Twin-Slits Experiment - Photons no longer...

Quantum Mechanics

Study matter and its motion through spacetime...

Moderators: Calilasseia, ADParker

Twin-Slits Experiment - Photons no longer...

#1  Postby Beelzebub » Jun 03, 2011 3:05 pm

...seem to pass through both slits simultaneously?
From BBC

The article states that
"While they were able to easily observe the interference pattern indicative of the wave nature of light, they were able also to see from which slits the photons had come, a sure sign of their particle nature."


So, it seems that Photons actually do pass through only one slit at a time, not both simultaeously, as had been thought?
So what happens to the idea that individual Photons 'interfere' with themselves, as they pass through both slits at the same time?
If this result is corroborated, why do interference patterns still emerge, even when only one Photon at a time traverses the experiment?

What do the more knowlegable members here think of this? :think:
User avatar
Beelzebub
THREAD STARTER
 
Posts: 66

Print view this post

Ads by Google


Re: Twin-Slits Experiment - Photons no longer...

#2  Postby Calilasseia » Jun 03, 2011 3:32 pm

Bugger, you beat me to it. I was just about to post this!

This is probably going to involve entangled quantum states between the real photon and a virtual partner.
Signature temporarily on hold until I can find a reliable image host ...
User avatar
Calilasseia
RS Donator
 
Posts: 22011
Age: 57
Male

Country: England
United Kingdom (uk)
Print view this post

Re: Twin-Slits Experiment - Photons no longer...

#3  Postby Evolving » Jun 03, 2011 4:32 pm

I don't think that can be right, Cali; though I freely admit that, having read just the abstract of the article in Science, it is not entirely clear to me what they have done:

Abstract
A consequence of the quantum mechanical uncertainty principle is that one may not discuss the path or “trajectory” that a quantum particle takes, because any measurement of position irrevocably disturbs the momentum, and vice versa. Using weak measurements, however, it is possible to operationally define a set of trajectories for an ensemble of quantum particles. We sent single photons emitted by a quantum dot through a double-slit interferometer and reconstructed these trajectories by performing a weak measurement of the photon momentum, postselected according to the result of a strong measurement of photon position in a series of planes. The results provide an observationally grounded description of the propagation of subensembles of quantum particles in a two-slit interferometer.
How extremely stupid not to have thought of that - T.H. Huxley
User avatar
Evolving
 
Name: Serafina Pekkala
Posts: 11593
Female

Country: Luxembourg
Luxembourg (lu)
Print view this post

Re: Twin-Slits Experiment - Photons no longer...

#4  Postby Prof. Faust » Jun 03, 2011 5:06 pm

Can I not calculate the average trajectory of a classical wave? I feel like I am missing something important here.
For a moment, consider the set of all sets that have never been considered.
User avatar
Prof. Faust
 
Posts: 234
Male

United States (us)
Print view this post

Re: Twin-Slits Experiment - Photons no longer...

#5  Postby twistor59 » Jun 03, 2011 9:29 pm

It's a bit of a pain that the paper isn't freely available, so it's hard to know exactly what they've done. There is a short paper (one of the authors here is on the paper in question) talking about something similar

http://www.aip.org.au/Congress2010/Abstracts/Monday%206%20Dec%20-%20Orals/Session_3E/Kocsis_Observing_the_Trajectories.pdf
A soul in tension that's learning to fly
Condition grounded but determined to try
Can't keep my eyes from the circling skies
Tongue-tied and twisted just an earthbound misfit, I
User avatar
twistor59
RS Donator
 
Posts: 4966
Male

United Kingdom (uk)
Print view this post

Re: Twin-Slits Experiment - Photons no longer...

#6  Postby Lion IRC » Jun 03, 2011 10:03 pm

Calilasseia wrote:Bugger, you beat me to it. I was just about to post this!

This is probably going to involve entangled quantum states between the real photon and a virtual partner.


:popcorn:
Glued to the set for THIS one.

entangled. I like it.
FORMAL DEBATE - Lion IRC (affirmative) vs Crocodile Gandhi (negative)
Topic - Gay marriage should not be legalised in society.
Moderator - Durro
Now Showing HERE.
User avatar
Lion IRC
Banned Troll
 
Posts: 4077

Print view this post

Re: Twin-Slits Experiment - Photons no longer...

