A Hydrogen maser, also known as hydrogen frequency standard

Frequency standard

A frequency standard is a stable oscillator used for frequency calibration or reference. A frequency standard generates a fundamental frequency with a high degree of accuracy and precision. Harmonics of this fundamental frequency are used to provide reference points.Since time is the reciprocal of...

, is a specific type of maser

Maser

A maser is a device that produces coherent electromagnetic waves through amplification by stimulated emission. Historically, “maser” derives from the original, upper-case acronym MASER, which stands for "Microwave Amplification by Stimulated Emission of Radiation"...

 that uses the intrinsic properties of the hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 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 .

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 leaks a controlled amount of gas into a discharge bulb. Molecular hydrogen consists of pairs of atoms bound together. The molecules are disassociated in the discharge bulb into individual hydrogen atoms by an arc. This atomic hydrogen passes through a collimator

Collimator

A collimator is a device that narrows a beam of particles or waves. To "narrow" can mean either to cause the directions of motion to become more aligned in a specific direction or to cause the spatial cross section of the beam to become smaller.- Optical collimators :In optics, a collimator may...

 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

Quartz

Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

. Its inside is coated with Teflon

Polytetrafluoroethylene

Polytetrafluoroethylene is a synthetic fluoropolymer of tetrafluoroethylene that finds numerous applications. PTFE is most well known by the DuPont brand name Teflon....

, allowing many collisions of the atoms with the wall without perturbation of the atomic state, and slows 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

Microwave cavity

A microwave cavity is a closed metal structure that confines electromagnetic fields in the microwave region of the spectrum. Such cavities act as resonant circuits with extremely low loss at their frequency of operation...

 made from a precisely machined copper or silver-plated ceramic cylinder. This cavity is tuned to the 1420 MHz resonance frequency of the atoms. A weak static magnetic field is applied parallel to the cavity axis by a solenoid to separate the magnetic Zeeman sublevels. To decrease the influence of changing external magnetic field 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

Q factor

In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....

for the cavity. However, in advanced microwave cavity made out of silver-plated ceramic, the gain factor is 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. E.g., the CH1 75 model made by PTF weighs 90 kg and consumes about 100 watts of power. Its long-term frequency accuracy is about ±5×10⁻¹⁶ over five years. The iMaser 3000 model has enhanced short-term frequency stability of 1.2x10⁻¹³ at 1 second and a very low phase noise of -130 dBc/Hz at 1 Hz from a 5 MHz reference. Its intrinsic frequency drift demonstrates as low as 7x10⁻¹⁶ per day even without implementing the Automatic Cavity Tuning system. When used as a clock, it will gain or lose a second in about 63 million years. With the advent of Ethernet technology, new generation of active masers can be self-diagnosed, controlled and monitored remotely for easy maintenance of complex operations.

In the passive hydrogen maser, the cavity is fed from an external 1420 MHz 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. The CH1 76 model also made by PTF weighs 55 kg and consumes about 90 watts. Its long-term frequency accuracy is much worse than the active maser at about ±1.5×10⁻¹² per year.