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Microscopy: two lasers are better than one

Stefan Hell and co-workers at Gottingen have adapted a technique known as fluorescence microscopy. In this form of microscopy the specimen is irradiated at a wavelength which excites either natural or artificially introduced fluorescent molecules known as fluorochromes. The sample is then studied through a filter that transmits at the fluorescence wavelength, but absorbs light at the excitation wavelength.

In the Gottingen experiments two lasers are shone on the sample: a green laser is used to excite the fluorescence, while a near-infrared laser is then used to “turn off” the fluorescence from the edges of the fluorescent spot. The second laser does this by exciting the fluorochromes to higher-still levels that decay to the ground state without fluorescing. This reduces the size of the spot by a factor of six beyond the diffraction limit length-wise, and by a factor of two in the radial direction. A further advantage is that the resulting fluorescent spot is more spherical than the spots produced with the traditional technique.

Sir Mark Oliphant dies

Oliphant was born in Adelaide in 1901 and attended the local university before going to Cambridge University in 1927, where he worked on nuclear physics experiments with Ernest Rutherford. In 1937 he was elected a fellow of the Royal Society and moved to Birmingham University to be head of department.

During his time at Birmingham Oliphant was a key figure in many of the scientific developments during the second world war. In 1940 two of his team, John Randall and Harry Boot, invented a new type of magnetron that was crucial in the development of radar. Oliphant also played a key role in getting the UK government to realize the significance of the Frisch-Peierls memorandum, which explained that only a relatively small amount of uranium was needed to make an atomic bomb. And with Ernest Lawrence, he played a similar role in the US. Oliphant actually worked at Los Alamos from 1943.

Oliphant was horrified by the bombing of Nagasaki and Hiroshima and later became one of the founders of the Pugwash peace movement. In 1950 he returned to Australia to help set up the Australian National University in Canberra as director of its research school of physical sciences. Although ANU went on to become Australia’s foremost research university, Oliphant was unable to fulfil his dream of building the world’s most powerful accelerator in Canberra. His machine was rather cruelly dubbed a “White Oliphant”.

Oliphant remained associated with the ANU after his retirement in 1963, continuing to campaign for nuclear power and against nuclear weapons, and serving as governor of South Australia from 1971 to 1976.

Nuclei show their soft side

Helium-6 is a typical neutron-skin nucleus, with two of the neutrons forming a “skin” around the alpha-particle core. Different types of dynamics are possible in such nuclei. In the soft dipole resonance observed by the Japanese group, the alpha particle and neutrons oscillate in opposite directions. In a so-called giant dipole resonance, on the other hand, the protons and neutrons oscillate in opposite directions. This type of behaviour has been observed in lithium-6, which contains three protons and three neutrons. And in a spin-dipole resonance, neutrons and protons with spin “up” move in one direction, while those with spin “down” move in the opposite direction.

S Nakayama and colleagues collided a beam of lithium-7 nuclei from the Ring Cyclotron of the Research Centre for Nuclear Physics at Osaka University onto a lithium-6 target. Some of the collisions resulted in the creation of helium-6 and beryllium-7, with the latter being ejected from the target. By analyzing the beryllium nuclei, Nakayama and colleagues identified a strong candidate for a soft dipole resonance with its energy, cross-section and other characteristics in agreement with predictions.

Superconducting gains

The device has three electrodes: a niobium injector electrode; a common electrode that consists of a layer of niobium and a layer of aluminium; and a niobium detector electrode. The electrodes are separated by insulating barriers, or junctions, through which current can pass by quantum tunnelling. At the operating temperature of 4.2 Kelvin, all the niobium layers are superconducting. In the absence of any injected current, the aluminium layer is also superconducting, as a result of the proximity effect. In a superconducting metal the electrons form pairs, and an amount of energy equal to the superconducting energy gap is needed to split a pair.