#7  Postby Beelzebub » Jun 04, 2011 2:14 am

Calilasseia wrote:Bugger, you beat me to it. I was just about to post this!

This is probably going to involve entangled quantum states between the real photon and a virtual partner.


I'm a bit unsure about this - in what way are the individual Photons 'entangled'? Where does a virtual partner come into play?

As I understand it (Say Wikipedia), virtual particles are extremely constrained in both space and time - either spontaneously (and very, very briefly) appearing as a virtual pair of particles within a static field, or appearing as a virtual mediator between two 'real' particles.

The twin-slit experiment seems to involve distances and times far larger than one would expect for virtual actions/interactions. I'm afraid I just don't 'buy' the notion of a single real Photon approaching a slit, deciding to spontaneously split into two virtual Photons, one going through one slit and the other going through the other slit, and then recombining on the other side to 're-constitute' a real Photon again. My main problem with this, is that the quantum state of the 'before' and 'after' Photon would be the same (Conservation laws), so this scenario resolves nothing.

Having said all this, we still have an interference pattern - even with only single Photons traversing the apparatus at any one time - that seems to need an explanation, or mechanism, of some sort.

As Twistor59 said, it is a shame the paper is behind a pay-wall. :(
User avatar
Beelzebub
THREAD STARTER
 
Posts: 66

Print view this post

Ads by Google


Re: Twin-Slits Experiment - Photons no longer...

#8  Postby twistor59 » Jun 04, 2011 7:30 am

The critical component in their experiment seems to be the concept of a weak measurement.

Hrvoje Nikolic, in this link clarifies:

To conclude, in a Ballentine style: A strong measurement reveals a property of an individual system, but a weak measurement only reveals a property of a large STATISTICAL ENSEMBLE of equally prepared systems. A weak measurement says nothing about properties of an individual system. All weirdness of weak values results from attempts to interpret properties of an ensemble (2.6 children) as properties of an individual system (a family).


(Ballentine is the author of a well known book on QM)

Edit And if you're bored this weekend here is a paper with more detail on weak measurements.
A soul in tension that's learning to fly
Condition grounded but determined to try
Can't keep my eyes from the circling skies
Tongue-tied and twisted just an earthbound misfit, I
User avatar
twistor59
RS Donator
 
Posts: 4966
Male

United Kingdom (uk)
Print view this post

Re: Twin-Slits Experiment - Photons no longer...

#9  Postby Beelzebub » Jun 04, 2011 1:33 pm

twistor59 wrote:The critical component in their experiment seems to be the concept of a weak measurement.

Hrvoje Nikolic, in this link clarifies:

To conclude, in a Ballentine style: A strong measurement reveals a property of an individual system, but a weak measurement only reveals a property of a large STATISTICAL ENSEMBLE of equally prepared systems. A weak measurement says nothing about properties of an individual system. All weirdness of weak values results from attempts to interpret properties of an ensemble (2.6 children) as properties of an individual system (a family).


(Ballentine is the author of a well known book on QM)

Edit And if you're bored this weekend here is a paper with more detail on weak measurements.


I had a quick look through this paper (Whilst listening to the Cricket!) - although its content is way beyond my meager abilities :tongue:
I wonder if the Science paper has taken account of Stephen Parrott's objections to weak measurements - namely that the weak values are not unique?
User avatar
Beelzebub
THREAD STARTER
 
Posts: 66

Print view this post

Re: Twin-Slits Experiment - Photons no longer...

#10  Postby Beelzebub » Jun 04, 2011 1:38 pm

---duplicate post---
(Why is posting/reviewing so damned slow, to the point of continuously timing-out? :scratch:
User avatar
Beelzebub
THREAD STARTER
 
Posts: 66

Print view this post

Re: Twin-Slits Experiment - Photons no longer...

#11  Postby Evolving » Jun 04, 2011 3:00 pm

Twistor: yes, that does seem to be the key: you do a "weak measurement", meaning you measure something else, after the event, but influenced by the event, and repeat that many times; and then you work backwards to determine what, on average, the event looked like. Since particles are only clouds of probability, there is no difficulty, in principle, in calculating averages over those probabilities.

But if that's all it is, I don't see what the fuss is about!

Clearly I'm missing something.
How extremely stupid not to have thought of that - T.H. Huxley
User avatar
Evolving
 
Name: Serafina Pekkala
Posts: 11593
Female

Country: Luxembourg
Luxembourg (lu)
Print view this post

Re: Twin-Slits Experiment - Photons no longer...

