How does one emit a single photon?

[Greylorn]

The double-slit experiment produces its mysterious interference pattern only when photons (or electrons) can be sent through the slit pair one at a time. I'd like to find an explanation of the experimental apparatus which is guaranteed to emit only single photons, at time intervals which are significantly greater than c (such a millisecond apart).

The question is relevant because if two or more photons are emitted simultaneously, or nearly so, the double-slit experiment ceases to be interesting.

 

[atyy]

Not quite the double slit experiment, but it addresses the same question.

http://arxiv.org/abs/quant-ph/0402001
http://arxiv.org/abs/0801.0979

 

[Cthugha]

Usually one uses some transition of a single emitter, which is blocked for a while after the emission of a photon. As an easy picture imagine an atom in an excited state, which emits a photon. After the emission it will take some time for the atom to get into the excited state and decay again, so there is a certain dead time.

For a closer look, you might look here:http://www.sciencemag.org/cgi/content/abstract/290/5500/2282
or here:http://www.sciencemag.org/cgi/content/abstract/303/5666/1992

 

[cesiumfrog]

In practice, for the single slit experiment, you would just turn down the intensity (and add neutral density filters to further attenuate the beam) until statistically there is almost never two photons at the same time.

 

[f95toli]

In practice, for the single slit experiment, you would just turn down the intensity (and add neutral density filters to further attenuate the beam) until statistically there is almost never two photons at the same time.

No, modern single photon sources ARE really single photon sources; they are not just "sources with low intensity". I.e. in theory they fire ONE photon everytime they are triggered and at a well definied time.
In reality they "misfire" occasionally (i.e. emitt a photon even when there is no trigger) and occasionally they will fire more than one photon; so they are not perfect (yet) but since e.g. quantum crypthography relies on such sources there is a lot of money and effort being put into their development.

When the photons are generated using parametric downconversion one can also use a scheme where the experiment is gated; due to conservation of momentum the two photons travel in different directions meaning you can use one of them to gate the other; i.e. you let the "extra" photon hit a detector which in turn generates a signal that can then be used to to tell when a photon reached the experimental apparatus; then you discard any "extra" photons. This is how "slit experiments" (single,double, tripple...) are usually done nowadays.

it is possible to test single photon sources by using e.g. a Hanbury-Brown and Twiss interferometer and measuring the second-order correlation function (known as g(2) ); low intensity light would have a different signature than a true single photon source (since the latter is in a number state).

See also the review article by M Oxborrow, AG Sinclair - Contemporary Physics, 2005

 

[cesiumfrog]

f95, I'm not disagreeing, just pointing out that there is also a much cheaper and more practical way to accomplish this simple experiment.

 

[Cthugha]

In practice, for the single slit experiment, you would just turn down the intensity (and add neutral density filters to further attenuate the beam) until statistically there is almost never two photons at the same time.

Well, one can do so, but I think this completely misses the interesting point of using single photons for a double slit experiment. By attenuating a beam you just shift the average value of the Poisson or Bose-Einstein distributed photon number, but everything else remains the same: You have several emitters, so you also have several fields, which are superposed. So - from a classical point of view - you have contributions to the intensity, which arise from the field of one emitter alone and you have contributions, which arise from the combination of the fields of two different emitters. Now the interesting point in using real single photon sources for a double slit experiment is, that those mixed contributions are not present anymore, which makes the double slit experiment even more interesting in my opinion.

 

[f95toli]

f95, I'm not disagreeing, just pointing out that there is also a much cheaper and more practical way to accomplish this simple experiment.

No, because it is not the same experiment. An attenuated laser is NOT the same thing as a single photon source.
Specifically because the light from a laser is always Poissonian(coherent) and if you measure the second order temporal correlation function for a laser you will ALWAYS find
$g^{(2)}=$1 for all times (regardless of the intensity) wheras a true single photon source has $g^{(2)}(\tau=0)=0$
Hence, there is a fundamental difference between two and and they are not -in general- equivalent from an experimental point of view.

