Piezoelectric oscillators today do not make it possible to obtain frequency stabilities better than 5.10^-14 over a few seconds of integration, which imposes limitations for certain very demanding applications.

Cryogenic gallery-mode microwave oscillators, initially developed in the 1980s by Australian teams for the detection of gravitational waves, can now generate signals with spectral purities and short-term frequency stabilities one to two orders of magnitude better than those of quartz oscillators.

In these oscillators, the oscillating electromagnetic field at frequencies of the order of 10 GHz, is localized on a sapphire monocrystal surface contained in a microwave resonant cavity. The field localization on the crystal surface according to a gallery mode makes it possible to considerably minimize the losses by absorption, thus leading to extremely high-quality factors of the resonance. This principle is identical to the one observed in the “whispering galleries”, including the very famous one in Saint Paul’s Cathedral in London. In addition, these oscillators operate at cryogenic temperatures, i.e., at the liquid helium temperature, which is an asset to avoid thermal noise affecting devices operating at room temperature.

These oscillators performance record is a frequency stability of 7.10^-16 for integration times from 1 second to a few hours.