#12  Postby Evolving » Jun 04, 2011 3:09 pm

Unless this is about interpretation: the realism debate. Are they saying that their averaging calculations show that each particle is, in fact, passing through just one slit, i.e. the system has non-probabilistic qualities, independently of whether they are measured? Which would be the EPR claim that was disproved by Bell (involving, indeed, entangled particles, in that case).
How extremely stupid not to have thought of that - T.H. Huxley
User avatar
Evolving
 
Name: Serafina Pekkala
Posts: 11593
Female

Country: Luxembourg
Luxembourg (lu)
Print view this post

Re: Twin-Slits Experiment - Photons no longer...

#13  Postby twistor59 » Jun 04, 2011 7:16 pm

Evolving wrote: Are they saying that their averaging calculations show that each particle is, in fact, passing through just one slit, i.e. the system has non-probabilistic qualities, independently of whether they are measured?


Maybe, I dunno, I'd like to see the paper !!

I don't see how they could deduce that from a weak measurement though.....
A soul in tension that's learning to fly
Condition grounded but determined to try
Can't keep my eyes from the circling skies
Tongue-tied and twisted just an earthbound misfit, I
User avatar
twistor59
RS Donator
 
Posts: 4966
Male

United Kingdom (uk)
Print view this post

Re: Twin-Slits Experiment - Photons no longer...

#14  Postby my_wan » Jun 04, 2011 9:37 pm

Evolving wrote:Unless this is about interpretation: the realism debate. Are they saying that their averaging calculations show that each particle is, in fact, passing through just one slit, i.e. the system has non-probabilistic qualities, independently of whether they are measured? Which would be the EPR claim that was disproved by Bell (involving, indeed, entangled particles, in that case).

Yes it is about interpretation, but such interpretations can play pivotal roles in more complete theories, as well as inventing applications that are not so obvious and cannot be intuitively dreamed up prior to running the math on a situation that has not been thought of yet.

Whether or not EPR was disproved by Bell hinges on whether you accept the *operational* meaning of causality defined in EPR. To articulate the difference between a definition and an *operational* definition consider an experiment that demonstrates that event A empirically leads to event B. Then did A cause B or did a cause C cause A then B. If C caused A then B then a third observer in a different frame that sees B then A is not the reversal of causality that the assumption that A caused B would make it seem to be.

The counterfactuals the Bell's theorem hinges upon to claim a "proof" is exquisitely dependent on the assumptions of specifically what caused what, while the measurement problem dictates that we cannot know what causal factors are involved between measurement A and measurement B. You cannot factually know what you cannot see, and between measurements you *by definition* do not see what is happening.

Nice piece of work in that paper.
User avatar
my_wan
 
Posts: 967
Male

Country: US
United States (us)
Print view this post

Re: Twin-Slits Experiment - Photons no longer...

#15  Postby twistor59 » Jun 05, 2011 7:50 am

my_wan wrote:

Nice piece of work in that paper.


Have you seen the paper ? If so, would you be able to summarise it in a few sentences.

From what I can piece together from second hand accounts, it is an ingenious way to do weak measurements on photons, and produces a bunch average trajectories that you'd get regardless of your chosen interpretation of quantum mechanics. However it reveals absolutely nothing new about quantum mechanics.
A soul in tension that's learning to fly
Condition grounded but determined to try
Can't keep my eyes from the circling skies
Tongue-tied and twisted just an earthbound misfit, I
User avatar
twistor59
RS Donator
 
Posts: 4966
Male

United Kingdom (uk)
Print view this post

Ads by Google


Re: Twin-Slits Experiment - Photons no longer...

#16  Postby my_wan » Jun 05, 2011 1:32 pm

I will need to make a trip into Chattanooga to get a complete copy, but the basics are fairly easy to understand from the sources given. The key to recovering the probable path information was achieved by passing the source photon through a calcite mineral. Thus only weakly perturbing the path as opposing to absorption and reemission required in strong measurements. This caused the photons that go one way to be nudged at a slightly sharper angle than the ones going the other way. This created a slight offset that can 'statistically' be recovered from the pattern on the detector screen. This means that if you divide the photon detections into a left and right group then some of the photons will be in the wrong group. Yet we know that more often than not they are correctly identified with the proper path information.

