Quantum cryptography protocol doesn’t require shared reference frames (Update)

Mar 09, 2010 by Lisa Zyga weblog
Two scenarios that could use a quantum key distribution protocol that is reference-frame-independent are (left) earth-to-satellite quantum communication and chip-to-chip quantum communication. Image credit: Laing, et al.

(PhysOrg.com) -- Quantum cryptography, which enables two parties to communicate with each other with unconditional security, has begun to be implemented by some governments, banks, and other corporations with high-security requirements. However, certain applications of quantum cryptography, such as satellite links, have proved to be challenging, partly due to a key requirement of quantum key distribution: that the two parties must have a shared reference frame.

But in a new study, physicists Anthony Laing and coauthors from the University of Bristol and the National University of Singapore have developed a protocol that is reference-frame-independent. By generating a secure between two parties without the need for aligning their reference frames, the new protocol could extend the advantages of into new domains. Possibilities include earth-to-satellite , in which the satellite is rotating and orbiting the Earth, as well as chip-to-chip quantum communication inside a computer and between different computers.

The new protocol requires the two parties, Alice and Bob, to share many pairs of entangled particles. The correlations between particles provide the secret shred key; the trick of the protocol is that the correlations also allow Alice and Bob to measure the purity of their entanglement. Too much impurity alerts them to the possible presence of an eavesdropper, Eve. The useful property of this kind of safeguard against Eve is that the purity should be quite robust in a reference frame that is unknown or even one that varies slowly compared with repetition rate of the quantum signals. This key advantage gives the protocol an edge in other situations such as an environment of intermittent rapid fluctuation where the key is exchanged during the periods of relative stability without the need to realign the reference frame.

As the physicists explain, the new technique greatly simplifies secure quantum encryption in situations that involve moving objects. For example, a beam that connects a satellite and a ground station may encode information in circular polarization, which remains stable. But if the satellite is rotating with respect to the ground station, the linear polarizations will vary, making it difficult to establish a shared reference frame. Similarly, the microchips inside electronic devices are constantly moving around relative to the wavelength of light, and would benefit from a protocol that doesn’t require a shared reference frame.

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User comments : 7

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1 / 5 (2) Mar 08, 2010
..the extra photon serves to establish a third reference frame..
Currently, quantum cryptography is currently only used for sending encryption. But it's well known already, by increasing of information carriers we could increase the robustness of encrypted transmission. Of course, it opens back door for attempts to hijack this session just by using of this redundant information.

If you're not whispering, don't be surprised, you're listened.

3.7 / 5 (3) Mar 09, 2010
seneca -

None of the comments you leave on this site ever make any scientific sense. You have completely missed the point of quantum encryption: it's the entangled SUPERPOSITION of states that are the information carriers. You don't need to "increase information carriers" because each "carrier" is, effectively, a qubit. There is no "back door", b/z the moment you try, you've ruined entanglement and, therefore, disrupted the information stream. This is why companies like Phillips, Mitsubishi, et al. keep pushing the limit for entangled states (currently at about 135km when I last checked).
3 / 5 (2) Mar 09, 2010
In the above scenario, any potential "Eves" can be easily monitored, as the article indicates, with respect to (borrowing from current technology) the number of packets lost :D
5 / 5 (1) Mar 09, 2010
@solidspin: You sure got that right. My own theory is that seneca/alexa/flaredone are actually all the same person, none of whom has any idea what they are talking about. In nearly every comment they make they claim that the research has "been done already years ago," etc.

Back on topic: I've heard solidspin's comment of "the moment you try, you've ruined entanglement and, therefore, disrupted the information stream" a number of times. Even if you can't evesdrop, couldn't you just attempt to and ruin the data stream for the intended receiver? If observing a quantum encryption destroys its information, couldn't you just do that and prevent the transmission of data at all? That seems almost equally effective.
1 / 5 (1) Mar 09, 2010
Hi, Danman -

Yes, you're right. If the optics aren't properly shielded, etc. you can. The extent to which these groups are guarding against that, however, is pretty remarkable: extremely high mu-metal jackets to prevent magnetic field interference; running these fiberoptics underground; heavy twisted wire ground shielding to prevent stray RF interference, etc., ad nauseam.

I'm almost done w/ my PhD in solid-state chemistry, using nuclear magnetic resonance as a primary investigative tool, so I love this stuff!! I hope to defend this August.
not rated yet Mar 09, 2010

Thanks for that information, that's very interesting! I had never seen anyone address that point in any other articles. Good luck with your thesis!
not rated yet Mar 10, 2010
those scientists decided that if the static foundries for the commchips are uncompromised then they can imprint keys into the hardware themselves in a round-robin approach, smart enough considering forging would be an expensive endeavour

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