The time according to atoms

The construction and marketing of high-performance hydrogen maser atomic clocks remains the privilege of the Americans and the Russians. The Swiss also market this type of clock, but with a Russian heart. Europe would like to become completely autonomous in this sector, in particular to meet the needs of its Galileo navigation system. Professor Thierry Bastin’s atomic physics unit took up the challenge with the Liège-based company Gillam-FEi within the framework of the Walloon “Marshall Plan”. The first prototype has just been completed. The aim of the next step is to divide the weight of these devices by ten so that they can be put on board the satellites at a low cost. This would be a world first for this type of maser.

KlepsydraThe history of measuring time is probably as long as that of humanity. Or, to be more precise, the history of measuring time intervals because that is what we have always measured. As a result, Man turned naturally towards phenomena that showed great regularity such as the earth’s rotation on its axis or that of the moon around the earth. He then attempted to create instruments capable of objectivising these observations and measuring such intervals: sun dials or sandglasses for instance. Following Galileo’s work on the pendulum, the 17th century was to experience a decisive advance: the development of the pendulum clock. “Ultimately, what’s a clock?” asks Professor Thierry Bastin, director of the Atomic Spectroscopy and Cold Atom Physics Department at the University of Liège. "A system that oscillates regularly over time. Never mind what it is that oscillates, providing that it is as regular as possible.” In mechanical clocks, the period of oscillation depends on the length of the pendulum and the acceleration of gravity.  In other words, if the length of the pendulum changes (because of the temperature, for instance), the clock will lose its accuracy; the same is true if the acceleration of gravity changes (for example, by gaining altitude). Clockmakers thus continued to successfully perfect their systems: as early as 1759, John Harrison had made a clock whose precision was a tenth of a second per day! But it wasn’t until 1918 – at least in terms of the principle; the actual clock wouldn’t appear until the 1930s – that a new scientific breakthrough occurred:  the quartz system. This time, it was no longer a mechanical system that was oscillating but electrical tension. The mechanical vibration of the crystal induces an electrical field that oscillates at a precise frequency and, in particular, a much higher one than that of pendulums (several million times per second). The greater the number of oscillations within a given time interval, the more precise the measurement is likely to be. Hence, it isn’t difficult to imagine that these quartz clocks, as they were called, had a degree of precision and invariability that was more than sufficient for everyday applications. But while physicists could explain what happened in everyday life, they also had to distance themselves from it. The search for greater precision and reliability thus led them towards the development of atomic clocks.

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