Are clicks proof of single photons?

[meopemuk]

Just a question. Let's say that I radiate some form of energy into a group of many people. Sometimes, casually, one of those persons feels "excited" because of that energy and jumps for some seconds. Where is the "collapse"?

This is actually a good analogy of the photoelectron emission process as described in Chapter 9 of Mandel & Wolf. The collapse (in my non-standard terminology) happens when a person feels excited for no good reason. If there is no deterministic explanation of this excitation, then we have an unpredictable truly random effect, which I call collapse.

Before the collapse the state of this person could be described by probability, i.e., the chance to be excited is X, the chance to be not excited is 1-X. After the excitation has materialized this probability distribution has collapsed to a certain state.

Eugene.

 

[meopemuk]

Of course, you could call any statistical element ''collapse'', but this is not the standard way of using the term. If you use your personal terminology, the only result is that nobody understand you anymore.

I use the word "collapse" every time when a probability distribution (i.e., incomplete knowledge) is converted to an actual event (complete knowledge).

I wouldn't use the word collapse in the case of coin tossing, because the coin movement is described by classical mechanics, which is capable of predicting the outcome with 100% certainty if the initial state if fully specified. Yes, this is difficult to do in the case of a coin or turbulence or other seemingly untractable "chaotic" classical systems. But "difficult" or "impractical" does not mean "impossible". So, in my understanding, there is no probability associated with classical coin tossing or turbulence. So, there is no collapse.

Quantum mechanical systems are fundamentally different from the tossed coin. When electron passes through a single-slit or a double-slit there is absolutely no way to predict where it will land. This is a truly unpredictable system. Before actual landing on the screen the electron is described by a probability density (square of the wave function) which collapses after the observation is made.

Eugene.

 

[A. Neumaier]

I use the word "collapse" every time when a probability distribution (i.e., incomplete knowledge) is converted to an actual event (complete knowledge).

Compare this with the conventional mainstream meaning:
''wave function collapse (also called collapse of the state vector or reduction of the wave packet) is the phenomenon in which a wave function—initially in a superposition of several different possible eigenstates—appears to reduce to a single one of those states after interaction with an observer. '' http://en.wikipedia.org/wiki/Wavefunction_collapse

I wouldn't use the word collapse in the case of coin tossing, because the coin movement is described by classical mechanics, which is capable of predicting the outcome with 100% certainty if the initial state if fully specified. But "difficult" or "impractical" does not mean "impossible". So, in my understanding, there is no probability associated with classical coin tossing or turbulence. So, there is no collapse.

But your definition that ''a probability distribution (i.e., incomplete knowledge) is converted to an actual event (complete knowledge)'' fully applies in practice. So your definition of collapse seems inconsistent.

 

[lightarrow]

Lightarrow... do we meet again? Are you the man I knew from the Naked Scientists?

Hello QuantumClue!

To answer your question, if it was possible for some quantity of energy to be tranferred, rather than simply radiated into the body of another person, then collapses may occur if there are decoherences in the stucture of the other person. These simple decoherences are collapse-like state systems.

I used that metaphor to express the idea that a quantum description is not needed for that effect.

It's a bit of an odd question, but if you are the man I remember, then it's not a great surprise :)

But quantum physics is odder, isnt'it?

 

[meopemuk]

But your definition that ''a probability distribution (i.e., incomplete knowledge) is converted to an actual event (complete knowledge)'' fully applies in practice. So your definition of collapse seems inconsistent.

I don't think our knowledge is incomplete when we are tossing a coin. Yes, it is incomplete in practice, because we are too lazy to specify all initial conditions exactly and to perform all necessary calculations.

On the other hand, when we are sending a polarized photon through a filter, the result is unpredictable. No matter how careful we are in preparing their state, the photons will behave unpredictably.

This is why coin tossing can be described (in principle, but possibly not in practice) by classical mechanics, and in order to describe photons or electrons we need quantum mechanics.

If you don't want to recognize this difference between classical and quantum mechanics, then you represent the "hidden variables" interpretation camp.

Eugene.