The only way I know to make sense of it is to assume the wavefunction is physically real, at least in an indirect sense. Something very roughly analogous to the self interference of a droplet of silicon oil or "walker" on water when passing through a slit. This was done in a purely classical system.
(PDF) http://oharas.com/PH/pdf78650511.pdf

The two main QM features this mimics is the path deviation when passing through the slit, due to spacial constraints on the waves, and interference patterns that build up after many oil droplets have passed through separately. In spite of these similarities a taking this analogy too literally simply does not work.

I suspect that the critiques will deny the validity of the path information gained this way, much as they did with the Afshar experiment. But we will have to wait and see what the critiques come up with. This experiment used thin wires to block light in the interference bands to show that the wires were not blocking any significant light, thus the interference pattern must exist to avoid the wires. Then instead of a detector screen it was passed through lenses to retrieve path information. You can read about it with diagrams and the criticism on the wiki page.
http://en.wikipedia.org/wiki/Afshar_experiment
http://www.irims.org/quant-ph/030503/

At this point I am more curious what kind of objections will soon surface.
User avatar
my_wan
 
Posts: 967
Male

Country: US
United States (us)
Print view this post

Re: Twin-Slits Experiment - Photons no longer...

#17  Postby Beelzebub » Jun 05, 2011 3:15 pm

my_wan,
Thanks for the links - I shall have a good look (Whilst listening to more Cricket! :grin: )
It is difficult to assertain what the Science article is actually saying, without reading it (Maybe my local library has copies? I'll check it out tomorrow), but it would be interesting to see comments and reviews of this article from others working in this field.
User avatar
Beelzebub
THREAD STARTER
 
Posts: 66

Print view this post

Re: Twin-Slits Experiment - Photons no longer...

#18  Postby twistor59 » Jun 05, 2011 4:22 pm

It looks to me like the experiment delivers the following information:

"If I have a whole bunch of photons which went, one at a time, through the 2 slit apparatus, then the average trajectories of the photons in the ensemble would look like the ones shown."

i.e the average trajectories look just like what you'd expect from a classical wave. Which is what prof. Faust said above.

An individual photon doesn't have a trajectory (which needs simulateneous values of position and momentum) as we all know, but you can measure (using the clever setup they have) the probability distributions of these variables at each point. If you then take the average at each point, the classical wave trajectories are what you end up with. Interpreted this way, it's a lovely confirmation of the predictions of quantum mechanics.
A soul in tension that's learning to fly
Condition grounded but determined to try
Can't keep my eyes from the circling skies
Tongue-tied and twisted just an earthbound misfit, I
User avatar
twistor59
RS Donator
 
Posts: 4966
Male

United Kingdom (uk)
Print view this post

Re: Twin-Slits Experiment - Photons no longer...

#19  Postby my_wan » Jun 05, 2011 9:13 pm

Twistor59 pretty much summed it up again in the quote he gave. It also, as he notes, provide a lovely confirmation of the predictions of QM. Something people often miss or confuse is that the interpretations of QM, which these experiments sometimes call into question, is NOT QM. QM works just the same with or without any interpretation whatsoever, and invalidating a given interpretation does not effect the predictions of in the least.

If you want to see some photon behavior that QM cannot predict see:
The shadow of light: further experimental evidences
This is far more straightforward to understand the experimental to, just by comparing the diagram to the raw results block (figure 5 to table 3). Though the details of why it is so out there may take a bit more looking under the hood of QM.

Here is another version without the nice neat results block to compare directly to the diagram.
The shadow of light: evidences of photon behaviour contradicting known electrodynamics

It looks to me like these experiments may actually be measuring the energy associated with the wavefuction itself, which if true would mean that so called interaction free measurements are not interaction free.
User avatar
my_wan
 
Posts: 967
Male

Country: US
United States (us)
Print view this post

Re: Twin-Slits Experiment - Photons no longer...

#20  Postby my_wan » Jun 07, 2011 4:23 pm

Here is a blog that does an excellent job of explaining "weak measurements" and what it means in terms of th OP subject:
http://scienceblogs.com/principles/2011 ... re_obs.php
User avatar
my_wan
 
Posts: 967
Male

Country: US
United States (us)
Print view this post

Next

Return to Physics

Who is online

Users viewing this topic: No registered users and 1 guest