Due to my lack of time to maintain this weblog, it will only be accessible as an archive until further notice. In the meantime, I can be reached through my e-mail afshar[at]rowan.edu.
Thank you for your cooperation and understanding.
Shahriar S. Afshar
Hi Ian - it appears to me that this forum has become dormant - Prof. Afshar is not responding to questions, and Quantum Mirror is no longer here (?). Let me try to answer your questions - I am also an amateur “non-formal” QM aficionado:
1) It would be helpful if you reproduce Fenyman’s statement on page 81.
2) I don’t believe Prof. Afshar has carried out his experiment with matter waves. The outcome should not be different.
3) I wholeheartedly agree with your assessment. A meme had befallen the physics community and some mystical followers for a few decades that, based on the Schroedinger Cat thought experiment, claimed that reality does not exist without “conscious” observers. That there is no planet on “51 Cygni” until somebody on earth 45 light years away discovers it, and then all of a sudden it gets formed. In the past 2 decades, the advent of Decoherence theory (Zeh, Zurek) has put to rest such irrational notions of self-centricity and solipsism.
It is not an observation or measurement that results in collapse. According to decoherence, as I understand it, it is simply an interaction that results in loss of coherence, which precipitates collapse. This usually is simply a photon hitting an object with many degrees of freedom. The issue is not whether there is an observer or not. The issue is whether the information “is there in principle” or not.
4) Multiple collimated photons do produce interference. Also for entanglement, you need multiple particles - however it can be argued that in full entanglement, the multiple particles are in reality a single particle. So yes, quantum phenomena do appear across multiple unrelated particles. Any (?) interaction between two (coherent) particles will produce a quantum phenomena.
5) I think you are pinpointing the central mystery to QM. If there is a second aperture open, the trajectory is affected by that, even though the trajectory does not go through the second aperture. If one insists in viewing this phenomena in particle and trajectory terms, indeed such mystery will arise. Under this interpretation, we have a paradox. That is why the wave interpretation of light has been proposed, and the wavefunction model can explain away this mystery (but produces another mystery, namely collapse and the interpretation of the wavefunction).
6) Perturbations are not limited to electrons and photons. Any system will do. What is important is the “degrees of freedom". An electron/photon interaction with the self-interfering object produces an entanglement that will kill the self-interference, through the production of the anti-fringe. So you raise a very good question.
Assume we have a matterwave interferometer. The massive molecule has passed a double slit and would produce an interference pattern if it hits the screen. But before it hits the screen, we shoot a photon at it that bounces off, and is now entangled with the molecule. So we will lose the self-interference (without correlation selection). We now break the entanglement by absorbing the photon by a system with large degrees of freedom.
Question is: would the self-interference pattern reappear, or is it irrevocably lost? I understand that Scully has shown that the self-interference is irrevocably lost (i.e. cannot be recovered without correlation selection). Therefore, there is no “reset” effect, and you are correct that self-interference will not preserve across an interaction.
I appreciate receiving feedback from Prof. Afshar and others.
Dear Professor Afshar,
I have just read of your experiment and viewed your November 2005 presentation. Although not a physicist, I venture the following comments/questions:
1. Your experiment contradicts a statement in Richard Feynman’s book “QED: The Strange Theory of Light and Matter”, page 81. Consequently the single particle experiments he refers to and any subsequent ones must have a flaw, which I presume is the issue of perturbation of the particle as it is measured passing through the aperture.
2. Has your experiment yet been carried out for particles with mass, as you have suggested should be done?
3. Presumably one of the crucial points to draw from the experimental results is that quantum measurement (or indeed the possibility of measurement) does not necessarily change/make reality, and consequently the concept of reality as existing separate to an observer, and without requiring one, can be restored?
4. Does the totality of experimental physical evidence indicate that quantum particle interference might never be related to having more than one particle (i.e. it is always single particle interference), or are there plenty of examples of definite multiple particle quantum interference?
5. I guess from your comments that you would disagree with the following statement:
The results of single particle self-interference experiments are -
• A single particle sent to a single aperture goes through as everybody would expect (with a single real trajectory seemingly randomly selected according to the appropriate single aperture wave function);
• A single particle sent to a double aperture also goes through one aperture only, but after exiting that aperture follows a modified single real trajectory (for some yet to be explained reason) but this time seemingly randomly selected from the two aperture wave function.
