Can a Single Photon Be Directly Observed?

[MargaretePS]

Is it possible a single photon to be seen by using any existing scientific equipment?

I mean seen , after it is caught and enlarged on a photo or computer monitor, the way we can see a very small object.
I mean a photon directly , not a picture of effect it may produce when interact with the environment...
I understand that photons behave both as a particle and wave and
I also understand this question is weird and silly
but perhaps someone will spare some time to answer me and most importantly, if my question is fundamentally wrongly formulated, to tell me why.

Thank you in advance!

Margarete

 

[chill_factor]

you can detect a single photon. you cannot see a single photon. indeed you can't see photons at all while they're in motion.

 

[Crazymechanic]

I would say that we can see and we indeed measure and see the effect of the single or many photons or the interaction that they make when it or they hit a metal plate for example( see he photoelectric effect) http://en.wikipedia.org/wiki/Photoelectric_effect

But directly to see a photon like a ball or a pencil in front of you.NO we can't do that.
When we say that we have detected a photon or photons it usually is when we see some other particles being emitted and then we make measurements of their energies and calculations and then we get the estimate of the incoming photon energy.Something like that.
If it helps to understand it is sort of like similar to get to know a person by his friends not seeing the person in face.

 

[georgir]

Remember, we do not actually see objects around us, but we see the photons that bounce off them and hit our eyes.

So, in a sense, no, a photon can not 'look' like anything other than complete blackness in the same sense that other objects 'look' like images to us. Because photons do not bounce off of photons.

In a sense, yes. But only when it hits our eye. Once again, our eyes are simply photon detectors. We normally need lots and lots of photons to get an image, so I doubt if we'd be able to notice a single photon... but maybe if it had enough energy... If we were, it'd look just like a momentarily flash of light at the single point in our vision that it hits, with brightness and size dependent on its energy, and color dependent on the type of receptor that it got absorbed by.

Not quite what you meant by your question?

Detection implies interaction.
Interaction, for the case of photons, is more or less limited to absorbtion. So, any photon detector we may have currently is going to absorb the photon. Then it can beep, or tell you 'here was a photon' or even show you an artist's impression of what that photon might have looked like, which might or might not be just a single colored point on a black background... But the fact remains, after our detection, there is no photon any more, and nothing left to show.
We can even re-emit the photon so that we can pretend our artist's impression is about something still existing...

In theory, one day in the far future it might be possible to detect the gravity, or spacetime curvature, caused by a photon without affecting it (much). And then we can draw you a representation of that curvature tell you that this is how a photon 'looks'. In a sense.

 

[Crazymechanic]

theoretically we should be able to detect a single photon.By the emission of a single electron from the age old photoelectric effect experiment.
And the only way of telling photons apart from each other is by their wavelength/frequency hence their corresponding energy.

 

[ZapperZ]

We have single photon detectors. This is well established, or else Hamamatsu has lied with their photo detectors.

Zz.

 

[MargaretePS]

Thanks a lot

Thanks a lot for all this serious and quickly sent information guys!
And well explained - even a non-specialist like me can understand it!

Much much appreciated!
M.:!)

 

[Curious3141]

Since the OP used the word "see", there's a bit of biology to be addressed here, even if it had not been the intention of the original question. It's actually a good question to ask what the visual threshold for photon detection is. And it's answered here: http://www.lifeslittlemysteries.com/2426-human-eye.html

Synopsis: under ideal conditions, viz. using photons from the most sensitive part of the visible spectrum (blue-green), incident upon the most rod-dense and therefore sensitive area of the retina (periphery) under fully dark-adapted conditions (most sensitive), it takes 54 to 148 photons hitting the eyeball to register a "hit" 50% of the time. Only about 10% of those photons are actually being "detected" by rod cells in the retina, so that's 5 to 14 or so.

More to the point of the OP's question, it's exceedingly unlikely that a single incident photon hitting the eyeball will be detected by a normal human visual system.

 

[ZapperZ]

Since the OP used the word "see", there's a bit of biology to be addressed here, even if it had not been the intention of the original question. It's actually a good question to ask what the visual threshold for photon detection is. And it's answered here: http://www.lifeslittlemysteries.com/2426-human-eye.html

Synopsis: under ideal conditions, viz. using photons from the most sensitive part of the visible spectrum (blue-green), incident upon the most rod-dense and therefore sensitive area of the retina (periphery) under fully dark-adapted conditions (most sensitive), it takes 54 to 148 photons hitting the eyeball to register a "hit" 50% of the time. Only about 10% of those photons are actually being "detected" by rod cells in the retina, so that's 5 to 14 or so.

More to the point of the OP's question, it's exceedingly unlikely that a single incident photon hitting the eyeball will be detected by a normal human visual system.

I've mentioned this already:

https://www.physicsforums.com/blog.php?b=4118

However, I think you are confusing different things here.

Saying that something can detect single-photon is not the same as saying that that thing can detect ALL single photons. Let's be clear on that.

We have single-photon detectors. These detectors are sensitive enough to detect at the level of single photon. However, the quantum efficiency (QE) of these detectors are not 100%. In other words, there WILL be single-photons that they will miss. But still, it certainly is capabe of detecting at the level of single-photons, and that single photon can trigger the necessary electronics to indicate a signal.

Zz.

 

[sophiecentaur]

Is it possible a single photon to be seen by using any existing scientific equipment?

