Around the world, hundreds of atomic clocks are continuously compared to establish International Atomic Time (TAI). In Germany, the Physikalisch-Technische Bundesanstalt (PTB) contributes with two of the world’s leading caesium fountain clocks. Keeping these running, however, depends on sophisticated technology to cool clouds of atoms with lasers to a few microkelvin and perform Ramsey spectroscopy. AWG cards by Spectrum Instrumentation play a key role in the latest set-up of the atomic clocks. The caesium fountain clocks CSF1 and CSF2 steer the national reference time scale UTC (PTB) and constitute the basis for PTB’s time services and the legal time for all of Germany, synchronising everything from clocks and trains to air traffic, communications, broadcasting, computer systems and much more.
Infrared photo of the vacuum chamber showing the fluorescing cloud of caesium atoms. (Photo courtesy of Mareike Bernien)
Caesium fountain clocks, such as those operated at PTB, rely on the quantum behaviour of caesium atoms. One second is defined as the duration of exactly 9,192,631,770 periods of the radiation corresponding to the transition between the two energy levels of the ground state of the caesium-133 atom. In a vacuum chamber, an ultracold cloud of these atoms passes twice through a Ramsey microwave cavity before their final state is measured using laser-induced fluorescence spectroscopy.
How to cool down the caesium atoms
Dr Johannes Rahm, a member of PTB’s Time and Frequency department, explains: ‘Before the measurement can take place, a cloud of ultracold atoms must be loaded. This is realised by means of a magneto-optical trap (CSF1) or an optical molasses (CSF2). In both cases, lasers in specific geometric alignment are detuned slightly below an optical transition of caesium-133. Due to the residual thermal motion, this laser light is Doppler-shifted to the transition frequency and by absorption of photons from the laser and conservation of momentum, the atoms in the cloud are slowed and therefore cooled down.’
A working cycle of the caesium fountain clock
A small frequency difference between the lasers then pushes the cloud upwards, like tossing a ball into the air. This creates the ‘fountain’ effect: the atoms pass through the Ramsey cavity once on the way up and again on the way down under gravity. After passing through the Ramsey cavity the second time, the transition probability is determined by means of fluorescence spectroscopy.
Dr. Johannes Rahm inspecting the caesium fountain clocks, which run continuously 24 hours a day. (Photo courtesy of PTB)
During each passage through the Ramsey cavity, a microwave pulse is applied. The frequency of this pulse is tuned so that the resulting transition probability is about 50%. With each fountain cycle, which consists of the loading phase, the launch phase, the Ramsey interrogation, detection and ballistic flights between the phases, the sign of the microwave frequency detuning is reversed, so that the left and right sides of the symmetrical transition probability structure are sampled. The true transition frequency is then given by the mean of the two steered frequencies set for the 50% transition probability. Once the measurement is complete, after about 1.2 seconds, the cycle starts again and continues 24 hours a day.
Evolution of the clocks
The caesium fountain clock requires careful control of the lasers. This was originally implemented using custom analogue circuitry developed by an earlier generation of PTB engineers and scientists. Maintaining such a complex system over the long term becomes increasingly difficult. So, like the experiment itself, the supporting electronics must evolve, leading Dr Rahm to explore how to replace its analogue system with a digital alternative. His research led him to integrate AWG (Arbitrary Waveform Generator) cards in PXIe format from Spectrum Instrumentation. These cards generate the RF frequencies required to manipulate the laser frequencies for cooling, launching and detection of the atoms using acousto-optic modulators.
Spectrum M4x.6622 Arbitrary Waveform Generator card in PXIe format. (Photo by Spectrum)
A swift response from Spectrum support
However, a challenge arose during implementation. Although the M4x.6622-x4 AWG cards met the accuracy requirements extremely well, none of the possible signal generation modes matched the specific requirements at PTB. For an efficient design of the caesium fountain clock cycle, each laser needs a specific set of waveforms at a specific point in the cycle. Dr Rahm contacted Spectrum Instrumentation’s support team, where enquiries are handled directly by the design engineers. In just two days, the software engineers developed a new feature called ‘Sequence Restart Mode’. Dr Rahm’s tests were successful, and the new mode works exactly as envisaged.
Spectrum Instrumentation has since released Sequence Restart Mode as a free-of-charge upgrade for all AWGs in the 65xx and 66xx series. The new mode can be enabled easily by installing the latest drivers.
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