Double slit with time measurement

[Cthugha]

According Wikipedia lasers can have coherence lengths of more the 100 m, so that cannot be a principle problem.

What do you mean by that? The coherence length of the initial laser is simply not important for the spectral width of the PDC. It just determines the width of the sum energy of the two photons. It is not really related to how asymmetric the distribution is, which gives you the spectral width of the PDC.

If you would have an ideal PDC, what would be the minimal spectral width caused by the uncertainty relation? Is that Planck constant compared to the photons energy?

What do you mean by ideal? Usually you do not want too narrow spectral width as that kills entanglement somewhat. If you want long (first order) coherence times you can always filter your light spectrally. However, that of course lowers the information you get from coincidence counting. The relative time delay between the photons in the photon pair is just known to be smaller than the respective coherence times.

 

[DParlevliet]

What do you mean by ideal? Usually you do not want too narrow spectral width as that kills entanglement somewhat. If you want long (first order) coherence times you can always filter your light spectrally. However, that of course lowers the information you get from coincidence counting. The relative time delay between the photons in the photon pair is just known to be smaller than the respective coherence times.

With ideal I mean that the PDC output photons are each exactly 50% (of the laser). Then the spectral width of each beam (signal en idler) will be the spectral width of the laser plus the quantum uncertainty of generating photons. So what adds the quantum uncertainty to the spectral width?

 

[Cthugha]

With ideal I mean that the PDC output photons are each exactly 50% (of the laser). Then the spectral width of each beam (signal en idler) will be the spectral width of the laser plus the quantum uncertainty of generating photons. So what adds the quantum uncertainty to the spectral width?

There is no intrinsic mechanism that would favor a 50/50 splitting over any other splitting ratio. The way a crystal is cut determines the energy ratios which allow for phase matching. The better the phase is matched, the higher the probability that a certain splitting ratio is realized. If you just want narrow spectral lines, just place narrow spectral filters in both beam paths and you are done.

 

[ZapperZ]

This thread has been going around in enough number of circles to create several crop circles on many fields.

I don't know whether this is too obvious, but why can't you just go out and DO the darn experiment already? The equipment being described in here have been shown to be available even for undergraduate advanced labs. So it isn't that difficult to find! You could have saved pages and pages of argument simply by showing the experimental results!

Zz.

 

[ZapperZ]

Speaking of experiment, why haven't people look at the recent paper by Rueckner and Piedle in AJP? (AJP v.81, p.951 (2013))? It appears that they set up their experiment in a similar fashion (double slit, which-way, and quantum eraser).

Zz.

 

[Cthugha]

Speaking of experiment, why haven't people look at the recent paper by Rueckner and Piedle in AJP? (AJP v.81, p.951 (2013))? It appears that they set up their experiment in a similar fashion (double slit, which-way, and quantum eraser).

Because it is a somewhat misleading paper. It is not as bad as other papers in the field as it explicitly explains the shortcomings in section IV.a:
"In our previous paper we pointed out that, strictly speaking, we are not detecting single photons of light but rather single photoelectrons liberated by the light impinging on the detector; this is still true in the present experiment. Furthermore, the detection of a photoelectron does not necessarily imply that a single photon arrived."

I am aware that AJP is not your average journal and aimed at physics teachers and has an educational purpose. Nevertheless, I think it is of highest importance to use clear language from the very beginning in order to avoid students having to forget and relearn things. The term "single photon" is well defined and reserved for light sources showing antibunching (g2(0)=0). This paper operates in the very low intensity regime, but not with single photons, so it is a misnomer. I am aware that this is often done in order to simplify things and because the properties of real single photon sources are a nightmare you do not want to operate in an educational lab, but I strongly object to simplifying physics to the point where wrong claims are made. A paper using that kind of loose terminology would not make it to a research-oriented journal like PRL or PRA and rightfully so.

 

[ZapperZ]

Because it is a somewhat misleading paper. It is not as bad as other papers in the field as it explicitly explains the shortcomings in section IV.a:
"In our previous paper we pointed out that, strictly speaking, we are not detecting single photons of light but rather single photoelectrons liberated by the light impinging on the detector; this is still true in the present experiment. Furthermore, the detection of a photoelectron does not necessarily imply that a single photon arrived."

