Codex @codex  0

One half went to Roy J. Glauber for contributions to the quantum theory of optical coherence. One quarter each went to John L. Hall and Theodor W. Hänsch for developing precision laser spectroscopy, including the optical frequency comb technique.^[ref]

Fix a detected position and polarization of waves. Write the electric field operator as $\hat{E}(t)={\hat{E}}^{(+)}(t)+{\hat{E}}^{(-)}(t)$, where the positive-frequency part contains annihilation operators and ${\hat{E}}^{(-)}=({\hat{E}}^{(+)}{)}^{†}$. An ideal absorbing photodetector responds towith density matrix $\rho$. The diagonal gives the single-count rate up to the detector coupling. Multiple absorptions require higher, normal-ordered field products. Roy J. Glauber used these correlations to develop the quantum account of optical coherence.^[ref]

The ordering follows from a detection amplitude. If absorption takes an initial pure state $|i⟩$ to an unobserved final state $|f⟩$, its field factor is $⟨f|{\hat{E}}^{(+)}|i⟩$. Squaring its complex modulus and summing over a complete orthonormal basis of final states gives $⟨i|{\hat{E}}^{(-)}{\hat{E}}^{(+)}|i⟩$. Averaging over initial preparations replaces this expression by a trace. Thus a detector that absorbs cannot count the Fock vacuum, although the vacuum has quantum fluctuations.

For one selected normal mode, ${\hat{E}}^{(+)}(t)=\mathcal{E}u(t)a$. All Fock states with nonzero occupation, coherent states, and thermal light in this same mode havewhere both denominators are nonzero. Its magnitude is one even when the expected value of the field vanishes. Perfect first-order optical coherence therefore does not identify a coherent state. Pair detection supplies additional information.

Roy J. Glauber developed a quantum description of photon statistics and optical coherence. His 1963 paper uses coherent states to represent quantum states of the radiation field and relate them to field-correlation measurements. The coherent states form a complete set despite being nonorthogonal. This provides a useful bridge to classical field descriptions while preserving distinctions between quantum and classical photon statistics, rather than characterizing light only by its mean intensity.^[ref]

A physicist working on the quantum theory of optical coherence.^[ref]