Photodetection
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In his historic paper entitled "The Quantum Theory of Optical Coherence," Roy J. Glauber set a solid foundation for the
quantum electronics Quantum optics is a branch of atomic, molecular, and optical physics and quantum chemistry that studies the behavior of photons (individual quanta of light). It includes the study of the particle-like properties of photons and their interaction w ...
/
quantum optics Quantum optics is a branch of atomic, molecular, and optical physics and quantum chemistry that studies the behavior of photons (individual quanta of light). It includes the study of the particle-like properties of photons and their interaction ...
enterprise. The experimental development of the optical
maser A maser is a device that produces coherent electromagnetic waves ( microwaves), through amplification by stimulated emission. The term is an acronym for microwave amplification by stimulated emission of radiation. Nikolay Basov, Alexander Pr ...
and later
laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...
at that time had made the classical concept of optical coherence inadequate. Glauber started from the quantum theory of light detection by considering the process of photoionization in which a photodetector is triggered by an ionizing absorption of a photon. In the quantum theory of radiation, the electric field operator in the Coulomb gauge may be written as the sum of positive and negative frequency parts : E (\mathbf, t) = E^(\mathbf, t) + E^(\mathbf, t) where : E^(\mathbf, t) = E^(\mathbf, t)^\dagger One may expand E^(\mathbf, t) in terms of the normal modes as follows: : E^(\mathbf , t) = i\sum_\left(\frac\right)^\hat_\mathbf_e^ where \mathbf _ are the unit vectors of polarization; this expansion has the same form as the classical expansion except that now the field amplitudes \hat_ are operators. Glauber showed that, for an ideal photodetector situated at a point \mathbf in a radiation field, the probability of observing a photoionization event in this detector between time and + d is proportional to W_(\mathbf , t)d, where : =\langle \psi \mid \cdot \mid \psi \rangle and , \psi\rangle specifies the state of the field. Since the radiation field is a quantum-mechanical one, we do not know the exact properties of the incident light, and the probability should be averaged, as in the classical theory, to be proportional to : \langle \hat_^\dagger\hat_ \rangle where the angular brackets mean an average over the light field. The significance of the quantum theory of coherence is in the ordering of the ''creation'' and ''destruction'' operators \hat_^\dagger and \hat_: : hat_, \hat_^\dagger= 1 Since \hat_^\dagger\hat_ is not equal to \hat_\hat_^\dagger for a light field, the order makes the quantum statistical measurements (such as photon counting) quite different from the classical ones, i.e., the nonclassical properties of light, such as photon antibunching. Moreover, Glauber's theory of photodetection is of far-reaching fundamental significance to
interpretation of quantum mechanics An interpretation of quantum mechanics is an attempt to explain how the mathematical theory of quantum mechanics might correspond to experienced reality. Quantum mechanics has held up to rigorous and extremely precise tests in an extraordinarily b ...
. The Glauber detection theory differs from the Born probabilistic interpretation,M. Born, Z. Phys. 37, 863 (1926). For an English translation, see Quantum Theory and Measurement ed. J. A. Wheeler and W. H. Zurek, Princeton Univ. Press, New Jersey, 1983, pp. 52-55. in that it expresses the meaning of physical law in terms of measured facts (relationships), counting events in the detection processes, without assuming the particle model of matter. These concepts quite naturally lead to a relational approach to quantum physics.


References

{{reflist Quantum optics