I am aware that AJP is not your average journal and aimed at physics teachers and has an educational purpose. Nevertheless, I think it is of highest importance to use clear language from the very beginning in order to avoid students having to forget and relearn things. The term "single photon" is well defined and reserved for light sources showing antibunching (g2(0)=0). This paper operates in the very low intensity regime, but not with single photons, so it is a misnomer. I am aware that this is often done in order to simplify things and because the properties of real single photon sources are a nightmare you do not want to operate in an educational lab, but I strongly object to simplifying physics to the point where wrong claims are made. A paper using that kind of loose terminology would not make it to a research-oriented journal like PRL or PRA and rightfully so.

But this is why I pointed it out. Notice that my earlier comment on why people just don't try out the experiment mentioned that the type of equipment one needs to make a quick test of this can be available at the undergraduate-level lab! That is what this paper was aimed for, because the equipment being used are meant for such a lab. So this experiment is "simpler" than a full-blown research-front equipment that someone like Zeilinger would use!

I don't think the shortcoming with the "single-photon source" will be an issue here because I think it will not be a significant factor in affecting the outcome of the result.

Zz.

 

[Cthugha]

But this is why I pointed it out. Notice that my earlier comment on why people just don't try out the experiment mentioned that the type of equipment one needs to make a quick test of this can be available at the undergraduate-level lab! That is what this paper was aimed for, because the equipment being used are meant for such a lab. So this experiment is "simpler" than a full-blown research-front equipment that someone like Zeilinger would use!

Yes, without any doubt the experiment is most suitable for that kind of lab.

I don't think the shortcoming with the "single-photon source" will be an issue here because I think it will not be a significant factor in affecting the outcome of the result.

That depends a bit on the point one tries to make. When going from many photons/classical waves to single photons, there are two main questions students may ask:
1) Are there any non-linearities involved? The common interference pattern you see is a single photon interference pattern. The experiment in question shows very well that the instantaneous intensity present in the setup does not matter. This is perfectly in line with more complicated papers like Science Vol. 329 no. 5990 pp. 418-421 (2010) which show that Born's rule is perfectly valid and there are no higher-order interference effects to be found.

2) A coherent state is the closest thing to a classical state one can get in qm. An attenuated coherent state still is pretty close and classical. Fock states are intrinsically quantum and closer to what the layman would consider a particle. While it is true that this is not a significant factor at all in first-order interference measurements, I can at least understand people who do not want to accept this equivalence bona fide. While "learn QED" would be an appropriate answer to those, it is not a very satisfying one. The paper above does not help with this question. There are papers following that question (like http://arxiv.org/abs/1304.4943), but I am not sure whether it has been published yet, so I hesitate to cite it in discussions.

 

[DParlevliet]

So that was another circle. I cannot reach the article so cannot comment.
I have no labatory at all and Cthugha explained that the measurement is not as easy as one might think.

There is no intrinsic mechanism that would favor a 50/50 splitting over any other splitting ratio. The way a crystal is cut determines the energy ratios which allow for phase matching. The better the phase is matched, the higher the probability that a certain splitting ratio is realized. If you just want narrow spectral lines, just place narrow spectral filters in both beam paths and you are done.

Alright, so long coherence lentghs are possible, anyway in theory. What is now the remaining argument agains measuring the path and having an inteference pattern at the same time?

 

[Cthugha]

So that was another circle. I cannot reach the article so cannot comment.
I have no labatory at all and Cthugha explained that the measurement is not as easy as one might think.

Still the basic point that was raised stands. It would be very interesting to know what exactly you are aiming at. This discussion indeed is pretty stiff.

Alright, so long coherence lentghs are possible, anyway in theory. What is now the remaining argument agains measuring the path and having an inteference pattern at the same time?

In a nutshell: Long coherence times mean that the photons in each arm are pretty distributed over a long time range and the relative time delay between the two photons also is less well defined. By the time you get this time scale long enough to see interference/get overlap, the relative time delay gets so undefined that you cannot tell which path the photon took anymore. This is really just basic optical coherence theory and a bit of knowledge about Fourier transform. An introductory book on these topics might be very beneficial at this point.

