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Where do I post this question? And, hi! This is my first post!

  1. Aug 5, 2010 #1
    Hi! I'm ZxWhatever (Short for Zxblqcktptydfkjsplkn, my username elsewhere). This is my first post here. I'm going in to high school next year, and physics has always fascinated me, specifically quantum physics.

    You may think this is WAY to early to be thinking about a science fair, but I want to get my project done early this year. I have never won any sort of award at any science fair. The judging in my district is done by parents, stupidly. I had done several projects that I considered excellent, but I think dying Barbie's hair with different dyes, including soy sauce, and seeing which lasted the longest was more impressive. Sadly, this was an actual winning project, from the seventh grade. Well, physics is my best area of science, and science is my best area in school, so I knew that project was garbage. However, my project simply didn't have the wow factor required.

    I was doing some reading on Wikipedia (Please don't tell me it's a bad resource. I know I should never use information from it, so this was for my own interest.), and I came across a page on Quantum Key Distribution. I was intrigued, so I did some more research. I finally decided I want to do a project on it. It seemed like, if I could pull it off, a demonstration of QKD would be a great, possibly winning project.

    The problem is, I need a very low-intensity source of photons. I was thinking of using a very small amount of a positron emitter such as cesium. The annihilation of electrons and positrons would release small amounts of light, which could be polarized and used to demonstrate QKD. However, I am afraid that I won't be allowed to have even the milligram or less this would require because people would be terrified of even a micro-curie source of radiation, and would not even the tiniest bit of antimatter into the hands of a minor. Even though the computer I'm typing this on is almost certainly more radioactive than the source I would require, I doubt I would be allowed to have any material labelled "Radioactive." After doing more research, I discovered that commercial QKD uses an extremely low-intensity laser, but I couldn't find where to buy one.

    I have two questions for you. First, where can I get an extremely low-intensity laser? It needs to emit one or two photons in bursts I can control, and can't cost more than about $150, as that's my budget. Second, if the answer to my first question is it is impractical or impossible to get one in that price range, how can I get a positron emitter? Do I need some sort of license? Where would I buy it? I have parental permission, will be supervised by an adult (an adult with a PHD in particle physics, no less), and will keep the thing encased in lead, if required. Third, is there any other low-intensity source of photons I can use? I want to go with the most cost-effective, least objectionable option, so if there are any other options, I'd like to know.

    If you need any other information, just ask. I'm sorry if this is poorly written, poorly edited, and rambling, but it's late and I'm going to bed. I don't care to stay up any later that I have to to write this. Thanks for reading!
  2. jcsd
  3. Aug 5, 2010 #2


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    Welcome Zx! Wow, a lot on you plate for your age!
  4. Aug 6, 2010 #3


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    So I'm guessing after hours of struggling and failing to find a username that wasn't already taken, you mashed your keyboard in frustration and this is the end result? :biggrin:
    I can't really help you with your troubles, but I can welcome you to PF with open arms! Welcome, young one.

    p.s. wikipedia is a terrible source of information.

    just kidding hehe
  5. Aug 6, 2010 #4


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    One would need a license for possession and utilization of a radionuclide.

    See - http://hps.org/publicinformation/ate/q6996.html [Broken]

    A positron emitter will produce some moderate energy (0.511 MeV = 511 keV) gamma photons when the positron annihilates with an electron.

    A low level photon emitter would be possible, e.g., a light emitting diode (LED).


    http://www.edmundoptics.com/onlinecatalog/Browse.cfm?categoryid=239 [Broken]

    Otherwise one could use a laser pointer, but the intensity is much more than a few photons/s.
    Last edited by a moderator: May 4, 2017
  6. Aug 6, 2010 #5


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    Hi, Zx!
    If you were to buy some fish for some of that money, what sort of fish would you like to buy? :smile:
  7. Aug 6, 2010 #6
    I would like to buy some lobster. I say that's a fish, even though it isn't.

    I sort of assumed I couldn't get anything radioactive.

    I made a mistake in my original post. I thought of a laser first, but, when I couldn't find one with a low enough intensity, I thought of positrons.

    Astronuc, I've already searched for lasers. I couldn't find one. If you could provide a link to a specific laser, that would be great.

    I could just starve a standard laser for voltage by using resistors. That might work.

    It really needs to be a single photon at a time, or else there is no advantage to using QKD. I'll outline the process as I understand it here. If it's right, it should explain why I need a single photon. If it's wrong, please correct me. Also, I'll probably get some terminology wrong. Don't hesitate to correct it.

