The analysis of the preceding Chapter was for a constant magnetic field. From Exercise 14.10, we see that the magnetic field is very large (>1 T) for the Zeeman splitting to exceed the thermal energy. This is desired to prevent unwanted thermal excitation between the Zeeman split energy levels from affecting our qubit states. However, it is difficult to change such large static B-fields rapidly enough for quantum gates. Also, the resulting Larmor frequency ωo is very fast (>>GHz). We would like to have a slower rotation to get better control. To use smaller fields and slow down the precession, we use a technique called “electron spin resonance” (ESR).