[McQueen (OP)]

I was browsing the web when I came across this article on “Quantum Encryption” published by New Scientist :
http://www.newscientist.com/article.ns?id=dn4914 (P.s If you see an empty space at the head of the page , scroll down. ) Can anyone tell me what is happening  ? Here is my take on things. Two quantum entangled photons are made through a process known as “parametric down conversion”. Since the photons are “entangled” they posses identical but opposite polarization. One of the photons is sent to a different location (B) , where its polarization is detected , the photon which remained at (A) must now have the opposite polarization. If the photon sent to (B) is interfered with , then the polarization of the photon at (A) will no longer correspond and it is immediately apparent that someone has tried to interfere with the transfer. But what does this achieve ? If the time and duration of the transfer is known together with the intervals at which the photons are sent , then even a child could break the encryption . So why not just use morse code instead , the only advantage is that with entangled photons , one would be aware of any attempts to interfere with the transfer process. Am I right in thinking this , or have I missed something. Further what does it prove about non-locality ?

[Dr. Praetoria]

Doestn't it seem that when dealing with nonlocality and particles, one is at the subatomic level and such particles indicate a "tendency" for not occurring with certanity--at a definite time or ways, but instantaneously?

[McQueen (OP)]

quote:Originally posted by Dr. Praetoria

Doestn't it seem that when dealing with nonlocality and particles, one is at the subatomic level and such particles indicate a "tendency" for not occurring with certanity--at a definite time or ways, but instantaneously?

That seems to be exactly the crux of the question and it is why everything is so exciting at the moment , because we are on the verge of finding out if this is actually true or not. A large part of Quantum mechanics deals with the wave function , the collapse of which should imply faster than light communication of some sort. This is actually an off-shoot ( to a certain extent ) of any wave theory which predicts that the group wave will follow the same velocity as the object while the actual wave , would move faster. In the case of light this implies FTL interactions. So , is the wave-particle theory true or not. It is quite a momentous occasion , the ability to manufacture entangled photons in a fairly dependable manner should allow this question to be answered. In the EPR thought experiment , two entangled photons with opposite spin , are separated and sent to different destinations  A and B which are spatially separated.If the polarization of the photon at B is changed , does it effect the polarization of the photon at A. Notice that this is completely different from merely detecting the polarization of the photon at B as given in the first example of this thread , which only proves that entangled photons have opposite polarization and does nothing to show that this polarization can be effected at spatially separate distances.

[McQueen (OP)]

This is interesting , I have been giving the problem of  devising an experiment to ascertain the wave – particle duality   some more thought and have come to the conclusion that it should be relatively simple to do. Just as in the “Quantum Encryption “
Setup described at the beginning of this thread , a pair of entangled photons is created using the parametric down conversion method. It should be noted that at this time the photons can have any polarization , all that is certain is that they will have opposite polarizations. (This has been amply demonstrated by the fact that such a quantum encryption has been carried out.) One of these photons  is sent to a spatially separated location ( i.e., a location sufficiently distant that light takes a perceptible time to reach it) at B  and the other photon is kept at A. The photon kept at A maybe made to travel ( through an optical cable etc., ) for a distance longer than the distance to B. The photon at B is polarized using a vertical filter , and then its presence or absence is detected and a similar vertical polarizer and detector is placed in front of  A.  There are now only two possibilities , if the photon at B is detected then the photon at A will not be detected and vice versa. Here is the important thing , if after 100 or so repetitions there is a 100% correspondence (i.e., photon at B no photon at A and vice versa etc., ) then wave particle duality , the wave function and everything else in Quantum Mechanics is validated. If however there is an appreciable discrepancy in the results , then it proves once and for all , that wave particle duality is false , that the wave function at least to do with photons does not exist and that faster than light transactions and non locality are a myth. The question is , since the equipment already exists to carry out such an experiment , why has it not been done and why have the results not been widely disseminated? After all if the results are positive it would be the greatest vindication of Quantum mechanics ever.

[gsmollin]

quote:Originally posted by McQueen

I was browsing the web when I came across this article on “Quantum Encryption” published by New Scientist :
http://www.newscientist.com/article.ns?id=dn4914 (P.s If you see an empty space at the head of the page , scroll down. ) Can anyone tell me what is happening  ? Here is my take on things. Two quantum entangled photons are made through a process known as “parametric down conversion”. Since the photons are “entangled” they posses identical but opposite polarization. One of the photons is sent to a different location (B) , where its polarization is detected , the photon which remained at (A) must now have the opposite polarization. If the photon sent to (B) is interfered with , then the polarization of the photon at (A) will no longer correspond and it is immediately apparent that someone has tried to interfere with the transfer. But what does this achieve ? If the time and duration of the transfer is known together with the intervals at which the photons are sent , then even a child could break the encryption . So why not just use morse code instead , the only advantage is that with entangled photons , one would be aware of any attempts to interfere with the transfer process. Am I right in thinking this , or have I missed something. Further what does it prove about non-locality ?