    Alice has a source of single photons, and a photon polarizer. Bob has a polarity detector. Alice chooses to polarize each photon, independently, diagonally or rectilinearly at an angle of 0° or 90°. She can send a single bit, a 1 or 0. A 1 on a diagonal basis would be 45°, while a 0 on a diagonal basis would be 315°. A 1 on a rectilinear basis would be vertical, while a 0 would be horizontal. She selects a one or a zero and a polarization basis randomly with each photon she sends. Bob picks a random polarization basis for his detector with each photon he receives. If his basis agrees with Alice's, he will get the same value as Alice for the bit. If his basis is different, he will get a random 1 or 0. This is because a quantum bit can only be measured as a 1 or 0. It can actually be any combination of the two, but this only determines the probability it will be measured as a 1 or 0, not the value measured. So, since the angle is always between the two measured angles if the bases don't agree, the value will be random. Then, Bob sends the position of his detector to Alica over standard communication. Eve sends back a single bit stating whether this value is right or wrong, once again, over standard communication. If Alice says the bases were the same, the values are kept, along with the value of the bit sent and received. This is repeated several thousand times to generate a good key. Then, using the values they kept, each person calculates an error correction code. These codes are compared over standard, unencrypted communication. If they are the same, the generated key can be used. If they aren't the same, the transmission was either intercepted or there was an error, and the key must be re-submitted.

    The reason this is useful is that if Eve tries to intercept the key, she changes it. If her detector is in the same position as Eve's polarizer every time, she can intercept the key perfectly and simply re-submit her measured value to Bob. However, if it is wrong, she changes the basis. This is simply a fundamental property of quantum mechanics. By measuring it, she changes the polarity basis to what her detector was set to. With each photon sent, she has a 50% chance of being wrong. If Bob is wrong, too, this doesn't matter, since the bit will be thrown out. However, since Bob has a 50% chance of being right, Eve has a 25% chance of being wrong when Bob is right, meaning she changes the basis 25% of the time with a bit that won't be thrown out. Then, since Bob has a 50% chance of getting a value different from Eve when the basis is different, there is a 12.5% chance that Eve will be detected with each photon sent, since the value would be different without the basis of polarity being different. Since several thousand photons would be sent, the odds of Eve being caught become astronomical very quickly.

    Is that how it works? Please, if I made any mistakes, tell me what they are.

    If there's a better forum to post this in, please move it, Mod.
  8. Aug 6, 2010 #7
    Haha, are you gonna be the next Thiago Olson? [ <--- google him ]
  9. Aug 6, 2010 #8


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    Hi and welcome Zx, seems pretty advanced for a man at your age... :bugeye:

    First obvious question: Will the parents really appreciate your project? Dying Barbie's hair with soy sauce seems a bit far from quantum key distribution...? :smile:

    Maybe you should first run a "market survey" by letting the parents decipher this message:
    yhpargotpyrc mutnauq

    Anyhow, if you decide to proceed, you must first know that Quantum Cryptography is still under development, and second, "a very low-intensity source of photons" will not do the trick. If your budget is $150, I’m afraid it’s a dead end.

    But if you are still interested, read everything you can about http://en.wikipedia.org/wiki/Bell%27s_theorem" [Broken]. That’s where it all starts.

    Your outline of the process is almost correct, I guess, but I would explain it like this:
    You could send 1000 entangled photons to Alice & Bob, where both have their polarizers at 0º. The measured result will be 100% random, but perfectly correlated between Alice & Bob. Now Alice could openly send the first 100 results to Bob for verification, and if all is okay, they start using the other 900 results as a secure key for cryptography.

    Problem: Eve could be doing eavesdropping between Alice & Bob, and retransmit a polarized copy of the received photon. Alice & Bob could never tell the difference.

    Solution: Alice & Bob let their polarizers change randomly between 0º, 120º and 240º. After measuring 1000 entangled photons, they exchange openly the settings on their polarizers during the measurement. Then Alice & Bob use Bell's theorem on the results where the angles differ, and they can tell with 100% certainty if Eve is listening, or not.

    When Alice & Bob are sure the transmission was okay, they use the results where the angles was identical, knowing that these result are 100% secure and 100% correlated for the crypto key.​

    Neat, isn’t it!? :rolleyes:

    If you can explain this to the parents using something cheaper than laser and BBO crystals, I think you will win! :wink:

    Finally, here’s an introduction video for EPR-Bell experiments:

    https://www.youtube.com/watch?v=<object width="480" height="385"><param name="movie" value="http://www.youtube.com/v/c8J0SNAOXBg&hl=en_US&fs=1&rel=0&color1=0x006699&color2=0x54abd6"></param><param [Broken] name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/c8J0SNAOXBg&hl=en_US&fs=1&rel=0&color1=0x006699&color2=0x54abd6" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="480" height="385"></embed></object>

    This explains everything in detail: http://arxiv.org/abs/quant-ph/0205171" [Broken]

    Good luck!
    Last edited by a moderator: May 4, 2017
  10. Aug 7, 2010 #9


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    *Smacks ZX on the head with a 50 pound lobster, CRUNCH!*

    You are now properly initiated at PF! :smile:
  11. Aug 7, 2010 #10
    You can probably get a smoke detector containing this amount of Americinium-241 anywhere they sell for $10-$20.