 

[PhilDSP]

So, there can be only two legitimate interpretations of quantum mechanics. One is the "hidden variable" interpretation, which basically says that QM is just a branch of classical mechanics, where everything is deterministic and predictable. No probabilities involved and no collapse. The other interpretation is that quantum events are truly random. Then the collapse is needed. There is no third way.

Isn't that perspective a bit too limiting? One should be allowed the option of approaching every problem with the aim of finding at least one "hidden variable". Sometimes, maybe often, those variables will be so sensitively balanced that chaotic conditions pertain to the result. But in searching and possibly finding such a variable one might ultimately find some other characteristic that isn't deterministic. In other words, we recognize the difference between deterministic-predictive variables, deterministic-chaotic variables producing effectively unpredictable results and fully non-deterministic variables where anyone situation may may be composed of variables of each type.

 

[A. Neumaier]

I don't think our knowledge is incomplete when we are tossing a coin. Yes, it is incomplete in practice, because we are too lazy to specify all initial conditions exactly and to perform all necessary calculations.

No, because it is impossible. We cannot even specify single real numbers to infinite precision (and rounding them to a rational creates already enough uncertainty to make chaos apply after a very short time.

On the other hand, when we are sending a polarized photon through a filter, the result is unpredictable. No matter how careful we are in preparing their state, the photons will behave unpredictably.

This is unpredictable only in a particle picture. In a field picture, polarization is very easy to understand. The qubit was understood classically almost 50 years before Planck discovered the first hint to quantum mechanics - see slides 6-15 of my lecture http://arnold-neumaier.at/ms/optslides.pdf

The only reason why the Schroedinger equation wasn't found by Stokes in 1852 was that there was no incentive to do so...

This is why coin tossing can be described (in principle, but possibly not in practice) by classical mechanics, and in order to describe photons or electrons we need quantum mechanics.

Of course we need quantum mechanics to describe photons and electrons and dice.
A die is a quantum mechanical object - but it behaves approximately classically to such an extent that we hardly ever regard it as a quantum object. But a correct account of its falling behavior would require that.

On the other hand, we don't need a collapse to predict the laws of elasticity and classical motion of a die from a quantum mechanical basis. We only need Ehrenfest's theorem.

 

[harrylin]

[..] If you don't want to recognize this difference between classical and quantum mechanics, then you represent the "hidden variables" interpretation camp.

Eugene.

Why do you want to push people in "camps" that they may not have joined?

 

[Q-reeus]

..You could as well claim that the fact that a shower emits tiny rays of water is proof that water is composed of discrete rays...The corrections obtained by using QED instead of the classical external field are very tiny and can be neglected...
Then, how on Earth can the tiny dots be interpreted as an indication that light is a flow of discrete particles rather than a continuous wave? The external field used to generate the pattern _is_ a continuous wave, as one can trivially verify by inspecting the model...This is enough to make the conclusion invalid that the tiny dots must be regarded as proof of a discrete particle structure of the incident radiation...These discussions revealed to me that real-life photons are something very different from what superficial discussions seemed to suggest.

That was from #14. Still not clear just how the QFT and classical EM pictures differ. In some places you speak of 'more or less localised' photon states, elsewhere that 'the wave spreads out beyond the slit' etc. Unless there are two or more distinctly different models of what constitutes a photon/photon state in QFT, I am assuming localization is simply the result of wave interference as for instance in an antenna array or multi-mode cavity resonator. Wrong? Consider a specific case. An excited atom in vacuo undergoes spontaneous decay into ground state, emitting a field quanta (avoiding the 'p' word). Classically, the emission might be described as a weighted ensemble of multipole fields that propagates as a spherical pulse with superposed multipole angular distributions. At large r the field gets very tenuous but never becomes 'granular'. To what extent is the QFT picture different?

 

[A. Neumaier]

That was from #14. Still not clear just how the QFT and classical EM pictures differ. In some places you speak of 'more or less localised' photon states, elsewhere that 'the wave spreads out beyond the slit' etc. Unless there are two or more distinctly different models of what constitutes a photon/photon state in QFT,

The quantum field is described by a state, defining expectation values of the field operators. By the Ehrenfest theorem http://en.wikipedia.org/wiki/Ehrenfest_theorem , these expectations correspond exactly to the classical e/m field, independent of the photon content of the state. Measurements respond to this field and/or to the associated coherence fields, given by the expectations of certain bilinear expressions in the fields - among them is the energy density of the field. To see the nonclassical character of a quantum field, one needs to make correlation experiments that exhibit the deviating statistical properties.

