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 , which corresponds to the 21 cm line in hydrogen spectrum.
Hydrogen masers are very complex devices and sell for as much as . 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.
In the active hydrogen maser, the cavity oscillates by itself. This requires a higher hydrogen atom density and a higher quality factor for the cavity. However, with advanced microwave cavities made out of silver-plated ceramic, the gain factor can be much higher, thereby requiring less hydrogen atom density. The active maser is more complex and more expensive but has better short-term and long-term frequency stabilities.
In the passive hydrogen maser, the cavity is fed from an external 1.420 GHz frequency. The external frequency is tuned to produce a maximum output in the cavity. This allows the use of lower hydrogen atom density and lower cavity quality factor, which reduces the cost.
Passive Hydrogen Maser used as a clock in Galileo satellite navigation system.
The storage bulb is a quartz bulb about 20 cm tall which is coated inside with Teflon to control the recombination rate of atoms into molecules. The quartz bulb is located in a precisely machined pure copper cylinder which acts as a microwave resonant cavity for the 1.420 GHz frequency of the 21 cm hydrogen line. Once the atoms enter the resonance cavity, they find other atoms radiating and they fall in step. They "start to talk to each other" and echo what they hear. This produces a highly coherent oscillation. This is the signal to which the crystal oscillator is phase-locked. All of that is packaged in cabinets with power supplies, temperature control, and magnetic shielding.