 

[DrChinese]

What is now the remaining argument agains measuring the path and having an inteference pattern at the same time?

The argument against is: It won't work! When are you going to acknowledge the obvious? You can't have which-path AND interference. You are the only person asserting this, no experiment has ever shown the slightest evidence otherwise. It is not a part of theory either.

It is only your misreading of material that leads us here. At this point, the question is asked and answered.

 

[DParlevliet]

After seeing other experiments, a new version:

The source is a <5 ns pulsed laser with narrow bandwidth (coherence length >20 m). A 50% mirror reflects half of the photons to a detector, which starts a timer. Then a strong absorber degrades the beam to a 10% change of 1 photon per laser pulse. This photon enters the interferometer. The output detector stops the timer. So by measuring the time one knows which path the photon (particle property) used. On the other hand the waves are not disturbed, so there should also be interference at detector 2.
Right?

 

[Cthugha]

The source is a <5 ns pulsed laser with narrow bandwidth (coherence length >20 m).

This gives you a coherence time longer than the pulse duration (roughly 70 ns), which is pointless.

A 50% mirror reflects half of the photons to a detector, which starts a timer. Then a strong absorber degrades the beam to a 10% change of 1 photon per laser pulse. This photon enters the interferometer. The output detector stops the timer. So by measuring the time one knows which path the photon (particle property) used. On the other hand the waves are not disturbed, so there should also be interference at detector 2.
Right?

The statistical spread in detections will always be of the same order as the coherence time. If you have a real pulse corresponding to the coherence time, it will have a duration of 70 ns and you will not get any which-way information, but interference. If you just have a 5 ns pulse, the coherence time is of course limited by the pulse duration and you will not get any overlap and no interference.

And let me emphasize that again:

The argument against is: It won't work! When are you going to acknowledge the obvious? You can't have which-path AND interference. You are the only person asserting this, no experiment has ever shown the slightest evidence otherwise. It is not a part of theory either.

It is only your misreading of material that leads us here. At this point, the question is asked and answered.

 

[DParlevliet]

Is that a modern scientific (forum) attitude, irritation about challenges to find errors in new proposals? Simple saying "It won't work"? If you not interested, don't answer. Otherwise be serious.

So in your opinion a single photon has a coherence time of 5 ns? There is only a wave within this 5 ns, and no wave outside this time?

 

[Cthugha]

Is that a modern scientific (forum) attitude, irritation about challenges to find errors in new proposals?

In these forums it is also not acceptable to post 25 variations of perpetuum mobiles in order to ask why they do not work. It is the same thing here. You are introducing microscopically different versions of the same experiment all showing the same flaw.

If you want to know in detail what is acceptable here, have a look at the rules of this forum:
https://www.physicsforums.com/showthread.php?t=414380

So in your opinion a single photon has a coherence time of 5 ns? There is only a wave within this 5 ns, and no wave outside this time?

No. This is not what I wrote. I wrote that applying the concept to regions without any intensity is completely meaningless. The contrast in an interference pattern is the quantity of interest. You can easily take the simple formula for interference in a double slit or Mach-Zehnder interferometer and calculate the interference pattern you get when you have one arm or even two arms with no intensity at all. Also, as has already been pointed out, your understanding of "wave" and "particle" as two different entities is outdated by at least 50 years.

 

[DParlevliet]

I follow the Copenhagen interpretation and wave-particle duality, because according Wikipedia this is still most widely accepted. Probably you have another opinion. So the crucial question is: in according Copenhagen terms how does the wave look like in this experiment. There is much discussion how the wave can collapse in the whole universe when the photon is absorbed, so my logical conclusion is that the wave is infinite long, so at least very long coherence length (with itself).

 

[Cthugha]

I follow the Copenhagen interpretation and wave-particle duality, because according Wikipedia this is still most widely accepted. Probably you have another opinion. So the crucial question is: in according Copenhagen terms how does the wave look like in this experiment.

