FROM 10¹⁰ SPINS TO A SINGLE SPIN.

It is widely accepted that the upper sensitivity limit in conventional (CW X-band) ESR is 10¹⁰ electron spins. There are two basic reasons for this relatively small sensitivity: The first one is the small thermal population difference. The second reason is the fact that low energy photons are detected which leads to small detector efficiency and to relatively high thermal noise level. While optical photons can be detected with an efficiency near unity, many microwave photons are required to exceed the noise level. In the first part of this chapter, a very brief and partial description of the different solutions to the sensitivity problem will be given…