In the past few years, optical atomic clocks have made spectacular progress, becoming 100 times more precise than the best caesium clocks. So far, their precision has been available only locally, since frequency transfer via satellite cannot provide sufficient resolution. This has recently changed thanks to a new direct optical connection between France and Germany, established by joint work of Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Systèmes de Référence Temps-Espace (LNE-SYRTE) in Paris, and Laboratoire de Physique des Lasers (LPL) in Villetaneuse. High-precision optical frequencies can now "travel" through a 1400 km optical fibre link between LNE-SYRTE and PTB, where the most precise optical clocks in Europe are operated.
The first comparison between the French and German optical strontium clocks confirms the high expectations placed in the connection. It represents the first frequency comparison of its kind across national borders: the fully independent clocks agree with an unrivalled fractional uncertainty of 5 × 10e-17. The scientists report their results in the current issue of Nature Communications. Their successful collaboration is a first step towards a European network of optical clocks providing ultrastable high-precision optical reference signals to diverse users. This will benefit various research areas, with applications in fundamental physics, astrophysics and geoscience.
Comparisons of clocks at the highest resolution allow a wide range of very sensitive physical experiments, for instance, the search for time-dependent changes of fundamental constants. Also, the apparent rate of a clock depends on the local gravitational potential: comparing two clocks measures the gravitational redshift between them, and thus yields their height difference. Such measurements provide data points for the geodetic reference surface, the so-called "geoid". This research approach is pursued jointly by physicists and geodesists in the Collaborative Research Centre 1128 ("geo-Q") of the German Science Foundation (DFG).
Today's most precise atomic clocks are based on optical transitions. Such optical clocks can provide a stable frequency with a fractional uncertainty of only a few 10e-18. This is approximately 100 times more precise than the best caesium fountain clocks, which realize the unit of time, the SI second. However, clock comparisons using frequency transfer via satellites are limited to a frequency resolution near 10e-16.
For this reason, scientists from PTB and from two French institutes in Paris (Systèmes de Référence Temps-Espace, LNE-SYRTE and Laboratoire de Physique des Lasers, LPL) have been working for several years on an optical fibre connection between the German and the French national metrology institutes, PTB and LNE-SYRTE. The 1400 km long link is now completed: it is based on standard telecom optical fibres and optical power losses of 200 dB (10e20) are compensated by means of specially developed amplifiers. Furthermore, frequency fluctuations added during the propagation along the fibre are actively suppressed by up to 6 orders of magnitude. This allows the transmission of optical signals with very high stability.
The German part of the link uses commercially rented optical fibres and facilities of the German National Research and Education Network (DFN). The French part of the link uses the network for Education and Research RENATER, operated by the GIP RENATER. Approximately midway, signals from LNE-SYRTE and PTB meet at the IT Centre of the University of Strasbourg, so that the clocks of the two institutes can be compared there. The partners involved are: Physikalisch-Technische Bundesanstalt (PTB), Institut für Erdmessung (IfE) der Leibniz-Universität Hannover, Laboratoire de Physique des Lasers (Université Paris 13/Sorbonne Paris Cité/CNRS), LNE-SYRTE (Observatoire de Paris/PSL Research University/CNRS/Sorbonne Université/UPMC Univ. Paris 6/Laboratoire National de Métrologie et d'Essais), and the GIP RENATER (CNRS, CPU, CEA, INRIA, CNES, INRA, INSERM, ONERA, CIRAD, IRSTEA, IRD, BRGM, and the MESR).
In a first comparison using the most stable optical clocks of PTB and LNE-SYRTE, the link lived up to the high expectations. Frequency fluctuations between the two strontium optical lattice clocks of less than 2×10e-17 were observed after only 2000 s of averaging time, and the link itself supports fast clock comparisons with an uncertainty below 10e-18. As both clocks are based on the same atomic transition they should theoretically supply exactly the same frequency – except for the gravitational redshift due to the 25 m difference in height between the two institutes. This was indeed confirmed within the clocks' combined uncertainty of 5×10e-17, corresponding to a height uncertainty of only 0.5 m.
The partners consider this successful collaboration the first important step towards a European network of optical clocks connected by optical fibre links which could successively be joined by the optical clocks of further European metrology institutes. This should place them in a leading role for the dissemination of optical reference frequencies. As a long-term perspective, such a network may provide ultrastable high-precision optical reference signals (like those currently available from metrology institutes) to a broad range of users. Various research areas will benefit from this, including fundamental research (to test the fundamental laws of physics), geoscience and, last but not least, metrology. This work also clears the path towards a redefinition of the unit of time, the SI second, through regular international comparisons of optical clocks.