Also, note that even the imperfect sources I mentioned above are much "better" than a laser beam; some of the sources that are now becoming available exhibit $g^{(2)}(\tau=0)$ very close to 0.

 

[cesiumfrog]

In what way is the double slit experiment "more interesting" if the photons (which you count one at a time reaching the screen) were created by an ideal single-photon source rather than a mundane lamp (with attenuation and zeroeth slit for coherence)? How would I tell the patterns apart? (It seemed as though the OP's aim was just to eliminate the possibility that all interference is due to interaction of multiple separate photons..)

 

[Cthugha]

In what way is the double slit experiment "more interesting" if the photons (which you count one at a time reaching the screen) were created by an ideal single-photon source rather than a mundane lamp (with attenuation and zeroeth slit for coherence)? How would I tell the patterns apart? (It seemed as though the OP's aim was just to eliminate the possibility that all interference is due to interaction of multiple separate photons..)

The surprising point of the double slit experiment with single photons is, that one sees wave-like behaviour contrary to the almost ballistic naive picture of the photon the layman usually has. A layman expects a particle, which travels from some source towards the screen. The emission of a single photon source is very close to this naive view.
Coherent light is already a different topic. You now usually have an ensemble of emitters. Due to coherence, emitted photons inside a coherence volume are indistinguishable anyway. You can never know, which one of the emitters was the source for a detected photon. Even worse, you do not for sure, whether your single photon is a "product" of a single emitter or already the "product" of interference between several emitters.

Usually people do not ask, what a double slit pattern looks like, if photons are detected one at a time, but what a double slit pattern looks like, if photons are sent through the slits one at a time, which is just a small difference, but opens up a few more loopholes and makes the situation a bit more complicated, if you have several emitters and interferences. Using real single photon sources simplifies the situation in my opinion.

 

[cesiumfrog]

The surprising point of the double slit experiment with single photons is, that one sees wave-like behaviour contrary to the almost ballistic naive picture of the photon the layman usually has.

Right. But provided we measure only one photon per minute reaching the screen detector (so that we can be confident there was never two different photons going through both slits simultaneously) is there any reason for the layman to care whether the source of the photon was a true single-photon source (triggered once per minute) or even just very well-attenuated sunlight (with another slit preceding the double slit as is normally always the case)?

To the layman, who obviously lacks the QM to fully scrutinise the former "black box" source, wouldn't the latter source actually be preferred?

 

[Cthugha]

Right. But provided we measure only one photon per minute reaching the screen detector (so that we can be confident there was never two different photons going through both slits simultaneously) is there any reason for the layman to care whether the source of the photon was a true single-photon source (triggered once per minute) or even just very well-attenuated sunlight (with another slit preceding the double slit as is normally always the case)?

Well, this does not change the problem. The attenuation process is still a superposition of statistically independent random absorption/transmission processes, so you will just achieve an average time span of 60 seconds between two photons, but there will be events, when there are 2 minutes in between and there will be moments, when two photons arrive simultaneously, although those will not occur very often.

To the layman, who obviously lacks the QM to fully scrutinise the former "black box" source, wouldn't the latter source actually be preferred?

Well, I agree, that when a layman really wants to build a double slit at home attenuated light is the easier and better way for a do-it-yourself experiment. But if you just want to give a theoretical explanation, I don't see why the usage of a strongly attenuated light source should be easier to understand than a device, which fires a single photon every once in a while.

I just do not see, why a wrong explanation should be used when a correct explanation is not more complicated. Quantum optics is such a huge and alive field nowadays, that in my opinion one should not ignore its results.

 

[cesiumfrog]

there will be moments, when two photons arrive simultaneously, although those will not occur very often.
[..]
I just do not see, why a wrong explanation should be used when a correct explanation is not more complicated.

To be precise, we can make those coincidences arbitrarily rare; it is not merely a simplification if we ignore that fraction of detections completely.