The notion of photon is commonly used with two different meanings:
1. as a localized wave packet of approximate frequency omega and approximate total (integrated over time) energy omega*hbar, in some cases generated by a single atomic event;
2. as synonymous to a 1-photon state. The latter are in 1-1 correspondence with classical solutions of the Maxwell equations, but they are Fock states with very nonclassical properties.
This explains why the classical and the quantum field descriptions are quite similar, even when talking about single photons.

I am assuming localization is simply the result of wave interference as for instance in an antenna array or multi-mode cavity resonator. Wrong?

Interference creates the pattern. Localization is associated with the fact that a single, localized electron responds (according to a process whose rate is proportional to the intensity pattern).

Consider a specific case. An excited atom in vacuo undergoes spontaneous decay into ground state, emitting a field quanta (avoiding the 'p' word). Classically, the emission might be described as a weighted ensemble of multipole fields that propagates as a spherical pulse with superposed multipole angular distributions. At large r the field gets very tenuous but never becomes 'granular'. To what extent is the QFT picture different?

The field expectation values are precisely what the classical picture suggests.

The quantum nature is reflected by the knowledge that in this particular situation (far from easy to produce experimentally to good accuracy and with high efficiency) the quantum field state is a 1-photon state. This is usually completely inconsequential, but can make a significant difference in special correlation experiments.

 

[Q-reeus]

..The notion of photon is commonly used with two different meanings:
1. as a localized wave packet of approximate frequency omega and approximate total (integrated over time) energy omega*hbar, in some cases generated by a single atomic event;...

And this usage is legitimately part of standard QFT? Does it imply a non-spreading entity that propagates soliton-like to any distance?

2. as synonymous to a 1-photon state. The latter are in 1-1 correspondence with classical solutions of the Maxwell equations, but they are Fock states with very nonclassical properties.
This explains why the classical and the quantum field descriptions are quite similar, even when talking about single photons.

Err... chasing around a bit like at http://en.wikipedia.org/wiki/Nonclassical_light, get the idea Fock state has this undefined phase thing, in contrast to say a coherent state. Otherwise, to say '..1-1 correspondence with classical EM...with very nonclassical properties.' leaves me scratching pate.

At any rate, taking this to mean overall that we have a physical, objectively real and continuous field whose space and time evolution is essentially classical (in most situations), this only reinforces my misgivings about detector clicks for extremely attenuated light.

Let's consider the usual 2-slit setup, but where the detection screen is a wide and very narrow strip, total area being orders of magnitude smaller than say a hemisphere whose radius is that from twin-slit plate to detection strip. This means orders of magnitude smaller cross-section than a single field quanta (as spreading wave) presents to the screen. I share your view there is no possibility of instantaneous physical collapse of such a field quanta - what the screen 'sees' is what the screen 'gets'. OK then - let the light be so attenuated on average only one field quanta passes the slits every minute or so. Previously you have stated the detection screen electrons form a chaotic system with no memory (meaning I assume no ability to either accumulate incident energy, or retain knowledge of the intensity distribution for any reasonable length of time - ie. dissipative system). All the foregoing strongly suggests to me that by the continuous field view there will never be any clicks, or on the rare occasion a statistical fluctuation in number density allows one, there will be no final correlation with the expected interference pattern. None of this poses a problem for the corpuscular model (not necessarily 'point' photons, but at least highly localized wave packet photons). Probability of a click drops simply in direct proportion to the screen area, and the interference pattern is unaffected. And there is a ready QFT counter-argument, or have I completely misinterpreted the system?

 

[meopemuk]

This is unpredictable only in a particle picture. In a field picture, polarization is very easy to understand. The qubit was understood classically almost 50 years before Planck discovered the first hint to quantum mechanics - see slides 6-15 of my lecture http://arnold-neumaier.at/ms/optslides.pdf

The only reason why the Schroedinger equation wasn't found by Stokes in 1852 was that there was no incentive to do so...

