Entanglement, one of the underlying principles of quantum mechanics, may hold the key to the future development of cyber-information. GianCarlo Ghirardi explains why.

Entanglement and Encryption

GianCarlo Ghirardi

Entanglement is something that you might want to avoid--except, of course, if you're a theoretical physicist. Austrian-born Erwin Schrödinger, who in 1933 shared the Nobel Prize with Paul Dirac, cited Verschrankung (German for entanglement) "as the most characteristic trait of quantum mechanics, the one characteristic that enforces its entire departure from classical line of thoughts."
What is entanglement? It's a phenomenon in quantum mechanics by which an individual particle or system does not have a precise state of its own but exists only as a part of a larger composite system. In effect, the very possibility of considering a particle or system as possessing objective properties depends on its entanglement with another particle or system. In fact, in many instances the particle does not possess any property at all.
Here's an example. Consider a system composed of two distant and noninteracting photons, in an appropriately entangled state, moving in opposite directions. Now let's place polarizing filters in their path. Each photon has an equal probability of either passing through or being absorbed by the filter. In fact, according to quantum mechanics, it is impossible to know whether a photon will pass through or be absorbed by the filter. The individual events, simply put, are totally random.
Yet, Albert Einstein devised a situation that came to be known as the EPR paradox. He called attention to the fact that while it was impossible to know whether a single photon would pass or be absorbed by a filter (even more, that the situation forbids us to think that there is some feature determining in advance the fate of a photon), paired photons subject to the same polarization tests always act in tandem--that is, they either both simultaneously pass through or both are simultaneously absorbed by the filters, a consequence of the two photons being entangled.
In more down-to-earth terms, what the world's greatest physicist implied by entanglement is a situation analogous to the following: Although when tossing a coin time and again you cannot predict whether the next coin toss will turn up heads or tails, if you toss two coins simultaneously, when entanglement occurs, either both coins turn up heads or both turn up tails. More importantly, when you know what took place for one of the coins you know what took place for the other despite the fact that they are far apart and in no way interacting.
That's exactly how photons behave when entangled--a situation that has subsequently taken on greater weight and reliability thanks to the studies of John Stewart Bell and the experiments of Alain Aspect.
Now here's one of the most intriguing aspects of this 'entangled' story. By allowing us to overcome difficulties related to the secure transfer of information and by spurring further advances in computer efficiency, the mind-bending abstraction inherent in studies of entanglement may prove to be of critical value to real-world technological improvements in cryptography and encryption, as well as in computer technology.
Specifically, relying on entanglement, we can devise a system by which scrambling and unscrambling digitized information is a task that both the senders and intended recipients can easily perform but which is impossible for others to intercept or decipher. In brief, we can devise a full-proof mechanism for exchanging secret information in cyberspace.
Relying on entanglement, moreover, could place us on the threshold of applying the counterintuitive principles of quantum mechanics to make an incredible jump in the efficiency of our computers.
Quantum mechanics meets quantum computers: a marriage that could propel the informatics revolution well beyond its existing horizons.

GianCarlo Ghirardi
ICTP Consultant, High Energy Physics Section
Department of Theoretical Physics
University of Trieste

For a more detailed account, please see "Entangled States Allow Radical Change," CERN Courier, March 2002, pp. 20-23.

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