I do not see why you think either explanation (of the source of particles in a classic "one particle through the apparatus at a time" double slit experiment) is actually "wrong". I'm familiar with experiments which do require ideal single photon (on-demand) sources, but this seems not to be one of them. Please correct me. I was even under the impression that true single photon sources did not exist yet at the time in history when the results of these classic experiments convinced physicists that each single particle must follow a superposition of multiple paths?

 

[Cthugha]

To be precise, we can make those coincidences arbitrarily rare; it is not merely a simplification if we ignore that fraction of detections completely.

Yes, you can, but in practice this will also mean, that most pulses will be empty. As your detectors need to be switched on during every pulse, this really spoils your signal to noise ratio. If pseudo single photon sources are used, the mean photon number per pulse is almost never below 0.1 due to this.

I do not see why you think either explanation (of the source of particles in a classic "one particle through the apparatus at a time" double slit experiment) is actually "wrong". I'm familiar with experiments which do require ideal single photon (on-demand) sources, but this seems not to be one of them. Please correct me.

Well, whether this description is wrong or not, depends a bit on what you are actually trying to say. If you just want to show, that there is an interference pattern, when photons are detected one at a time, it works. But if you want to show, that there is an interference pattern when photons are present one at a time, it gets more complicated. As an example now you already have to assume perfect detectors to rule out, that there are other photons present, which are just not detected. The number of loopholes just increases.

I was even under the impression that true single photon sources did not exist yet at the time in history when the results of these classic experiments convinced physicists that each single particle must follow a superposition of multiple paths?

Good question. I always thought this was shown with electrons first as it is rather easy to get single fermions, but I must admit, that I don't know.

 

[lightarrow]

So, now the question turns into: how does one know to have fired a single photon (regardless of detection)?

 

[f95toli]

So, now the question turns into: how does one know to have fired a single photon (regardless of detection)?

Measure $g^{(2)}$ using a HB&T interferometer, a perfect single photon source will exhibit perfect anti-bunching, i.e. $g^{(2)}(\tau=0)=0$.

 

[edguy99]

In practice, for the single slit experiment, you would just turn down the intensity (and add neutral density filters to further attenuate the beam) until statistically there is almost never two photons at the same time.

Could you view this experiment as testing the intensity of light needed to set off the detector? If so, it seems there could be a lot of lower intensity photons going through that are not being detected.

 

[lightarrow]

So, now the question turns into: how does one know to have fired a single photon (regardless of detection)?

Measure $g^{(2)}$ using a HB&T interferometer, a perfect single photon source will exhibit perfect anti-bunching, i.e. $g^{(2)}(\tau=0)=0$.

Sorry, but this means that you can't say to have fired a single photon before detection.

 

[Cthugha]

Sorry, but this means that you can't say to have fired a single photon before detection.

No, but the second order correlation function usually depends just on the source. Once you have shown, that a photon source emits nonclassical light and shows perfect antibunching, you would assume that it will still do so, when you put it in front of a double slit. The good thing in $$g^{(2)}$$ measurements is, that a non-ideal detector efficiency is no problem anymore as you can clearly distinguish between Poissonian and sub-Poissonian statistics.

 

[DrChinese]

Could you view this experiment as testing the intensity of light needed to set off the detector? If so, it seems there could be a lot of lower intensity photons going through that are not being detected.

There is no such thing as "lower intensity photons" that are not being detected. Certainly no detector can be claimed as being of perfect efficiency, but the scenario you imagine has been ruled out experimentally. Entangled beams are used by detector manufacturers to rate their detector efficiency, which is extremely high these days. So for every detection at Alice, there is essentially always one at Bob.

The conclusion is that "lower intensity photons" are no photons at all. Otherwise, we would often have detections at Alice without a matching detection at Bob (and vice versa), but that doesn't happen.

 

[edguy99]

There is no such thing as "lower intensity photons" that are not being detected. Certainly no detector can be claimed as being of perfect efficiency, but the scenario you imagine has been ruled out experimentally. Entangled beams are used by detector manufacturers to rate their detector efficiency, which is extremely high these days. So for every detection at Alice, there is essentially always one at Bob.

