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Qubits and Crypto

December 6, 2010

Torah Kachur

Secrets and lies define the government and military, that and being led by bumbling fools.  There is no doubt that some military information should be kept secret like technological advances, battle locations and strategies and George W. Bush's IQ.  For secrets to be kept away from Wikileaks, cryptography is essential.  The new type of cryptography that is being tested by the US military research division, DARPA, is quantum cryptography.  Because if codes like DaVinci's Last Supper weren't complicated enough, might as well throw in the most complicated scientific issue in the world today - quantum theory. 

 

Quantum cryptography is based on using electromagnetic waves like light to carry information.  Photons carry information as qubits (not Q-bert...unfortunately) and light can be polarized by only allowing a particular magnitude and phase of light through a specialized filter.  This polarization gives light a specific binary (0 or 1) property depending on how it is polarized.   You can polarize light at different angles and wavelengths thus the photons carry information.   If two parties that send and receive the polarized light have detectors to receive and interpret the phase of photons, then a code is born. 

 

            

 

The major advantage of using quantum bits for encryption is the Heisenberg Uncertainty Principle which says that if you try to intercept a quantum-coded message, the act of trying to measure one property of a quantum state will disturb another.  In other words, if you touch this message (01001100) it will self-destruct in T-minus 1 second.

 

To add to the complexity, encryption can come in the form of quantum entanglement.  Entanglement is the phenomenom that says that two or more separate but similar objects can be linked and respond to changes in eachother, even over a distance.  So if a photon is split into two lesser energy photons (like UV into infrared) then those two photons are linked over a theoretically infinite distance.  If point A (commonly called Alice) and point B (Bob) are separated but carry entangled photons then they can communicate by changing the properties of one photon and expecting its twin to respond similarly.

 

Most of this encryption isn't sending long-winded messages about military targets or whether or not Bill inhaled, instead the information sent and received is usually the key to decode encrypted messages sent over regular communication channels.   So far, scientists have been able to send 1Mbit/s of information along an optical fiber 20kms in length.  The hope is that transmission distances can be extended to even communicate through space to the International Space Station.  

 

 TResearchBlogging.orghe newest rage in quantum encryption is using photons to encode more than just 0's and 1's.  Now, in a recent paper published in Science, researchers have been able to measure more than just the phase of the photon but also the angle, which means that the possibilities are infinite for encryption.  All of a sudden, entangled photons can communicate both the 0's and 1's and also an entire alphabet of angular information.

 

 

 

What will this mean to Bond, Langdon and Gagdet?  Faster and more secure encryption for the military which means more secrets.  It isn't clear if these secrets will include the "Don't Ask. Don't Tell" policy.

 

Leach J, Jack B, Romero J, Jha AK, Yao AM, Franke-Arnold S, Ireland DG, Boyd RW, Barnett SM, & Padgett MJ (2010). Quantum correlations in optical angle-orbital angular momentum variables. Science (New York, N.Y.), 329 (5992), 662-5 PMID: 20689014

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