The operation of the device can be understood in terms of electrons that are injected into the common electrode. When the injection current is small, the aluminium layer remains superconducting, with an energy gap that is lower than that of niobium. The electrons become trapped in the aluminium layer, but each one is capable of splitting at most one pair of electrons, so relatively few electrons reach the detector electrode. Thus, the current gain is low.

But this changes when the injected current is increased. The aluminium layer is then no longer superconducting, so its energy gap vanishes and the injected electrons increase the temperature enough for a large number of electrons to tunnel through the detector junction. Indeed, the Naples-Oxford team observed current gains of more than 50 and signal power gains as high as 1000.

Such devices could be exploited in a wide range of cryogenic particle and radiation detectors in both astrophysics and particle physics (N E Booth et al. 1999 Supercond. Sci. Technol. 12 538).

Laser control of X-ray output

In harmonic generation a short pulse of intense radiation is focussed into a gas of atoms. The laser-atom interactions are highly nonlinear, and a number of the input photons effectively combine to generate a single output photon with a correspondingly higher energy and shorter wavelength. In most experiments a range of so-called harmonics is produced. Kapteyn and co-workers from JILA, Delft University of Technology in the Netherlands, and Sofia University in Bulgaria, have shown that the strength of a particular harmonic can be increased by carefully controlling the rise and fall in intensity of the laser pulse with time.

They start with a 18 femtosecond long laser pulse which they shine into a gas or argon atoms. A CCD camera is used to measure the strength of the different harmonics produced by this interaction. This information is then fed into a micromachined deformable mirror that can change the shape of the pulse. The mirror is able to change the pulse shape because it is located at a position inside the laser system where the different wavelength components of the beam are spatially separated. By varying the pathlengths of the various wavelengths before they recombine to form the pulse, it is possible to change its shape.

An evolutionary algorithm is used to find the best input laser shape for a desired X-ray output. The shape of the pulse is defined by 19 inputs to the mirror and the algorithm generally finds the best pulse shape after about 50 iterations. The technique allows the team to increase the intensity of any harmonic within the output spectrum.

What’s hot – the ten best-selling physics books

Having just scooped this year’s Aventis Prize for Science Books, Brian Greene’s The Elegant Universe shot back to the top of the best-seller list of physics books for the month of May at Amazon.co.uk, the on-line bookstore. First published over a year ago, Greene’s masterful introduction to string theory was praised by the chairman of the judging panel Lewis Wolpert as “Science as magic. At last somebody has managed to explain a new fundamental theory of everything.”

Richard Feynman makes two appearances in the top 10 – with The Pleasure of Finding Things Out in second spot and The Meaning of It All in seventh place. Also in the top 10 is Stephen Hawking’s ever-popular A Brief History of Time, which has sold an estimated 25 million copies since it was first published over 12 years ago. Another thoroughbred still selling well since it first hit the bookstores in 1984 is John Gribbin’s lucid survey of quantum theory In Search of Schrödinger’s Cat, while the ever-popular Chaos continues to earn royalties for science writer James Gleick 13 years after it first appeared.

Newcomers in the list include The End of Time, in which the stay-at-home Oxfordshire physicist Julian Barbour argues that time does not exist, and The Odd Quantum – a non-mathematical introduction to quantum theory by the late Sam Treiman. Writing in Physics World Asher Peres called it “a wonderful guided tour through quantum mechanics, [which I recommend] without hesitation to every physicist”.

