Photon bunching

Under photon bunching (from English Bunch - accumulation) refers to the occurrence of temporal correlations of individual photons from the same source, typically a thermal light source . To put it clearly, thermally generated photons “clump”; two detectors are more likely to measure photons from the source at the same time.

Photon bunching was first observed by Robert Hanbury Brown and Richard Twiss and is therefore also known as the Hanbury Brown-Twiss effect (HBT effect).

history

Hanbury Brown and Twiss' original interest in 1955–56 was in measuring the apparent size of stars. They set up two photomultipliers at a variable distance d (up to approx. 180 m) and measured the spatial correlation of light from a star that fell on both detectors. Due to conceptual difficulties as to how the spatially separated statistical quantum processes in the two detectors could be correlated, they decided to test the experimental setup in the laboratory as well. In a laboratory test, the time correlation of the light from a mercury lamp was measured, which was split into two beams by a half-silvered mirror. Further details on the HBT experiment can be found in the article on the intensity interferometer .

overview

It can be shown that with photon bunching the correlation function of the second order

${\displaystyle \gamma (\tau )={\frac {\langle I_{1}(t)I_{2}(t+\tau )\rangle }{\langle I_{1}(t)\rangle \langle I_{2}(t)\rangle }}}$,

for is greater than 1. The term second order correlation indicates that the intensity is already the product of two wave functions and that this is not a two-point function , but a four-point function. ${\displaystyle \tau \to 0}$

The variance is generally greater for photon bunching than for the Poisson distribution , which is why it is also referred to as super Poisson statistics.

The role of photons can be taken over by any other boson , thermally distributed bosonic atoms at extremely low temperatures also show bunching . The tendency of bosons to clump into packets is an important part of the theory of the Bose-Einstein condensates .

Comparison with other statistics

With a coherent laser light source, the times of detection follow the Poisson distribution and apply to all . The first explanation of this difference came from Roy J. Glauber , who was awarded the Nobel Prize in Physics in 2005 for his contributions to the quantum theory of optical coherence . ${\displaystyle \gamma (\tau )=1}$ ${\displaystyle \tau }$

Photon antibunching with , however, only occurs if a time interval exists between the photons (such as, for example, in the single-photon source ), and is a quantum effect. It usually occurs with sub-Poisson statistics. ${\displaystyle \gamma (0)<1}$

Individual evidence

1. ^ ^{A b c} Mark Fox: Quantum Optics: An Introduction . Oxford University Press, 2006, ISBN 978-0-19-856673-1 ( limited preview in Google Book Search).
2. ^ R. Hanbury Brown; RQ Twiss: Correlation between photons in two coherent beams of light . In: Nature . No. 177 , 1956, pp. 27 .

literature

• Harry Paul : Photons: An Introduction to Quantum Optics . Vieweg + Teubner Verlag, 1999, ISBN 978-3-519-13222-6 .
• Pierre Meystre , Murray Sargent III : Elements of Quantum Optics . Springer, 2007, ISBN 978-3-540-74209-8 .

Web links

• Horst Hübel: Hanbury Brown / Twiss effect. In: Würzburg quantum physics concept. Retrieved May 22, 2009 . 
• Horst Hübel: Classic and non-classic light. In: Würzburg quantum physics concept. Retrieved May 22, 2009 .