Original URL: http://www.theregister.co.uk/2012/11/20/quantum_kilogram/

Glorious silicon globes could hold key to elusive PERFECT kilogram

El Reg drills into why we need an ultra-accurate mass

By Gavin Clarke

Posted in Science, 20th November 2012 11:01 GMT

Feature There's a piece of metal more than a century old just outside Paris causing men and women of science a lot of bother.

It's considered so important to the world it's kept on land designated international territory, so that no single country can claim it, in a maximum security vault maintained by the International Bureau of Weights and Measures (BIPM).

This smooth precious cylinder of alloy is preserved in conditions defined in 1921: the block is stored inside three glass bell jars, polished using a special mixture of alcohol and ether, and cleaned with pressurised steam prior to use. It can only be removed from its container, located on the outskirts of Paris, in the presence of an official from the French government among other select individuals.

It is the International Prototype Kilogram (IPK) and, as its name suggests, its mass is equal to the official definition of the kilogram in the International System of Units (SI). It was put to use calibrating weights soon after it was made in 1879, and was cast from an alloy of 90 per cent platinum and 10 per cent iridium.

The IPK was crafted by Victorians busying themselves with bringing science and engineering to the Western world. It was during a time when Thomas Edison was driving back the night's darkness with electric street lamps, the French were about to construct the Suez Canal, and telegraph networks connected far flung corners of America and Europe.

Fast forward to today, an era in which electronic chips have features mere billionths of a metre in size, and the IPK is no longer a pinnacle of precision: thanks to modern technology, scientists have found that its mass and the mass of its six working copies are changing.

SI units are the most widely used system of measurement for commerce and science. There are seven base units - the metre, kilogram, second, Kelvin, ampere, mole and candela - the kilogram is the only one still defined by a physical artefact.

The kilogram mass with bell jars

The official kilogram protected from contamination under three bell jars

Since the 1970s scientists have been working towards moving from a physical object to a kilogram based on fundamental constants of nature. This week, their efforts moves a step closer to fruition.

The key thing to grasp is that the kilogram is defined by a lump of metal locked away in a vault; we rely on it to calibrate the world's weights - and if it changes for whatever reason then scientists face a headache as all the world's kilogram weights drift ever so slightly from the official standard.

But if the unit of mass was defined as a mathematical equation, using a universal constant, then any number of kilogram weights can be precisely made, verified and calibrated without the original artefact.

Contrast this to the definition of, say, the metre: one metre is equal to the distance travelled by light in vacuum in a 299,792,458th of a second. The speed of light in a vacuum is constant, the above fraction is constant, the metre is therefore constant - there is no vault somewhere in the world with the official metre defined as the length of a plank of platinum-iridium alloy.

How are the world's top minds going to crack this problem?

On 21 and 22 November, experts from the around the world will gather at the BIPM at an invitation-only meeting to discuss how to define the new SI kilogram mass using using Planck's constant, which is used in the field of quantum mechanics. Another approach up for discussion involves counting the number of atoms in a 1kg sphere of material.

The debate follows an agreement at the General Conference on Weights and Measures (CGPM) - an international body that oversees metric units and directs the BIPM - to redefine the kilogram in terms of Planck's constant. This is a value that defines the ratio between the energy of, say, a photon of light and the frequency of said radiated light. The constant is about 6.626 x 10-34 joules second but with some chicanery it can be expressed in kilogram metres-squared per second, establishing a relationship with mass.

But there's a hitch: the constant's precise value is not known, and two key instruments used by scientists to calculate Planck's constant don't agree and can't as yet produce sufficiently reliable results. These instruments, called watt balances, determine the power required for an electric coil to support the weight of an object; this power - in joules per second - can be used to determine Planck's constant.

The difference between the measurements of the two machines is tiny; we're talking fractions of a grain of sugar.

One balance is installed at the Institute for National Measurement Standards (INMS) in Canada and the other at National Institute of Standards and Technology (NIST) in the US. Scientists operating the devices were set a target of eradicating the difference by 2015, but it's not clear they'll be able to do that.

Worse, more watt balances are being built elsewhere, which could further delay proceedings if they also produce readings that disagree with the others. That could push the project back to 2019.

It's vital for the results from the balances are as accurate as possible with a margin of error too small for anyone to detect or worry about.

Ian Robinson, a fellow at the UK's National Physical Laboratory (NLP) that built the watt balance for the INMS, told The Reg: "Our job is to make sure that nobody notices the differences - if somebody notices, we have failed and that's one of the reasons it hasn't happened until now."

Splitting a speck of dust

No one knows why Robinson's balance in Canada and the NIST balance in the US can't agree.

"Most of the checks have been carried out - everybody has done the straight forward stuff, it's now down to some very subtle things in the apparatus," Robinson said.

The differences between the results are infinitesimal yet signficant. Earlier this year Robinson achieved a value of 6.626071 x 10-34 joule seconds using the balance now at INMS. The NIST value, from 2007, is 6.626068 x 10-34 joule seconds.