The entangled photons are used only to transmit a cryptographic key to two separated locations. The value of the key is random, but it is known in two separated locations, and the transmission of the key was secure. Now this key is used in one location to encrypt a message. Then the message is sent conventionally to the other location, where the key is used to decrypt the message. This key is used only once. A new key is transmitted by entangled photons for the following message.

"F = ma, E = mc^2, and you can't push a string."

[DoctorBeaver]

My brain hurts! [xx(]

[McQueen (OP)]

quote:Originally posted by gsmollin

quote:
The entangled photons are used only to transmit a cryptographic key to two separated locations. The value of the key is random, but it is known in two separated locations, and the transmission of the key was secure. Now this key is used in one location to encrypt a message. Then the message is sent conventionally to the other location, where the key is used to decrypt the message. This key is used only once. A new key is transmitted by entangled photons for the following message.

Thank you for a clear explanation of how quantum encryption is actually carried out. More important than this however is the subject I had referred to in the previous  post in this thread , which could clear up a controversy that has been raging for the past 300 years , namely the question of whether light is a wave or a particle. The results of the experiment if positive would be a crushing vindication of the Quantum Mechanics theory of wave-particle duality. Has it been done ? Are there any references to such an experiment wherein  a quantum entangled photon pair is spatially separated and then one of them is polarized , affecting the other photon’s polarization.

[gsmollin]

The entangled photons are oppositely polarized as they separate. Of course we are using mixed metaphors when we say that. The direction of spin is the proper term for a photon. Polarization is the matching property of the photon's wave function.

I cannot answer your question, however. As far as I understand the process, querying the spin of the photon requires its destruction. How this affects the entangled photon is unclear. It should result in a matching destruction if spin is to be conserved, but how would the matching photon be read at the other location if that happened?  This is intriguing, and I would like to study the details, assuming they are public.

"F = ma, E = mc^2, and you can't push a string."

[McQueen (OP)]

quote:Originally posted by gsmollin

The entangled photons are oppositely polarized as they separate. Of course we are using mixed metaphors when we say that. The direction of spin is the proper term for a photon. Polarization is the matching property of the photon's wave function.
I cannot answer your question, however. As far as I understand the process, querying the spin of the photon requires its destruction. How this affects the entangled photon is unclear. It should result in a matching destruction if spin is to be conserved, but how would the matching photon be read at the other location if that happened?  This is intriguing, and I would like to study the details, assuming they are public.

“What a tangled web we weave ….”  Take a look at this web page
http://www.cs.caltech.edu/~westside/quantum-intro.html and tell me what you think.  First note that the web page is apparently from Caltech and should therefore ( normally ) ,  be an unimpeachable source. I am referring specifically to the Diagrams marked A and B .  As far as I can make out in Diagram A the photon , once it reaches the beam splitter , has an equal chance of going either to the detector at (A) or (B) . If one detector is activated the other is always silent. Now in Diag. B adding two mirrors and another beam splitter , results in the photon always going to the detector at (A). This is weird enough , but if one of the paths is blocked by an (absorbent) material , then the photon once again has an equal chance of going to either of the detectors (A) or (B). The reason is apparently that in keeping with Quantum Mechanics and the theory of superposition , the photon is actually taking both paths. As far as I am concerned this means one of two things. (a) Either the data and consequently the information about the experiment is being deliberately distorted OR (b) we are entering the Harry Potter era in earnest. I had tried to trace the paper by Deutsch and Ekert and have a look at the original diagram , without success. Could you help out. It would mean a lot.  P.S The only possible explanation I can think of is Fermat’s Principle “ A light ray chooses the path of minimum time in traveling from one point to another.” In which case the photon in the first diagram should also always activate detector (A). P.P.S I know this is not exactly what your post was about but it is still interesting. P.P.P.S I found the original paper and the original diagram and the key sentence Is:" imagine an experiment shooting a photon into a half-silvered mirror.........."

[gsmollin]

Wave-particle duality is the original shocking result of QM. The double-beam-splitter experiment you reference shows this shocking result. The detector at A gets all the photons because constructive interference of the wave functions gives an anti-node of amplitude there. This is analogous to the bright line in a classical interference pattern. At detector B we have a node of amplitude, or a dark space in the classical double-slit experiment.

The crazy part is that the interference is happening with a single photon. Feyneman though the photon took all possible paths, which he called a "sum of histories". So the photon splits at the mirror, and two entangled photons go on both paths. They recombine at the second mirror where their wave functions interfere. Any attempt to measure one of the entangled photons collapses the entanglement, and the photon is measured at only one place. The photon is always only measured at one place. It can only be in two places at once if we are not looking.

It is really weird.

"F = ma, E = mc^2, and you can't push a string."

[DoctorBeaver]

quote:So the photon splits at the mirror,...

Please excuse my ignorance on this subject, but that quote caught my eye. I thought that splitting particles either required or released a tremendous amount of energy; surely far more than just hitting a mirror.

We learn from history that we do not learn from history.
(Georg Hegel)