1 The Elegant Universe (Amazon, Amazon UK)
Brian Greene (Vintage) pb

2 The Pleasure of Finding Things Out (Amazon, Amazon UK)
Richard Feynman (Allen Lane The Penguin Press) hb

3 A Brief History of Time (Amazon, Amazon UK)
Stephen Hawking (Bantam) pb

4 The End of Time (Amazon, Amazon UK)
Julian Barbour (Phoenix Press) pb

5 Hyperspace (Amazon, Amazon UK)
Michio Kaku (Oxford Paperbacks) pb

6 In Search of Schrödinger’s Cat (Amazon, Amazon UK)
John Gribbin (Corgi) pb

7 The Meaning of It All (Amazon, Amazon UK)
Richard Feynman (Penguin) pb

8 Chaos (Amazon, Amazon UK)
James Gleick (Minerva) pb

9 Lucifer’s Legacy (Amazon, Amazon UK)
Frank Close (Oxford University Press) hb

10 The Odd Quantum (Amazon, Amazon UK)
Sam Treiman (Princeton University Press) hb

Thomas Kuhn’s best-selling philosophy

Thomas Kuhn is famous for writing the surprise best-seller The Structure of Scientific Revolutions. Who would have thought that a book published in 1962 on the history of science would turn out to be what Steve Fuller claims is the best-known academic book of the second half of the 20th century? So well known is this book, indeed, that Physics World asked a sociologist (me) to write a review of a philosopher’s analysis of the book because, as they told me, Kuhn is someone whom even physicists know about.

Let me say straight away that Fuller’s book is not really for physicists. It is a book aimed at people who have taken inspiration from Kuhn’s book for their own analyses of science. The sociologists (such as myself) and the cultural-studies types who say that “science is a social construct” – and who thus make themselves easy targets for “science warriors” such as Alan Sokal or Lewis Wolpert – mostly started out with Kuhn. That is not to say that Kuhn, who trained as a physicist, provided much in the way of real ideas that we could use; these ideas came either from the philosopher Ludwig Wittgenstein (for the sociologists) or from various French radicals (for the cultural-studies people).

It is not even to say that Kuhn was all that original. In the 1930s a Jewish medical doctor called Ludwik Fleck published a book called Genesis and Development of a Scientific Fact, in which he re-described his own research on syphilis in a way that today would merit attack from any red-blooded science warrior. Fleck, by the way, was forced by the Nazis to make a typhus vaccine for the German army and managed to make it impotent – his superb book has been available in an English translation since 1979, with a foreword by Kuhn himself (University of Chicago Press).

But Kuhn’s book appeared at the right time – at the start of the 1960s, which was the first affluent post-war decade when everything cultural, political, sexual and conscious-altering suddenly loosened up. Kuhn essentially said that science progresses not by orderly accumulation of knowledge, but by sudden shifts of fashion after which the whole scientific world, including data, suddenly takes on a new meaning. He talked of “scientific paradigms”, which provided the framework for the way we design experiments and interpret their results. He also spoke of scientific revolutions, in which existing paradigms are overthrown and replaced. Notoriously, he said that, say, Newtonians and Einsteinians “live in different worlds”. What Kuhn did with his paradigms and scientific revolutions was to loosen up science – to turn it, as many of his critics claimed, into mob psychology.

In 460 or so pages of hyperactive prose, Fuller embarks on taking the wind out of the sails of the science-analyst radicals by telling us that Kuhn was not a radical at all but a Cold War conservative. Fuller has plenty of evidence regarding Kuhn himself. First we can look at his published output. Kuhn’s second “great work” – which seems to be the book that he set out to be remembered by – was a thick treatise on black-body radiation entitled Black-body Theory and the Quantum Discontinuity:1894-1912 (1978 Oxford University Press).

As astonished and disappointed Kuhn groupies noted at the time, the index of this massive and scholarly work contained no references to either “paradigm” or “scientific revolution”. This was just the climax to a post-1962 academic lifetime throughout which Kuhn continually condemned and distanced himself from his radical interpreters – telling us, as Fuller points out, that he preferred his critics to his followers. Furthermore, Kuhn was influenced by the Harvard Society of Fellows, which was in part set up, according to Fuller, to make popularized science an antidote to communism.

One has to admire Fuller for taking one of our best-ever radicals and telling us that Kuhn – and by implication we ourselves – are Cold War puppets. And there is a grain of truth in it. The politics of consciousness of the 1960s were a diversion from big political questions, and so was the analysis of culture. After all, if you were busy altering your mind or relativizing science, you were not blowing things up or writing big books in the name of communism.

