School of Physics and Astrophysics

A candid discussion about atomic clocks

The following is a candid discussion about atomic clocks, the UWA atomic lattice clock and the European Space Agency’s ACES mission.

First I’ll define a clock then define the atomic clock. Every clock needs an oscillating component (resonator + a power source) and a read out.  So a watch has a quartz crystal with electromagnetic radiation oscillating at say 32kHz and you can have this tied to a display of sorts; e.g., a LCD read out or  a dial face.

Good clocks are those where the oscillation rate (the frequency) remains constant.  An excellent example is the cryogenic sapphire oscillator that has been a core of research here at UWA for 20 years; rather than quartz, sapphire is used and the frequency of the electromagnetic radiation is about 10GHz.  This is a very pure signal and excellent in its own right for doing fundamental physics tests.  There is a slight drawback though, a couple in fact.  The resonant frequency, or optimum frequency, of these devices is never the same when a new one is made, and secondly, over long periods of time the frequency drifts (like wrist watches but at a far far slower rate).  So how do we circumvent this?  This is where the atom comes in.  The energy separation between certain transitions in atoms are almost immune to external influences (it’s never perfectly immune, but as long as one can keep track of various shifts then you have the ingredients for a good clock / frequency reference).  So these make good time standards.  Anything that doesn’t drift with time makes a good standard (you want reproducibility and something that doesn’t change with time — you also want practicality). So if an oscillating thing can be tied to this intrinsic immovable entity in the atom then we have an ideal frequency reference and from there a time standard.

So for 60 years or more there has been a big drive to make ever better frequency standards base on atomic transitions.   The research has been in two main areas (1) improving the quality of the oscillating part, e.g the cryogenic sapphire oscillator and (2) improving the behaviour or properties of the atoms that are used.

It has been a rapidly developing field:  orders of magnitude improvement with in a decade.  Much faster than Moore’s law (factor of 2 every 18 months).   The rate of improvement of optical atomic clocks has been more than a factor of 50 per year — sounds unbelievable (but this is the case according to the literature).  

Now that I have mentioned optical atomic clocks, I better say something about them: here the oscillating part is replaced with a very high quality, low noise, laser source (in the visible or close to the visible). And in the atom, sometimes it can be the same atom (element) as for a microwave clock, the transition chosen lies in the optical domain  (think of optical spectra, but any optical line you can readily see wont make for a good atomic clock — it’s too broad in frequency).  

Reasons for the rapid improvement:  a big part of it is having more cycles per unit time; i.e. per second.  In this case nearly one hundred thousand times more.  So the unit of time is divided up into 105 more bits.  But really that’s just the in-principle improvement.   To get that improvement  you have to work exceptionally hard on making a high quality laser source and preventing systematic frequency shifts to the atoms.  So this is where the effort lies (there is more about atomic clocks below).

Aims of the UWA lab:

  1. Set-up ground station to participate in an international clock comparison experiment via an atomic clock that is to be placed on the International Space Station (ISS).  The experiment/mission is called ACES = atomic clock ensemble in space.
  2. Explore aspects of cold-atom physics, of which there has been a lot, but we have already managed to add a small contribution to this tome of knowledge (Kostylev et al., JOSA B, 31, in press, July, 2014) .
  3. On a more practical level, we wish to find better or most cost effective ways of building a Yb-based lattice clock. It seems clear now that neutral Yb based clocks will play a big role in tomorrow’s Time and Frequency labs and most likely in future timescales. We also need to find ways to make them more reliable (operate continually).
  4. As a possible diversion, these devices can be used to explore more exotic physics such as searching for evidence of extra dimensions.  This requires some changes to the experiment and presents its own challenges, but it is an intriguing area of research.  There is one group in Paris (Paris Obs) attempting the task with Rb.  To our knowledge there is no other group trying to perform the experiment with Yb or any other element for that matter.

Aims of ACES

ACES — atomic clock ensemble in space: the early stages of development occurred at the Paris Observatory (Système de Référence Temps-Espace and the Laboratoire Kastler Brossel).  Which was then followed by development at CNES (le Centre National d’Etudes Spatial) in Toulouse, France.

Two main components of the mission: (1) the technical challenge of putting a cold-atom atomic clock in space (there are lots of atom clocks in space; e.g., GPS satellites, but the ACES ensemble is expected to be at least 1000 times more accurate).   One needs all the lasers systems and control systems to work.  (2) The second part is the physics experiments, which are mostly tests of the Einstein Equivalence Principle.

In a little more detail we can break the ACES aims into three main categories.

