Double Resonance Atomic Clock: Principle of Functioning and Block Diagram

Atomic clocks use the electron’s transition frequencies (the microwave signal that electrons in atoms emit when they change energy levels), as a frequency standard for its timekeeping element. This frequency is used to electronically stabilize a crystal oscillator’s RF frequency; therefore, the stability of atomic structure is transferred to the clock’s tick rate. Alkali metals (and in particular Rb and Cs) are commonly used as references in atomic clocks for a simple, practical reason: because of their single valence electron, their microwave frequencies are easily accessible. The MACQS project aimed to develop a Double Resonance (DR) Rubidium Compact Atomic Clock [67], [74]. This clock uses the transition between two hyperfine states of ⁸⁷Rb as frequency reference. The ground state of Rb is split into two hyperfine levels by the

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magnetic-dipole interaction between the single valence electron and the nucleus. These two hyperfine levels are labeled by the total angular-momentum quantum number F (F = 1, F = 2). Each hyperfine level is then further split into several Zeeman sublevels, labeled by the quantum number m_f. The so-called 0-0 transition, between the F = 2, mf.= 0 and F = 1, mf. = 0 states, with a frequency of υ0 = 6.8347 GHz, is the transition used in a Rb Double Resonance (DR) atomic clock to stabilize the oscillations of a crystal oscillator. This 0-0 transition is preferred because its frequency is not affected by stray magnetic fields. Figure 2. 4 illustrates this physical phenomenon.

Figure 2. 4: The ground state of 87Rb is split into two hyperfine levels, labeled by their total quantum number F. The hyperfine levels are further split into several Zeeman sublevels, labeled by

the quantum number m_f.. The transition between the two m_f.= 0 sublevels produces a radiation of 6.8347 GHz, used to stabilize a crystal oscillator’s frequency.

The double resonance atomic clock consists of several simple modules. First, a light source produces light resonant with the ⁸⁷Rb absorption line. The next part is the reference cell, which contains the alkali atoms in their metallic vapor state together with a buffer gas. The light is directed to the reference cell, and the alkali metal atoms are ground-state polarized. The clock transition is detected by applying a microwave field to the atoms; this is done via a microwave cavity (another fundamental element of the DR atomic clock), which is placed around the reference cell. The light power transmitted through the cell has a narrow dip, caused by the resonance itself, as shown in Figure 2. 4. The frequency of a voltage-controlled quartz oscillator is then locked to this narrow dip using phase-sensitive detection. Between the light source and the reference cell, an ⁸⁵Rb filter is placed, to filter out the ⁸⁵Rb absorption lines from the light which is then sent to the reference cell. This filter is particularly useful because, due to a coincidence in nature, the absorption lines of ⁸⁷Rb F=2 state are nearly degenerate with those of ⁸⁵Rb F=3 state. Thus, the ⁸⁵Rb filter can eliminate the F=2 spectral component from the ⁸⁷Rb light beam, so when the filtered light beam reaches the ⁸⁷Rb atoms contained in the reference cell, it preferentially excites atoms out of the F=1 state. The easy and efficient filtering of the light beam is one of the principal advantages that the use of Rb offers with respect to the use of Cs in the reference cell of

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an atomic clock. Cs has only one stable isotope, so for clocks based on this metal no filtering is possible. As a result, Rb clocks are more compact and cheaper than Cs cells, while Cs-clocks have a better stability. Figure 2. 5 illustrates the structure of a DR Rb atomic clock.

Figure 2. 5: The structure of a DR Rb atomic clock.

The atomic clock is a complex structure, and several are the engineering challenges involved in the fabrication of such device. The fabrication becomes still more challenging in the case of a Chip Scale Atomic Clock, because we add the complexity of being small in all the parts we develop and to fabricate a low-power consuming device.

Our laboratory, LPM (Laboratoire de Production Microtechnique), with a vast expertise in thick-film and LTCC technology, electronic packaging and microfabrication, was involved in the already described MACQS (Miniature Atomic Clock and Quantum Sensors) research project. This research project started with the goal to propose new techniques to batch-fabricate the key components of a Double Resonance (DR) Rubidium Compact Atomic Clock, and, ideally, put together the fabricated components to constitute a working atomic clock demonstrator. In this context, the research effort of LPM (and therefore my personal effort also) was focused on the following two objectives:

1. The development of the reference cell using solder sealing, a quick, low-temperature bonding technique which should minimize evaporation of the alkali metal during the sealing process;

2. The realization of the electronic packaging, interconnection and temperature control of the various parts of the atomic clock.

Several are the issues related with the fabrication of the reference cell. First, this system must be filled with non oxidized rubidium. Rubidium is a very reactive metal, especially with oxygen and water. This makes the handling of this metal very challenging and the fabrication becomes critical; moreover, the encapsulation of the alkali metal must be completely hermetic, avoiding any air penetration into the

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packaging, otherwise the rubidium will react with the oxygen and loose its properties.

Inside the cell, together with Rb, a buffer gas is commonly added. Since the alkali atoms loose their spin polarization when colliding with the cell walls, the function of the buffer gas is to reduce the mean free path.

Regarding the second objective, the packaging of the components of the atomic clock, the major issues were related with the temperature control: in fact, some parts must be stabilized at a very precise and well-defined temperature (for example, the reference cell works at 70°C), and high precision is wished, ideally 0.01°C. The difficulty is compounded by the fact that the different parts of the atomic clock have different working temperatures and, being a miniature system, the components are very close to each other. Therefore, efficient thermal insulation must be provided, so that the temperature of one part does not affect the temperature of the other parts of the system.