Illustrated Physics
physics, drawn
Concept

Antibunching — where it appears

The tendency of photons from a single emitter to arrive one at a time, so that the correlation between two detectors falls to zero at zero delay. No classical field can produce it, which makes the measurement the standard proof that a source is a single quantum object.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

Also named here as second-order coherence, thermal light — the same set of essays touches all of them, so they are one junction rather than several.

Three sources, and the number that separates them. The chance of detecting a second photon a delay τ after a first, divided by the chance if the two were independent, for three kinds of light. At long delay every curve is one, which is what independence means. At zero delay they are 2, 1 and 0, and those three numbers are three different physical situations. Thermal light is bunched: its intensity fluctuates, and a photon is more likely to be found where the intensity happened to be high, so it arrives with company for as long as the fluctuation lasts — 4 nanoseconds here. A laser is flat, because a coherent state has no intensity fluctuation to correlate with. And a single emitter is antibunched: it gives zero, exactly, because after emitting it is in its ground state and cannot emit again until it has been re-excited, which takes 12 nanoseconds. The zero is the important one. Every classical field, of every possible intensity distribution, has g²(0) at least one — the inequality follows from the fact that the mean square of a real positive quantity is at least the square of its mean. A measurement below one is not merely evidence for photons; it is a result no wave theory can produce.

The experiment a wave cannot pass

The photoelectric effect is offered everywhere as the proof that light is quantised, and it is not one — a classical wave falling on quantised matter reproduces every feature of it. The measurement that no wave can pass is a different one: send single photons at a beam splitter and count how often both detectors fire.

quantum · Photon The same source measured by amplitude and by intensity. The degree of coherence of a 47 milliarcsecond disc at 550 nm, and its square, against the separation of two apertures. A Michelson interferometer measures the upper curve, because fringe contrast is |γ|. Correlating the intensities at the two apertures instead measures the lower one, because the excess correlation of two thermal beams is |γ|² — the same information about the source, since one curve determines the other, and reaching zero at the same baseline of 2.94 m. What is lost is the phase of γ, which the intensity correlation never sees; what is bought is that a path error of many wavelengths does not matter, because the quantity being correlated is a slow fluctuation of brightness rather than a wave. The half-coherence baselines differ — 1.71 m against 1.25 m — which is the practical statement that the squared curve is the steeper one to measure against.

The correlation that survives what the phase does not

Two telescopes can measure a star's diameter by interfering the light, which requires holding two paths equal to a fraction of a wavelength through an atmosphere that will not hold still. Or they can throw the phase away entirely and correlate the brightness fluctuations, which needs the paths equal to a few metres and works.

optics · Coherence

Named alongside it

The objects these essays reach for when they reach for this one.

Second-order coherenceThermal lightAngular diameterBaselineBeam splitterCauchy schwarzCoherenceCoincidenceCorrelationDegeneracy parameterIntensity interferometryPhotoelectric effect

All concepts