HORACE : A Compact Cold Caesium Atom Clock for Next Galileo's Generation
Esnault, F.X.¹; Tremine, S¹; Holleville, D¹; Guerandel, S¹; Perrin, S¹; Dimarcq, N¹; Delporte, J²; Hermann, V^3
1LNE-SYRTE; ²CNES; ³Thales Electron Devices

Development of efficient atomic clocks is a very important stake for the GALILEO project and several clock technologies have already been selected for the first generation of GALILEO’s clocks, like rubidium clock and hydrogen maser.

For more than 15 years, LNE-SYRTE has been working on cold atom fountains and has showed improvements of about 2 orders of magnitude compared to thermal beam clocks, reaching level of a few 1E-14 at 1second. Operation of such devices in microgravity environment is suitable for the increase for the increase of interrogation time, leading to an improvement of frequency performances. This is the objective of ACES/PHARAO project to install in space a cold atom clock with frequency stability and accuracy at the 1E-16 level. In PHARAO experiment, atoms are cooled in a specific chamber then they are launched on a ballistic trajectory to the microwave cavity. After interrogation phase, atoms reach the detection zone. A miniaturisation of PHARAO concept is required for GALILEO. The HORACE clock is an elegant solution to these requirements.

We present a design of compact cold atom clock called HORACE, which is a very good trade off between space constraints and high performances. There are several advantages to this design. First of all, the vacuum chamber is very compact because all interactions (cooling, interrogation, detection) are performed at the same place. The second advantage is that cold atoms are not lost from a cycle to another as they remain in the cooling zone, this improves clock operation by reducing cooling time. The third advantage is that the special cooling technique used in HORACE doesn’t put stringent constraints on cooling light (polarization, power imbalances) leading to a great simplification of the optical bench, which is a very good point for space applications.

In our experiment, on earth, atoms fall with gravity, limiting interrogation time to about 50 ms. In microgravity environment, this interrogation time can be considerably increased.

We present the preliminary frequency stability of 5.5 E-13 at one second, reaching the1E-14 level at 3000s.

We will present a brief description of the clock’s operation and the preliminary results. We will discuss the current limitations and give some extrapolations for a microgravity environment operation. Work on miniaturisation of the physics package will be presented as well.