Chapter XI - Quantum nondemolition measurements of a resonator's energy

Summary

Review of methods of measurement

There are two different variants of the task of measuring electromagnetic energy: i) measuring the energy in a mode of an electromagnetic resonator, and ii) measuring the energy of a traveling electromagnetic wave. This chapter deals with the first variant; the next chapter, with the second.

The general principles underlying QND measurements of energy were formulated in chapter IV. Let us recall that the main principles are: i) the response of the measuring device must be directly proportional to the energy (and not, for example, to the strength of the electric field or the charge on the capacitor); and ii) the response must contain no information about the phase of the electromagnetic oscillations. Chapter IV analyzed the simplest example of such a device: a ponderomotive sensor that registers the electromagnetic pressure on the resonator's wall.

The weakness of the electromagnetic pressure produced by a small number of quanta makes it unlikely that this ponderomotive method can be realized in practice. Because of this, several other schemes for QND energy measurements have been proposed. Most are based on nonlinear effects in dielectrics. Their practical realization is a rather complicated task because, when the energy density is low, the nonlinear effects hardly work at all, and the response of the measuring device is correspondingly small.