Of Clocks and Time by Lutz Hüwel

Author:Lutz Hüwel
Language: eng
Format: epub
ISBN: 9781681740966
Publisher: IOP Publishing
Published: 2018-04-23T00:00:00+00:00

3.11 Fountain clocks—state-of-the-art time-keeping

No doubt other clocks will surpass and replace them, but for now cesium fountain clocks are the state of the art for time-keeping. Figure 3.6 already encapsulates the generic layout of atomic clocks, so that we can concentrate here on some details of the ‘physics package’. From the generic EM spectrum in figure 3.5 and the energy–frequency relation for photons, E = hf, it follows that the energy splitting corresponding to the microwave transition used in Cs atomic clocks is about 100 000 times smaller than a typical separation of quantum states involved in the emission or absorption of visible light. The energy of the Cs transition [25] is also about 1000 times smaller than the kinetic energy of cesium atoms moving at room temperature. That this energy is so minute stems from the general nature of the splitting, which is an example of the so-called hyperfine structure of atoms. If you hold two bar magnets with like poles adjacent to each other (N next to N, S next to S), you will notice that this configuration is not stable. It tends to flip the magnets into the lower energy arrangement of opposite poles near each other. Although quantum physics changes the details, something very similar gives rise to the energy levels associated with the atomic hyperfine structure. Both valence electron of the cesium atom and its nucleus have magnetic properties that are very similar to those of a bar magnet. Because of the weakness of the magnetic interaction in general and the large distance—relatively speaking—the electron and nucleus keep on average, the energy difference between the two orientations of the magnetic dipoles is as small as observed and involves microwave radiation in the emissions and absorption process. The magnetic nature of the states is also the reason that emission of a photon is an unlikely occurrence, in other words the lifetime of the higher energy state is long. But we have seen that the Heisenberg uncertainty relation connects long lifetime with a narrow frequency spread. Which is a good thing for a stable clock and partly explains the high performance of cesium atomic clocks. The energy–time uncertainty relation holds in general. Therefore, the frequency uncertainty also increases with decreasing length of the interrogation time of the quantum oscillator. In other words, the longer you observe, the more consistent the clock frequency becomes. If an atomic clock could run with a single atom, and if that atom could be immobilized, we could probe the atom for an arbitrary long time. And if we keep the environment of the trapped atom stable, the clock will be extremely stable. While such clocks are in principle feasible, they do not currently exist as viable timekeepers. Cesium fountain clocks operate with about 10 million atoms at a time, but manage to keep them all going with nearly the same slow velocity that facilitates a long read-out time. After the introduction of the first fountain clock in 1999, NIST now has a second-generation clock (NIST-F2, item M in figure 3.

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