To me this looks like a very unusual way of looking at quantum mechanics. Thanks for sharing.

Eugene.

 

[A. Neumaier]

To me this looks like a very unusual way of looking at quantum mechanics.

You can find the conventional way of looking at the same in the first Chapter of Sakurai's book.

It is classical optics made mysterious by pretending it is a particle effect...

 

[A. Neumaier]

And this usage is legitimately part of standard QFT? Does it imply a non-spreading entity that propagates soliton-like to any distance?[,QUOTE]
It is used quite a lot in practice. These photons spread, like any wave packet.

Err... chasing around a bit like at http://en.wikipedia.org/wiki/Nonclassical_light, get the idea Fock state has this undefined phase thing, in contrast to say a coherent state. Otherwise, to say '..1-1 correspondence with classical EM...with very nonclassical properties.' leaves me scratching pate.[,QUOTE]
Yes. The state vectors are in 1-1 correspondence but the normalized states are not.

At any rate, taking this to mean overall that we have a physical, objectively real and continuous field whose space and time evolution is essentially classical (in most situations), this only reinforces my misgivings about detector clicks for extremely attenuated light.

It is classical in the common situations like sunlight or laser light. It takes quantum optics ingenuity to create nonclassical states of light.

I share your view there is no possibility of instantaneous physical collapse of such a field quanta - what the screen 'sees' is what the screen 'gets'. OK then - let the light be so attenuated on average only one field quanta passes the slits every minute or so. Previously you have stated the detection screen electrons form a chaotic system with no memory (meaning I assume no ability to either accumulate incident energy, or retain knowledge of the intensity distribution for any reasonable length of time - ie. dissipative system).

The energy is absorbed collectively, of course, but the electron doesn't know that.

All the foregoing strongly suggests to me that by the continuous field view there will never be any clicks, or on the rare occasion

Rare occasion means one electron per minute, or so.

Probability of a click drops simply in direct proportion to the screen area,

No. The probability drops quadratically with the distance from the screen but grows linearly with the screen area (assuming the detector has constant thickness).

 

[Q-reeus]

It is used quite a lot in practice. These photons spread, like any wave packet.

Thanks for clearing that point up.

The energy is absorbed collectively, of course, but the electron doesn't know that...Rare occasion means one electron per minute, or so.

Not as per my scenario. Recall that the screen area was taken to be several orders of magnitude smaller than needed to fully capture an incident field quanta at some nominal distance from the slits - assuming screen effective cross-section equals it's area. For sake of argument make it a neat factor of 100 (let's not worry about interference fringes for the moment - just take an average screen intensity value). So each of the ~ 1-quanta per minute passing the slits can deposit no more than ~ 1% of their energy to the screen (I think we agree in such a setup the screen can in no way act as a resonant antenna - after all there is no monochromatic stream of radiation). Hence at best the average time between clicks will be around 100 minutes. But that's the real sticking point as I see it. This 'best case scenario' assumes the screen is not only capable of fully absorbing all incident radiation, but losslessly accumulating each hit for perhaps hours until sufficient energy is present to eject one electron. What's more in order to reproduce the interference pattern, a memory of the incident intensity is also dissipationlessly stored. This seems utterly incredible. I would expect very rapid dispersion and dissipation of incident energy to destroy any chance of even one click. This energy accumulation picture is also seemingly at odds with the known rather sharp frequency threshold for photoelectric effect. If arbitrarily small portions of a field quanta can be progressively absorbed and accumulated, shouldn't there be a very gentle dependency on frequency, with no particular cutoff frequency? But being no expert here, will defer to your much greater knowledge in this area. Is there no limit to how long this storage/accumulation/memory 'magic' can persist for?

No. The probability drops quadratically with the distance from the screen but grows linearly with the screen area (assuming the detector has constant thickness).

We actually agreed on this minor point - i was assuming fixed radius, and only screen area as variable.

 

[lightarrow]

Let's consider the usual 2-slit setup, but where the detection screen is a wide and very narrow strip, total area being orders of magnitude smaller than say a hemisphere whose radius is that from twin-slit plate to detection strip. This means orders of magnitude smaller cross-section than a single field quanta (as spreading wave) presents to the screen. I share your view there is no possibility of instantaneous physical collapse of such a field quanta - what the screen 'sees' is what the screen 'gets'. OK then - let the light be so attenuated on average only one field quanta passes the slits every minute or so.