Why is the above wrong?
6. Coming back to perturbation during measurement. Is the only perturbation possible electron/photon interaction? Do you think that this then must/may reset the particle’s self-interfering capability? That is particle self-interference is only possible between interaction events and not across them.
I see the problem in any case should be addressed to detection of singnals from sorces. Could everyone in world measure with precise mathematical value which will not be changed by repetition of the further the same experiment for example what we call in optic significant physical characteristic is photocurrent. It s clear that we would like to say that some value has fluctuation and we should apply some approximation model.The argument is that we live in reality with there time developing natural laws. The detector will give us only some approximated by our model the answer on what happens with source.
$1000 challenge - addendum
To the extent that the question in Afshar’s experiment is not “which detector” (future tense), but given a detection, “which pinhole” (past tense), one must perform an interpretation, insofar as such questions (in terms of QM) don’t make sense - literally.
The “particle” of which way questions, when translated into QM terms, could have been the probability of a particle, at the nominated junture (pinhole). But given the detection, this probability no longer exists. The probability of where the photon might be found or otherwise theorised - at an earlier time - would be everywhere zero, insofar as the photon detection is already given.
Since a which way question is asking something of the past, (rather than the future) we can construct a wave function from the downstream data, and propagate it backwards through the apparatus, in order to reconstruct what might be a suitable answer to a which way question.
To be consistent with the principles that saw each detection as the result of a wave distributed to both detectors, we must, therefore, use detections from both detectors to reconstruct the which way answer.
We will see (of course) that two pinholes are reconstructed - not one.
If we limit our downstream data to that produced in one detector our reconstruction will be, accordingly, just one pinhole. It is the choice of detector which determines which pin hole is reconstructed. Not the wave function. The wave function is constructed from data in both detectors.
The reconstruction of just one pin hole should tell us something is amiss. Where is the other pin hole?
It was edited out, insofar as the data that would have otherwise reconstructed it, was edited out.
Since the reconstruction of one pin hole, rather than two, is not based on the wave function but a decision regarding which data to use, the corresponding result does not constitute the identification of which way the wave/particle went. The result just tells us which detector was chosen to otherwise represent/imagine which way the wave/particle went.
22 January 2006
The $1000 challenge.
The following either answers the challenge, or says the challenge can’t be answered. I leave that up to you.
The wave function used to predict the interference patterns in each detector (or the probability of where a particle might be found in each detector) is computed across both detectors.
This is consistent with conventional applications of quantum theory.
The computed wave function defines an equal probability of finding a particle in one detector as it does the other. This probability is not affected by the introduction of the pinholes. The two waves emerging though the pinholes are the same wave function. Wavelets. That we can direct one wavelet to one detector, and the other wavelet, to the other detector, by means of the same lens (or different lenses) still doesn’t change this, or the probabilitys.
Until the photon is measured, it still has an equal probability of being measured in one detector as it does the other. Again, this is consistent with conventional quantum theory.
That we find a particle in one detector, and not the other, does not mean the wave (otherwise used to predict that particle’s location) came only through one pinhole. The same wave also went through the other pin hole, to the other detector.
The same wave.
Now the 50/50 probability of a particle being found in one detector vs the other is not defined by the wave per se, but the question - which detector? Each detector has been prepared with equal access to the same wave so the probability - in terms of the question - is 50/50.
21 January 2006
Hi Danko (if you’re out there)
To the extent that Afshar’s experiment is positioned as a violation of Copenhagen principle(s) we should provisionally set aside the math otherwise used to represent such principles. For example, the math that otherwise embodys Relativity Theory can’t be used to prove Newton incorrect anymore than the statement 2+2=4 can.
But through careful experiments, theoretical arguments, philosophical debates, a day in the art gallery, and the math, we can allow ourselves to adjust Newton’s equations to match those of Einstein’s. Otherwise we could just use Newton’s equations and say: see Einstein’s done the math wrong, he is therefore wrong.