Furthermore, if you look at the way “science studies” has evolved, you do find that it is struggling to tell everyone what a timid and unfrightening little thing it is. “Wimps,” says Fuller. “Let’s get back to some real radicalism. Why? Let’s reopen those big questions about whether science is the right way to spend the resources of nations and ask about the effect of the kind of science we do on the world-wide distribution of wealth and power.”

The other interesting thing about Fuller’s book is that it tries to switch the debate about Kuhn from what he said to the social context that caused him to say it. That is to say, Fuller does a sociology-of-knowledge job on the kind of knowledge that the sociologists of knowledge rely on. This is salutary for people like me because it helps us understand what the scientists we analyse have to put up with.

When I look at, say, the debate about whether gravitational waves have been detected, I discuss the social pressures that impinge on the scientists involved, not the scientific content of their arguments. Now Fuller is also concentrating on the forces that led Kuhn to say what he said and on its social meaning – rather than on its content – and I feel about as frustrated as the gravitational-wave scientists must feel about my conclusions. (To appreciate what I mean, read Kuhn’s brilliant 1961 paper “The Function of Measurement in Modern Physical Science” (ISIS 52 162) and remind yourself that in Fuller’s analysis we must ignore the content and concentrate on the context.)

Fuller gives the impression of writing at high speed – the Hunter Davies of science studies – but with immense scholarly breadth. Unfortunately, this approach carries the risk of carelessness. For example, in scything through some of the modern debates in science studies, he neglects to mention that many of his positions and insights – such as the call for science studies to be used in science education, or the way French philosophical radicalism turns full circle into common sense (something that Alan Sokal does not seem to have noticed) – are already there in the work of those he is analysing. This is important because the scythe gets blunted when one knows what the protagonists actually said.

It is also a shame that Fuller claims that social studies of science have concentrated wholly on big science in an attempt to gain respectability. This is exactly opposite to the notorious claim by Lewis Wolpert that science studiers never look at any serious science at all. Neither claim is true.

Schrodinger’s cat comes into view

In his original thought experiment, Schrodinger imagined that a cat is locked in a box, along with a radioactive atom that is connected to a vial containing a deadly poison. If the atom decays, it causes the vial to smash and the cat to be killed. When the box is closed we do not know if the atom has decayed or not, which means that it can be in both the decayed state and the non-decayed state at the same time. Therefore, the cat is both dead and alive at the same time – which clearly does not happen in classical physics.

The SUNY-Stony Brook experiment uses superconducting quantum interference devices (SQUIDs). These are ring-shaped devices in which persistent currents, made of billions of pairs of electrons, can circulate in either a clockwise or an anti-clockwise direction without decaying. Their device is made from niobium, which is superconducting at the temperatures of 40 millikelvin used in the experiment, and aluminium oxide, which acts as a barrier. A palladium-gold shield protects the device from interactions with the environment that would otherwise wipe out the quantum superpositions being studied.

The system can be represented as a potential well with two minima, both of which contain several bound states, separated by a barrier. Friedman and co-workers start with a current of about 1 microamp flowing in, say, the clockwise direction. Next they illuminate the SQUID with microwaves which excite the system to a clockwise state with higher energy. The system can now tunnel from the clockwise state into the anti-clockwise state, and back.

The question is essentially whether the system remembers or forgets its quantum state as it tunnels. To answer this the Stony Brook team measures the probability of finding the current flowing in the anti-clockwise direction as the shape of the double-well potential is changed. The results are exactly as predicted by assuming that the system is in a macroscopic superposition of states. The difference between the two states corresponds to a current of 2 to 3 microamps or a magnetic moment of 10 billion Bohr magnetons, which is “truly macroscopic” according to Friedman and co-workers.

