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You know it's the future when collecting and tossing a ball of supercooled gas up a tube and catching it again using lasers and measuring time with it to a part

You know it's the future when collecting and tossing a ball of supercooled gas up a tube and catching it again using lasers and measuring time with it to a part per ten quadrillion is the old and busted method.

adrian_b 5y ago
While the fountain clocks use atoms that fall slowly in a chamber, the optical clocks use ions (a single ion or more ions) or neutral atoms (usually a relatively large number disposed regularly in a so-called lattice) which are trapped in fixed positions in vacuum, using various combinations of lasers and electromagnetic fields. So the control over the ions or atoms is even more advanced than in fountains, because they stay fixed (except for thermal vibrations) like in a solid, even if they are widely spaced in vacuum. The reason why a real solid is not used is that the atoms or ions are too near one from the other in a solid, and their interactions modify the atomic resonance frequencies. When a kind of artificial solid is made, where the atoms or ions stay in vacuum, in fixed positions, but which are much more widely separated than in a natural solid, the interactions between the atoms or ions can be minimized. Because the atoms or ions are cooled to very low temperatures, their movements are reduced to relatively slow vibrations, so there are no large frequency deviations caused by the Doppler effect, like when the atoms or ions are free to move in a gas.
- rnhmjoj 5y ago
  > The reason why a real solid is not used is that the atoms or ions are too near one from the other in a solid, and their interactions modify the atomic resonance frequencies. Why don't we use the frequency of a solid transition, then? I'm sure there are ways, like the Hartree-Fock method, to compute energy levels of atoms in a solid, maybe the accuracy is not good enough?
  - adrian_b 5y ago
    The quality factor of resonant frequencies in solids is too low to be competitive, i.e. the spectral lines of solids are not narrow lines but wide bands. When 2 atoms come very near, what happens is that the frequency of each spectral line splits into 2 lines, 1 with a slightly lower frequency and 1 with a slightly higher frequency. When there are N atoms close to each other, each spectral line splits into N lines with slightly different frequencies, which can no longer be discriminated by detectors, so they look like a continuous band. So solids cannot be used to stabilize the frequency of an oscillator to a very precise value. The highest possible quality factor, i.e. the narrowest possible spectral line is achieved when using a single isolated ion or atom. There are atomic clocks which use a single ion trapped in a fixed position in vacuum. Unfortunately, while this has the narrowest spectral line, it also is the worst from the point of view of the sensitivity to noise. The signal that can be obtained from a single ion or atom is extremely weak, so it is drowned in the ambient noise. The atomic clocks with single ions need a very long integration time to average the effect of the noise, e.g. a half of day or even more, up to many days. Therefore such atomic clocks must be used together with oscillators which have an exceptional frequency stability for long times, up to a day or more, because their frequency can be compared with the resonance frequency of the single ion at most a few times per day, so it must not have unpredictable changes between comparisons. The solution to overcome the noise problem is to use a large number of neutral atoms or ions, e.g. a few thousands, to provide a much greater amplitude of the signal, allowing the reduction of the integration time to 1 hour or less. Those many neutral atoms or ions must be kept separated by large distances, so that the electrons will continue to move around them like around isolated ions or atoms, not influenced by their neighbors, thus maintaining the same differences between the energy levels of various states, so the same resonant frequencies as in isolated ions/atoms.

adrian_b 5y ago

While the fountain clocks use atoms that fall slowly in a chamber, the optical clocks use ions (a single ion or more ions) or neutral atoms (usually a relatively large number disposed regularly in a so-called lattice) which are trapped in fixed positions in vacuum, using various combinations of lasers and electromagnetic fields. So the control over the ions or atoms is even more advanced than in fountains, because they stay fixed (except for thermal vibrations) like in a solid, even if they are widely spaced in vacuum. The reason why a real solid is not used is that the atoms or ions are too near one from the other in a solid, and their interactions modify the atomic resonance frequencies. When a kind of artificial solid is made, where the atoms or ions stay in vacuum, in fixed positions, but which are much more widely separated than in a natural solid, the interactions between the atoms or ions can be minimized. Because the atoms or ions are cooled to very low temperatures, their movements are reduced to relatively slow vibrations, so there are no large frequency deviations caused by the Doppler effect, like when the atoms or ions are free to move in a gas.

rnhmjoj 5y ago
> The reason why a real solid is not used is that the atoms or ions are too near one from the other in a solid, and their interactions modify the atomic resonance frequencies. Why don't we use the frequency of a solid transition, then? I'm sure there are ways, like the Hartree-Fock method, to compute energy levels of atoms in a solid, maybe the accuracy is not good enough?
- adrian_b 5y ago
  The quality factor of resonant frequencies in solids is too low to be competitive, i.e. the spectral lines of solids are not narrow lines but wide bands. When 2 atoms come very near, what happens is that the frequency of each spectral line splits into 2 lines, 1 with a slightly lower frequency and 1 with a slightly higher frequency. When there are N atoms close to each other, each spectral line splits into N lines with slightly different frequencies, which can no longer be discriminated by detectors, so they look like a continuous band. So solids cannot be used to stabilize the frequency of an oscillator to a very precise value. The highest possible quality factor, i.e. the narrowest possible spectral line is achieved when using a single isolated ion or atom. There are atomic clocks which use a single ion trapped in a fixed position in vacuum. Unfortunately, while this has the narrowest spectral line, it also is the worst from the point of view of the sensitivity to noise. The signal that can be obtained from a single ion or atom is extremely weak, so it is drowned in the ambient noise. The atomic clocks with single ions need a very long integration time to average the effect of the noise, e.g. a half of day or even more, up to many days. Therefore such atomic clocks must be used together with oscillators which have an exceptional frequency stability for long times, up to a day or more, because their frequency can be compared with the resonance frequency of the single ion at most a few times per day, so it must not have unpredictable changes between comparisons. The solution to overcome the noise problem is to use a large number of neutral atoms or ions, e.g. a few thousands, to provide a much greater amplitude of the signal, allowing the reduction of the integration time to 1 hour or less. Those many neutral atoms or ions must be kept separated by large distances, so that the electrons will continue to move around them like around isolated ions or atoms, not influenced by their neighbors, thus maintaining the same differences between the energy levels of various states, so the same resonant frequencies as in isolated ions/atoms.