Afshar’s experiment is a “which way” experiment. Such experiments already violate Copenhagen principles before they’ve even been performed. To follow Afshar’s experiment/argument is to (if only provisionally) employ a violation of the principle in preparing the experiment.
We should, against our better judgement, allow this.
The most famous “which way” experiment of all time is EPR. It begins as a violation of Bohr’s principle, before it is even performed. And it demonstrates (itself as) a violation.
Bohr’s comeback is to show how a correlation of the remote measurements, required to complete the demonstration, can not be performed without relocalising the remote measurements, ie. bringing them together. In the interim period, the information that would otherwise demonstrate the violation (through localisation) has yet to occur. The violation, in a sense, has yet to be demonstrated. It is in “waiting” so to speak.
Are we allowed to use this future event (localisation) to say what is taking place prior to such? Why not. We can just imagine ourselves in two places at once, performing the correlation, and thus predicting the conclusion which is otherwise demonstrated when done locally.
Bohr is in a good position here. He knows (to the extent that we can grant him such insight) that such a superpositioned observer can’t be localised any earlier than the measurements. The information otherwise held by our imaginary observer is as inaccessible (and therefore not information) as the remote measurements.
But insofar as we are allowed to imagine the violation, albeit retrospectively, we can imagine we’ve violated the Copenhagen principle.
And that is all that seems to matter in “which way” experiments.
But if a “which way” experiment could physically demonstrate a violation of Copenhagen principles, ie. before they could physically do so, only then would the principle be actually violated.
Obviously, if only by definition, this is impossible.
If, on the other hand, we read Bohr as advocating some limit on what we might imagine, or otherwise theorise as “there” prior to the demonstrated correlation, we could argue that Bohr is wrong.
We can, obviously, imagine anything.
But any serious analysis of Copenhagen (Bohr, Heisenberg, et al) would not see it as imposing any limits on our imagination per se. Afterall, the probability wave is an imaginary wave and forms the basis for predicting interference patterns.
‘Which way” measurements are at cross purposes to the principles of Copenhagen. Copenhagen is about what can, in fact, be measured. “Which way” measurements are about what could have been measured if it wasn’t for those pesky Copenhagen principles.
21 January 2006.
Greetings Prof. Afshar and Quantum Mirror - I am happy to see several new developments regarding the Afshar Complementarity experiment happen, ever since I was last here about a year ago (as “Unconscious Observer"). As my training is not in physics but rather computer science, I am not sure if the entanglement thought experiment proposed below will help shed any more light on this issue. Please view:
I believe this faithfully reproduces the experiment conducted by Mandel at Rochester. C1 and C2 are spontaneous parametric down converters. A1 and B1 are entangled. So are A2 and B2. A1 and A2 are coherent. So are B1 and B2. (Pls. note the use of the term coherent here is slightly different from Afshar’s usage in the preprint.)
I understand that in this experiment Mandel observed no IP at D0 (a CCD array). Thus V = 0 and D1 or D2 provides us WWI, hence K = 1.
He then removed D1 and D2, and with the help of mirrors combined B1 and B2 together and decohered the combination so that WWI was lost. As a result, he observed an IP at D0, hence V = 1 and K = 0.
My question is, what happens if we instead move both D1 and D2 far off to a distance, so that the length of the B paths are much longer than the A paths. Will we see V = 1 restored while maintaining K = 1 (after a short delay) and thus produce a violation of PC?
If not, then what if we remove D1 and D2 altogether so that B1 and B2 never decohere, even though they are not combined? Will we get V = 1 (as K = 0)?
I wonder if such entanglement can be used to confirm the Afshar experiment? I would appreciate your opinion on this thought experiment.
Dear Prof Afshar
Thankyou for publishing this talk, it was excellent to see phyics people talking the talk for all to see. It seems that you still have some detractors out there but it seemed to go very well in the main.
Here’s the link for the streaming video of my talk at the Perimeter Institute along with some related slides. It was a talk in the “PIQudos” seminar series.