But are you really able to say: "it has been sent a photon, then it passes through the slits, let's wait for a detection event...no, it was not detected", or you can only say that a single photon has been sent because you have a detection event?

 

[A. Neumaier]

Not as per my scenario. Recall that the screen area was taken to be several orders of magnitude smaller than needed to fully capture an incident field quanta at some nominal distance from the slits - assuming screen effective cross-section equals it's area. For sake of argument make it a neat factor of 100 (let's not worry about interference fringes for the moment - just take an average screen intensity value). So each of the ~ 1-quanta per minute passing the slits can deposit no more than ~ 1% of their energy to the screen (I think we agree in such a setup the screen can in no way act as a resonant antenna - after all there is no monochromatic stream of radiation). Hence at best the average time between clicks will be around 100 minutes.

OK, this is not the usual scenario when making double-slit experiments. Then wait 100 minutes. You still have full agreement with experiment. The mean waiting time is inversely proportional to the total energy reaching the screen (assuming 100% absorption by electrons).

But that's the real sticking point as I see it. This 'best case scenario' assumes the screen is not only capable of fully absorbing all incident radiation, but losslessly accumulating each hit for perhaps hours until sufficient energy is present to eject one electron. What's more in order to reproduce the interference pattern, a memory of the incident intensity is also dissipationlessly stored. This seems utterly incredible.

No storage is needed. Just a rate of response. That's the nature of probability - even tiny rates accumulate to an almost sure success over a long enough time.

It is like repeatedly throwing 10 dice together. Getting 10 sixes is utterly improbable, but it happens if you wait long enough on the average a number of 1/p throws where p is the tiny probability. The dice don't store their history; indeed, the success might happen the very first throw if the dice are in the right mood.

 

[Q-reeus]

But are you really able to say: "it has been sent a photon, then it passes through the slits, let's wait for a detection event...no, it was not detected", or you can only say that a single photon has been sent because you have a detection event?

Well the latter one would correspond surely to the photon-as-particle picture - deposits all of it's energy in one go - there is a click or scintillation. What I have been trying to distill is the process involved in the continuous field quanta model of QFT. As instantaneous physical collapse of a spherically expanding quanta wave that could be light-years in radius is eschewed, I see no alternative than an accumulation of possibly tiny fractions of a single field quanta per screen 'hit', until an event finally transpires. My view is there are huge problems with that. However the last posting in #107 seems to suggest no accumulation of energy occurs - it's all just a probabilistic thing. So I'm still not clear on how to view it all!

 

[Q-reeus]

OK, this is not the usual scenario when making double-slit experiments. Then wait 100 minutes. You still have full agreement with experiment. The mean waiting time is inversely proportional to the total energy reaching the screen (assuming 100% absorption by electrons).

At first sight this suggests a time extended energy accumulation scenario, but from your later remarks that's evidently not what you are meaning.

No storage is needed. Just a rate of response. That's the nature of probability - even tiny rates accumulate to an almost sure success over a long enough time.

Well if there is no energy storage between 'hits', the implication seems to be either an actual collapse of all the energy into the screen of what could be a very extended single field quanta, or a purely probabilistic event where energy conservation is just a time-averaged thing. Please clarify.

It is like repeatedly throwing 10 dice together. Getting 10 sixes is utterly improbable, but it happens if you wait long enough on the average a number of 1/p throws where p is the tiny probability. The dice don't store their history; indeed, the success might happen the very first throw if the dice are in the right mood.

No problem with dice throwing, but it's the energetics and statistics of interaction between a screen of small cross-section and very infrequent and tenuous field quanta as continuous wave that I still cannot quite picture. When an electron is ejected, can we say anything definite about the transfer mechanism beyond an appeal to stats?

 

[A. Neumaier]

Well if there is no energy storage between 'hits', the implication seems to be either an actual collapse of all the energy into the screen of what could be a very extended single field quanta, or a purely probabilistic event where energy conservation is just a time-averaged thing. Please clarify.