John Pratt, group leader of NIST's fundamental electrical measurement group, said the gap between the two devices translates to a difference in the measured mass of 250 parts per billion; the CGPM committee wants 20 parts per billion. To give an idea of scale, the kilogram masses in Paris have changed by 50 parts per billion in just over 100 years.

Pratt, who succeeded NIST physicist Richard Steiner on the project, told us 40 parts per billion would be "fine", adding: "At 20 parts per billion, it becomes more difficult to transport the test mass around the world and have it be reliable. It's a dust speck on the mass at that level."

NIST Watt Balance copy right Robert Rathe, © Robert Rathe; courtesy NIST

Steiner adjusting the NIST's watt balance. Credit: Robert Rathe, NIST

In a watt balance, a coil of wire is suspended in a magnetic field, and electric current is passed through the coil - this causes the coil to move downwards with a force proportional to the current. The coil is attached to one end of a balancing beam and the mass under test is attached to the opposite end, so that the coil's downward force acts as a counterweight to the normal gravitational pull on the test object. When the force, plus the weight of the coil, matches the weight of the object, the current can be recorded.

In the second phase of the experiment, the mass is removed and the coil glides through the same magnetic field at a constant speed, and the voltage generated in the coil by this movement is measured. Planck's constant can be calculated from these two electrical values.

You can get more details on the maths involved from the NPL website here and the NIST website here.

There, though, the similarities between the devices end. Robinson's balance used wires to support the coil while the other has solid arms; the NIST machine is huge - spanning two floors - while the NPL-designed machine sits neatly on a work surface, two metres tall and long, and one meter wide; NIST employs a superconducting solenoid to generate its magnetic field while the NPL-designed balance uses a permanent magnet made of neodymium-boron - a type of rare earth used in electrical motors and hard-drive heads. Also, Robinson's machine, now at INMS, measured a 0.5kg mass not 1kg, supposedly for greater accuracy.

"The trouble is if anybody actually knew what the problem is, they could fix it. The problem is quite deeply buried in the apparatus," Robinson said. "That means they have to make as many checks as possible to see where the problem might lie."

Robinson, who led NPL's watt balance work, said that, for example, he identified small unwanted motions of the balance and its support as the test mass was placed in and removed from its holding pan; these are the sorts of tiny mechanical problems that the scientists must mitigate or remove completely.

Building a watt balance is an expensive process; one will set you back $1m according to NIST, and Robinson made two with NPL colleague Bryan Kibble, who first proposed the watt balance system in 1975. A Mark-III model of the measurement machine was designed by NPL between 2003 and 2006, but it was not built as the lab closed its experiment in 2007 and sold the equipment to the Canadian institute in 2009.

INMS has since implemented changes suggested by Robinson: the balance arm was tilted and shortened to change the way the mass is raised and lowered. Also, rods replaced the wires to suspend the coil.

NIST has pressed on: boffins removed superfluous wiring and inspected connections to track down and close holes where current may leak. New team members were also brought in a year ago. "This experiment is a point where it needed fresh eyes," Pratt said. "Extremely talented and experienced fresh eyes."

From Russia with love: the perfect silicon spheres

The watt balance is not the only effort to modernise the kilogram: work is also underway using a method that employs the Avogadro constant, which is used to calculate the number of atoms in a substance. Thus, once one has worked out how many atoms should exist in a 1kg sphere of silicon-28 isotope, the balls can be manufactured and X-rays scanners used to ensure the correct number of atoms are present.

Production of the ultra-pure 93.6mm-diameter spheres began in 2004 in Russia using a centrifuge at the Central Design Bureau of Machine Building in St Petersburg.

Watt balances, though, are where the action is at: scientists are building balances at France's Laboratoire National de Métrologie et D'essais (LNE), China's National Institute of Metrology (NIM) and New Zealand's Measurement Standards Laboratory (MSL) in addition to the BIPM's very own balance and work in Switzerland and South Korea. NIST has also plans for a second-generation balance to be operational by 2020.

If the NPL and NIST machines can agree, or if the level of uncertainty narrowed, then their data will be assessed by a committee. Data from the Avogadro device will also be combined. At November's BIPM meeting teams from each group will present their work and field questions in an attempt to finally bring everything out into the open and scotch doubts.

Ian Robinson Watt Balance

Robinson with the second-generation watt balance he helped build

Professor David Inglis, watt balance group leader for Canada's institute, warned against more balances being powered up before the existing equipment reaches an accord.

"It's important we come to an early decision or else all the countries will want to get involved," Inglis told us. "If they get so involved that they feel they need to be contributing it will slow everything down."

He reckoned NIST and INMS should know by next summer how well improvements are progressing; meanwhile his next move is to measure a full kilogram and weigh different materials to validate the results next spring.

