Best comparison worldwide between distant optical clocks confirms excellent quality of the connection
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 | EurekAlert!
Move over, lasers: Scientists can now create holograms from neutrons, too
21.10.2016 | National Institute of Standards and Technology (NIST)
Finding the lightest superdeformed triaxial atomic nucleus
20.10.2016 | The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences
Researchers from the Institute for Quantum Computing (IQC) at the University of Waterloo led the development of a new extensible wiring technique capable of controlling superconducting quantum bits, representing a significant step towards to the realization of a scalable quantum computer.
"The quantum socket is a wiring method that uses three-dimensional wires based on spring-loaded pins to address individual qubits," said Jeremy Béjanin, a PhD...
In a paper in Scientific Reports, a research team at Worcester Polytechnic Institute describes a novel light-activated phenomenon that could become the basis for applications as diverse as microscopic robotic grippers and more efficient solar cells.
A research team at Worcester Polytechnic Institute (WPI) has developed a revolutionary, light-activated semiconductor nanocomposite material that can be used...
By forcefully embedding two silicon atoms in a diamond matrix, Sandia researchers have demonstrated for the first time on a single chip all the components needed to create a quantum bridge to link quantum computers together.
"People have already built small quantum computers," says Sandia researcher Ryan Camacho. "Maybe the first useful one won't be a single giant quantum computer...
COMPAMED has become the leading international marketplace for suppliers of medical manufacturing. The trade fair, which takes place every November and is co-located to MEDICA in Dusseldorf, has been steadily growing over the past years and shows that medical technology remains a rapidly growing market.
In 2016, the joint pavilion by the IVAM Microtechnology Network, the Product Market “High-tech for Medical Devices”, will be located in Hall 8a again and will...
'Ferroelectric' materials can switch between different states of electrical polarization in response to an external electric field. This flexibility means they show promise for many applications, for example in electronic devices and computer memory. Current ferroelectric materials are highly valued for their thermal and chemical stability and rapid electro-mechanical responses, but creating a material that is scalable down to the tiny sizes needed for technologies like silicon-based semiconductors (Si-based CMOS) has proven challenging.
Now, Hiroshi Funakubo and co-workers at the Tokyo Institute of Technology, in collaboration with researchers across Japan, have conducted experiments to...
14.10.2016 | Event News
14.10.2016 | Event News
12.10.2016 | Event News
21.10.2016 | Health and Medicine
21.10.2016 | Information Technology
21.10.2016 | Materials Sciences