The conclusion is that "lower intensity photons" are no photons at all. Otherwise, we would often have detections at Alice without a matching detection at Bob (and vice versa), but that doesn't happen.

I used the wording of the quote I was responding to. Perhaps a better wording would be: Do the filters take out the lower energy photons or does the detector simply not detect them? Remember we are talking about a light source like the sun or a lamp with enough filters in front of it (or a very dim source) to make the detector only go off "occassionally". In your entangled beam test, I agree there are no lower energy photons.

Please go easy, its just a question I do not know the answer to. Thanks.

 

[Greylorn]

Thank you for the excellent replies. I'm willing to operate under the assumption that single photons separated in time intervals large enough to eliminate quantum effects can be generated using modern techniques. I doubt that the original double-slit experiments were performed with similar rigor. So, next question: has the double-slit experiment been reproduced with current technology?

If so, given an inexplicable result from a relatively primitive and easily reproducible on-planet experiment, shouldn't we resolve it before speculating about things upon which we cannot experiment, such as the origin of the universe.

Simple, fundamental experiments have been physics' backbone. Galileo did fundamental work and found one of Newton's laws by rolling balls down wooden ramps. Without a tiny hole in a hot box, we wouldn't have QM. Yet we've allowed physics to progress as if the double-slit experiment had never been performed, because we cannot resolve it.

What if this simple experiment is telling us something fundamental about the nature of space?

Has anyone tried moving the single-photon emitter incrementally closer to one of the double slits, so as to determine the point at which the "interference" effect disappears?

 

[DrChinese]

If so, given an inexplicable result from a relatively primitive and easily reproducible on-planet experiment, shouldn't we resolve it before speculating about things upon which we cannot experiment, such as the origin of the universe.

Actually, the opposite is true. The thing you call "inexplicable" is actually necessary to make sense of many other phenomena. All kinds of objects have been sent through double slits and shown to yield diffraction patterns. You might be surprised to learn that molecules as large as fullerene (C-60) exhibit interference patterns consistent with theory.

So what we have is a situation where theory and experiment match. That is considered a suitable foundation for making assessments for other situations in which theory would normally be applied. Not sure how that relates to the Big Bang exactly as there are a variety of theoretical issues also involved outside of QM.

 

[Greylorn]

Actually, the opposite is true. The thing you call "inexplicable" is actually necessary to make sense of many other phenomena. All kinds of objects have been sent through double slits and shown to yield diffraction patterns. You might be surprised to learn that molecules as large as fullerene (C-60) exhibit interference patterns consistent with theory.

So what we have is a situation where theory and experiment match. That is considered a suitable foundation for making assessments for other situations in which theory would normally be applied. Not sure how that relates to the Big Bang exactly as there are a variety of theoretical issues also involved outside of QM.

Dr. Chinese,

Thank you for the clarification. I am indeed surprised that buckyballs exhibit interference behavior when passed through paired slits, because I never thought to wonder about what would happen as the size or momentum of things sent through paired slits increases. Baseballs and BB's should then do the same, although the effect would be impossible to measure.

I wish to learn more. Obviously I've missed some things. My most egregious miss is the explanation of the double-slit experiment, which I've been curious about for since the late sixties. The question I proposed which initiated this thread, approximately, 'How do we know that we have actually emitted single photons' (or whatevers) was based upon my apparently mistaken belief that the "interference" pattern produced by singly-emitted whatevers had yet to be resolved.

Kindly point me to a source of this resolution. In other words, where do I find the theory which matches the experimental data? Thanks!

 

[Peter Morgan]

Suppose we think for a moment in terms of the Copenhagen interpretation. Then we couldn't say anything about whether photons are emitted one-by-one. We observe events, but we cannot say anything about whatever we say cause these measurement events, except at the point of measurement. So nothing about before the measurement. Duh, that includes the time of emission. Indeed, all we can talk about are statistics of the times (and places) at which events are observed. We cannot from the empirical statistics of observed events conclude that the statistics of unobserved emitted events are the same. It takes two, preparation and measurement, to QM.

