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**Time and frequency

Up to 1967, the second, that constitutes the base unit of the SI in the field of time and frequency was dependent on astronomy. Up to 1956 indeed, the time unit was defined in relation to the Earth rotation; then between 1956 and 1967, because of the irregularity of the Earth motion, it was connected to earth traversing movement around the Sun (time ephemeris). However, despite the regularity of this traversing movement, long-term stability proved to be insufficient (some 10-9). That’s why, in 1967 during the 13th General Conference on Weights and Measures (CGPM), a fresh definition was adopted. This latter, which is still topical, is based on a microwave transition, with frequency of 9,192 631 770 GHz, between the two hyperfine levels of the ground state of caesium133 atom (133Cs).

In France, the LNE-SYRTE (in English Time-Space reference System) at the Observatoire de Paris (in English Observatory of Paris) is the National Metrology Institute (NMI) in the field of time and frequency.

Specific studies have also been carried out by the LNE-INM (in English National Institute of Metrology) at the Conservatoire national des arts et métiers (CNAM).
In addition, two laboratories located in Besançon, the LPMO department (in English Laboratory of Physics and Oscillators Metrology) of the LNE-FEMTO-ST (Franche-Comté Electronique, Mécanique, Thermique et Optique – Sciences et Technologies) and the LNE-OB (in English Observatory of Besançon), are associated with the LNE for the calibrations and the connection in the field of time and frequency.

Frequency

In 1982, under the scientific policy of the Bureau National de Métrologie BNM (in English National Office of Metrology) which missions were transferred to the LNE in 2005, it was decided to realize a primary thermal atomic beam frequency standard. Since this realization, several methods that allow cooling and confining the atoms using lasers have surfaced. And thanks to these, the 1997 Nobel Prize for Physics was awarded to Frenchman Claude Cohen-Tannoudji together with Americans Steven Chu and William D. Phillips. These cooling methods have allowed the development of the cooled atoms frequency standard.

Optically Pumped Beam - (OPB)

OPB is the primary frequency standard that is based on a thermal beam of caesium atom that is optically pumped and detected. This is the first primary laboratory standard developed in France.

It operates according to the following principle: a furnace produces a caesium atom beam containing as much atoms in both levels of the ground state of caesium. Using a laser beam with wavelength adapted to a of the transitions of caesium atom, and a static magnetic field, the atoms are distributed in the different Zeeman sub-levels of the first hyperfine level of the ground state of caesium.

These atoms will then cross the Ramsey cavity which is a U-shaped wave guide, where the two cavities are separated by some distance. There is a microwave field (created by a generator) in these cavities, which adjustable frequency is provided by a quartz oscillator. This microwave field will allow exciting the atoms of the hyperfine transition. At the outlet of the Ramsey cavity, the atoms will enter in the detection zone where they will be subjected to a laser beam tuned to a specific transition of caesium atom. The atoms that have made the microwave transition will then be subjected to a large number of absorption-emission cycles and will thus emit photons detectable via fluorescence. The quartz oscillator frequency will be modified until the amplitude of the detection signal reaches the maximum. The OPB’s accuracy is evaluated at 6,3 10-15 for which it is classified among the best of its category, and its stability, expressed by the Allan standard deviation is of σy=5 10-13t-1/2, i.e. the best performance achieved by beam standards.

The OPB primary frequency standard
The OPB primary frequency standard

Atomic fountains

The principle of atomic fountains is identical to the one of the OPB standard but for the fact that the atoms are confined in an area that creates optical molasses, which means they are cooled by six convergent laser beams tuned to a frequency slightly lower to the frequency of cycling transition used for cooling. This method which consists in reducing as much as possible the thermal agitation speed, allows attaining better stability at short term and above all, better accuracy. Using the vertical and symmetrically detuned laser beams, atoms are then launched towards the microwave cavity which frequency is produced by a cryogenic oscillator with sapphire resonator and controlled by an hydrogen maser. Under the effect of gravity, atoms will cross again the cavity then the atoms that have made the transition will be detected.

Atomic fountain
Atomic fountain;
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The LNE-SYRTE keeps three atomic fountains called FO1, FO2 and FOM. In 1995, FO1 was the first standard with cool caesium atoms in the world capable to deliver a signal with ultimate precision.
FO2 is a double atomic fountain that can operate with two different atoms (caesium 133Cs and rubidium 87Rb which hyperfine frequency transition is 6, 834 682 613 GHz). The interest in having a double atomic fountain is to seek a possible variation over time of the fine structure α constant that characterises the intensity of the electromagnetic interaction at low energy.
Finally, FOM is a mobile atomic fountain deriving from the prototype of the spatial clock PHARAO tested in gravity zero.

