ability of a rubidium oscillator is much better than a quartz oscillator, which is very helpful property when the clock loses its reference signal. Rubidium oscillators are also quick to settle and reach a stable operation after only a short warm-up period. This stability, combined with reasonable cost, means that operators deploy them as an affordable option to increase holdover performance (see the following section on "Holdover Mode"). The main shortcoming of rubidium oscillators is that they are somewhat temperature sensitive, SO they can only be deployed where there are reason- able environmental controls (although, to be fair, atomic clocks are hardly ruggedized devices). Cesium oscillators use a beam of excited cesium atoms, which are then selected by magnetic fields. The device transmits a microwave signal to resonate with the cesium atoms and cause them to alter their energy state. Using a detector to determine the number of altered atoms, the device "tunes" the frequency of that signal to maximize the number of altered atoms, which increases the closer the microwave approaches the 9,192,631,770-Hz resonance frequency. This is all done in combination with a high-quality quartz oscillator. A well-designed cesium oscillator is the most accurate reference available, with almost no deviation over time (see the upcoming discussion of aging), although other environmental factors (motion, magnetic fields, and SO on) can cause small frequency shifts. It requires no adjustment and is quick to warm up and settle to a stable output. Cesium oscillators are used by standards organizations to define the SI second. A hydrogen maser is a resonator that uses hydrogen atoms that have had their energy levels elev- ated to certain specific states. Inside a special cavity, the atoms lose their energy and release mi- crowaves with a resonant frequency of 1,420,405,751.786 Hz. This signal is then used to discipline a well-designed quartz oscillator. The process is maintained by a continuous feed of more hydrogen atoms. Maser i