Talk:Hanbury Brown and Twiss effect

message: Note that many of the first radio interferometers in the 1940s were intensity interferometers. Also note that all astronomical interferometers use light which is not temporally coherent, so the lack of temporal coherence for the light in the first optical intensity interferometers was not unusual. Rnt20 13:26, 27 September 2005 (UTC)
thread_title: temporal coherence

message: The page at it currently stands gives me the impression that there is anti-correlation between the signals at the two detectors.  The whole point of the Hanbury Brown - Twiss experiment, with light from a high-pressure mercury arc lamp is that the signals at the detectors are correlated, not anti-correlated.  This is only practically measurable, as I understand it, with narrowband incoherent light.  It is hard or impossible to detect any correlation with a laser.  Robert Hanbury Brown, in his 1972 book "The Intensity Interferometer" rejects the particle notion of light.  I think he uses "photon" to mean the generation and deposition of (implicitly unquantized) electromagnetic radiation (emr) by matter, not to indicate that light can be regarded as a stream of "photon" particles.  On page 7 he writes: "These difficulties about photons troubled physicists who had been brought up on particles and had not fully appreciated that the concept of a photon is not a complete picture of light.  Thus many people are reluctant to accept the notion that a particular photon cannot be regarded as having identity from emission to absorption."  
On page 6 he refers to the work of Forrester, Gudmundsen and Johnston (FGJ), also in 1955, (http://prola.aps.org/abstract/PR/v99/i6/p1691_1)  who used the interference between two closely spaced wavelengths of a Zeeman split emission line to generate an electrical beat frequency on a photo-cathode.  This seems to be a very clear instance of two emr signals summing to produce a modulated total signal.  The FGJ experiment creates a modulated electromagnetic wave at the photo-cathode, leading to an electrical signal, at about the 10GHz resonance of their 3cm cavity. This is not explicable by two separate beams of "photons", with different frequencies, since such photons in the pure particle sense are not, or were not, regarded as being capable of interfering with each other - the two beams should both produce a continuous current independently.
It seems to me that the Hanbury Brown - Twiss effect, which is correlation between the signal of photo-detectors receiving a split version of a narrowband incoherent light source, can't be explained with conventional "photons", and can only be explained with the emr being a classical wave.  Both the FGJ experiment and the Hanbury Brown - Twiss experiment constitute arguments against the conventional view of emr being quantitized as "photons".  I think that the current state of this page does not reflect this, and by explaining the effect solely in terms of particles (photons, which are bosons) hides the fact that there is a debate and presents one view as if there was no debate.
Robin Whittle
:You are right to complain that the article did not make explicit that Hanbury-Brown and Twiss themselves observed a correlation.  Is the current revision better?  A positive correlation was also observed in the Mercury experiment you described (referenced in the page as Morgan and Mandel 1966).  However, the experiment pictured does in fact result in an anticorrelation, as described in the 1986 paper.  This result can be intuitively understood as simply demonstrating the fact that a single photon can only be observed at a single detector.  Anticorrelation, unlike the correlation found by Hanbury-Brown and Twiss, cannot be understood classically.
:There is perhaps an interesting discussion of the history of Dirac's famous statement that a photon interferes only with itself.  Although the development of quantum field theory has made it clear that photons can interfere with each other, it was at one time widely believed that this was not the case.  Hence the trouble introduced by the simple interference effects (FGJ) you describe.  I'm not particularly well versed in this history - maybe there should be another page for it.  If you accept that photons are able to interfere with one another, HBT and FGJ are suddenly much more accessible from the quantum point of view.  You might imagine that two nearby photons, being bosons, tend to interfere constructively, causing their wave packets to be amplified in the place where they overlap.  For this reason, two photons which are close to each other tend to be detected right next to each other, since that's where their amplitudes are greatest.  Similarly, you might imagine that fermions tend to destructively interfere, so that the amplitudes of their wave packets are smallest where they overlap.  This is just the Pauli exclusion principle.  That's all very informal, but I hope it helps conceptually.
:I think the section titled "Wave Mechanics" does a good job explaining the effect purely as a classical one.  I don't think there's a need to emphasise it any more.  Finally, I think I ought to emphasise that there is no debate concerning the existence of photons.  They are not some construct whose existence is controversial in any way, they are established physics.  In fact, a careful consideration of the century-old photoelectric effect should essentially convince you that the energy of light must be delivered in a corpuscular manner, and that each corpuscle must have energy hf.  Particularly note the fact that increasing the intensity of the incident light does not increase the energy of the emitted electrons, but increasing the frequency does.  Since the classical energy of light is given only by its intensity and not its frequency, it's hard to believe that this can be explained classically without heavy modification.  Combined with Planck's solution for the blackbody radiation curve and the anticorrelation effect described in the 1986 paper, I hope you are convinced.
:--Hyandat 08:44, 4 March 2006 (UTC)
replies: text: The 'quantum' section states that Ugo Fano showed correlation between amplitudes, but doesn't give any actual formulas, which makes it hard to understand.  The formulas are needed.  The beat-frequency for the zeeman effect is also a rather interesting phonomenon, deserving exposition here or elsewhere. User:Linas (talk) 20:10, 1 December 2013 (UTC)

