ANYONE interested in computers will be familiar with 鈥渂its鈥� of digital
information, represented by 0 or 1 in a binary code. But researchers in Austria
and the US have now employed the laws of quantum mechanics to encode a digital
message as 鈥渢rits鈥濃�攔epresented as 0, 1 or 2. Their experiment is the
first
demonstration of quantum communication.
These researchers are not the first to have suggested ways of using the laws
of quantum mechanics to encode a digital message. But Klaus Mattle, Harald
Weinfurter and Anton Zeilinger of the University of Innsbruck, working
with Paul
Kwiat of the Los Alamos National Laboratory in New Mexico, have won the race to
bring these ideas to life.
The researchers encoded a message by manipulating the polarisation of light.
In classical physics, each photon of light can carry only one bit of
information
in its polarisation. To send a message, a physicist can send a beam of light
through a polarising filter which can be flipped between its horizontal and
vertical settings. The recipient of the message then passes the arriving light
through another filter that lets through only horizontally polarised
light. When
a photon emerges, the recipient registers a 1. Each gap in the signal is
registered in the form of a 0.
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Mattle and his colleagues squeezed an extra level of information out of each
photon by passing ultraviolet light through a crystal with 鈥渘onlinear鈥� optical
properties. This crystal converted every photon of light travelling through it
into a pair of photons, each with half the energy of the original photon.
The crystal also polarised the photons, but due to the uncertainties of the
quantum world, each photon emerged with an element of both horizontal and
vertical polarisation. Because the photons were produced in pairs, the
polarisation states of the two photons within each pair were linked: in quantum
terms, they were 鈥渆ntangled鈥�.
To send a message, the physicists sent one photon within each pair directly
to an observer, while the other one reached the observer after passing
through a
device that could delay its horizontally polarised component to varying
extents.
As a result, each altered photon received by the observer was in one of four
possible states relative to its unaltered partner. In practice, however, two of
the altered photon states were indistinguishable, which meant that each altered
photon effectively encoded one of three states鈥攖he 0, 1 or 2 of a
trit.
To prove the point, the experimenters sent a message consisting of three
characters, 鈥淜鈥�, 鈥淢鈥� and 鈥溌扳��. In the ASCII format used by today鈥檚
computers,
these characters are represented by strings of binary code containing
eight bits
of information each, giving a total of 24 bits. Using the quantum system,
however, the message needed only 15 trits (Physical Review
Letters, vol
76, p 4656). 鈥淚t鈥檚 the first experiment which demonstrates a communication
scheme using pure quantum states,鈥� says Weinfurter.
The researchers鈥� next plan is to refine their system to achieve quantum
鈥渢eleportation鈥�. This scheme uses pairs of entangled photons to transmit
information about the state of a third photon. This information can then
be used
by an observer to recreate a photon in exactly the same state (This Week, 3
April 1993, p 12).
The work could also lead to the development of powerful computers that
exploit the uncertainties of quantum mechanics to conduct many calculations
simultaneously.