UK loses ground in optics boom

Optics is widely seen as a key technology for the 21st century. The exponential growth of Internet traffic, for instance, presents an enormous opportunity for the companies that build and sell the lasers, amplifiers, optical fibres that make up the world’s communications infrastructure. Indeed, it is estimated that the market for fibre-optic components will be worth over $20bn in three years’ time. Lasers and other optical technologies will also play a crucial role in healthcare, manufacturing, and the defence and aerospace industries.

The pervasive influence of optics was spelt out in a 1998 report by the US National Research Council (NRC) called “Harnessing Light”. Many countries around the world have grasped the message contained in this report and invested accordingly. However, although the UK has made major contributions in many areas of optics in the last 15 years, many scientists and engineers feel that Britain could fall behind in the optics industry. Indeed, in a recent review of physics in the UK (Physics World June p5), an international panel of physicists expressed concern at the UK’s level of investment in optics.

With Internet traffic doubling every 100 days, it will not be long before the current capacity, or “bandwidth”, is exceeded. In order to increase bandwidth, telecoms companies must either increase the rate at which they can send data along optical fibres, or replace the electronic components that limit the performance of optical networks.

The complete replacement of electrons by photons – the shift from optoelectronic to photonic technology – is a holy grail in communications. But this shift will require the development of new materials with high optical nonlinearities and fast response times. There are many materials that offer one of these properties but none offer both. Many in industry, therefore, believe that the answer to the Internet’s potential bottleneck lies with fundamental research.

One class of materials currently undergoing intense research is “photonic crystals”, a material with a photonic band gap – the optical equivalent of the energy band gap found in semiconductors. Photonic crystals would allow light to be directed around much sharper bends than is currently possible with fibres. Microelectromechanical devices (MEMS) are another active area of research. A MEMS device would use tiny mechanical mirrors to switch optical signals.

Many in industry and academia, however, think that such blue-sky research is being stifled in the UK. David Hanna of the Optoelectronics Research Centre (ORC) at Southampton University believes that good research ideas in optics are not being funded quickly enough. “I think we are doing extremely well with what we are given,” he says, “but we are being held back by limited resources.” Since optical technologies are so pervasive, they usually only plays a secondary role in larger systems, and Hanna believes that their economic importance is often not fully recognized. Another problem is that optics research in universities is carried out in a range of different departments – physics, chemistry, materials, electronics, engineering and computing. “No one involved in the field has the full view,” says Hanna, “and so optics does not get the share of the funding that it should get.”

A recent review of UK photonics said that it is not appropriate for the Engineering and Physical Sciences Research Council (EPSRC) to impose lists of preferred technologies when selecting research proposals. Universities should provide the scientific knowledge base that companies can then draw on to select the most appropriate technologies, according to the report. The review panel recommended that EPSRC should increase funding of its photonics research by at least 30%, and ideally by 50%, from its present level of about £70m every year, within the next two years.

UK industry has a poor record of investing in optics. Although the UK produces about half of Europe’s optical components, most of the industry is owned by foreign companies such as Nortel, JDS Uniphase and Agilent. And earlier this year, BT sold its world-leading photonics research centre to US firm Corning. However, there are some promising signs – last year Marconi separated from its parent company GEC to concentrate on telecommunications, and the number of photonics start-up companies is on the rise.

“Traditionally the UK has had a strong science and technology base in photonics, with good inward investment, but the difficulty has been converting research into production,” says Andrew Rickman, chief executive of Bookham Technology, a UK optical components company. “But there is now a change in tide: people are now prepared to invest in start-up companies.”

Indeed, last month a spin-off company from the ORC attracted the largest ever investment for a university start up. Southampton Photonics received £37m of venture capital to design and manufacture fibre-optic components, and says it will create 200 skilled jobs in the UK over the next 18 months. Several other UK universities have produced start-up companies, including several in southern Scotland.

To encourage more collaboration between universities and industry, the Department of Trade and Industry and EPSRC last month put £11m into a new “LINK” scheme, which will run over five years. “The UK has a world-class reputation in optical technologies,” says the UK science minister Lord Sainsbury, “and the new LINK programme will do much to ensure that this research is exploited by UK-based companies for the benefit of the UK economy.”

