The first step consists of selecting the hydrogen atoms in their emitting state. Once this is achieved, a cloud of these atoms is sent to a cavity called a cavity resonator. There, they are subjected to an electromagnetic wave of the appropriate frequency. When the atoms are excited, they emit a signal (very low, approximately a tenth of a picowatt!) which is captured by a detector. But an atom can be easily disturbed by a magnetic field. Therefore, it is necessary to eliminate all the effects due to residual fields (for instance, the earth’s magnetic field), which requires surrounding the system with a magnetic shield. A vacuum must be created so that the hydrogen atoms are isolated in the cavity, hence the presence of vacuum pumps. Since the cavity can only accommodate one wave at a particular frequency, its size depends on the wavelength that excites the atoms, i.e., the one they have chosen. For hydrogen, a cavity of 20 cm in diameter is required (the cavities for rubidium clocks are smaller, but these clocks are less stable). A size that cannot vary in the least, especially under the effect of temperature, otherwise this will influence the frequency and disrupt the system. Therefore, it is necessary to stabilise everything to ten-thousandth of a degree! Not to mention a system that first splits the H2 molecules in the hydrogen gas into H atoms followed by all the signal detection electronics. In short, a hydrogen maser is a relatively voluminous and heavy system, which subsequently increases the size and weight (and therefore the launching cost!) of satellites when they have to be sent into space.

Hence, Thierry Bastin’s and Gillam-FEi’s intention is to miniaturise such a clock. Relatively small hydrogen maser clocks already exist but they perform less well because they function in passive mode.  Liège’s goal is to miniaturise the system without decreasing performance (while maintaining the active nature of the clock). “We submitted a project within the framework of the Marshall Plan in 2008”, Thierry Bastin explains. “Our goal was to create a prototype to be taken on board a satellite. But we were starting from zero, and we therefore chose to begin by building a traditional atomic clock with the help of someone who already had specific experience in this type of clock, Dr. Cipriana Mandache to be exact. This is what we did and the clock now functions to our complete satisfaction. The following stage is miniaturisation. And here, we’re just at the beginning..." While the basic principle of the maser has been well known for decades, actually making one is quite another matter.  And miniaturisation comes up against another problem... size: the size of the cavity matters and is linked to the wavelengths used. Therefore, how can this cavity be reduced without the system really noticing it? “We have to devise another design for the cavity”, Thierry Bastin explains. “But it’s important to realise that we don’t know everything, there isn’t a ready-made formula for such small clocks and the simulations require enormous computer capacities; we still don’t know how to simulate everything and our work sometimes has to be empirical in certain respects when it comes to optimising the components." Let's talk about it again in two or three years time to find out whether they have succeeded!