Dr Mark Oxborrow, from the National Physical Laboratory tells us about his collaboration with French researchers in Besançon.
In a nutshell: the project facilitated interactions, in the area of microwave frequency standards1, between students and staff of the Laboratoire de Physique et Métrologie des Oscillateurs (LPMO)-CNRS, Institut FEMTO-ST, Besançon and ourselves at the National Physical Laboratory (NPL) in Teddington, London.
Our host at the LPMO was Vincent Giordano, Directeur de recherches au CNRS.
I shall now succinctly describe our exchange activities in 2004, with some concrete results that issued from them identified along the way. By way of background/explanation, I ought first explain that the National Physical Laboratory's facilities and research programmes are managed on behalf of the British government by a subsidiary of a public limited company. It has been my creative task, as project leader for the PN 04.059 on its British side, to reconcile the NPL's commercial pressures and contractual obligations with the objectives and spirit of the Alliance Programme. Upon reading the whole of this report, I trust the reader will judge that I have just about succeeded.
A revision of NPL's own technical 'milestones' in the late Autumn of 2003, meant that it no longer made any sense to engage in the development of a microwave synthesizer, as detailed in the project's original (June 2003) application to the British Council. My project partner (Vincent) and I agreed to collaborate instead on a different yet allied and equally challenging task, namely the design and installation of a microwave power-control servo loop2 at the LPMO. In addition, as it turned out, we grabbed and ran with an extremely exciting serendipitous discovery, which has resulted in the two conference submissions appended to this report. Vincent and his PhD student, Pierre-Yves Bourgeois, visited the National Physical Laboratory here in Teddington (a town in the suburbs of southwest London) from 29th March to 2nd April 2004, just prior to the European Time and Frequency Forum (EFTF), 5th-7th April 2004, held at the Univ. Surrey in Guildford, which we also all attended together.
We paid two visits to France:
Visit 1 Giuseppe Marra and I (Mark Oxborrow) visited the Laboratoire de Physique et Métrologie (LPMO), Besançon for the week of May 31st to 4th June 2004, when one of the LPMO's sapphire oscillators3 was at liquid-helium temperature (i.e. ~4 Kelvin) and fully operational. We had imported a significant quantity of microwave components in our suitcases from the NPL. Explaining all these gadgets through the x-ray scanners at Waterloo's Eurostar terminal was quite an ordeal! We reconfigured their oscillator installing several of our components, including the power-control servo, with the hope of improving their oscillator's stability. The graphic below shows a version of LPMO's set-up ; the power servo loop is displayed in red. These NPL components were left installed on our departure, allowing Vincent and colleagues to evaluate them for several months.
(source : P-.Y. Bourgeois' PhD thesis)
I am afraid to report that our power-servo implementation did not actually improve the stability of their oscillator; there was a more dominant, unsuppressed source of instability, as yet to be identified. Mother Nature's shamelessly consistent (though not always uncharitable) application of the laws of physics had humbled us again –oh well!
Through arrangements made by our hosts, we got to see several other facilities, external to the LPMO, that were relevant to our research work in time and frequency metrology. We paid a visit to a nearby hydrogen maser4 and gained an understanding of the fibre-optical link that relayed this maser's output back to Vincent's own lab. These visits, including some impromptu conversations with local experts along the way, were all hugely informative from a technical standpoint. One scientific as well as cultural highlight for both Giuseppe and myself was a private tour of Besançon's old observatory, conducted by its modern director, François Vernotte. Among other, wondrously arcane instruments from the late 19th and early 20th centuries, François showed us an enormous lunette méridienne 5, still installed within its own grand building, that had been thoughtfully constructed to maintain the lunette's accuracy.
We actually stayed in Besançon until Sunday 6th, using the Saturday, when the LPMO was shut, to make a cycle tour along the river Doubs. As someone with little/no pre-existing knowledge of the Franche-Comté region, I must say that I found Besançon, especially the old city, with its encircling river embankments and castle towering above, surprisingly beautiful.
The four main participants in the exchange (from left to right); Giuseppe Marra (NPL), Vincent Giordano (LPMO), Mark Oxborrow (NPL), and Pierre-Yves Bourgeois (LPMO).
