Test & measurement
ATOMIC CLOCKS REFINED WITH AWG CARDS IN PXIE FORMAT
Germany’s caesium fountain clocks depend on precise laser control to keep time. Dr Johannes Rahm explains how Spectrum Instrumentation’s arbitrary waveform generator cards are helping PTB replace ageing analogue circuitry with a digital alternative.
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round 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.
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.”
The two caesium fountain clocks at PTB provide the legal time for the entire Federal Republic of Germany.
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 for the second time, the transition probability is determined by means of fluorescence spectroscopy. 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 per cent.
Infrared photo of the vacuum chamber showing the fluorescing cloud of caesium atoms. (Photo courtesy of Mareike Bernien)
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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 per cent transition probability. Once the measurement is complete, after about 1.2 seconds, the cycle starts again and continues 24 hours a day.
October 2026 Instrumentation Monthly
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