Superconducting microwave cavities as frequency standards

In contrast to the zero resistance state observed in superconductors at very low frequencies, ohmic losses in superconducting microwave cavities are not in general negligibly small. However, if the temperature is reduced to well below the transition temperature of the superconducting material from which the cavity is made, and if careful attention is paid to material purity and mechanical construction, it is possible to produce resonators with very high Q values (⩾10¹¹ ). This paper outlines the essential properties of a superconducting cavity which make it suitable as a highly stable frequency reference standard, summarising the results achieved so far and commenting also on some disadvantages of such systems. Work has been reported rather recently on high Q dielectric-loaded superconducting resonators which may circumvent some of the problems associated with simple superconducting cavities. Also the advent of high temperature superconductors may be expected to have considerable impact on the wider applicability of microwave cavity frequency standards. The paper also reports recent work at NPL and other laboratories in these two new fields, concluding with some speculation on likely future directions of this work