Select “PIQudos” from the list on the left by scrolling down, and then click on
View: Measurement without “measurement”: Experimental violation of Complementarity and its aftermath
Bohr’s Principle of Complementarity of wave and particle aspects of quantum systems has been a cornerstone of quantum mechanics since its inception. Einstein, Schrodinger and deBroglie vehemently disagreed with Bohr for decades, but were unable to point out the error in Bohr’s arguments. I will report three recent experiments in which Complementarity fails, and argue that the results call for an upgrade of the Quantum Measurement theory. Finally, I will introduce the novel concept of Contextual Null Measurement (CNM) and discuss some of its surprising applications.
Preprint (published in Proc. SPIE 5866, 229-244, 2005): http://www.irims.org/quant-ph/030503/
It is important to bear in mind that this was an internal PI seminar with battle-hardened expert audience. So, please put the rare temper flares during the Q&A session in that context. It is common among physicists to get a bit more passionate when serious issues are being discussed. Nothing personal of course!
Your comments are welcome.
P.S. Contents of the talk are copyright material and the source must be quoted. Preferably, you can wait until the unpublished material on CNM (currently under peer review) gets published.
Dear Professor Afshar,
Thankyou for your explanation. It has helped to make things clear. I appreciate your taking the time to respond.
Good to hear from you.
What is the feedback from the reviewers? Can you comment? Is it close?
Dear Shannon Rowe,
There is no doubt that single photons interfere with “themselves” and there is really no way that a photon affects another photon later on. Technically speaking, a coherent source cannot give you true “single” photons, but in that case, we use something called the coherence length of the photons to determine the flux below which the overlap between the two sequential photons is negligible. If your coherence length is small compared to the optical path of your experimental setup, you can easily achieve a condition in which at any given instant only one quantum of light is present in the system. In that case, you can say with a high certainty that the presence of two photons between subsequent detections is negligible (usually of the order of 10^-4).
So, to make a long story short, the “lingering wake” exists only at high flux where there is substantial overlap between the wavefunctions of photons, otherwise at low flux or short coherence lenghts (i.e. downconverted photon pairs) photons do not affect each other in the manner you discussed. This is well documented and is used everyday in the quantum optics labs.
I hope this helps!
P.S. Papers are still under review!!!
For those interested, here’s the link to my talk at the Perimeter Institute:
Yes, I’ve already pored over all the material for Wheeler’s delayed choice, and also Scully and Drühl’s quantum eraser. You’re correct in that they are similar, but from what I’ve seen, those delayed choice experiments are more about “catching the particle in the act” by activating the detectors midway through the particle’s run. I’m not aware of any experiments so far that have attempted to establish whether there is some sort of interference causality that lingers over time between particles that can influence subsequent particles. It seems to me that there has always been an unverified assumption that only immediate spatial causality matters, and I believe this assumption needs to be challenged.
The generally held conclusion seems to be that experiments with singly emitted particles indicates that those particles must be interfering with themselves, via wave-particle duality, because such experiments have assumed that only one particle is present at a time, in a spatial sense. However, correct me if I’m wrong, when a more basic two-slit experiment is performed with a continuous stream of particles, it is then generally claimed that the interference in those cases is caused by the stream of particles from slit A interfering with those from slit B, ie normal wave pattern interference. So interference, according to theory, can come either from other particles passing through the slits in the stream variation, or from a single particle interfering with itself in the single particle variation. This is all, however, based on the assumption that interference must be occurring at a spatial level only - ie that the stream of multiple particles, or the spread-out wave of the single particle, are passing through the slits at nearly the same time and causing “real time interference". This logic strikes me as having omitted time as a possible variable.
What I would like to see tested is that assumption of spatial causality, to try to prove or disprove whether time causality might also matter. Because if interference is actually something that can linger within an experiment over a time duration as well, like the ripple of a stone thrown into a pond taking time to spread out and fade, then that would put the theory of single particle self-interference into question. One hypothesis that could arise from such a finding would be that maybe there is a “quantum wake” lingering behind each particle’s passage and influencing subsequent particles; this could then potentially negate the need to claim self-interference as the explanation for single particle apparent interference - it would then be equally plausible that the wakes from previous particles were causing the disturbance. Furthermore, the “flushing” of these wakes caused by the use of detectors might go some way towards explaining the measurement problem. The nature of such a hypothetical quantum wake is of course another matter entirely, and I do have further thoughts about what that would be, but obviously there’s no point waving theories around unless an actual experiment indicates that there is in fact something anomalous to explain.
