dust particle of 0.4 mm diameter) • To precious to use! (only used for comparisons 3 times in more than 100 years) • The scale is disseminated with stainless steel masses • microscopically messy • Very large air buoyancy correction

2. SI definition of the Second • “The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.” Caesium Atomic Clock- best clocks have uncertainties ~ 10-16

2. The Second • Improvements in clock performance have been a consequence of the very productive research in atomic physics • linked to ~ several Nobel prizes (Ramsey, Phillips, Hall etc) • cooled ion and atom clocks, laser frequency combs • General Relativistic corrections are required • NIST time and frequency lab in Boulder is at an altitude of 1.6 km

2. History of the Second Was 1/86400 of the mean solar day but the earth is not a good clock: From NMIJ website

2. Leap Seconds leap second on 0h 1 Jan 2009

2. SI definition of the Kelvin • “The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.” • Realised by a triple point cell • Water in the form of vapour, liquid and solid in equilibrium (you can try this at home..) • In practice true thermodynamic temperature is very difficult to measure • ideal gas law, Johnson noise • Instead a practical scale based on the temperature of a platimum resistor is used (ITS 90) • The resistance ratio is defined at ~ six fixed points corresponding to melting transitions of pure metals (gallium, gold etc)

2. The Kelvin • Triple Point Cells are consistent internationally to ~ 40mK • A recent finding has been the need to define isotopic composition of the water used in the cell (variations in 2H, 17O and 18O can cause shifts of 100mK)

2. The Kelvin • In practice true thermodynamic temperature is very difficult to measure • ideal gas law, Johnson noise • Instead a practical scale based on the temperature of a platimum resistor is used (ITS 90) • The resistance ratio is defined at ~ 8 fixed points corresponding to melting transitions of pure metals (mercury, gallium, silver, gold etc)

2. SI definition of the Candela • “The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.” • Realised by a Cryogenic Radiometer

2. The Candela • Cryogenic Radiometer is actually a measurement of electromagnetic power • Use in an electrical substitution mode • Compere the heating effects of a beam of light with that generated by a current through a resistance heater • The most difficult part is accounting for the loss of light in enetrng the cryostat • Limits accuracy to ~10-5

2. SI definition of the Mole • “1. The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. • 2. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.” • This defines Avogadro’s constant • NA = 6.02214179 x 10-23 /mol • Resolved differences between chemistry and physics • But is a counting base unit necessary?

I d I 2. SI definition of the Ampere • “The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length.” • The present definition of the ampere fixes the magnetic constant m0 (the permeability of vacuum) at 4p x10-7 H/m • Realised by force balances

2. MSL’s Base Units in Summary • Mass: three stainless steel 1kg- calibrated at the BIPM • Length: I2 stabilised He-Ne Laser- internationally agreed lines • Time: three HP Cesium Clocks- contribute to global average maintained at BIPM • Electricity: 10 V Josephson Array, Calculable Capacitor, Quantum Hall Resistor • Temperature: Various Fixed points- agreed practical temperature scale (IPTS 90) • Radiometry: Cryogenic Radiometer • As well as many derived units and scales (power, impedance, humidity, pressure etc)

3. General Comments • In practice the SI definition of the ampere has proved to too difficult to implement at sufficient accuracy • For many years only ~ 0.2 ppm • It is possible to make electrical measurements with more precision than can be shown to be consistent with the rest of the SI units • e.g. Capacitance ~ 0.001 ppm • Two important discoveries of quantum phenomena have enabled the electrical quantities of voltage and resistance to be obtained with precision better than 0.01 ppm • current is a more difficult quantity to measure directly • Field of quantum metrology • The standards for electrical quantities have progressed in a self consistent way well beyond their SI traceability “The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length.”

V +1 -1 n = 0 I 3. The Josephson Effects • The ac Josephson effect • Junction between two superconductors • Apply ac current at frequency f (microwave frequency) • Get constant-voltage steps • h/2e  2 V/GHz • Josephson constant KJ 483 597.9 GHz/V V R Ic I

3. Quantum Metrology – the Josephson Volt • Initially realized by a single Josephson junction radiated by microwaves • The key technical advance in the 1990’s was to put thousands of them in an array – and out pops 10 V • About 20 systems now in use by industry and the military in North America Brian Josephson (1973) "for his theoretical predictions of the properties of a supercurrent through a tunnel barrier, in particular those phenomena which are generally known as the Josephson effects"

3. The Impact of the Josephson Effect

3. Aside: Counting • Note that the most accurate SI quantities involve counting (time and length) • The most accurate because counting is a simple and robust measurement method - can count with very high precision • Ideally would like to convert all measured quantities into a frequency (or vice versa – the Josephson Volt converts a frequency into a voltage)

3. Quantum Metrology – the Quantum Hall Effect • 2 dimensional electron gas • formed at the boundary of a hetrostructure • At high magnetic fields electrons condense into Landau levels • At certain field values there is an energy gap between levels which cannot be overcome at low temperatures (< 4 K) • The Hall resistance (transverse voltage/ longitudinal current) becomes quantised Klaus von Klitzing (1985) “for the discovery of the quantized Hall effect”

3. Quantum Metrology – the Quantum Hall Effect Laughlin, Stomer and Tsui (1998) “"for their discovery of a new form of quantum fluid with fractionally charged excitations" ”

3. Measuring the QHR • How do we relate the QHR to real resistors without losing accuracy? • Use a Cryogenic Current Comparator (CCC) • precise dc current ratio device • uses a SQUID (superconducting quantum interference device) as a null detector • Used in a Bridge configuration • bridge Techniques are powerful because they cancel out common effects (e.g. variation in applied voltage) • matched components, bridge balance sensitive to any differences (e.g. Wheatstone Bridge) • Used in many transducer applications (e.g. strain gauges)

3. Cryogenic Current Comparator • CCC – an almost ideal current ratio device • Set of ratio windings (1,2,4,8,….4001 turns) • Carefully shielded by a superconductor (lead) in an overlapping tube construction (“a snake swallowing its tail”) • Net flux is coupled to a SQUID (resolution is 10-5 of a flux quantum F0) • Can achieve current ratios of ~10-11

3. CCC Bridge • Two balances to ensure definition is accurate: • Voltage balance across resistors with high impedance null detector • Current balance via flux balance in CCC • SQUID senses total flux in CCC

Precision Measurement Techniques