QM claims energy conservation only in the ensemble average, which is usually equated (with practical success but no fully convincing demonstration) with the time average.

I don't know how precisely the energy balance can be tested experimentally - it is very difficult to account for efficiency losses. I believe (without having checked that) that the energy fluctuations in a typical solid state device at room temperature is far bigger than the uncertainty in the energy due to a fraction of a photon's energy.

It would perhaps be interesting to know what happens with the experimental photodetection statistics at very low temperature. Maybe Cthugha knows of work in this direction?

 

[Q-reeus]

QM claims energy conservation only in the ensemble average, which is usually equated (with practical success but no fully convincing demonstration) with the time average.

I don't know how precisely the energy balance can be tested experimentally - it is very difficult to account for efficiency losses. I believe (without having checked that) that the energy fluctuations in a typical solid state device at room temperature is far bigger than the uncertainty in the energy due to a fraction of a photon's energy.

It would perhaps be interesting to know what happens with the experimental photodetection statistics at very low temperature. Maybe Cthugha knows of work in this direction?

Many thanks for clarifying on that - I had been arguing the wrong model all along, but has taken till now to realize that! I imagine this viewpoint will generate some contention! Bed time. :zzz:

 

[zonde]

It would perhaps be interesting to know what happens with the experimental photodetection statistics at very low temperature.

Lowering detector temperature leads to lower rate of dark counts i.e. spontaneous clicks but it does not affect rate of incident photon detection.

I would say that for coincidence counting cases particle approach to photons is way more natural than wave approach.
Say when you increase detection efficiency coincidence rate increases too and it tends toward 100% coincidence rate as you extrapolate detection efficiency towards 100% efficient.
You would not expect such tendency for waves.

 

[A. Neumaier]

Lowering detector temperature leads to lower rate of dark counts i.e. spontaneous clicks but it does not affect rate of incident photon detection.

I would say that for coincidence counting cases particle approach to photons is way more natural than wave approach.
Say when you increase detection efficiency coincidence rate increases too and it tends toward 100% coincidence rate as you extrapolate detection efficiency towards 100% efficient.
You would not expect such tendency for waves.

You would not expect such tendency for classical waves. Indeed, coincidence counts reveal the limitations of the semiclassical picture, but only when fed with nonclassical light. In this case, one needs the corrections from quantum field theory. This together with coherence accounts for nonclassical, nonlocal correlations.

Coincidence counting in case of Bell-type experiment probing the quantum nature on a fundamental level is unnatural in _any_ semiclassical picture, whether in terms of particles or in terms of waves. But interference experiments (which don't need special quantum technology for their performance) remain unnatural for particles, while they are completely natural for light. This let's me prefer the field intuition over the particle intuition (which fails for photons anyway since they are not localizable).

 

[zonde]

Coincidence counting in case of Bell-type experiment probing the quantum nature on a fundamental level is unnatural in _any_ semiclassical picture, whether in terms of particles or in terms of waves.

I do not exactly agree with this.
Photon Bell-type experiments can be analyzed using semiclassical picture in terms of particles.
You have to drop fair sampling assumption in turn making Bell theorem not applicable to photon Bell-type experiments.

But interference experiments (which don't need special quantum technology for their performance) remain unnatural for particles, while they are completely natural for light. This let's me prefer the field intuition over the particle intuition (which fails for photons anyway since they are not localizable).

Resolving photon Bell-type experiments in semiclassical particle picture provides approach for interference experiments too.
It might be less natural than wave approach but that should be Ok if testable predictions specific (or more natural) to particle approach are made.

And I prefer semiclassical particle approach because it conserves energy at the level of single photon.

 

[A. Neumaier]

I do not exactly agree with this.
Photon Bell-type experiments can be analyzed using semiclassical picture in terms of particles.

Of course. I only claimed that it is not natural to think of particles - without any properties between creation and detections.

Resolving photon Bell-type experiments in semiclassical particle picture provides approach for interference experiments too.
It might be less natural than wave approach but that should be Ok if testable predictions specific (or more natural) to particle approach are made.

And I prefer semiclassical particle approach because it conserves energy at the level of single photon.