I mean seen , after it is caught and enlarged on a photo or computer monitor, the way we can see a very small object.
I mean a photon directly , not a picture of effect it may produce when interact with the environment...
I understand that photons behave both as a particle and wave and
I also understand this question is weird and silly
but perhaps someone will spare some time to answer me and most importantly, if my question is fundamentally wrongly formulated, to tell me why.

Thank you in advance!

Margarete

Actually, you have it in a nutshell, there. All you can ever say about a photon is that it 'did' interact with the environment - either in the form of energy emitted or energy absorbed or a momentum change. You cannot really attribute any other properties to it without risking serious misunderstandings unless you dive right into QM.

 

[Crazymechanic]

If the discussion goes about photons there had to be sophiecentaur in it.He always attends these :D Hi sophiecentaur.


""""You cannot really attribute any other properties to it without risking serious misunderstandings unless you dive right into QM. """
Well I would like to add that even diving into QM won't do you no good as that is basically hard math and theoretical models that try to tell what is the reality like but in the OP question sense about trying to physically see and "feel" a photon I believe QM has nothing more to offer than what you just said yourself.
If we exclude the math and models.The only (ontic) real things we know about photon is from it's interaction with the environment.And even the frequency we don't measure directly off of a photon.So...

 

[sophiecentaur]

Someone rattled my cage. ;-)

 

[Curious3141]

I've mentioned this already:

https://www.physicsforums.com/blog.php?b=4118

However, I think you are confusing different things here.

Saying that something can detect single-photon is not the same as saying that that thing can detect ALL single photons. Let's be clear on that.

We have single-photon detectors. These detectors are sensitive enough to detect at the level of single photon. However, the quantum efficiency (QE) of these detectors are not 100%. In other words, there WILL be single-photons that they will miss. But still, it certainly is capabe of detecting at the level of single-photons, and that single photon can trigger the necessary electronics to indicate a signal.

Zz.

What was confused, or confusing, about what I posted?

I never said that a single-photon detector (which the eye decidedly is NOT) is the same thing as detecting ALL single photons. Please show me where I claimed that.

In fact, the source I quoted stated a threshold for incident radiation at a 50% hit rate. That's a fair coin-toss. Hardly a perfect detector.

 

[Cthugha]

What was confused, or confusing, about what I posted?

I never said that a single-photon detector (which the eye decidedly is NOT) is the same thing as detecting ALL single photons. Please show me where I claimed that.

It is not confusing. It is just wrong. ;)
The rods in the human eye have single photon sensitivity. This is discussed for example in F. Rieke and D. A. Baylor, "Single-photon detection by rod cells of the retina", Rev. Mod. Phys. 70, 1027–1036 (1998).

Under normal ambient conditions the neural filters fortunately will not allow these signals to come to the brain - that would really be too much for the brain to cope with, but the rods in the eye indeed have single photon sensitivity, although that may seem strange. Even your link implies that: It is the redundancy of having a signal from several rods which makes the signal go to the brain, but the single rods obviously need to show a response already at the single photon level.

 

[Curious3141]

It is not confusing. It is just wrong. ;)
The rods in the human eye have single photon sensitivity. This is discussed for example in F. Rieke and D. A. Baylor, "Single-photon detection by rod cells of the retina", Rev. Mod. Phys. 70, 1027–1036 (1998).

Under normal ambient conditions the neural filters fortunately will not allow these signals to come to the brain - that would really be too much for the brain to cope with, but the rods in the eye indeed have single photon sensitivity, although that may seem strange. Even your link implies that: It is the redundancy of having a signal from several rods which makes the signal go to the brain, but the single rods obviously need to show a response already at the single photon level.

I'm aware that the rods respond to single photons. When it gets to them, that is.

It's fairly explicit in the link I provided too. Here:

Thus, the absorption of 5 to 14 photons, or, equivalently, the activation of just 5 to 14 rod cells, tells your brain you're seeing something.

One photon, one rod.

Of course, there are multiple reasons why a single photon incident on a whole eyeball (or even the aperture of a dilated pupil will fail to fall onto a rod cell. And even if a photon evokes a photochemical reaction in a rod (rhodopsin-based phototransduction), the signal may not reach "conscious awareness" as you pointed out. I'm aware of these intricacies.

I do accept that the "tells your brain you're seeing something" phrasing in the article overstates the case.

 

[Cthugha]

Ok, then it is just confusing me. I thought you meant to say that there is no single photon sensitivity in the eye. But yes, I agree that it is not easy to make the distinction at what stage 'seeing' actually begins.

It would be interesting to know, whether there are people suffering from the savant syndrome which actually have signal delivered to their brain even for amounts of light so small that no signal would be created for most people.

To the OP: The single photon detectors are usually spads (single photon avalanche diodes). They are sensitive to single photons, but are usually binary detectors, so they cannot distinguish between one or many photons being present. Photon number resolving detectors also exist, but are still relatively new. Generally speaking, there is always some trade off. Efficient detectors are usually not fast and vice versa. Low dark count rates often come at the cost of having to use low temperature operation or rather long dead times. But as has been pointed out already: these detectors have single photon sensitivity.

 

[Crazymechanic]

"""But yes, I agree that it is not easy to make the distinction at what stage 'seeing' actually begins."""

Hmm , from a human level I would say seeing begins when you start to see... (DUH) :D:D

 

[sophiecentaur]

There are none so blind as those who will not see.

 

[dipole]

In the land of the blind, the man equipped with a single-photon detector is Mayor.

 

[berkeman]

It looks like this physics thread has run its course.