The Copenhagen interpretation has gone out of style, but it concentrates the mind on what experiment can be imagined to justify. There are indeed reasons why the Copenhagen interpretation went out of style (inter alia, I think people really don't like to be so minimalist about causality), but there are also reasons why it remained pre-eminent for 50 years. It's dangerous to say "The Copenhagen interpretation would say ...", because Copenhagen is rather multifaceted, but, for here: the quantum state associated with a given preparation apparatus does no more than usefully summarize what statistics of event timings we will observe with various different kinds of measurement apparatus.

To briefly mention Bible Thumper's "sending an electron directly at another, then recording the resultant (260nm UV) photon that's emitted", how could we know when the electrons are emitted, without measuring them, hence changing the experiment?

Those who have seen earlier posts of mine will know that I am willing to go beyond the Copenhagen interpretation's rather minimal commitments. The double-slit experiment is a consequence of interference of a field (of a specialized, probabilistic kind, called a random field, because how should this stuff be trivial?). Events are caused by the field acting on the thermodynamically nontrivial "measurement" apparatus, which has been tuned to go from a metastable thermodynamic state to a different thermodynamic state that can be macroscopically distinguished. The inverse problem of constructing a random field model (or a quantum field model) from a finite number of event timings is of course underdetermined unless we make quite strong auxiliary assumptions. So it's best just to worry about times of emission unless you have a strong constitution.

 

[sirchasm]

I find it fascinating that any measurement always depends absolutely on the 'preparedness' of some system to act as a channel that conveys a signal, or a message.
Quantum expectation isn't fundamentally different to classical expectation
If we prepare a medium, and we know it can 'carry' something like a signal then we know we can 'send' certain signals. Expectation and uncertainty are then just because of 'measurements' we are allowed to make, assuming the channel exists, and we know about how much it can carry, etc.

Probability seems to rule the way we can get any information or expect any, in a more or less universal way, it's just down to finding a way to send a signal, ultimately. You always need a start and a end, you can't send something across a non-existent gap, any gap always has two edges.

 

[borgwal]

I just want to add something to the discussion in this thread before it got hijacked:

That it is single photons interfering with themselves, rather than one photon with another was shown in an experiment from 1909, which clearly did not make use of a single-photon source nor of a laser beam, but of a very weak light beam:

G.I. Taylor, Proc. Camb. Phil. Soc. 15, 114 (1909).

 

[Peter Morgan]

I just want to add something to the discussion in this thread before it got hijacked:

That it is single photons interfering with themselves, rather than one photon with another was shown in an experiment from 1909, which clearly did not make use of a single-photon source nor of a laser beam, but of a very weak light beam:

G.I. Taylor, Proc. Camb. Phil. Soc. 15, 114 (1909).

True enough, I did. I apologize that I can give only a half-apology. I admit that I don't understand Sirchasm's response to the thread, despite looking at his earlier posts on other subjects (I think he has a way to go to define his ideas, but to me it's hopeful that he's thinking so differently about the question, and apparently not naively, and, I think, not in any way that's easily identifiable as something that's been tried before and failed), and I suppose few people may understand my reaction to the thread enough to respond to it, but there is a real empirical question to my comment that is not answered by a discussion that talks of "single photons interfering with themselves". Swathes of recent published academic discussion of quantum theory would find problematic language that so strongly implies a particle ontology as a way to discuss the quantized electromagnetic field, however many papers can be cited that talk about "photons" in a simple-minded way. 1909, needless to say, was long before the formalism of quantum optics became mature. How to engineer the emergence of particles from a quantum field formalism -- which quantum optics is -- is a deep question that is so far from answered that it is barely discussed.

It's the sad feeling that I occasionally, perhaps always, hijack threads that results in me coming to PF only for a few weeks, then leaving it for a long while. So, you can look forward to the next time someone says something about photons, in few weeks or a month, when I won't be here to say something about it. Anyway, people can always do just as you have done, refer to earlier posts that they understand and have a response to, ignore stuff that they don't, whether for good or bad reasons. Don't worry at bad posts, just ignore them.