Working Group 4.41
Unit of Time
Scheme of operation and typical realisations of atomic clocks
Atomic clock scheme
 Atomic clocks make use of the ability of atoms to emit or absorb electromagnetic waves of a characteristic oscillation frequency f0 during transitions between two energy states. The value f0 corresponds to the energy difference between the two states, divided by Planck's constant. Atomic clocks make use of transitions between such energy levels whose natural lifetime is long and whose energy is not significantly affected by electric or magnetic fields or other pertubations. Suitable atoms are, for example, the alkalis with their hyperfine structure splitting of the ground state.
The figure shows the functional scheme of a so-called passive atomic clock.
An electromagnetic field of frequency fp (with fp ~ f0) is produced by means of a quartz oscillator VCXO (Voltage-controlled X-tal oscillator). This field is coupled into the resonance apparatus where the atoms are exposed to the field. With a certain probability this will excite transitions between the energy levels involved. In order to observe a signal reflecting the atomic excitation, the majority of the atoms must be prepared in one of the energy levels before excitation. The transition probability due to interaction with the oscillating field is then inferred from the change in the population of the energy levels. The probability is maximum if fp and f0 are identical. A resonance-like reaction of the atoms is observed, which is converted into a detection signal ID with a spectral "linewidth" W, W = 1/T, with T being the interaction time of the atoms with the radiation field. The signal ID thus contains the required information whether the frequency fp and the transition frequency f0 are identical. ID is further processed so that a control signal UR for the VCXO frequency is produced. The frequency variations of the VCXO signal are suppressed within the bandwidth of the control loop, and the stability of the atomic resonance determines the long-term frequency stability of the output signal. The VCXO emits a standard frequency fN (usually 5 MHz) which is further processed depending on the intended application.
If an electric pulse is generated after 5 million periods of fN, the time interval between successive pulses is one second. A prerequisite for this is that conversion from fN to fp takes place with the correct multiplication factor k.
For caesium clocks, k.5 MHz = 9192.631770 MHz;
for rubidium clocks, k.5 MHz = 6384.682 6128 MHz.
Caesium clocks with thermal atomic beam
Since 1967, the caesium clock has been used for the realization of the unit of time. It works on the principle illustrated in the figure. An atomic beam emerges from the orifice of an oven which contains some grams of 133Cs metal. The beam passes a first magnet, called polarizer, which deflects only atoms of energy state E2 into the desired direction so that a state-selected atomic beam enters the microwave resonator. In the two end parts of the resonator the atoms are irradiated by the microwave field, and if its frequency is in resonance the atoms are transferred to the state E1. The analyser magnet then directs these atoms to an ionizer which is formed by a heated wire. The Cs+ ions pass through a magnetic mass filter and are directed to an secondary-electron multiplier.
The detector signal ID as a function of fp is called atomic resonance signal (low left-hand corner of the figure). The resonance line has a width in the range of 50 to 500 Hz which is determined by the time of flight T of the atoms along the resonator length L.
Some properties of caesium make it especially suitable for the realization of an atomic clock according to the scheme described here. One of its advantages is that only a single stable isotope of the element exists in nature. Moreover, even at an oven temperature of not more than 100 °C, the vapour pressure is high enough to furnish an intense atomic beam. Finally, the ionization efficiency of the detector is practically 100 %: The ionization energy of the caesium atom is smaller than the work function of the platinum-iridium alloy typically used as detector wire.
Caesium clocks have been commercially available since the end of the 1950s. According to some estimates 100 to 200 clocks are manufactured worldwide every year. All clocks use the scheme described above but differences in certain details lead to somewhat different properties. The best clocks realize the SI second with an uncertainty of a few 10-13 s. They are used for navigation, geodesy, space exploration, telecommunications, and in time institutes such as PTB. At present, PTB has six caesium clocks from industrial manufacture at its disposal.
If an accuracy higher than that achievable by the commercial products is aimed at, limitations imposed on the commercial product (price, weight, energy consumption, etc.) must be avoided. In order to achieve a particularly high accuracy over long operation intervals, the atomic clocks CS1, CS2, CS3 and the fountain clock CSF1 were developed and built at PTB.
Rubidium vapour frequency standard
The rubidium-vapour frequency standard (Rb clock) makes use of the transition between the ground-state hyperfine sublevels of the 87Rb isotope at a frequency f0 = 6 384 MHz. The principle is illustrated in the figure. Optical excitation is used to pump the 87Rb atoms to one of the ground-state sublevels, and to detect changes of the level populations due to the microwave excitation. First, light from an 87Rb lamp is transmitted through a filter cell containing 85Rb vapour. The light then excites 87Rb atoms in an absorption cell filled with a buffer gas. The cell is placed in a microwave resonator. The buffer gas - a mixture of light noble gases - prolongs the time of interaction T of the atoms with the microwave radiation by reducing the collision rate of the atoms with the wall of the cell. The filtered lamp light selectively depopulates the lower ground-state hyperfine level of the 87Rb atoms which reduces the absorption of the pump light. As soon as microwave radiation of the frequency fp ~ f0 acts on the atoms, the lower level is repopulated and some absorption is observed in the signal ID of the photodetector. The resulting resonance linewidth W is typically in the range of 500 Hz.
Rubidium clocks can be manufactured with compact dimensions and at a low price. A considerable number (several 1000 per year) is produced and used in the fields of telecommunications, energy supply (supervision of the state of the power distribution networks) and for calibrations carried out by industry. A highly sophisticated Rb clock is used in the latest generation of GPS satellites.
It may be noted that the Rb clock scheme described here has some serious deficiencies. Relative differences of about 10-9 occur between the resonance frequency observed in operation and the f0 value of undisturbed 87Rb atoms. These differences are mainly caused by the magnetic field inside the cell, by collisions of the Rb atoms with the buffer gas, and by the simultaneous interaction of the atoms with light and with the microwave field. In addition, thermal effects or aging change the spectrum and the intensity of the Rb lamp as well as the composition of the gas in both the filter cell and in the absorption cell. This limits the achievable long-term frequency stability and prevents the use of uncalibrated Rb clocks for the realisation of the unit of time; it is thus called a secondary frequency standard.
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