adding time of flight measurements to double slit experiment

I guess shutter timing of 100ps would be acceptable in some experiments for TOF, so wouldn't be comparable to a femtosecond pulse in that case.

cheers, Jamie

The question is, what's different about a single photon that has passed through a shutter? It would have been in the same place and time whether the shutter existed or not.

The only thing I can imagine is that somehow passing through a shutter randomizes the wavelength of single photons. That's about the only gotcha I can imagine that might make TOF measurement and interference mutually exclusive.

John

Hi,

From what I read, Young's double slit experiment was originally used to give evidence for the wave nature of light, and then later it was used to give evidence for the wave-particle duality of light. However in this second case, it is assumed that light can be sent through the experiment as particles. This seems like a circular definition to me, so I think its important to really look at the source of the light t see if it truly is a wave or particle before any experiment is analyzed. The interpretation of the results of the experiment depend on the initial assumptions, and the results are designed to answer the assumptions (whether light is a wave or particle).

cheers, Jamie

As I understand it (probably not very well...) it is the chopping up in time that destroys the coherence and therefore the fringes too.

AIUI Yes, if the switch enables a ps width gate.

--

John Devereux

Hi,

Here's another thought experiment from the view point of the copenhagen interpretation of quantum mechanics. For a star in space emitting photons outwards spherically at the speed of light, this creates a probability wave for each photon, and when the photon is absorbed at 1 light year from the star, the 1 light year radius probability wave "collapses" for that specific photon, and it cannot be absorbed at any other point in space. So the thought experiment is to put a (very!) large number of telescopes looking at that star at the 1 light year radius geometrically distributed all on the same side of the star. According to the copenhagen interpretation of quantum mechanics, these telescopes will reduce the number of available photons to be absorbed on the other side of the star at the 1 light year radius, so the star will then appear dimmer from that location.

cheers, Jamie

Hi,

Woops, the experiment would only work if it was limited to one hemisphere of the sphere, since visible light can only be seen from one hemisphere.

cheers, Jamie

If you are all keen on a definitive answer post it to sci.physics.research

--
Dirk

http://www.neopax.com/technomage/ - My new book - Magick and Technology

The photon can interfere with itself provided that the path length difference is not too great compared to the energy dispersion of the nominally monochromatic photons. Any amplitude modulation necessarily causes sidebands in the frequency domain and so uncertainty in the energy of the photons you get.

The photons can and do all interfere provided their wavefunctions along different paths get a chance to overlap on the target. However, to build up visible fringes the photons all have to have very similar energy and wavelength or the path length must be relatively short.

That is classic quantum mechanics. The photon wavefunction obeys essentially the same sort of wave equation (with minor differences) as electromagnetic waves. It should not come as a great surprise that it shows interference fringes with just one photon in the experiment.

However, to build up a recognisable interference fringe pattern from there individual photons you require that their energy dispersion is not too large compared with the path length difference.

Fair enough so far.

You really are making a meal of this and still apparently completely failing to grasp the basic rules of quantum mechanics as exemplified by the Heisenberg Uncertainty Principle.

The timings you will see when there are strong interference fringes will be two overlapping ranges representing the pulse width at the input to the experiment and the energy dispersion of the photons. If you try to constrain one you automatically lose control of the other.

Once you can distinguish which path the photon must have taken unambiguously the interference fringes are gone and you can pretty much apply classical ray optics and billiard balls for photons. Your idea of what happens is right only at the extreme classical limit.

Actually someone has done exactly this experiment using a biphoton emitter to trigger a second fast shutter to modulate the wavefunction of the outgoing second photon. It might help you to think about the photon in transit as a wavefunction representing the probability of finding the particle as a function of time and space.

The rough wavefunction is probably not to far off a carrier wave with amplitude modulation that is Gaussian or Napoleons hat in shape. The fancy shutter was used to null the centre part leaving just the start and end tails by blocking out the middle.

The problems you are having seem to stem from interpreting a photon in motion as a billiard ball and then applying an inconsistent mixture of classical mechanics and Larkin "common sense" to it.

A black box of that type has indeed been made to create photons with a modulated wavefunction that only shows interference fringes for particular ranges of path difference in the interferometer. I posted a link to it earlier - you obviously do not read anything that conflicts with your own prejudices about how the world works.

The problem with "monochromatic photon" is that it is an oxymoron. You cannot ever know the wavelength precisely and it is the bound on the uncertainty in the energy and time or if you prefer its total length and momentum that determines whether or not you can observe fringes.

The simplest way would be to take a coherent laser light source and have two filters. One is a very dense neutral density filter that lets a random photon through at say a rate of 1 in every 10^15. The other is a very very fast shutter that lets bits of photon wavefunction through for

1fs every 1s. The results of these two sampling methods are very different in terms of the energy distribution of the sampled photons.

The 1fs time gated photon will contain a sample of less than one wavelength of the originally monochromatic visible light source. Accordingly it has a huge uncertainty in its energy.

Regards, Martin Brown

There are often articles in the American Journal of Physics that treat these problems, in the context of descriptions of experiments for physics students.

An interesting article is for example "A hands-on introduction to single photons and quantum mechanics for undergraduates", Brett J. Pearson, David P. Jackson, Am. J. Phys. 78 (5), May 2010, pp471-483. The article describes why simple 'photon counting' experiments do not conclusively prove that light is quantized. It then describes progressively more sophisticated setups towards showing that it finally *is* quantized, but argues that the mental picture of a photon as a tiny massless particle travelling at C is wrong.