There is another theory that chasing a perfect Planck's constant is wrong and that the physicists working on the project are pushing too hard in the wrong direction. Robinson dismisses this idea.

"The choice of the Planck constant for defining mass in the new SI is a decision that, in my opinion, is independent of the watt balance and Avogadro arguments. I feel that it is the right decision because it has a number of advantages including the effect of unifying the SI by bringing the extremely precise quantum electrical units, which were somewhat outside the SI, into the heart of the SI," he said.

"I feel that the difference between NIST and INMS results will be resolved by careful checking of each apparatus and that process is already underway. In addition other countries are building watt balances, which will add to our knowledge."

The effort to break the kilogram from its original physical artefact has taken the best part of forty years, gobbled a lot of money, and could end in deadlock - all for a breakthrough the vast majority of the planet won't even notice. Not even devotees of BBC2's The Great British Bake Off, who've fully embraced digital kitchen scales, will benefit from the existence of a super-accurate kilogram.

Where it will make a difference is in national laboratories; institutions that rely on absolute accuracy and finely calibrated measuring equipment. The standard mass in Paris, an old-world lump of metal kept under three bell jars, is fickle and unpredictable - relatively speaking, of course.

BIPM principal research physicist emeritus Richard Davis said the rules surrounding the handling of the standard mass are slavishly adhered to for no clear purpose.

Let's not go the way of the leap second, eh?

Take the method of cleaning the cylinder using alcohol, ether and steam: "We've tried not to change the cleaning process from what's been done historically in case that has some unforeseen change," Davis said. And yet the mass might have changed.

Then there's the security, details of which he refused to divulge.

"We have security arrangements - but we don't go into them," he said. These do, however, include restricted access to the artefact: just the president of the international committee of weights and measures and a representative of the French government can be present when the mass is removed from its glass jars.

"This is one major reason to replace the definition based on a constant of nature," he said.

Essen and Parry with Ceasium resonator, pic: Courtesy of NPL

The Earth still relies on astronomical seconds instead of atomic seconds, defined by Louise Essen (R) and Jack Parry at NPL

The focus on Planck's constant is an attempt to replace the voodoo with something scientific - something that's rational, logical and constant.

"Assuming we have a new definition, if the watt balances agree with each other, then we will work out how to maintain the unit of mass and disseminate it to the public and to the national labs," Davis said.

The goal for this week's meeting is to decide the next steps in telling the world how to build the perfect kilogram, should the results be accepted. Davis hopes a draft resolution will be approved that can be taken to wider consultation; Robinson's view is that more watt balances should be built to calibrate the perfect "electric kilograms".

Judging by history, though, a standard backed by quantum mechanics is not guaranteed global adaption just because it's more logical than a lump of alloy worshipped in Paris.

A CIPM meeting in 1967 changed centuries of our approach to time and accepted a new definition for the second that was not based on the rotation of the Earth.

That definition is based on the movement of an atom of caesium-133: one second is the time it takes for 9,192,631,770 transitions between two particular states. It's implemented today using atomic clocks - the first was built at NPL in 1947.

The atom's movement is constant; the Earth's rotation is not - it's slowing down. That mean that clocks must occasionally be adjusted to compensate. Today we have a global time standard called International Atomic Time (IAT) that is set using a network of 300 atomic clocks.

The world, however, remains on Universal Coordinated Time (UTC), set by the rotation of the Earth. UTC is used in servers, network devices, aircraft - almost everything. But because the Earth's rotation is slowing, a leap second must be added to UTC every few years to keep it on track with TIA, specifically within 0.9 of an atomic second.

That leap second has a habit of tripping up unprepared Linux servers - as Australian carrier Qantas discovered this year when a leap second was added to UTC and the Linux servers running the Amadeus-based reservation system Qantas relies upon were not updated. It caused a massive malfunction hours into a new day this year causing reservation and check-in chaos for a short period of time at Qantas. The leap second is considered such a serious problem, that space missions do not take off on the day that leap seconds are added.

Yet we do not let go of UTC. A similar unwillingness to relinquish the mechanical past of the kilogram could mean that the metal lump in Paris gets the last laugh over the men and women working on the SI kilogram standard. It's something BIPM wants to avoid not just for all the uncertainties associated with the current mass but also because the kilogram affects other SI standards.

The kilogram as a function involving Planck's constant is tricky to explain, and that could pose a problem in encouraging its wider adoption given the atomic second is relatively easy to grasp yet shunned. "Understanding Planck's constant is a very difficult concept," Davis said.

"There's something very appealing to say we have a lump of metal. But because of the interconnectness of physics, and the impact it has on other fields - it should be replaced as soon as possible."

The lump of metal in Paris, it seems, will enjoy its steam baths for the foreseeable future.

Bootnote

Join Ian Robinson and other Reg readers for a Live Chat on November 23 at 2pm GMT, 9am Eastern, 6am Pacific. He'll update us on the BIPM meeting and talk more about his work and the challenges of defining the electric kilogram. ®