These atomic fountains have accuracies from 5 to 7.10-16.

FOM mobile atomic fountain
FOM mobile atomic fountain

Atomic fountain
Atomic fountain

Time scales

Since 1967, a time scale has been realized by the caesium atomic clocks that, similarly to the frequency standards, generate the SI second.
The International Atomic Time (TAI) is the uniform time scale established by the International Bureau of Weights and Measures (BIPM) using data from some 250 atomic clocks working in some fifty organizations, in accordance with the definition of the second.

However, despite its metrological qualities, the TAI cannot be directly disseminated. That’s why the Coordinated Universal Time (UTC) has been attached to the TAI. UTC differs from TAI by a variable whole number of seconds, chosen so that the gap between the UTC and the astronomic time (UT1 ), based on the rotation of the Earth, remains within the limits of ± 0,9 s. Since the 1st January 2006, there were 33 leap seconds leading to 33-second shift between TAI and UTC. However, the UTC is a paper time scale that is not available in real time (it is calculated from a algorithm with thirty days delay). A physical realization in real time of the UTC is established by the NMIs, it is the UTC(k), k being the laboratory’s acronym.

National references of time

In France, two time references are established by the LNE-SYRTE.

  • One is the legal reference time UTC(OP) (OP stands for Observatoire de Paris) that is materialized by the more stable (by study of the long-term stability in the past) of the seven commercial caesium clocks (clocks of the type HP 5071 A equipped with a high performance tube) stored in a air-conditioned room, in the basement of one of the Observatoire de Paris’s buildings.
  • The other one, is the French Atomic Time TA(F), which focuses on qualities such as stability and accuracy. It is the reference for scientific time that is established on the basis of several clocks: commercial clocks located in the LNE-SYRTE and clocks located in eight other French laboratories, meaning twenty five clocks all together. These clocks operate freely, meaning in agreement with the operating recommendations given by the manufacturer and without any voluntary frequency correction.

The aim of the UTC (OP) is to remain at ± 100 ns of UTC even it means debasing its stability at one month, whilst the objective of the TAF is to generate a signal with the best stability at one month.

Dissemination of the UTC (OP)

According to the accuracy sought, UTC (OP) is available through different means:

  • For an accuracy of 20 ms in continental France, UTC (OP) is accessible via the speaking clock of France Telecom. The first speaking clock in the world was inaugurated on 14 February 1933 at the Observatoire de Paris. Since then, three other speaking clocks have succeeded it and the last one put into in service on 18 September 1991 is fully electronic. In practice, time dissemination is ensured by one array of four speaking clocks (each one is made up of one “generator of announcements ”) controlled by a comparator.

Operating room of time
Operating room of time

  • For an accuracy of 1 ms in Metropolitan France, UTC (OP) is broadcast since 1982 by encoding on the radio carrier wave of France Inter which long waves transmitter (about 162 kHz) is located at Allouis in the Cher area. To pick up the time that flows into the signal of the radio station, requires a receiver decoder that is calibrated on the basis of its implantation point.
  • For better synchronization than the millisecond, the user can access UTC(OP) through the reception of hour signals by GPS or by LORAN-C. The GPS (Global Positioning System) is a system which primary function is positioning but which is also used to broadcast time and for comparison between the atomic clocks. The GPS system is based on a constellation of 24 NAVSTAR satellites located on orbits at 20 200 km of altitude on 6 orbital planes inclined at 55° to the equatorial plane. Each satellite takes on an atomic clock allowing it to emit a very precise hour signal. The LORAN-C (Long Range Navigation) is a system of mean range, based on the emission of low frequencies ground waves. It is made up of 34 channels primarily set up in the northern hemisphere, each channel includes one master station and secondary stations (up to five stations).

Associate laboratories

The two associate laboratories, the LNE-OB and the LNE-FEMTO-ST/LPMO, carry out activities of calibration and connection in the time and frequency area.

The LNE-FEMTO-ST/LPMO thus carries out activities of approved calibration, covering the measurements of phase noise for frequencies ranging up to 18 GHz, and activities involving short-term stability of oscillators for frequencies ranging from 5 MHz and 1 GHz. The LNE-OB intervenes on the statistical analysis of the behaviour of oscillators and of the different types of noise affecting them, on the time and frequency transfers through GPS link with common view and on the dissemination of the national frequency references. The LNE-OB also keeps three commercial caesium clocks (clocks of the type HP 5071 A ) that participates in the elaboration of the national and international time scales.