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Mark (back) and Pierre-Yves (front) playing with their
microwave gadgets on the top of a liquid-helium
cryostat (the big blue cylinder) |
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During our visit, Vincent told us about a strange 'bistability' phenomenon6 that he and colleagues (Yann Kersalé, Pierre-Yves Bourgeois) had witnessed, and for which they had no complete explanation. On the last day of our visit (in the lab at the LPMO), they demonstrated the effect to us; to be honest, I was somewhat doubting its existence. On the train back to England, I mulled over what I had seen and, two weeks later or so, I began to come up with a physical theory that could explain the experimental facts. These musings, after several revisions with Vincent and colleagues, have now led to two joint conference abstracts (Annex II) and a (to be peer-reviewed) journal paper that we are in the process of writing (to be submitted probably to Applied Physics Letters),
Visit 2: I visited the LPMO alone from Monday 13th December to Wednesday 16th December 2004; the primary reason for this visit was Pierre-Yves' thesis defence ('soutenance'), for which I had been invited as a member of the jury. His defence was actually also quite an 'experience' (in perhaps both the word's English and French meanings) for myself; I had never participated nor witnessed such a spectacle before. I interrogated the candidate politely with a series of technical questions, which, for someone with a dusty French 'O' level and fading neurons, was either brave, foolhardy or a bit of both. The candidate passed. After recovering from some quite serious après-soutenance festivities, Vincent, Pierre-Yves and I worked on how the bistability phenomenon could most appropriately be written up and presented for publication. My trip back from Besançon to London by train was also quite eventful. A huge crackling/sparking plume of copper-green smoke rose above my carriage, and soon afterwards the TGV broke down very much en pleine voie. Having been turfed out into a freezing, muddy field, rescued by a fleet of coaches from the same, put onto a regional train (at Dijon), I got to the Gare de Lyon some 4+ hours late. The unreservedly positive opinion I once held about French infrastructure has now been duly modified!
Between the scientific work, there were several opportunities to socialise and, in particular, to discuss the pros and cons of the systems and organizations, at various levels, that affect our working lives. The differences were really quite fundamental and shocking. We explained to them what 'contractualization' meant in practice, the UK's specific exemptions from (and hence its workers' general complete ignorance of) the European working-time legislation, not to mention the French 35-hour week, as well as the distinction between 'money-purchase' and 'final-salary' pension schemes. Vincent and his colleagues in turn explained to us –and in some detail– how the ongoing (attempted) reorganizations of both the French university system and the CNRS could affect their research careers and how their time might in future be divided differently between teaching, administrative duties, and actual research work in the lab. My overall – perhaps still naïve– impression was that being a researcher in France, at least within the CNRS, was still not a bad lot. Both Vincent and I each hosted an evening party in our respective homes for all visitors and hosts connected with the exchange. We both must thank our wives for preparing most of the food and for both politely tolerating several hours of 'merry' technical babble. In conclusion: Both partners have gained some significant, perspective-changing experiences from the interaction. We are both now in a stronger position to apply jointly for European funding as opportunities (perhaps related to future improvements to the 'Galileo' global positioning system) arise. My French is a tad better. Thanks to good old scientific serendipity, at least one piece of truly novel, publication-quality research has come out of the exchange. En plus, not many Englishmen can claim to have eaten morning toast spread with a mixture of both Marmite and cancoillotte 7–though I can't honestly say I recommend it!
Dr. Mark Oxborrow
Principal Research Scientist
National Physical Laboratory
Quantum Detection Group
Queens Road
Teddington, Middlesex
United Kingdom TW11 0LW
mo@npl.co.uk
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1A general, historical introduction to time and frequency standards can be found at http://physics.nist.gov/GenInt/Time/time.html. The fundamental concept of frequency stability is explained, pedagogically, at http://www.boulder.nist.gov/timefreq/phase/Properties/main.htm .
2An electronic circuit for actively maintaining the power of a microwave signal at a constant level. It contains both a device for measuring the power and one for altering it. Between them lies the circuit’s ‘brain’ –hence the word servo– in the form of an (active) electronic filter, which continually calculates the correct adjustment to be applied.
3A sapphire oscillator generates a sinusoidal “AC” electrical signal. Compared to regular 50-Hertz mains electricity, the signal’s frequency, i.e. the number of cycles per second, is almost a billion times higher (typically around 12 GigaHertz) and, most importantly, the frequency is extremely constant over time. At the heart of the oscillator lies a squat, cylindrical piece of high-purity, monocrystalline sapphire, about the size and shape of a small tin of tuna. When cooled to cryogenic temperatures, this sapphire cylinder supports an extraordinarily narrow electromagnetic resonance, upon which an extremely frequency-stable oscillator can be built.
4A hydrogen maser is another, quite different, device for producing stable frequencies. In contrast to a sapphire oscillator, the resonance on which it is based is the 1.42 GHz transition between two (‘hyperfine’) quantum levels in the spectrum of the hydrogen atom. The long-term stability of a sapphire oscillator can be assessed by comparing it against such a maser. See, for example, http://tycho.usno.navy.mil/maser.html .
5A lunette méridienne is an optical instrument for precisely determining the moments in time when stars and planets in the sky cross a defined astronomical meridian (as in the Greenwich Meridian). Before the establishment of more accurate electronic and atomic time scales in the 20th century, instruments of this sort were used to calibrate clocks – see http://www.obs-besancon.fr/article.php3?id_article=109
6The observed microwave power would suddenly switch between two distinct levels.
7A local cheese – of viscous spreading consistency – see http://www.interfrance.com/en/fc/ga_la-fromagerie.html
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