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Masters
Quantum Key Distribution Over a Free-Space Optical Link
Last updated: February 20, 2007 @ 12:24am
The new field of quantum computing has brought with it many new computational capabilities, one of the most notable among them is the ability to factor large numbers efficiently. While as innocuous sounding as this is, it actually has some very profound consequences for the field of cryptography. More precisely, a quantum algorithm was discovered which solved the hidden subgroup problem, of which factoring is an example. An efficient algorithm for the hidden subgroup problem effectively breaks most of the practical cryptographic protocols which we employ today (for example, being able to factor the large number in the popular RSA algorithm into its two component prime factors allows the easy decryption of any message encoded using the large number as the key).
Fortunately, even though quantum computing breaks many of the current cryptographic protocols it also gives us new tools to build new, more secure, cryptographic protocols. Quantum Key Distribution is a new kind of encryption scheme which uses the rules of quantum mechanics to distribute a secure, secret, random key to two parties (commonly called Alice and Bob) which they can then use to encrypt and decrypt messages sent between each other. The security of the scheme no longer rests on the assumed computational complexity of certain mathematical problems (such as factoring) but instead relies solely on the validity of quantum mechanics (a rather well tested theory to say the least!).
In the QKD system described here, the key is encoded into a pair of polarization-entangled photons (particles of light). The entanglement means when Alice measures one photon from the pair and Bob measures the other photon from the pair they are guaranteed to get correlated results. Security is assured by the fact that measurement disturbs a quantum system and any eavesdropper (commonly called Eve) attemping to gain information about the photons (and thus the keys) by interacting with them in any way NECESSARILY will disturb their state and ruin the perfect correlations which Alice and Bob would otherwise observe. Eve's influence is observed when Alice and Bob calculate and error rate between their measurements (without Eve they would expect to see an error rate close to 0%, but not exactly zero because this is a real life system with imperfections). Mathematicians have then come up with security proofs which give you a threshold for the quantum bit error rate (QBER) below which the key which Alice and Bob generate is PROVABLY secure.
Project News
The full 2 free-space link entangled QKD system is operational!
Go to the QKD News page for a bit of a timeline for the QKD project.
The what is QKD? page will be completed soon.
Go to the Pictures page for some pictures of the QKD system.
Project Files
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The poster describing the project. | pdf
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My Masters thesis! | pdf
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A very accessible article in Phys13 News (Waterloo's physics news sent to high schools) | pdf
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Our first article detailing the full 2 free-space link system and our experimental results | arXiv
Group
Current Members:
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Professor: Gregor Weihs | info
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Professor: Raymond Laflamme | info
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Graduate Student: Chris Erven | info - ME!
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Post Doc: Christophe Couteau | info
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- For his never ending supply of optics help and advice, and all his night time experimenting help!
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Undergraduate Student: William Sellier | info
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- For lots of programming help with the error correction algorithms, offline analysis, and polarization controllers!
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Past Members:
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I owe the following people a huge amount of thanks for all their help on the project! THANK YOU! :)
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Undergraduate Student: Peter Forbes | info
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- For tons of great experimenting help, odd jobs, and a great job on the polarization controllers!
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High School Student: Robbie Irwin | info
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- For lots of help with the comm programming
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Undergraduate Student: Immo Soellner | info
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- For tons of odd jobs in the machine shop and a great variable pinhole design and some night time experimenting help
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Summer Student: Erwann Bocquillon | info
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- For tons of great experimenting help
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Undergraduate Student: Nikoline Ilic
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- Alignment lasers on the source board design and lots of experimenting help
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Undergraduate Student: Ben Schmidt
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- Some great programming on the error correction and privacy amplification routines and some jobs in the machine shop
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High School Student: Travis Gerhardt
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- Help debugging the original code
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Undergraduate Student: Jordan Thompson
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- Characterization of the source and lots of odd jobs in the machine shop
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Research Assistant: Matthew Peloso
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- Initial detector box and telescope design
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Undergraduate Student: Paul McGrath
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- Initial lab setup and ordering for source
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