  1. Technological: test whether cold-atom atomic clocks can be successfully operated in space. Will the usually delicate optics survive the rocket journey getting it into space? (in this case they have been made robust and put through many rigorous tests).   Even without the physics the successful operation of a cold atomic clock in space will be a big achievement.
  2. Fundamental physics: test various aspect of Einstein equivalence principle. (it is from the EEP that we deduce that gravitation is the effect of curved spacetime. ) - one of the goals of ACES will be to look for any cracks in this extremely strong foundation of physics.  (It’s a robust and elegant theory, but we know that quantum mechanics and general relativity are not a happy couple in certain regimes and according to the experts is GR that is expected to give way).
  3. Practical physics: test characteristics of the upper-spheres, stratosphere, ionosphere: measuring the total electron content etc..  things I know very little about, but who knows this may be where we learn the most.  It may well help with aspects of space weather.

Einstein Equivalence Principle (EEP) tests

  1. test the gravitational redshift (different clock rates at different altitudes, experiencing different gravity potentials) — test the different ticking rate between the Cs clock on the ISS and the Cs clocks on the surface of the earth. The ACES clock should tick/cycle ever so slightly faster (a 1m change in height causes a 10-16 fractional frequency shift).
  2. Search for a velocity of light spatial dependence;  e.g., are the timing of signals between ACES and a clock in France the same as that for a clock in the USA — using the same type of clock.
  3. Measuring frequency ratios between Cs clocks and various optical clocks, and checking to see of the ratio remains constant over the course of the mission.  If they drift then it could point to a systematic effect or that some fundamental parameter of physics is changing very steadily (e.g. the electromagnetic coupling constant, the Fine Structure Constant — alpha).  This information — these frequency ratios —will also be useful for later ground clock comparisons.  Frequency ratios between clocks are particularly useful in the search for fundamental constant variations.  Once you have made one measurements that’s your reference and you can compare to that forever more into the future — looking for changes in the ratio.

Our work at the University of Western Australia

UWA Yb Clock
If you thought that was technical it becomes even more so now.

It is important to immobilize the atoms as much as possible — any motion appears as a Doppler shift, so if there are lots of atoms buzzing around then the central frequency is different for each, which is why slowing down atoms with laser beams was a boon. Not only can you slow them down but they can be trapped into a very small volume of space — a diameter less than 1cm typically.  You can use a combination of laser light and magnetic field gradients to slow and trap moving atoms.  With microwave atomic clocks  the temperatures of the atoms are about 2 micro-K  (above absolute zero, vmp ~ 1.6cm/s, compare with velocities at room temperature of hundreds of meters per second).

In optical atomic clocks,  the constraining of motion has been brought to another level.   Here the atoms have been constrained to a distance less than the wavelength of light that is trapping them; e.g. about 380nm.  This trapping can be done in 1, 2 or 3 dimensions.  It is starting to appear that trapping in one direction is sufficient for making a super accurate clock, which means the atoms are very tightly constrained in one direction but are free to move around a bit in the orthogonal plane. The space they occupy could be described as a very thin pancake or crepe.  And the layers of pancakes we call a lattice, though strictly, it probably shouldn’t be.

With this in place, motional effects become insignificant.  More important becomes the shift in the clock frequency due to the light that is trapping the atoms.  As it turns out there are certain wavelengths of light that shift the lower and upper states of the atomic transition by precisely the same amount...and this is referred to as the magic wavelength.  This needs to be known to about parts in 107 or more (we did this with Hg in Paris...)

Once this magic wavelength is known, one can produce spectral lines that are 10Hz wide or less which means a quality factor of 1014 (it’s a bit hard to appreciate what this means, but as an analogy, if one could make a bell, a sounding bell, with the equivalent quality factor, once struck it would continue to ring for more than 1000 years)

Other effects you have to look out for in testing an atomic clock:

  • magnetic field effects:  1st and 2nd order shifts are relatively easy to deal with.
  • temperature of the surroundings, the thermal radiation surrounding the atoms can perturb the atomic transition.  This is more difficult to deal with.
  • atomic density / number of atoms per lattice site.
  • and the work goes on.

The state-of-the-art atomic clocks are now demonstrating stabilities and accuracies in the 10-18 range, making the clocks sensitive to height changes of just a few centimeters.   This opens up new possibilities for geodesy and geophysics.   As a crude interpretation of what 10-18 accuracy means: if one were to fill a cube that is 10km on each side with fine beach sand, an equivalent counting mechanism could determine exactly how many sand grains there are down to the very last grain.

I’ll end the discussion here for now.  For further information, feel free to contact me: John McFerran

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Last updated:
Tuesday, 3 June, 2014 10:14 AM