Now this may all be just conjecture. Most likely there isn’t any such “lingering wake", and wave-particle duality will still hold firm as per the standing theory, but it seems to me as though this long-held assumption should at least be tested, if possible, because I haven’t seen any evidence that the notion of causality has been properly considered or disproved so far. The self-interference explanation has become accepted because it is the only logical explanation when you look at the findings from a spatial point of view only. But what if the issue of time (and quantum effects that linger over time) has to date been ignored? Professor Afshar’s findings that seem to defy the measurement principle certainly suggest that something is wrong with the Copenhagen model. Perhaps it is the inherent artificial laboratory nature of the two-slit experiment that has blinded us all to ticking off all the assumptions involved, and we have so far neglected the idea that time might be a factor. At any length, it is always good to re-examine and re-test all of our “concrete assumptions” with a fresh eye. This is why Professor Afshar’s work has been so valuable, in challenging some of those assumptions and finding their flaws.
Thankyou for your feedback, Peter.
Have you looked into the Wheeler delayed choice experiment? It sounds very similar to what you are proposing.
where is quantum mirror ?
Also What is the latest status on Afshars paper being published in PRL? Is it PRL A? etc.
Dear Professor Afshar,
I would like to propose the following, even simpler, experiment which may be useful in further confirming your theories. Utilising a standard two-slit single-particle-at-a-time apparatus (ie a stock textbook setup, unmodified with the wires):
Step 1: Fire single particle while detectors are turned OFF.
Step 2: Turn detectors ON for a short period of time, do nothing, and then turn detectors OFF again.
Step 3: Repeat from Step 1, looping until sufficient particles have been fired to establish (or not) interference.
The idea is that the particles are always fired during an OFF cycle of the detector, with the detectors only turned ON and then OFF in between (never during) each firing in order to “flush” the experiment. According to regular quantum theory, as I interpret it, such an experiment should still produce an interference pattern because each individual particle is not actively being measured at the time it is fired, and according to complementarity, is allegedly interfering with itself.
If however, an interference pattern does NOT occur, then this opens up a whole new area of investigation along similar lines to your own findings. Were that to happen, one theory may then be that each particle is somehow tangibly affecting the progress of the next particle. Which would lead to the question: what IS interference, really? Maybe interference is NOT caused by a given particle’s wavefunction interfering with itself after all, as Copenhagen would have it, but is actually a result of some sort of lingering “quantum wake” trailing behind the previous particles that have passed through that same space. In other words, the interference could be due instead to a barely perceptible “wake” or “ripple effect” left behind in that region of space by the passage of previous particles - a wake that can be smoothed out or “flushed” somehow by, in this case, the flashing of the detectors. As a visual example, imagine the wash from a speedboat causing turbulence to affect any vessels following behind it. And since it’s hard to extend that analogy to include the flushing mechanism, to illustrate that aspect, imagine footprints in the snow being filled in by fresh snowfalls.
At any length, whether the above seems foolish or not, I would imagine that the experiment as outlined above would be rather easy for you to test in your lab, given the equipment you have on hand. I don’t recall having seen any other researchers consider the issue of wake causality, and I imagine such an experiment would still be valuable whatever the outcome. If the result was that it still produced the expected interference, then at least the assumption used in all two-slit experiments - that previous particles do not affect later ones, and that there is no causal lingering effect - could be successfully continued and to some extent verified. However, if interference is NOT present, then it would be one further nail in the coffin of complementarity, and a whole new can of worms would be opened, also adding a great deal more support to your conclusions.
You can use: http://www.irims.org/quant-ph/030503/
I wonder why there is no search result for you in arXiv. Could you please write for us the information of the details of your publications.
Amendment - when I say only a wave can interfere with itself I meant that only waves can produce interference.
Ashars experiment only “infers” (non perturbative ?) waves is that what you are saying ?
Only a wave can interfere with itself and produce dark fringes that restore the image to almost its full intensity whilst at the same maintaining through the lens WWI/WPI. Is this not the case and if not why not ?
Schrodinger had it right with his cat paradox when speaking of complemantarity. Not everyone was enamoured of Bohr it would seem.