The result is the same in any interpretation.

There is much discussion how the wave can collapse in the whole universe when the photon is absorbed, so my logical conclusion is that the wave is infinite long, so at least very long coherence length (with itself).

This entirely depends on the emission process. If you can keep the emitter and the light field in superposition for a long time (for example by shielding it perfectly from the environment or placing it in a resonator) you can get long coherence lengths. If the emission process is localized in time, so is the photon and you get a short coherence length. Collapsing the wave function in the whole universe is kind of a pop sci simplification. If the wave has no amplitude at some position (for example in a region causally disconnected from the emitter), collapse does not really change anything there.
However, that is a whole subfield of physics on ebetter learns as a whole. Mark Fox' book on quantum optics is a good low-level way to get into the topic.

 

[DParlevliet]

So how does the wave of the photon look like in this experiment?

 

[Cthugha]

The envelope of the pulse follows your target temporal shape. The way you have drawn it, it looks like a rectangular pulse. The coherence time is given by the spectral density of your light field (the spectrum). Note that there is some minimum spectral width you get by the Fourier transform of your temporal pulse shape. You cannot get a narrower width. Coherence time is more or less the time scale over which you have a fixed phase relation. It is, however, also limited by the pulse duration. At times where you have no intensity (outside of your rectangle), you will just have the vacuum state. I am not sure, how much quantum optics you know. If you know what a Wigner function is, you can have a look at the Wigner function of the vacuum state and will find that its phase is undefined, but I suppose that is a bit too advanced. However, as a rule of thumb one can say that if you have a lot of photons at some instant in time, you can get a narrower phase distribution and thus a phase that is better defined.

However, that is a topic that is too difficult to learn at some physics forum, even a good one. Learning that thoroughly needs a good book.

 

[DParlevliet]

The envelope of the pulse follows your target temporal shape.

So that is what I mentioned before: during 5 ns a wave, outside 5 ns no wave, more or less. And how long would this envelope be for photons generated by heat or fluorescence? Brian Green in The fabric of the Cosmos: "according quantum theory the propability wave spreads out over the whole universe". What is the source of these photons?

 

[Cthugha]

And how long would this envelope be for photons generated by heat or fluorescence? Brian Green in The fabric of the Cosmos: "according quantum theory the propability wave spreads out over the whole universe". What is the source of these photons?

You need to distinguish between the envelope and the microscopic picture. The envelope roughly follows the mean values in the ensemble average or averaged over many equivalent time windows. However, microscopically within each repetition the intensity and phase distributions may be noisier. The typical time scale of this noise is the coherence time. So for fluorescence or thermal emission you get a more or less constant envelope, but the coherence time is very short. Typical time scales are nanoseconds for atoms, picoseconds for semiconductors, femtoseconds for light from the sun.

That pop-sci Brian Greene stuff is not really the appropriate way to learn real physics. This is rather like the famous point-like cow in vacuum. There is no light source which emits single photons as the ones Green talks about. The closest thing to what he says that can happen is that you get a single atom emitting a single photon. Detecting the photon somewhere collapses the wave function (if you follow an interpretation with collapse - and the concept of wave functions for photons is pretty questionable, too) over the whole sphere.

 

[The Dog Star]

I follow the Copenhagen interpretation and wave-particle duality, because according Wikipedia this is still most widely accepted. Probably you have another opinion. So the crucial question is: in according Copenhagen terms how does the wave look like in this experiment. There is much discussion how the wave can collapse in the whole universe when the photon is absorbed, so my logical conclusion is that the wave is infinite long, so at least very long coherence length (with itself).

Since you mention the Copenhagen Interpretation it's probably a good idea to make sure we are on the same page, asking what the wave looks like seems to fundamentally jar against the Copenhagen Interpretations "dogma" or ethos if you will.

It is impossible to know what the wave looks like because that is unknown it is possible to observe the results however which are a distribution of dots on a screen or a single incidence on a detector at a time related to distance; the wave function itself importantly is a complementary analogue of the quantum picture expressed in probabilistic terms, it is not however a pictorial representation in that if we drew the outputs of the maths we would get the image of the wave length if we extended the time line backwards from the results to the origin.

