Tunneling photons travel faster than light?

[lmoh]

I recently found an interesting read on the mysteries of quantum mechanics, here :

Raymond Chaio, of the University of California, Berkeley, and his colleagues have actually been measuring a different, but related, kind of tunneling. They have devised an experiment in which two photons (particles of light) are produced simultaneously in a source, and travel on parallel paths. One photon goes straight to a detector; the other is confronted by a barrier which would reflect the light of the photons obeyed the laws of classical, "Newtonian" physics. But according to quantum theory there is a high probability that some of the photons arriving at the mirror will tunnel straight through, and go on their way to the detector.

Sure enough, that is what happens. The barrier is 1.1 micrometers thick, so anything traveling through it at the speed of light would take 3.6 femtoseconds (3.6 thousand million millionths of a second) on the journey. But the new experiment is so sophisticated that it can compare the arrival times of pairs of photons, one of which has gone past the barrier and one through it, and shows that the one which goes through the barrier arrives first. It tunnelled through the barrier faster than the speed of light, in less than 3.6 femtoseconds. As the researchers put it, "it is as though the particle 'skipped' the bulk of the barrier". But don't ask them, or anyone else, what it means -- in the words of Richard Feynman, "nobody understands quantum mechanics".

Don't know about you, but I am personally confused by this result. How can a photon travel faster than itself? If it can travel faster than itself, than what does that mean for the universal speed limit?

My own uneducated guess is that this phenomenon, called quantum tunneling, only applies in limited cases (such as the one established for the second set of photons). Tunneling as far as I am concerned is similar to teleportation, in that you jump from one place to another without crossing the intervening space (in this case, the barrier). But this discrete motion is different from continuous motion, which is what we usually associate with the speed of light. In terms of the latter, the photon is the fastest and nothing can surpass it. So most of the time, a photon would be moving continuously at the speed of light, but in cases where it encounters a barrier, then it can jump places and reach its destination faster than it is supposed to. This would probably be what distinguishes the first photon from the second in the experiment.

What do you guys think?

 

[San K]

Imoh,

I don't know much about tunneling, however found this paper...

According to the author of http://arxiv.org/ftp/arxiv/papers/0708/0708.3889.pdf

In summary, what is measured in tunneling time experiments is the time it takes for the energy stored in the barrier to leak out of both ends of the barrier. It is identical to the dwell time and should not be used to calculate a propagation velocity. The measured group delay is the photon lifetime within the stop band. This lifetime can be arbitrarily short for a highly reflective barrier. It does not imply that anything is traveling faster than light. My conclusion is that photons do not tunnel with superluminal group velocity.

I recently found an interesting read on the mysteries of quantum mechanics, here :
Don't know about you, but I am personally confused by this result. How can a photon travel faster than itself? If it can travel faster than itself, than what does that mean for the universal speed limit?

My own uneducated guess is that this phenomenon, called quantum tunneling, only applies in limited cases (such as the one established for the second set of photons). Tunneling as far as I am concerned is similar to teleportation, in that you jump from one place to another without crossing the intervening space (in this case, the barrier). But this discrete motion is different from continuous motion, which is what we usually associate with the speed of light. In terms of the latter, the photon is the fastest and nothing can surpass it. So most of the time, a photon would be moving continuously at the speed of light, but in cases where it encounters a barrier, then it can jump places and reach its destination faster than it is supposed to. This would probably be what distinguishes the first photon from the second in the experiment.

What do you guys think?

 

[ZapperZ]

I recently found an interesting read on the mysteries of quantum mechanics, here :

How old is this "news"?

This is why we strongly do not recommend citing a webpage as valid sources or references. You must provide exact published, peer-reviewed citation if you wish to create a discussion on here.

As has been mentioned, Winful and others already had a number of papers published that directly dealt with this issue:

H. Winful, PRL v.90, p.023901 (2003)
M. Buttiker and S. Washburn, Nature v.422, p.271 (2003)
H. Winful, Phys. Rep. v.436, p.1 (2006)

Read those first before progressing any further.

Zz.

 

[lmoh]

Actually, I was just looking for the paper mentioned on the website, and I think I have found it on arxiv: http://arxiv.org/abs/quant-ph/9811019. If you want to know the date, it was made in 1998.

Here is a bit from the abstract:

Experiments have shown that individual photons penetrate an optical tunnel barrier with an effective group velocity considerably greater than the vacuum speed of light. The experiments were conducted with a two-photon parametric down-conversion light source, which produced correlated, but random, emissions of photon pairs. The two photons of a given pair were emitted in slightly different directions so that one photon passed through the tunnel barrier, while the other photon passed through the vacuum. The time delay for the tunneling photon relative to its twin was measured by adjusting the path length difference between the two photons in a Hong-Ou-Mandel interferometer, in order to achieve coincidence detection. We found that the photon transit time through the barrier was smaller than the twin photon's transit time through an equal distance in vacuum, indicating that the process of tunneling in quantum mechanics is superluminal. Various conflicting theories of tunneling times are compared with experiment.