Although Colin Webb of Oxford University, and president of the UK Consortium for Photonics and Optics, welcomes the LINK scheme, he believes it provides only about a quarter of the funding needed. “One wonders whether the UK’s investment will have the desired effect,” he says, “as similar government support schemes have done in the US and Germany.”

Many within the photonics community are worried whether the UK will be able to supply industry with the qualified people it needs to expand. Rickman hopes that Bookham will eventually employ thousands of people, but is uncertain about where the company will find the personnel in the long run. He says Bookham has had to work very hard to recruit engineers, and describes the training required to provide employees with the relevant technical knowledge as “effectively starting up your own university”.

The chairman of Southampton Photonics, David Payne, shares this sentiment, saying that the problem boils down to a lack of students taking science and technology subjects at university. Indeed, many science and engineering PhD students in the UK are from abroad. “The UK has a serious problem with engineering – it is still seen as a second-class profession. Students going through university all want to be in finance or the media rather than manufacturing.” He points out that in the US studying science and engineering is seen as a stepping stone to becoming a rich entrepreneur.

To try and combat the shortage of people trained in optics, the panel reviewing EPSRC’s photonics research recommended that some of the additional funding proposed for the area should be devoted to increasing the number of project studentships. The panel also suggested that there should be more interdisciplinary research between physicists, engineers, materials scientists, chemists, mathematicians and those working in IT. This is something recognized by EPSRC’s chief executive, Richard Brook, who says that EPSRC may in the future devote more attention to its “interdisciplinary research collaborations”.

One such collaboration that has just been set up in the physics department at St Andrews University in Scotland could lead to new ways of increasing Internet bandwidth. The department has received a grant of £10.5m to work with five other universities and eight companies on the application of ultrashort (femtosecond) optical pulses. They will carry out research on novel materials, including organic compounds and small-scale structures in semiconductors. The team hope to achieve a three to four orders of magnitude improvement in data rates in what they say will be “an aggressive research programme with a long-term ‘blue skies’ content”.

Theory, experiment and fine structure

Spectroscopy – the measurement of the properties of light emitted or absorbed by matter – is one of our most powerful tools to study nature. When a prism is used to separate the light from a flame in which salt has been sprinkled, distinct yellow lines become visible in addition to the usual colours of the rainbow. The wavelength of these yellow lines is directly related to the energy levels of the sodium atom, and in this way spectroscopy opens a window onto atomic structure.

The spectrum of any particular atom or molecule acts as a kind of fingerprint. For example, the measurement of previously unknown lines in the spectrum of the Sun led to the discovery of helium even before it was detected on Earth. A recent experiment on the spectrum of helium by Eric Hessels and co-workers at York University in Canada bears on one of the most fundamental constants of nature, the fine-structure constant (C Storry et al. 2000 Phys. Rev Lett. 84 3274).

The study of the spectra of the two simplest atoms – hydrogen and helium – has played a central role in the development of modern physics, especially in quantum mechanics and quantum field theory. Bohr’s initial planetary model of hydrogen was built on its successful description of the Balmer formula, a simple formula for the spectrum of hydrogen that was developed in 1885. However, the model failed to describe the spectrum of helium, which became one of the crucial tests of the “new” quantum mechanics introduced in 1925.

The theory of relativity was already 20 years old when quantum mechanics was introduced. The natural step of merging the two theories into the first relativistic quantum field theory, quantum electrodynamics (QED), was quickly taken, although certain mathematical problems were not fully resolved until the late 1940s.

An important feature of atomic spectra that arises from relativity is fine structure – so called because what at first appears to be a single line in a spectrum is actually seen to consist of two or more closely spaced lines when analysed with high precision. Fine structure arises because the electron has an intrinsic angular momentum or spin that interacts with the magnetic field that is produced, for example, as the electron orbits the nucleus at relativistic speeds. The strength of this interaction is characterized by the fine-structure constant, a, given by e²/2e₀hc, where e is the charge of the electron, e₀ is the permittivity of free space, h is Planck’s constant and c is the speed of light. This dimensionless constant has a value of around 1/137. The equations for fine structure contain a factor of a², so the spacing between the lines is suppressed by over a factor of 10 000.