Since i/hbar (probability distribution resulting from squaring the eigenvalues) is introduced in the equation the wavefunction is only a figurative description of the energy concerns in the beam of whatever it happens to be, there is no classical exact description that maps 1 to 1 as far as can be known, the results simply exhaust the physical probabilities with a limit of infinity (renormalisation) to indicate a realistic freedom of movement rather than an unrealistic one where the "object" in question can instantaneously appear at the other end of the universe.

the closest analogue to what actually the wave function looks like is a mathematical expression hence, but in no way does it pictorially represent the exact appearance of the wave before it is measured.

eg the Dirac equation:

In essence that is paraphrasing what Bohr himself envisioned Copenhagen to mean.

Since any energy concern has infinite extent in essence the wave is infinite, but it is probably only of use to think of infinite extent in field theories.If you want to understand the philosophical nuts and bolts of Copenhagen or at least how Bohr envisioned the concepts, I recommend this source.

5. Misunderstandings of complementarity

Complementarity has been commonly misunderstood in several ways, some of which shall be outlined in this section. First of all, earlier generations of philosophers and scientists have often accused Bohr's interpretation of being positivistic or subjectivistic. Today philosophers have almost reached a consensus that it is neither. There are, as many have noticed, both typically realist as well as antirealist elements involved in it, and it has affinities with Kant or neo-Kantianism. The influence of Kant or Kantian thinking on Bohr's philosophy seems to have several sources. Some have pointed to the tradition from Hermann von Helmholtz (Chevalley 1991, 1994; Brock 2003); others have considered the Danish philosopher Harald Høffding to be the missing link to Kantianism (Faye 1991).

But because Bohr's view on complementarity has wrongly been associated with positivism and subjectivism, much confusion still seems to stick to the Copenhagen interpretation. Don Howard (2004) argues, however, that what is commonly known as the Copenhagen interpretation of quantum mechanics, regarded as representing a unitary Copenhagen point of view, differs significantly from Bohr's complementarity interpretation. He holds that "the Copenhagen interpretation is an invention of the mid-1950s, for which Heisenberg is chiefly responsible, [and that] various other physicists and philosophers, including Bohm, Feyerabend, Hanson, and Popper, hav[e] further promoted the invention in the service of their own philosophical agendas." (p. 669)

Most recently, Mara Beller (1999) argued that Bohr's statements are intelligible only if we presume that he was a radical operationalist or a simple-minded positivist. In fact, complementarity was established as the orthodox interpretation of quantum mechanics in the 1930s and therefore often regarded as a consequence of a positivistic outlook. During the 1930s Bohr was also in touch with some of the leading neopositivists or logical empiricists such as Otto Neurath, Philip Frank, and the Danish philosopher Jørgen Jørgensen. Although their anti-metaphysical approach to science may have had some influence on Bohr (especially around 1935 during his final discussion with Einstein about the completeness of quantum mechanics), one must recall that Bohr always saw complementarity as a necessary response to the indeterministic description of quantum mechanics due to the quantum of action. The quantum of action was an empirical discovery, not a consequence of a certain epistemological theory, and Bohr thought that indeterminism was the price to pay to avoid paradoxes. Never did Bohr appeal to a verificationist theory of meaning; nor did he claim classical concepts to be operationally defined. But it cannot be denied that some of the logical empiricists rightly or wrongly found support for their own philosophy in Bohr's interpretation and that Bohr sometimes confirmed them in their impressions (Faye 2008).

Second, many physicists and philosophers see the reduction of the wave function as an important part of the Copenhagen interpretation. But Bohr never talked about the collapse of the wave packet. Nor did it make sense for him to do so because this would mean that one must understand the wave function as referring to something physically real. Bohr spoke of the mathematical formalism of quantum mechanics, including the state vector or the wave function, as a symbolic representation. Bohr associated the use of a pictorial representation with what can be visualized in space and time. Quantum systems are not vizualizable because their states cannot be tracked down in space and time as classical systems'. The reason is, according to Bohr, that a quantum system has no definite kinematical or dynamical state prior to any measurement. Also the fact that the mathematical formulation of quantum states consists of imaginary numbers tells us that the state vector is not susceptible to a pictorial interpretation (CC, p. 144). Thus, the state vector is symbolic. Here “symbolic” means that the state vector's representational function should not be taken literally but be considered a tool for the calculation of probabilities of observables.