The problem I have with this article is that the authors finally conclude that light is truly quantized from the results of an experiment involving parametric down-conversion, which according to them is a purely quantum-mechanical process. That seems to imply that non-linearity is necessarily of quantum-mechanical origin, which is a notion I haven't yet fully assimilated.

I still believe, albeit without being able to properly argue my case, that the semi-classical theory, where light is a wave but its interaction with matter is quantized, is sufficient.

Jeroen Belleman

Depends on the wavelength and the precision with which the frequency/energy of it is known.

Your second question is unanswerable beyond saying that is what the mathematics of quantum mechanics predicts and it *is* observed. Moreover it is also observed for silver ions and buckyballs.

Its wavefunction has to be non-zero over a range that is larger than the path length difference if you are to see interference fringes.

You are stuck because of your billiard ball model of "Larkin photons".

NB it doesn't stop interference fringes occurring if this energy spread condition is not met it just means that for longer path lengths the fringes have been smeared out by convolution over the energy spectrum.

You can always see the constructive interference and something of the first minima at the zero path difference white light fringe even with a broadband source. As you increase the path difference you have to narrow the energy bandwidth to still be able to see fringes.

Regards, Martin Brown

On a sunny day (Tue, 07 Jun 2011 23:59:06 +0200) it happened Jeroen wrote in :

Exactly!

On a sunny day (Tue, 07 Jun 2011 16:16:31 -0700) it happened John Larkin wrote in :

You really have no clue, maybe Jim is right.

Looks like you will never have a clue.

On a sunny day (Tue, 07 Jun 2011 17:24:01 -0700) it happened John Larkin wrote in :

Wrong,

Einstein was an idiot, and giving awards to yourself is the Microsoft way.

I win the Jan Panteltje Best Electronics Award very year.

A lot of physicist objected Einstein getting the photon, look it up. It was a POLITICAL decision, just like giving that Chinese trouble maker a Nobel peace prize.

Get over it,

And you Jewish who occupy that Syrian land should not shoot unarmed people who only try to return to THEIR land.

But not to worry, the strongest will walk away from all this as the 'right and victorious'. And I have an idea it will not be Israel this time. Tick tac toe could go like this: Israels surrounding countries unite and start to overrun its current claimed borders. Israel gets desperate and starts to nuke (pick any place they do not like, that could be anywhere in the world :-) They accidently hit a place Russia or China does not like.

tac, that country retaliates, Tel Aviv, maybe Jerusalem glassified. Now US has to show it really keeps its promise and toe, nukes a place in Russia or China. That of course is not accepted and I see those missiles flying over my head from east to west and west to east. Andreas fault COULD be triggered in the process, but the destruction would be so big it would not make much of a difference. Add to that GLOBAL WARMING and for sure the end of days is here LOL

What on earth makes you think you can totally ignore causality in this situation? You cannot invoke FTL travel arbitrarily like that!

There are things in quantum mechanics related to entangled states where measurement of one of a pair forces the other entangled one into the corresponding opposite state at the same instant no matter how far away it is. This seems to be borne out experimentally too.

It probably highlights a weakness in our current theories in much the same way as Newtonian gravity having to act instantaneously did.

However, the fact remains that quantum theory works and makes useful predictions. Particles can and do behave like waves and you can obtain the correct observed results by treating them as waves.

As you have also proved conclusively you can become horribly confused by mixing classical physics and common sense in handwaving arguments about the behaviour of photons inside these experiments.

The mathematics and experimental verification of it is trustworthy.

Regards, Martin Brown

All true. And if you do that and can observe two independent peaks you are back in the domain of classical physics. Otherwise you see times that overlap and photons that could have come by either path with fringes. It all depends critically on the path lengths and the energy spread of the photons whether you can see the fringes or not.

Exactly.

You have unwittingly made a measurment of it in time and space by forcing it to be in the very narrow time gate so that its energy uncertainty is greatly increased propagating forward in time.

Exactly. A femto second shutter would let through less than one wavelength of visible light with sharp discontinuities too. Even if you could phase lock to get a single exact cycle zero crossing to zero crossing you would still have a huge chunk of sidebands created from that action of sampling the wave. It is a lot easier to see this in the wave formalism (either as QM or electromagnetic waves).

Regards, Martin Brown

Wavicle, smeared out to infinity:

--
Dirk

http://www.neopax.com/technomage/ - My new book - Magick and Technology

Jim can't do simple household repairs without asking for help.

What, specifically, is wrong with either statement?

John

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Pulse length is your shutter time... open optical shutter for 10 ns. It has nothing to do with your repetition rate.

How can a single photon possibly interfere with itself?

It's gotta overlap 'properly' in time and space.

The photons have 'lengths' that are greater than meters.

(This horse seems beaten to death.)

George H.

Thinking in terms of single photons and nanosecond gate times, the observable effect would be wavelength randomization of individual photons, as observed at the ultimate detector. The details could get interesting.

Even if you

I'm still thinking about measuring single photons, which would involve shuttering at low fluxes. If a shutter does indeed randomize the wavelength of single photons, a number of interesting experiments could be done with EO modulators.

So if my TOF experiment won't work because the shutter randomizes wavelengths, fringes would be restored by observing them through a narrowband optical filter. Which would lead to the verifiable observation that an optical filter randomizes TOF.

John