BIPM is in charge for generation of the International Atomic Time (TAI) on the basis of comparison of some 250 clocks present in different laboratories. These comparisons are obtained either by local comparison, or through radiofrequency techniques: common views of satellites of the GPS constellation, or two-way method called " Two-Way Satellite Time and Frequency Transfer - TWSTFT".
a. GPS common view time comparison
Each laboratory participating in the comparison simultaneously receives, thanks to a receiver, the time signals coming from the same GPS satellite. The travel time of signals is dated thanks to receiving station clock, whilst the emission time of these signals by any satellite is dated in the local time scale of this satellite establishes by a atomic frequency reference standard. A algorithm allows establishing the relation between the satellite time scale and the GPS time. By making one simple difference between the observations made at the same moments in both stations, the GPS time disappears and it is then possible to obtain the reading difference between the clocks of both stations.
b. Time comparison by TWSTFT
Since March 2003, the LNE-SYRTE has been equipped with one TWSTFT (Two-Way Satellite Time and Frequency Transfer) station operating in Ku band (frequency band 12 GHz-18GHz used in satellite communications) and dedicated to comparisons of time signals between atomic clocks (distant and located on ground) via radiofrequency links with a communication satellite. The time signals are transmitted at the same moment, signal from a station is received and measured by the other station. The exchange of measurement data allows calculating the difference between the two clocks. The result’s accuracy depends on the non-reciprocated residual effects on the propagation delays of the signals. The TWSTFT technique provides accuracy six times better than accuracy obtained with the GPS common view time comparison.
LNE-SYRTE’s TWSTFT station
EGNOS (European Geostationary Navigation Overlay Service) is a European system, realised by the European Space Agency (ESA), which goal is to complete and improve the American (GPS) and Russian (GLONASS) navigation systems. It is the equivalent of the American WAAS system or the Japanese MSAS system. In its navigation message, EGNOS will propose to users, notably civil aviation, a connection of its ENT reference time scale to UTC via one or more European UTC(k) among which UTC(OP) that supplies one station of the ground segment installed at the Observatoire de Paris, under the responsibility of the CNES (Centre National d'Etudes spatiales).
PHARAO (French acronym for project of atomic cold atom clock in orbit) is a space cold atom clock of the cold atom fountain type, developed by the LNE-SYRTE at the Observatoire de Paris and the Kastler Brossel Laboratory at the Ecole Normale Supérieure, under the European space project called ACES (Atomic Clock Ensemble in Space). The main aim of the ACES project is to install on the international Space Station (ISS), the PHARAO atomic clock, an hydrogen maser and a high performance link for time and frequency comparisons. The ACES project which should see the light of day in 2010, will allow PHARAO to operate in microgravity as well as get round the limitation imposed by the acceleration of gravity in the atomic fountains. The different objectives are to measure time with accuracy and stability never ever reached until to date, and carry out comparison of clocks and fundamental physics experiments (measurement of the gravitational red-shift, search for a possible anisotropy of the light speed, search for a possible drift in the fine constant structure α).
The LNE-SYRTE is developing a compact cold atom clock in which the atom cooling stages, the atomic state preparation, the microwave interrogation and the signal detection are carried out in the microwave interrogation cavity so that the payload is reduced. This clock, named HORACE (French acronym for atomic clock with cooled atoms in a cell) is likely to be taken on board the satellites that constitute the European positioning and radionavigation system GALILEO, and will therefore take advantage of operation under microgravity.
To date the ultimate performances of the atomic fountains have almost been reached, and there is no sign anticipated indicating significant improvement in accuracy or stability without causing degradation of either property.
Cold atom clocks based on transitions in the optical spectrum are undergoing development and seem very promising to improve in a significant way the accuracy and stability levels. Two types of optical clocks can be realised: clocks operating with trapped ion and clocks based on neutral atoms.
The LNE-SYRTE and the LNE-INM have decided to develop optical clocks based on trapped neutral atoms. The performance level sought with these optical clocks imply that the atoms are confined in an optical lattice in order to have perfect control over movement. On these so trapped atoms it is then possible to observe the clock transition.
The LNE-SYRTE’s choice has focused on a cold strontium atoms clock on the one hand, which clock transition is 698 nm, and a mercury cold atoms clock which clock transition is 265 nm on the other hand. The benefits brought by this latter clock are that the mercury atom exhibits a very low frequency displacement related to thermal radiation and very high sensitivity to the possible variations in the fine structure constant α.
The LNE-INM is itself constructing an optical clock based on hyperfine two photon transition in silver atom in the proximity of 661,2 nm, and the silver atoms are cooled in one magneto-optical trap.
The development of several clocks will allow in the longer term to realise mutual evaluations of accuracy at very low levels.
Cloud of cold Strontium atoms
The comparison of optical clocks and clocks operating in the microwave frequency domain, which frequency ranges are separated by several orders of magnitude, is a complex problem that has been recently solved with the appearance of the femtosecond laser. Before, these measurements were based on the use of non linear materials which implementation was very complex. To date, the phase mode-locked femtosecond lasers constitute the most reliable means for frequency measurement. Associated to Photonic-crystal fiber, these lasers generate a frequency comb with ranges extending from approximately 1100 nm to 450 nm, which allows measurements all over this spectrum. The longer term objective is to realize these comparisons with resolution under 10-16.
The recent development of atom cooling techniques has paved the way for new types of inertial sensors based on atomic interferometry, which principle is similar to a Mach-Zehnder interferometer in optics. Thus a gyroscope-accelerometer has been constructed by the LNE-SYRTE.
The source of the gyroscope – accelerometer developed at the LNE-SYRTE is a cloud of cold caesium atoms launched with an angle of 8° from the vertical axis defined by gravity, along a parabolic flight, then prepared in one well-defined quantum state. The symmetrical Ramsey-Bordé type interferometer is realised by creating splitter and mirrors using the three simulated Raman transitions via counter-propagating beams. The atoms in free fall constitute one intertial reference in relation to which the rotations and accelerations of the interferometer plane are detected by measuring the phase shift between the two arms of the interferometer. In addition, two counter-propagating thermal beams of cold caesium atoms are used to discriminate between rotation and acceleration contribution to phase shift.
« gyrondex.5 »: cold caesium atoms gyroscope -accelerometer
Schematic diagram of gyroscope (TF-schema-gyrometre)
To realise a fresh definition of the kilogramme, the experiment of the Watt balance must include one measurement of Earth’s attraction g with an accuracy of 10-9. To achieve this, the LNE-SYRTE is developing one absolute gravimeter which principle is to measure, via three laser pulses, the position of the cold atoms in free fall observed in three times.
" gravi ": gravimeter vacuum system
In order to achieve the ultimate performances of the atomic fountains, oscillators exhibiting a very high stability are required. That’s why the LNE-FEMTO-ST/LPMO is developing cryogenic sapphire resonator oscillator involving very low relative instabilities and which oscillations are maintained through a amplifier installed outside the cryogenic system. Moreover, these systems are associated with stabilizing systems designed for ensuring the frequency stability.
Cryogenic sapphire resonator oscillator
The LNE-FEMTO-ST/LPMO is also working on how to obtain a maser effect in a iron-doped cryogenic oscillator. Indeed the recent results show that crystal growth introduces impurities of which ferric ions Fe3+, that have three energy levels. A suitable frequency excitation allows the electrons to pass through the first energy level towards the third one, followed by relaxation on the second level. And hence, a sapphire resonator can be used in one maser operating mode with gallery mode.
The LNE-OB is developing a system "SYREF" based on a commercial GPS receiver, which provides a continuous link to the national time and frequency references.