Editing Maser (section)

==Uses==
Masers serve as high precision [[frequency reference]]s. These "atomic frequency standards" are one of the many forms of [[atomic clock]]s. Masers were also used as [[electronic amplifier|low-noise microwave amplifier]]s in [[radio telescope]]s, though these have largely been replaced by amplifiers based on [[field effect transistor|FET]]s.<ref>{{cite web |title=Low Noise Amplifiers – Pushing the limits of low noise |url=https://science.nrao.edu/facilities/cdl/low-noise-amplifiers |publisher=National Radio Astronomy Observatory (NRAO)}}</ref>

During the early 1960s, the [[Jet Propulsion Laboratory]] developed a maser to provide ultra-low-noise amplification of [[S-band]] microwave signals received from deep space probes.<ref>{{cite web |url=https://descanso.jpl.nasa.gov/monograph/series10/Reid_DESCANSO_sml-110804.pdf |title=Low-Noise Systems in the Deep Space Network |publisher=JPL |editor=Macgregor S. Reid |year=2008}}</ref> This maser used deeply refrigerated helium to chill the amplifier down to a temperature of 4&nbsp;[[kelvin]]. Amplification was achieved by exciting a ruby comb with a 12.0 gigahertz [[klystron]]. In the early years, it took days to chill and remove the impurities from the hydrogen lines. 

Refrigeration was a two-stage process, with a large Linde unit on the ground, and a crosshead compressor within the antenna. The final injection was at {{convert|3000|psi|MPa|order=flip|abbr=on}} through a {{convert|0.006|in|um|abbr=on|order=flip}} micrometer-adjustable entry to the chamber. The whole system [[noise temperature]] looking at cold sky (2.7&nbsp;[[kelvin]] in the microwave band) was 17&nbsp;kelvin. This gave such a low noise figure that the [[Mariner IV]] [[space probe]] could send still pictures from [[Mars]] back to the [[Earth]], even though the output power of its [[radio transmitter]] was only 15&nbsp;[[watt]]s, and hence the total signal power received was only −169&nbsp;[[decibel]]s with respect to a [[milliwatt]]&nbsp;(dBm).

===Hydrogen maser===
{{Main|Hydrogen maser}}
[[File:Hmaser.svg|thumb|right|200px|A hydrogen maser.]]
The hydrogen maser is used as an [[atomic clock|atomic frequency standard]]. Together with other kinds of atomic clocks, these help make up the [[International Atomic Time]] standard ("Temps Atomique International" or "TAI" in French). This is the international time scale coordinated by the [[International Bureau of Weights and Measures]]. [[Norman Ramsey]] and his colleagues first conceived of the maser as a timing standard. More recent masers are practically identical to their original design. Maser oscillations rely on the stimulated emission between two [[hyperfine structure|hyperfine energy level]]s of atomic [[hydrogen]].

Here is a brief description of how they work:

*First, a beam of atomic hydrogen is produced. This is done by submitting the gas at low pressure to a high-frequency [[radio wave]] discharge (see the picture on this page).
*The next step is "state selection"—in order to get some stimulated emission, it is necessary to create a [[population inversion]] of the atoms. This is done in a way that is very similar to the [[Stern–Gerlach experiment]]. After passing through an aperture and a magnetic field, many of the atoms in the beam are left in the upper energy level of the lasing transition. From this state, the atoms can decay to the lower state and emit some microwave radiation.
*A high [[Q factor]] (quality factor) [[microwave cavity]] confines the microwaves and reinjects them repeatedly into the atom beam. The stimulated emission amplifies the microwaves on each pass through the beam. This combination of [[amplifier|amplification]] and [[feedback]] is what defines all [[oscillator]]s. The [[resonant frequency]] of the microwave cavity is tuned to the frequency of the hyperfine [[energy level#Energy level transitions|energy transition]] of hydrogen: 1,420,405,752 [[hertz]].<ref>{{cite journal |url=https://www.nist.gov/pml/time-and-frequency-division/popular-links/time-frequency-z/time-and-frequency-z-h |title=Time and Frequency From A to Z: H|journal=NIST |date=12 May 2010 }}</ref>
* A small fraction of the signal in the microwave cavity is coupled into a coaxial cable and then sent to a coherent [[radio receiver]].
* The microwave signal coming out of the maser is very weak, a few [[picowatt]]s. The frequency of the signal is fixed and ''extremely'' stable. The coherent receiver is used to amplify the signal and change the frequency. This is done using a series of [[phase-locked loop]]s and a high performance [[quartz oscillator]].