Contacts for the strontium clocks:
Dr. Christian Lisdat, PTB Working Group 4.32 Optical Lattice Clocks,
phone: +49 (0)531 592-4320,
Dr. Jérôme Lodewyck, LNE-SYRTE, CNRS
phone: +33 (0) 1 40 51 22 24,
Contacts for the optical fibre link:
Dr. Gesine Grosche, PTB Working Group 4.34 Frequency Dissemination with Fibres,
phone: +49 (0)531 592-4340,
Dr. Paul-Eric Pottie, LNE-SYRTE
phone: + 33 (0) 1 40 51 22 22,
C. Lisdat et al.: A clock network for geodesy and fundamental science. Nature Comms. 7:12443 (2016), DOI 10.1038/NCOMMS12443
Dr. Christian Lisdat
Katika miaka ya hivi karibuni, saa za atomiki za macho zimefanya maendeleo makubwa, zikiwa sahihi mara 100 kuliko saa bora za sesiyamu. Hadi sasa, usahihi wao umepatikana tu mahali hapo, kwa kuwa uhamisho wa mzunguko kupitia setilaiti hauwezi kutoa azimio la kutosha. Hii imebadilika hivi karibuni kwa sababu ya uhusiano mpya wa moja kwa moja wa macho kati ya Ufaransa na Ujerumani, ulioanzishwa na kazi ya pamoja ya Physikalisch-Technische Bundesanstalt (PTB) huko Braunschweig, Systèmes de Référence Temps-Espace (SRTES) huko Paris, na Laboratoire de Physique des Lasers (LPL) huko Villeneuve. "Kupitia ""kusafiri"" kwa njia ya 1400 km fiber optic uhusiano kati ya LNE-SYRTE na PTB, ambapo saa ya macho sahihi zaidi katika Ulaya ni kuendeshwa." Kulinganisha kwanza kati ya saa za macho za Strontium za Ufaransa na Ujerumani inathibitisha matarajio makubwa yaliyowekwa katika uhusiano huo. Kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano, kwa mfano. Wanasayansi hao wanaripoti matokeo yao katika toleo la sasa la Nature Communications. Ushirikiano wao wa mafanikio ni hatua ya kwanza kuelekea mtandao wa Ulaya wa saa za macho zinazotoa ishara za kumbukumbu za macho za usahihi wa juu kwa watumiaji mbalimbali. Hii itasaidia maeneo mbalimbali ya utafiti, na maombi katika fizikia ya msingi, astrophysics na geoscience. Kulinganisha saa katika azimio la juu zaidi inaruhusu mbalimbali ya majaribio ya kimwili nyeti sana, kwa mfano, utafutaji wa mabadiliko ya muda-kutegemea ya constants msingi. Pia, kiwango cha kawaida cha saa hutegemea uwezo wa uvutano wa mahali hapo: kulinganisha saa mbili hupima kuhamishwa kwa rangi nyekundu ya uvutano kati yao, na hivyo hutoa tofauti yao ya urefu. Vipimo hivyo hutoa pointi za data kwa ajili ya uso wa kumbukumbu ya geodetic, inayoitwa "geoid". "Hii ni utafiti wa kwanza wa kisayansi uliofanywa na ""Jumuiya ya Mafunzo ya Jimbo la Ujerumani"" (JRC) katika mwaka wa 2000 na ""Jumuiya ya Mafunzo ya Jimbo la Ujerumani"" (JRC) katika mwaka wa 2000." Saa za atomiki za leo zenye usahihi zaidi zinategemea mabadiliko ya macho. Saa za macho zinaweza kutoa mzunguko thabiti na kutokuwa na uhakika wa sehemu ya 10e-18. Hii ni mara 100 zaidi ya saa za kisima cha caesium, ambazo zinajua kipimo cha wakati, SI sekunde. Hata hivyo, kulinganisha saa kwa kutumia uhamisho wa masafa kupitia satelaiti ni mdogo kwa azimio la masafa karibu na 10e-16. Kwa sababu hii, wanasayansi kutoka PTB na kutoka taasisi mbili za Ufaransa huko Paris (Systèmes de Référence Temps-Espace, LNE-SYRTE na Laboratoire de Physique des Lasers, LPL) wamekuwa wakifanya kazi kwa miaka kadhaa kwenye uhusiano wa fiber optic kati ya taasisi za kitaifa za metrolojia za Ujerumani