The way this works in helium can be illustrated by describing the transitions studied in the York experiment. Hessels and co-workers measured the transition between a state with two electrons in which the orbital angular momentum is zero (denoted 2³S₁) and a state with a single unit of orbital angular momentum that is split into three fine-structure levels (denoted 2³P_J, where J = 0,1 and 2 are the quantum numbers that label the fine structure).

Were it not for relativity, the three states with different J would all have the same energy, and the light emitted in the transition would have a single frequency of about 277 000 GHz. However, relativistic effects mean that the states have slightly different energies, and when this light is analysed carefully, splittings of the order of 10 GHz are seen.

Accurate values of the fine-structure constant can be obtained in a number of ways – some require solid-state physics, while others involve neutron-scattering experiments. The most accurate method comes from comparing the QED theory of the magnetic moment of the electron with experiment, and gives a result with an accuracy better than four parts per billion.

So why is there so much interest in determining a using the fine structure of helium? The measurement has the potential to provide the next-most-accurate determination of the fine-structure constant. If this value agrees with the best measurement, it will provide a powerful test of the internal consistency of QED. Disagreement, however, could signify possible new physics.

Hessels and co-workers have measured a particular transition in helium known as the small interval, n₁₂, which is the energy difference between the J = 2 and J = 1 levels. Their result of n₁₂ = 2291 174.0 kHz has an error of 1.4 kHz, which is well below the estimated theoretical uncertainty of 20 kHz.

The experiment involves first exciting a beam of helium atoms in the 2³S₁ state to the 2³P₁ state. This state is then “tickled” using microwave energy with just the right frequency to cause the transition to the higher 2³P₂ state. When the atoms are detected falling back down to the original 2³P₁state, this signals that the microwave frequency corresponded to the small fine structure interval n₁₂. A major complication with the measurement is a smearing out of the energy of the states connected with their decay. This so-called natural linewidth is about 3000 kHz, so very careful measurements are required to find the centre of the line to within 1.4 kHz.

If the spacing between the lines were directly proportional to a², the value for the fine-structure constant could immediately be determined from a comparison with theory. However, QED is a theory containing corrections that are powers of a, and in order to compare it with such an accurate experimental result requires understanding the theory to at least order a³. Unfortunately, while this level of understanding has been reached for hydrogen, only the order a² results are known for helium because of its more complicated atomic structure. (The reason hydrogen fine structure is not competitive for determining a relates to the more rapid decay of the corresponding states compared with helium, which leads to much more severe smearing-out problems.)

Theoretical activity towards a complete calculation at the next order has intensified, largely spurred by such high-accuracy experiments. The 20 kHz theoretical error mentioned above is an estimate of the size of the uncalculated terms. When the calculation is completed, agreement with this experiment will provide a check of the highly complex theory.

Once the theory has passed this test, its predictions for another interval known as the “large interval”, n₀₁, which is also being measured by the York group and others, should be reliable. If the experiments can reach the 1 kHz level of accuracy, a determination of a that is accurate to 17 parts per billion will result. Such a measurement will be second in accuracy only to the determination from the electron magnetic moment.

Because of the complexity of high-precision experiments such as this one, different experimentalists frequently find incompatible results that cloud the interpretation of the data. Thus it is gratifying to note that only one month after the publication of the York result, a second determination of the same splitting was reported by David Shiner and co-workers at the University of North Texas (J Castillega et al. 2000 Phys. Rev. Lett. 84 4321). Their result of 2291 175.9(1.0) kHz is in good agreement with the previous measurement, and establishes this splitting as one of the most accurately and reliable fine-structure splittings measured so far.

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