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# News from ICTP 101 - What's New

*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*

**E**ntanglement 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.
*