Time of flight of a single photon

[Len Moran]

In principle, could the time delay between the emission and absorption of a single photon in vacuum be measured?

There are some issues here that I am not sure about in achieving this, namely, uncertainty in the position of the photon at the detecting stage, the production of a single photon on demand and what exactly is meant by a trajectory of one photon in terms of its path length.

Very simplistically, I could envisage a vacuum chamber with a large matrix of single photon detector elements at one end and a single "photon on demand" source at the other. The time between the electronically detected photon and the electronic trigger would then provide a correlation between emission and absorption that could be used to measure the time of flight of a single photon.

I would suppose the uncertainty principle would not allow us to get a detectable hit every time, but given a sufficiently densely packed arrangement of detector elements and if we triggered the source enough times, presumably we would get at least one photon properly striking one detector element and thus allowing us to make the correlation.

As far as the photon on demand is concerned, there seems to be serious research being carried out in achieving this goal, specifically in the field of quantum computing. So presumably such a goal is considered to be a possibility even if it has not yet been achieved. However, is it truly possible to produce one photon on demand with a consistent correlation between the trigger and the actual emission of the photon?

[Ken G]

I would suppose the uncertainty principle would not allow us to get a detectable hit every time, but given a sufficiently densely packed arrangement of detector elements and if we triggered the source enough times, presumably we would get at least one photon properly striking one detector element and thus allowing us to make the correlation.

The uncertainty principle doesn't limit the detection rate so much as the detection time. What you are talking about is certainly possible, but check out the recent thread about the "amplitude of a photon", where I pointed out that "real photons" are not plane waves but rather wavepackets, which means they have a bandwidth in frequency. It is only that bandwidth that allows the observation you are describing-- you can measure the emission and arrival times of a photon only to within the time uncertainty of the reciprocal of the frequency bandwidth. So get a long enough distance and "sloppy" enough photons, and yes, you can do it. Typically the bandwidth of a photon from a low-density atom is about a hundred million inverse seconds, so you need a travel time much longer than 10^-8 seconds, or a distance much longer than 3 meters, to make what you are saying feasible.

So presumably such a goal is considered to be a possibility even if it has not yet been achieved.

Single-photon emission and detection has been achieved, and used to show that double-slit diffraction still occurs.

However, is it truly possible to produce one photon on demand with a consistent correlation between the trigger and the actual emission of the photon?

If you can accept an uncertainty of that inverse bandwidth, yes.

[Len Moran]

Thanks for that, it clarifies misconceptions I had regarding the uncertainty possibly preventing a meaningful correlation between the emission and absorption of a single photon. It was claimed that such a correlation would be impossible to obtain (because of the uncertainty principle) when I argued (not on this forum) the merits of such an experiment in relation to a particular view of the single photon in flight in vacuum.

I have known that single photon generators are routinely used in experiments, but my understanding was, given current technology, that there is the possibility of such devices producing a sequence of single photons for each initiating trigger pulse.

So just to confirm my understanding:

Assuming we can trigger a laser to produce one photon per initiating trigger pulse, there will be a time uncertainty between the trigger signal and the actual generation of the wave packet. Upon detection, there will also be a time uncertainty between the absorption of the wave packet and the detected signal. In the example you gave, this would amount to a combined uncertainty of 20ns (10ns for emission and 10ns for absorption) which equates to a length of 7m.

If we were able to construct a vacuum chamber much longer than this 7m, with a laser accurately aimed at the detector, then there would be a small uncertainty in the time correlation between the trigger signal and the detected signal (the longer the chamber, the smaller the uncertainty). This would signify a valid measurement of single photon delay (subject to the uncertainty and taking into account known measurement delays) between the emission and absorption of a single photon that involved no interaction with any medium on route to the detector.

[EvilEye]

What if you added 1 more detector?

What I mean is...

Since the primary emission time is uncertain, you could add a mirror/detector and just measure the time/distance between the mirror detector (which would be instant) and the final detector?

[Len Moran]

What if you added 1 more detector?

What I mean is...

Since the primary emission time is uncertain, you could add a mirror/detector and just measure the time/distance between the mirror detector (which would be instant) and the final detector?

Well remember I am just talking about the measurement of one photon in flight, any prior detection would absorb the photon at that point so there would be no final detection because there would be no photon.

[EvilEye]

Well remember I am just talking about the measurement of one photon in flight, any prior detection would absorb the photon at that point so there would be no final detection because there would be no photon.

As you stated before, you may have to do the experiment several times to get any detection. So the same applies to adding a mirror/detector/reflector.

If you can get just ONE photon to deflect off the mirror and end at the final detector, you have your measurement. (I think)

That is.. if the mirror is used to detect, but not absorb the photon. Like detect the presence or the "bounce"...or whatever.

Anyway... it's a real cool experiment, and I hope you figure it out.

[Ken G]

Assuming we can trigger a laser to produce one photon per initiating trigger pulse, there will be a time uncertainty between the trigger signal and the actual generation of the wave packet.

Yes.

Upon detection, there will also be a time uncertainty between the absorption of the wave packet and the detected signal.

Yes, although you can't actually disconnect these two statements of uncertainty, it's all the same thing.

This would signify a valid measurement of single photon delay (subject to the uncertainty and taking into account known measurement delays) between the emission and absorption of a single photon that involved no interaction with any medium on route to the detector.

Yes.

[Robert TG]

"In principle, could the time delay between the emission and absorption of a single photon in vacuum be measured?"

I'm sure I'm missing the point of your question here. I'm curious, could you please explain.

Are you simply talking about measuring the speed of light?

By measuring the time delay between emission and absorption of a photon over a known distance should give you the speed of light speed. Since you already know the distance and the speed of light in a vacuum is constant, this leaves you with an expected time of flight.

[Len Moran]

"In principle, could the time delay between the emission and absorption of a single photon in vacuum be measured?"

I'm sure I'm missing the point of your question here. I'm curious, could you please explain.

Are you simply talking about measuring the speed of light?

Glad to.

The question relates to a radical idea of light put forward by a group known as POAMS. I put forward a counter argument that involved the possibility of being able to measure the time of flight of a single photon in vacuum, a notion that I considered would question their basic stance. They insisted that such a measurement (even an approximate one) involving just one photon would be impossible to achieve because of the uncertainty principle. At the time I was unsure about this and I could find no information regarding any such measurement as having taken place, so I could not follow my argument through.

In the wake of the "amplitude of a photon" thread I thought it would be an opportune time to ask the question here, and the answer given is much what I though should be the case, although I had the uncertainty concept a little confused.

So, to answer your question, I am not simply talking about measuring the speed of light, I am specifically questioning what we can and cannot know regarding the time of flight of a single photon in vacuum between a source and sink. And given Ken's answer, I am now armed to resurrect my original argument with POAMS.