October 24, 2023


Today, on Earth, the most accurate and stable atomic clocks use laser-cooled caesium atoms. In an atomic clock, the duration of the coherent interaction between the atoms and the electromagnetic field is a fundamental limit to the resolution of the frequency measurement. This duration can be dramatically increased by using cold atoms and microgravity. 

A caesium clock operates with the hyperfine transition near 9.2 GHz used to define the SI unit of time, the second. Caesium atoms in a cloud are cooled to a temperature of 1µK corresponding to an average velocity of about 7 mm/s. This cloud is optically pumped to the required quantum state and then launched across a microwave field. The duration of interaction dictates the quality.

Analysis of the output cloud then indicates if the microwave frequency is perfectly tuned to the required frequency. On Earth, a cold caesium clock operates in a fountain geometry, where atoms are thrown upwards and then fall due to gravity. This time cannot exceed one second for a reasonably tall fountain. By contrast, in microgravity conditions the interaction time in PHARAO can increase up to 5-10 seconds with a simple and compact device. The principle of PHARAO is described in the figure on the left. 

The PHARAO instrument is made up of five subsystems:

  • a vacuum tube, also called caesium tube (TC), in which atoms are captured, launched, cooled, selected and detected after undergoing a microwave interaction inside a microwave cavity. The caesium tube provides ultra-vacuum conditions throughout the atomic path and applies a constant and extremely uniform magnetic field along this path, especially inside the microwave interrogation chamber. It also includes the ion pump high voltage supply (CV-THT), which is mounted separately on the ACES baseplate.
  • an optical bench, also called laser source (SL), which provides the various laser beams necessary for the capture, launch, cooling, selection and detection of the atoms.
  • the microwave source (SH), which supplies the signal to drive the interrogation and preparation cavities.
  • the on-board management unit (UGB), which processes the detection signal in order to command the frequency corrections to be applied to the microwave source in standalone mode or transmitted to ACES-XPLC in the other operational modes. It also synchronizes the different phases of the atomic cycle, handles acquisition of measurements and the remote-control systems in order to modify the instrument’s functional parameters.
  • the BEBA electronic unit, which controls the caesium tube magnetic coils and acquires analog signals from the caesium tube.

The Flight Software orchestrates this entire process, decoding and operating the micro-sequences tables, distributing for each launch of atom clouds the commands to the five subsystems.

Mass and volume of PHARAO elements

Unit Mass (kg)Size (L x l x h, mm)Power (W)
Caesium tube (TC)44990 x 336 x 4445
Laser source (SL)20529 x 330 x 17840
 Microwave source (SH)7300 x 270 x 11725
 Onboard management unit (UGB)6245 x 240 x 12026
 BEBA1134 x 118 x 85 
 Total (including harness, attachments and margin)911,000
at longest