At the cost of not being able to say anything about the field between the source and the target. In view of the fact that as the number of photons get larger, suddenly an approximate classical picture of the field appears out of nowhere, this seems to me very strange.

I prefer to have a reasonably realistic view of what happens at every point in space and time, which turns naturally into the classical picture as the intensity get large. This is possible with a field picture but not with a particle picture. It removes most of the weirdness of quantum mechanics with one single stroke.

 

[zonde]

Of course. I only claimed that it is not natural to think of particles - without any properties between creation and detections.

At the cost of not being able to say anything about the field between the source and the target. In view of the fact that as the number of photons get larger, suddenly an approximate classical picture of the field appears out of nowhere, this seems to me very strange.

I prefer to have a reasonably realistic view of what happens at every point in space and time, which turns naturally into the classical picture as the intensity get large. This is possible with a field picture but not with a particle picture. It removes most of the weirdness of quantum mechanics with one single stroke.

Hmm, no.
When I talk about semiclassical picture I mean that we have trajectories at macro level.
And we have more or less sharp volume that defines photon at micro level, but not point particles.

Maybe I should have said classical picture but that people tend to associate with some billiard ball like model.

 

[A. Neumaier]

Hmm, no.
When I talk about semiclassical picture I mean that we have trajectories at macro level.
And we have more or less sharp volume that defines photon at micro level, but not point particles.

How then does such a photon go through a double slit and create the interference pattern? Could you please describe the trajectories and explain why the resulting patterns are different with one slit closed and with both slits open?

And what happens when such a photon passes a beam splitter?

 

[zonde]

How then does such a photon go through a double slit and create the interference pattern? Could you please describe the trajectories and explain why the resulting patterns are different with one slit closed and with both slits open?

Photons have straight trajectories except at slit. But when photons reach detector (or filter if you have such present) they interfere with state of measurement device (or filter). That state is created by photon ensemble (previous photons) including those photons coming from other slit. Result of that interference in detector is that photon is either absorbed as thermal energy or produce avalanche.
In case of destructive interference on average more photons are absorbed as thermal energy but in case of constructive interference on average more photons produce avalanche.
When one slit is closed state of measurement device is different because photons coming from other slit are absent.

And what happens when such a photon passes a beam splitter?

Nothing. It either goes one way or another with equal probabilities for 50/50 beam splitter (with change in phase by half of wavelength when it undergoes front side reflection).
It's a bit more difficult with polarization beam splitter. Say when photon with polarization angle 30° encounters polarization beam splitter with outputs of 0° and 90° polarizations it undergoes -30° phase shift if it appears in 0° output but 60° phase shift if it appears in 90° output.

 

[A. Neumaier]

Photons have straight trajectories except at slit.

So which trajectory does the photon have after having passed the slit? Straight in a random direction?

But when photons reach detector (or filter if you have such present) they interfere with state of measurement device (or filter). That state is created by photon ensemble (previous photons) including those photons coming from other slit.

This is not enough to guarantee that the first few photons will not place themselves at a position of destructive interference. In a particle picture, the detector can know the inference pattern at best when enough photons had already arrived to display it.

Result of that interference in detector is that photon is either absorbed as thermal energy or produce avalanche.
In case of destructive interference on average more photons are absorbed as thermal energy but in case of constructive interference on average more photons produce avalanche.

When one slit is closed state of measurement device is different because photons coming from other slit are absent.[/QUOTE]
How could the detector know about the difference?

Nothing. It either goes one way or another with equal probabilities for 50/50 beam splitter

This is inconsistent if you apply different polarization filters to the two resulting beams and then recombine the beams.

 

[zonde]

So which trajectory does the photon have after having passed the slit? Straight in a random direction?

Straight according to probabilities of single slit diffraction.

This is not enough to guarantee that the first few photons will not place themselves at a position of destructive interference. In a particle picture, the detector can know the inference pattern at best when enough photons had already arrived to display it.

Yes. Detection probability for first photon arriving at detector will not show any signs of interference.

How could the detector know about the difference?

Detectors "remembers" phase of arriving photons. Or in other words it undergoes oscillations that affect detection probabilities of photons that arrive later.

This is inconsistent if you apply different polarization filters to the two resulting beams and then recombine the beams.

Would you care to explain this in more details.