text: Note: the HBT experiment with a sharp number state, n=1, does not imply anything about localized photon particles.  The lack of coincidence counts is required by conservation of energy.  After the first detection, n=0.  —Preceding unsigned comment added by 67.169.157.55 (talk) 08:11, 6 October 2008 (UTC)

text: Hi James, ... interesting read, but "those doing the actual editing" need to be expert to accomplish your hopes and goals for the article, which really means that only you are capable of getting this to happen. You should do the edits. User:Linas (talk) 20:29, 1 December 2013 (UTC)
thread_title:  Photons and HBT 

message: The page: Hanbury-Brown and Twiss effect (and associated pages)
The problem: Hanbury Brown is not hyphenated, but I don't know how to fix the heading of the article. Starrylady 23:54, 27 January 2007 (UTC)
thread_title:  Please amend the heading 

message: I suggest that photon bunching be merged into this article - as far as I can tell, photon bunching is just a simple case of the HBT effect?  (In any case, the photon bunching article is orphaned at the moment as well).  Akriasas (talk) 10:35, 8 February 2008 (UTC)
thread_title:  [[Photon bunching]] merger proposal 

message: With reference to the section on wave mechanics, the formula given for intensity is i1 = E^2 sin^2 wt. However, detectors (e.g. PMT) are generally not fast enough to detect frequency of optical waves. We are detecting instead the time average and as such both i1 and i2 should be just proportional to the square of the E-field instead of sin^2 wt.
However, since HBT effect is essentially a second-order correlation effect, the final form of the correlation formaula is correct. Perhaps it may be clearer if the intensity correlation be written as <E1* x E1 x E2* x E2> instead of <i1 x i2>  -- Tzu Hao Goldbeaver (talk) 03:29, 8 September 2008 (UTC)
replies: text: The problem goes deeper than this, and the final form of the correlation formula (in August 2011 at least) is not correct. The article states that the HBT effect is equivalent to a classical effect which supposedly allows the phase (of g^(1)) to be measured. That is, the final form of the correlation function, even with the averaging of sinusoidal oscillations, contains the optical phase phi. This is completely untrue, and it is untrue because it assumes that intensity measurements give a square-of-sinusoid signal. This is certainly untrue at optical frequencies using photon counters, which is what the HBT effect is all about. The HBT effect is a measurement of g^(2), which equals |g^(1)|^2 for thermal fields. The classical analgoue only makes sense for fields with fluctuating intensities, and for real *intensity* measurements, not E^2 measurements pretending to be intensity measurements. I hope to amend the article soon. Vortimer (talk) 02:19, 29 August 2011 (UTC)
replies: text: So, can someone fix this? User:Linas (talk) 19:50, 1 December 2013 (UTC)
thread_title:  Intensity formula is incorrect 

message: The article contains a false statement that the HBT effect is due to boson nature of photons: "...the results of a given experiment depend on whether the beam is composed of fermions  or bosons". Actually, the HBT effect is due to interference between waves of close frequencies, which results in intensity fluctuations. However, no wave effect, neither interference nor diffraction, may be derived from photon statistics, be it quantum (Bose-Einstein), classical, or intermediate statistics. It means that photon statistics has nothing to do with the HBT effect. For details see, for example, M.O.Scully and M.S.Zubairy, "Quantum Optics".  —Preceding unsigned comment added by 81.5.96.132 (talk) 10:26, 20 February 2010 (UTC)
replies: text: I disagree, in that it would make a difference if photons were fermions (but then there would be no classical limit of light as waves). Vortimer (talk) 02:13, 29 August 2011 (UTC)
thread_title:  Photon statistics and the HBT effect 

message: the external link * http://physicsweb.org/articles/world/15/10/6/1
is dead and from that citation it is difficult to figure out what it pointed at 
Netrapt (talk) 01:42, 16 April 2010 (UTC)
thread_title:  Dead link