I will try reading those papers, but as I said, I don't have any knowledge of QM. I am still open to other answers though, hopefully one that I can understand.P.S. : Zapper, can you provide the titles of those papers? It will help a lot if I can search for the actual papers rather than references.

 

[Brian Lakstins]

What if multiple barriers are used?
Will the photon reduce it's time by 3.6 femtoseconds for each barrier?
Can enough barriers reduce the travel time to a point where it is outside the probability location of the photon when it arrives at the collector?

 

[Jilang]

No, there is a dwell time before the barrier where the energy builds up and before it leaps across so to speak.

 

[Brian Lakstins]

I understood that quantum tunneling is about the probable locations of a particle, and that the tunneling occurs because there is a possible location for the particle on the other side of the barrier.

Does it work some other way?

If there is some kind of energy build up, where does that energy come from?

 

[ZapperZ]

I understood that quantum tunneling is about the probable locations of a particle, and that the tunneling occurs because there is a possible location for the particle on the other side of the barrier.

Does it work some other way?

If there is some kind of energy build up, where does that energy come from?

Why should there be an "energy build up"?

In ballistic tunneling, the energy of the particle that tunneled through is the same as the energy of the particle that went in. This is an elastic tunneling. This nothing more than an example of Fermi's Golden Rule.

Zz.

 

[Jilang]

If the particle is coming from the left, the energy builds up from the left. The final energy of the outgoing particle is the same as the incoming one though.
http://arxiv.org/pdf/quant-ph/0403010.pdf

 

[ZapperZ]

If the particle is coming from the left, the energy builds up from the left. The final energy of the outgoing particle is the same as the incoming one though.
http://arxiv.org/pdf/quant-ph/0403010.pdf

I'm sorry, but I still don't understand this. Can you show me exactly the mathematics/physics that shows this "energy build-up"? Is this build-up present in the elementary treatment of tunneling that we teach undergrads in intro QM?

Zz.

 

[Jilang]

Not in elementary QM, perhaps in queuing theory. The paper mentions the slowing down of the particles as they approach the barrier. Wouldn't that cause a bunching up?

 

[Brian Lakstins]

If the particle is traveling at the current limit of speed for the medium it is traveling in, how does it "know" the barrier is there so that it can slow down?
Is it because the barrier's effect on the medium?
Is it because the particle is a probability function and some of those probabilities are being taken away by the barrier so they have to "compress" away from the edge of the probability function until the particle actually "reaches" the barrier and the probability function is large enough to include probabilities on the other side of the barrier?

And thanks for entertaining this conversation with me. I'm not a physicist. I'm just a person who wants the world around me to make some kind of understandable sense.

 

[ZapperZ]

Not in elementary QM, perhaps in queuing theory. The paper mentions the slowing down of the particles as they approach the barrier. Wouldn't that cause a bunching up?

Would it? Will this happen for a square barrier, or an arbitrary barrier where you do a WKB-type approximation? If it is only for the latter, then the "slowing down" is not a function of tunneling, but rather due to the change in E-V, which will happen no matter if there is tunneling or not. Not only that, in "real life", such piling up will create a "space charge" effect if these are electrons, for example, which in turn will modify the single-particle potential field. Have we seen such effects? I haven't.

I still don't see this energy pile up in the physics of tunneling. I wish someone can just point this out to me.

Zz.

 

[vanhees71]

Why and where some energy should pile up? Have you ever done the calculations with potential wells and looked at wave packets (scattering states) running through them? Here are some animations (with German explanations only, sorry):

http://theory.gsi.de/~vanhees/faq/quant/node35.html
...
http://theory.gsi.de/~vanhees/faq/quant/node39.html

 

[ZapperZ]

Why and where some energy should pile up? Have you ever done the calculations with potential wells and looked at wave packets (scattering states) running through them? Here are some animations (with German explanations only, sorry):

http://theory.gsi.de/~vanhees/faq/quant/node35.html
...
http://theory.gsi.de/~vanhees/faq/quant/node39.html

Yes? And?

The probably density get messed up by the potential barrier. There's no disagreement there. But where is the energy build-up, and how is this manifested in our measurement?

Zz.

 

[vanhees71]

Which energy buildup. I need a definition of what you mean by that to hopefully being able to answer the question.

 

[ZapperZ]

Which energy buildup. I need a definition of what you mean by that to hopefully being able to answer the question.

Please read Post #7, #9, and #11 that I was responding to. I was not the one who made a claim of an "energy build-up". I merely questioned the presence of such. Your post appeared to respond to my last question, so I presumed you knew what it was.

Zz.

 

[Jilang]

Vanhees, this is my fault entirely. Considering the case of multiple barriers, if the overall energy flow needs to be on average less than c, isn't it logical to consider that the speed between the barriers must be somewhat less?
P.s I would be very interested to see the second animation on a different scale. It is quite hard to ascertain what is going on, unlike the first animation which is more clear.