# How does one emit a single photon?

by Greylorn
Tags: double slit
 PF Gold P: 48 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.
 Sci Advisor P: 7,915 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
 Sci Advisor P: 1,549 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/conten.../290/5500/2282 or here:http://www.sciencemag.org/cgi/conten.../303/5666/1992
P: 2,050

## How does one emit a single photon?

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.
PF Gold
P: 2,176
 Quote by 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.
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
 P: 2,050 f95, I'm not disagreeing, just pointing out that there is also a much cheaper and more practical way to accomplish this simple experiment.
P: 1,549
 Quote by 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.
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.
PF Gold
P: 2,176
 Quote by 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.
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.
 P: 2,050 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..)
P: 1,549
 Quote by 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..)
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.
P: 2,050
 Quote by Cthugha 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?
P: 1,549
 Quote by cesiumfrog 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.

 Quote by cesiumfrog 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.
P: 2,050
 Quote by Cthugha 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?
P: 1,549
 Quote by cesiumfrog 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.

 Quote by cesiumfrog 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.

 Quote by cesiumfrog 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.
 P: 1,504 So, now the question turns into: how does one know to have fired a single photon (regardless of detection)?
PF Gold
P: 2,176
 Quote by 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. $g^{(2)}(\tau=0)=0$.
PF Gold
P: 287
 Quote by 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.
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.
P: 1,504
Quote by 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. $g^{(2)}(\tau=0)=0$.
Sorry, but this means that you can't say to have fired a single photon before detection.

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