na Ufaransa, PTB na LNE-SYRTE. Uunganisho wa kilomita 1400 umekamilika sasa: inategemea nyuzi za kawaida za mawasiliano ya simu na hasara za nguvu za macho za 200 dB (10e20) zinalipwa kwa njia ya amplifiers maalum. Kwa kuongezea, kutofautiana kwa masafa yaliyoongezwa wakati wa kuenea kwenye nyuzi huondolewa kwa nguvu hadi kwa amri sita za ukubwa. Hii inaruhusu maambukizi ya ishara za macho na utulivu wa juu sana. Sehemu ya Ujerumani ya kiungo hutumia nyuzi za macho zilizokodishwa kibiashara na vifaa vya Mtandao wa Kitaifa wa Utafiti na Elimu wa Ujerumani (DFN). Sehemu ya Kifaransa ya kiungo hutumia mtandao wa Elimu na Utafiti RENATER, unaendeshwa na GIP RENATER. Karibu katikati ya njia, ishara kutoka LNE-SYRTE na PTB kukutana katika Kituo cha IT cha Chuo Kikuu cha Strasbourg, ili saa ya taasisi mbili inaweza kulinganishwa huko. Mashirika ya ushirikiano ni: Physikalisch-Technische Bundesanstalt (PTB), Institut für Erdmessung (IFE) der Leibniz-Universität Hannover, Laboratoire de Physique des Lasers (Université Paris 13 (Sorbonne) Paris-Cité (CNRS), LNE-SYRTE (Université de Paris Observatory) na PSL (Université de recherche Paris-Sorbonne). Mfumo wa Utafiti wa Utafiti wa Utafiti wa Utafiti wa Utafiti wa Utafiti wa Utafiti wa Utafiti wa Utafiti (LINURA) na GIP RENATER (CNRS, CEP, CEP, INRIA, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, INSERM, Katika kulinganisha kwanza kwa kutumia saa za macho thabiti zaidi za PTB na LNE-SYRTE, kiungo hicho kiliishi kulingana na matarajio ya juu. Mzunguko wa frequency kati ya saa mbili za strontium za macho ya chini ya 2x10e17 ilionekana baada ya sekunde 2000 tu za wastani wa wakati, na kiungo hicho yenyewe kinasaidia kulinganisha saa za haraka na kutokuwa na uhakika chini ya 10e-18. Kwa kuwa saa zote mbili zinategemea mabadiliko sawa ya atomiki, zinapaswa kutoa mzunguko sawa kabisa, isipokuwa kwa redshift ya mvuto kutokana na tofauti ya urefu wa mita 25 kati ya taasisi mbili. Hii ilithibitishwa ndani ya usahihi wa pamoja wa saa ya 5x10x17x10x10x10x17x10x10x17x10x10x17x10x10x17x10x10x10x17x10x10x10x17x10x10x17x10x10x10x17x10x10x10x17x10x10x10x10x10x17x10x10x10x10x10x10x17x10x10x10x10x10x10x10x10x10x10x17x10x10x10x10x10x10x17x10x10x10x10x10x10x10x10x10x10x17x10x10x10x10x10x10x10x10x10x10x10x10x10x17x10x10x10x10x10x10x10x10x10x10x10x10x10x10x10x10x10x10x10x10x10x10x10x10x10x10x10x10x10x10x Hii inapaswa kuwaweka katika jukumu la kuongoza kwa ajili ya kuenea kwa masafa ya kumbukumbu ya macho. Kwa mtazamo wa muda mrefu, mtandao kama huo unaweza kutoa ultrastable high-usahihi optical kumbukumbu ishara (kama wale sasa inapatikana kutoka taasisi metrology) kwa mbalimbali ya watumiaji. Maeneo mbalimbali ya utafiti yatanufaika kutokana na hili, ikiwa ni pamoja na utafiti wa msingi (kujaribu sheria za msingi za fizikia), sayansi ya dunia na, mwisho lakini si chini, metrolojia. Kazi hii pia inasafisha njia kuelekea ufafanuzi mpya wa kitengo cha wakati, sekunde ya SI, kupitia kulinganisha mara kwa mara kwa kimataifa kwa saa za macho. "Kampuni ya ""Straight Line"" ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG ya LNG." : mtandao wa saa kwa ajili ya geodesy na sayansi ya msingi. Mawasiliano ya Asili. (Kutoka 7: 24-43 (DOI 10.1038 NCOMMS 12443))