Hydrogen maser
A hydrogen maser, also known as hydrogen frequency standard, is a specific type of maser that uses the intrinsic properties of the hydrogen atom to serve as a precision frequency reference.
Both the proton and electron of a hydrogen atom have spins. The atom has a higher energy if both are spinning in the same direction, and a lower energy if they spin in opposite directions. The amount of energy needed to reverse the spin of the electron is equivalent to a photon at the frequency of 1,420,405,751.786 hertz (Hz), which corresponds to the 21 cm line in hydrogen spectrum.
Hydrogen masers are very complex devices and sell for as much as US$235,000. They are made in two types: active and passive.
In both types, a small storage bottle of molecular hydrogen, H2, leaks a controlled amount of gas into a discharge bulb. The molecules are dissociated in the discharge bulb into individual hydrogen atoms by an arc. This atomic hydrogen passes through a collimator and a magnetic state selector. The atoms are thereby selected for the desired state and passed on to a storage bulb. The storage bulb is roughly 20 cm high and 10 cm in diameter and made of quartz. Its inside is coated with Teflon, allowing many collisions of the atoms with the wall without perturbation of the atomic state, and slowing the recombination of the hydrogen atoms into hydrogen molecules. A durable Teflon & bulb coating technology allows for over 20-year lifetime. The storage bulb is in turn inside a microwave cavity made from a precisely machined copper or silver-plated ceramic cylinder. This cavity is tuned to the 1,420 GHz resonance frequency of the atoms. A weak static magnetic field is applied parallel to the cavity axis by a solenoid to lift the degeneracy of the magnetic Zeeman sublevels. To decrease the influence of changing external magnetic fields on the transition line frequency and be compliant to electromagnetic interferences, the cavity is surrounded by several nested layers of shields.
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