Can a Klyshko photon be blocked?

[.Scott]

Back in October, while contributing to a thread in this forum, I ran into an interesting description of a entanglement/double slit interference experiment. One of the contributors cited a 1999 "Reviews of Modern Physics" article. This remark is made on page S290:

This experiment can be understood intuitively if we carefully analyze what registration of a photon behind a double slit implies. It simply means that the state incident on the double slit is collapsed into a wave packet with the appropriate momentum distribution such that the wave packet peaks at both slits. By virtue of the strong momentum entanglement at the source, the other wave packet then has a related momentum distribution which actually is, according to an argument put forward by Klyshko (1988), the time reversal of the other wave packet. Thus, photon 1 appears to originate backwards from the double slit assembly and is then considered to be reflected by the wave fronts of the pump beam into the beam towards the lens which then simply realizes the standard Fraunhofer observation
conditions.

Let me see if I can explain this more simply. If we call that "time reversal" photon a "Klyshko photon", then what David Klyshko was "arguing" in some other 1988 article (that I have not located) was that before the big photon reaches the down-converter and splits into two entangled little photons, a little "Klyshko photon" is emitted from the experiment to meet the big photon at the down-converter. To be clear, this is a thoroughly bizarre notion that could not possibly exist in classical mechanics - and it's not clear that the author even considers it "real". In fact, I'm not even certain that David Klyshko considered it real.

But it is testable and I would love to know if anyone has ever checked it. If you put a shutter in the apparatus so that the path of the Klyshko photo is blocked only at the moment when the Klyshko photo would be reaching that shutter, would you still be able to retrieve the interference pattern that is collected in that experiment?

 

[DrChinese]

Not sure I follow exactly, but that K photon would be time reversed so that the arrival at the shutter would occur the same as one traveling forward in time. In other words, it would not lend itself to a testable prediction.

Down conversion is considered more or less time reversal of up conversion. And vice versa.

 

[.Scott]

Not sure I follow exactly, but that K photon would be time reversed so that the arrival at the shutter would occur the same as one traveling forward in time. In other words, it would not lend itself to a testable prediction.

Down conversion is considered more or less time reversal of up conversion. And vice versa.

The shutter would be some distance (perhaps 30cm) away from the converter. So you would close the shutter before the large photon reached the converter and open it before the regular small photon reached the shutter. You would need to be able to switch the shutter back on within about a nanosecond. I don't know if there are electronic shutters that operate that quickly. But if not, there are probably other ways to configure the experiment.

The fact that the shutter is closed only when the main photon has yet to reach the first part of the experimental setup is what would make the effect (if it exists) especially bizarre.

 

[DrChinese]

The shutter would be some distance (perhaps 30cm) away from the converter. So you would close the shutter before the large photon reached the converter and open it before the regular small photon reached the shutter. ...

The 2 arrive at that shutter at exactly the same time because one is time reversed (coming backwards from the future).

 

[.Scott]

The 2 arrive at that shutter at exactly the same time because one is time reversed (coming backwards from the future).

Ahhh. So the time reversal is only for the purpose of imagining the small photon is traveling in the right direction, carrying information back in time. It isn't to suggest that there is another virtual photon.

Thanks.

 

[Jilang]

You might like this article which is recent and based on weak measurements. The actual path of the photon appears to be a coincidence of a forwards in time wave and a backwards in time wave. Is this at the heart do the Born Rule I wonder?
http://arxiv.org/pdf/1304.7469.pdf
This site gives a nice summary of it if the paper gets hard going.
http://www.thefunisreal.com/2013/11...ories-quantum-mechanics-1000-pm-know-photons/

 

[Cthugha]

Vaidman already presented these results at a conference last year. While they are nice, his two-state vector formalism does not add anything new from the conceptional point of view. You get the same results from standard quantum mechanics (as Vaidman also acknowledges in that paper). The really good thing about his two-state vector formalism is that it is so simple you can teach it to your dog. Some people in quantum optics simply prefer it because their undergrads can evaluate a moderately complicated setup in a rather short time using the two-state vector formalism, while it is not so obvious what will happen when using the standard approach.

 

[Jilang]

Does it apply just to optics or to particles as well?

 

[Cthugha]

Particles as well. It is just one special kind of time-symmetric interpretation and goes back to the 1920s. If I remember correctly, Schottky suggested it first.