    I don't think reliable single-photon emitters are commercially available. These guys claim to use some in their product. Remembering a talk I attended, it wasn't necessarily one at a time, but it did strictly serve their product's application. Either way, they certainly know the subject :

    http://www.idquantique.com/true-random-number-generator/products-overview.html [Broken]
    Last edited by a moderator: May 4, 2017
  12. Aug 16, 2010 #11
    Just so you know, I'm using the BB84 protocol. It's slightly different than what you specified. And I know my explanation was horrible. I was writing to get my thoughts down, not for clarity. I will explain the fundamentals first, then the details, like you did, when actually presenting. Besides, I think better out loud.

    It really needs to be a single photon. If it isn't, it's just a really slow, insecure, unreliable way of distributing a key, rather than the uncrackable super-encryption the real deal is.

    My friend's dad works in optics. I'll ask him for a good supplier. They might have some.

    And Americium won't work. It's an alpha emitter, not a positron emitter.
  13. Aug 16, 2010 #12
    Americium releases gamma photons + alpha particles. The alphas are +, so in principle, you should be able to deflect them with an E field, and remain with a gamma source similar to e+ & e- annihilation. Although I leave it up to you to polarize gammas rays.
  14. Aug 17, 2010 #13


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    I dunno, I thought your explanation was pretty good (and accurate). Makes me wonder how you managed to find all that information about quantum cryptography before entering high school...

    But anyway: from what I've heard, real quantum cryptography has only been demonstrated in well-funded (i.e. O($105)) research labs. It's not the kind of thing you could pull off on a $150 budget, at least not unless you invent some radical new technique for producing and measuring the photons. (If you do, you will become very very rich in short order :tongue2:) Besides, it's kind of boring to watch - quantum key exchange involves hooking up a bunch of fiber optic cables and then sitting around while the computers do their stuff. You wouldn't really see anything happening.

    For a science fair, I think if you were able to construct some sort of demonstration using normal-sized physical objects, it would probably go over better (i.e. be more interesting), and it would definitely be cheaper than real quantum cryptography. Of course, normal-sized physical objects don't show quantum effects very well. Your challenge would be finding a way to get around that, perhaps by constructing something that can represent a photon (in the sense that a spectator or judge can partially, but not completely, measure its actual state).
  15. Aug 17, 2010 #14


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    Cool, I was wrong! You do not need entangled pair of photons! Always nice to learn new things and the BB84 protocol looks real clever.

    One thing that struck me: What happens when Alice and Bob broadcasts the basis of "sent & measured" over a public channel, if Eve is eavesdropping and modifying also this message, and settings to fit the results after Eve’s first eavesdropping on the polarized photons? Is this really 100% secure??

    Never mind, I think you should follow diazona’s very wise advice and make a macroscopic version so spectators really can see what happens. If you’re not in physics, this will look like a "magician trick" :smile: at the microscopic level, and at worst people are maybe going to question if you are cheating...

    Are you into programming and CG? Maybe a Flash movie or a 3D movie could work? Something like this (but much more informative):

    <object width="640" height="505">
    <param name="movie" value="http://www.youtube.com/v/NfvwY8pgqqw&fs=1&amp;hl=en_US&amp;rel=0&amp;color1=0x402061&amp;color2=0x9461ca"></param> [Broken]
    <param name="allowFullScreen" value="true"></param>
    <param name="allowscriptaccess" value="always"></param>
    <embed src="http://www.youtube.com/v/NfvwY8pgqqw&fs=1&amp;hl=en_US&amp;rel=0&amp;color1=0x402061&amp;color2=0x9461ca" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="640" height="505"></embed>
    Last edited by a moderator: May 4, 2017
  16. Aug 21, 2010 #15
    Actually, I was planning on using an application bundled with Apple's developer software for OS X called Quartz Composer. It can make animations, and I could use a program to generate this in real time.

    However, that does not show it would work. In order for this to be impressive, I need an actual working model. To do this, I need a low-intensity source of photons, a polarizer (is that a word?), and a polarity detector, as well as some fiber optic cable. However, I don't think I can build this in a reasonable price range. If you can point me toward any of these components, however, I would be much obliged.
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