Third, Bohr flatly denied the ontological thesis that the subject has any direct impact on the outcome of a measurement. Hence, when he occasionally mentioned the subjective character of quantum phenomena and the difficulties of distinguishing the object from the subject in quantum mechanics, he did not think of it as a problem confined to the observation of atoms alone. For instance, he stated that already "the theory of relativity reminds us of the subjective character of all physical phenomena" (ATDN, p. 116). Rather, by referring to the subjective character of quantum phenomena he was expressing the epistemological thesis that all observations in physics are in fact context-dependent. There exists, according to Bohr, no view from nowhere in virtue of which quantum objects can be described.

Fourth, although Bohr had spoken about "disturbing the phenomena by observation," in some of his earliest papers on complementarity, he never had in mind the observer-induced collapse of the wave packet. Later he always talked about the interaction between the object and the measurement apparatus which was taken to be completely objective. Thus, Schrödinger's Cat did not pose any riddle to Bohr. The cat would be dead or alive long before we open the box to find out. What Bohr claimed was, however, that the state of the object and the state of the instrument are dynamically inseparable during the interaction. Moreover, the atomic object does not posses any state separate from the one it manifests at the end of the interaction because the measuring instrument establishes the necessary conditions under which it makes sense to use the state concept.

It was the same analysis that Bohr applied in answering the challenge of the EPR-paper. Bohr's reply was that we cannot separate the dynamical and kinematical properties of a joint system of two particles until we actually have made a measurement and thereby set the experimental conditions for the ascription of a certain state value (CC, p. 80). Bohr's way of addressing the puzzle was to point out that individual states of a pair of coupled particles cannot be considered in isolation, in the same way as the state of the object and the state of the instrument are dynamically inseparable during measurements. Thus, based on our knowledge of a particular state value of the auxiliary body A, being an atomic object or an instrument, we may then infer the state value of the object B with which A once interacted (Faye 1991, pp. 182-183). It therefore makes sense when Howard (2004, p.671) holds that Bohr considered the post-measurement joint state of the object and the measuring apparatus to be entangled as in any other quantum interaction involving an entangled pair.

Finally, when Bohr insisted on the use of classical concepts for understanding quantum phenomena, he did not believe, as it is sometimes suggested, that macroscopic objects or the measuring apparatus always have to be described in terms of the dynamical laws of classical physics. The use of the classical concepts is necessary, according to Bohr, because by these we have learned to communicate to others about our physical experience. The classical concepts are merely a refinement of everyday concepts of position and action in space and time. However, the use of the classical concepts is not the same in quantum mecahnics as in classical physics. Bohr was well aware of the fact that, on pains of inconsistency, the classical concepts must be given “a suitable quantum-theoretical re-interpretation,” before they could be employed to describe quantum phenomena (ATDN, p. 8).

A small excerpt^

http://plato.stanford.edu/entries/qm-copenhagen/

I have to further add that I suspect from reading the rules if Brian Greene was a member of this forum he would not be for very long. :rofl:

He seems to favour the many worlds interpretation which although philosophical is hardly a scientific matter. Likewise his ideas of deterministic probability should be taken with a pinch of salt too.

His maths is of course theoretical when applied to experiment, if applied to philosophical interpretations such as Copenhagen vs Many Worlds then a large pinch of salt needs to be added before you disappear too far down the rabbit hole.

 

[DParlevliet]

You need to distinguish between the envelope and the microscopic picture. The envelope roughly follows the mean values in the ensemble average or averaged over many equivalent time windows. However, microscopically within each repetition the intensity and phase distributions may be noisier. The typical time scale of this nopise is the coherence time. So for fluorescence or thermal emission you get a more or less constant envelope, but the coherence time is very short. Typical time scales are nanoseconds for atoms, picoseconds for semiconductors, femtoseconds for light from the sun.

