March 20th, 2013 by Michael Ricciardi
The practical possibility of quantum cryptography (QC) — of sending a secure message using “entangled” quantum particles (like photons) — made a big splash back a few years ago, but experiments since then were hampered by a limit to the distance such particles can travel (89 miles, to be exact) due to loss of signal and/or atmospheric scattering. Even using fiber optic cables to conduct the photons has proven limited (250 miles).
The basic idea of QC is that anyone eavesdropping on (i.e., attempting to intercept and/or decode) a quantum-encrypted message (conveyed by those two entangled particles) would cause an alteration in the original message that would be instantly detectable by the receiver, who would then know that the message had been potentially compromised.
And although the “spooky” effect of entanglement has been experimentally verified for nearly 40 years, practical application has come up against this maximum range limitation.
The answer to this distance problem may be space itself, that is, “teleporting” an encrypted message — composed of entangled particles — from a satellite orbiting the Earth through the vacuum of space to one or more ground stations. Space, you see, while not truly “empty”, is vastly thinner than a planet’s atmosphere, and so, little scattering of the light particles would occur. This in theory would allow greater transmission distance for any encrypted message.
Well, that’s according to Thomas Jennewein at the University of Waterloo, Ontario, who is one of the top researchers in the field. Many other researchers are also working on the technology.
How Does QC Work?
Typical message encryption involves the use of “keys” — a string of numbers and/or letters that encode and then decode the message. In symmetric-key encryption (used here in quantum cryptography), the sender has the original (encoding) key, and the receiver has a copy of this same key used for decoding the message (note: in “public-key” encryption, the encoding key is made public, but only the receiver has the unique decryption key).
The idea behind such quantum cryptography is actually a combination of two theoretical (now proven) quantum phenomena: the Heisenberg Uncertainty (or Indeterminacy) principle and the Einstein – Podolsky – Rosen (ERP) effect.
The Heisenberg principle asserts that you can’t know key information about a particle’s state (like its momentum or exact position in space); merely attempting to observe the behavior of a particle will change the properties of the particle (and thus, presumably, its entangled particle “twin”).
Likewise, Einstein et al theorized (in 1935) that if you had two interacting quantum systems (such as two atoms from the same molecule) and then separated them, they would remain entangled (i.e., their properties would be linked) and measuring either particle would instantly create a change in the other, no matter how far apart one had separated them. This bizarre linking behavior prompted Einstein to refer to it as “spooky action at a distance” and he thus rejected it as anything other than theoretical, as it would mean that some faster-than-light-speed communication were at work (recall Einstein’s speed of light constant) in the universe.
In 1972, physicists proved that this “spooky action” was indeed possible — if only within their laboratory space (also: physicists assure us that the speed of light limitation in not violated in this “spooky”, purely quantum, effect).
So, using these principles, any attempt to intercept the quantum-encrypted transmission would alter the entangled state of the encrypting particles, and the receiver’s (symmetric) key would no longer work, thus alerting the communicators to the intercept and allowing them to switch to new keys. QC makes covert message-monitoring impossible, and thus a way of insuring secure communications between two parties.
A New Frontier for Quantum Cryptography
What Jennewein and many other scientists (and many private corporations) are proposing is the construction of a space-based quantum communications network; a system of satellite transmitters capable of entangling two particles (like photons), and then teleporting them to the Earth, unperturbed.
Jennewein’s lab is working on a system involving an entanglement device, housed on board a satellite, which would generate beams comprised of millions of pairs of entangled photons and then simultaneously transmit one from each pair to two Earth-based stations, each of which has the same encryption key.
According to this scheme, once the beam has been transmitted, computers at each station would compare the keys (which are encrypted along with the rest of the message). If the two keys are shown to be identical*, that would mean that the message had been transmitted securely (i.e., no one had attempted to intercept or change the message in transit). A conventionally encrypted message could then be transmitting via that (confirmed secure) channel.
But if an eavesdropping attempt were made, this would immediately altered the entangled state, and both key holders would know and thus select new keys and/or a new communication channel.
What’s Next for QC?
While this form of quantum “teleportation” is not quite the “beam me up, Scotty” breakthrough some sci-fi fans would hope for, it’s realization (if demonstrated experimentally) would be a true breakthrough for applied quantum science; it will allow secure teleportation of information…and,if you believe the theoretical speculations of Stephen Wolfram — that we are all just so much organized information — then perhaps some form of teleportation beyond mere messaging is possible.
Indeed, as Jennewein and othesr see it, while actual objects would not be teleported, information about the objects would be. That information, encoded by a quantum-entangled two-particle system, would vanish from one particle (or beam of particles) on Earth and reappear somewhere in space (hopefully, the satellite or intended target). This type of transmission would involve three particles: an input photon (whose information is to be teleported) and a pair of two entangled (but separated) photons.
Jennewein elaborates: “The input photon is correlated with one of the entangled ones, and thereby its quantum state is fully transferred onto the other entangled photon, which can be at a distance. The final photon is the new ‘original,’ and the initial photons completely lose their information.”
Currently, both public and private research facilities are actively working on some form of quantum cryptography satellite system. Beyond encryption experimentation, a quantum satellite system would allow scientist to test other quantum theoretical predictions at much greater distances.
*This comparison of keys would occur through a public (non-quantum encrypted, perhaps a public key) channel, presumably, insofar as sending an additional, encrypted message (via symmetric-key) to confirm the QC keys, would need to be likewise quantum-encrypted, and compared, etc.
Some source material for this post came from the Sci Am article: ‘Quantum Teleportation in Space Explored as Message Encryption Solution’ By Joel N. Shurkin and Inside Science News Service
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