The femtoseconds of sunlight is in the order of the wave lentgh. So a double slit with (filtered) sunlight will show only a small interference pattern? And Wheeler's astronomical experiment is impossible because you never get photons within a femtosecond coherence time?

Since you mention the Copenhagen Interpretation it's probably a good idea to make sure we are on the same page.

According Copenhagen when you measure with classical wave equipment, you will see wave behaviour. That is what I am using, nothing more.

 

[Cthugha]

The femtoseconds of sunlight is in the order of the wave lentgh. So a double slit with (filtered) sunlight will show only a small interference pattern? And Wheeler's astronomical experiment is impossible because you never get photons within a femtosecond coherence time?

Huh? That does not make any sense at all. A double slit measures spatial coherence and does not really care about coherence time (at least unless you have a point source placed off-center). You need a Michelson or Mach-Zehnder interferometer for measurements that are sensitive to coherence time.

According Copenhagen when you measure with classical wave equipment, you will see wave behaviour. That is what I am using, nothing more.

What is "classical wave equipment" supposed to be?

 

[DParlevliet]

Huh? That does not make any sense at all. A double slit measures spatial coherence and does not really care about coherence time (at least unless you have a point source placed off-center). You need a Michelson or Mach-Zehnder interferometer for measurements that are sensitive to coherence time.

The experiment above is about the same as a double slit. It uses path length (so time) difference to show an interference pattern. Your argument is that if the path (so time) difference is too long, both wave(packets) does not arrive at the detector at the same time, so does not interfere. In Wheelers experiment the wave(packets) follow each a different path around a galaxy or black hole. It is very unlikely that both waves arrives at Earth within their femtosecond coherence time

[/QUOTE] What is "classical wave equipment" supposed to be?[/QUOTE] A spatial detector which can show waves.

 

[Cthugha]

The experiment above is about the same as a double slit.

No, it is clearly not. A double slit measures spatial coherence. A Mach-Zehnder interferometer measures temporal coherence. These are two different quantities which are not "about the same".

A spatial detector which can show waves.

And what is that supposed to be? I do not think this thread is going anywhere and you do not seem to be willing to understand the things in detail or read up on them, so I am afraid this is getting pointless.

 

[DParlevliet]

No, it is clearly not. A double slit measures spatial coherence. A Mach-Zehnder interferometer measures temporal coherence. These are two different quantities which are not "about the same".

If a single photon from the sun goes throught a double slit, then the coherence time of each wave, going throught each slit, is in the femtosecond range?

 

[Cthugha]

It does not matter what you do to it. Unless you filter the light spectrally, you always get a coherence time of maybe 100 femtoseconds or something like that for light from the sun.

Whether you will see an interference pattern or not still depends on spatial coherence, though. So for example it matters strongly, whether you place a pinhole to filter the light.

 

[Bone234]

It does not matter what you do to it. Unless you filter the light spectrally, you always get a coherence time of maybe 100 femtoseconds or something like that for light from the sun.

Whether you will see an interference pattern or not still depends on spatial coherence, though. So for example it matters strongly, whether you place a pinhole to filter the light.

I would give up, the fact you haven't though strongly indicates your patience, so kudos.

 

[Bone234]

If a single photon from the sun goes throught a double slit, then the coherence time of each wave, going throught each slit, is in the femtosecond range?

It's in the Planck range actually but meh...

 

[DParlevliet]

It does not matter what you do to it. Unless you filter the light spectrally, you always get a coherence time of maybe 100 femtoseconds or something like that for light from the sun.

Of course a spectral filter is needed because one needs monochromatic light. It must be as much as possible be comparable with single photons, so not broad bandwith. Does that matter much?

 

[Cthugha]

Of course a spectral filter is needed because one needs monochromatic light. It must be as much as possible be comparable with single photons, so not broad bandwith. Does that matter much?

Eh? Single photons are Fock states with a fixed photon number of 1. That is all there is. They do not have to be monochromatic. They often are far from that in reality. Sorry, but Bone234 seems to be right. This discussion seems entirely pointless.

 

[Dale]

closed for moderation