The most economical way to send a message may be for the receiver to
send himself half of it, according to two American physicists. Charles
Bennett of IBM’s Thomas J. Watson Research Center, York-town Heights, New
York, and Stephen Wiesner hit on their bizarre communications system after
thinking about sending messages as streams of individual photons. They concluded
that it is possible to use a photon to carry two bits of information instead
of just one.
But intuitively, if you want to send a message in the form of an object
which can be put into one of N indistinguishable states, the maximum number
of different messages you can send is N. For example, a photon can have
two distinguishable polarisation states: ‘left-handed’ or ‘right-handed’.
So if you were to send a message by altering the polarisation of a single
photon and transmitting it, it seems obvious that at most you could send
two distinguishable messages: one by sending a left-handed photon and the
other by sending a right-handed one.
But Bennett and Wiesner say this intuition is false. They have found
that if your communications medium can be put into one of N distinguishable
states then, by using a special pre-transmission set-up, you can transmit
up to N2 different messages. This is equivalent to cramming a
two-bit message into each physical bit used for transmission. So the polarisation
of a photon can be used to send up to four different messages instead of
the two you might expect.
This magic is performed by harnessing a unique property of quantum particles
such as photons. The property is known as ‘non-locality’, and was proved
theoretically by the late John Bell of CERN, the European centre for particle
physics, and later corroborated experimentally by a group of physicists
led by Alain Aspect in France.
In classical physics, any physical system is completely described by
a description of its subsystems and how they interact. But this not true
in quantum physics. There are other properties of a system which, though
they are accessible to experiments on the system as a whole, are not reflected
in the outcome of any combination of separate experiments on its subsystems.
These properties are called quantum correlations.
Quantum correlations can exist between objects that are any distance
apart, which is why they are called non-local. Although the term ‘correlation’
is borrowed from probability theory, quantum correlations need not be probabilistic.
In some situations, they can carry information that, in principle, is retrievable
Bennett and Wiesner have devised a hypothetical quantum communications
system in which the receiver reads a two-bit message from two physical bits.
But, unusually, only one of those bits – the transmitted bit – has come
from the sender of the message. The other – the reference bit – never leaves
The diagram shows the sequence of events. Curiously, it is the receiver
who makes the first move. He prepares two bits in a state that contains
certain non-local quantum correlations, stores one as a ‘reference’ bit
and transmits the other to the sender. When the sender gets this pre-transmission
bit, he stores it.
To ensure that the non-local correlation between the two bits is maintained,
each bit must be kept isolated from its surroundings. When it is time to
send the message, the sender then performs one of four physical operations
on the stored bit before transmitting it back to the receiver (Physical
Review Letters, vol 69, p 2881).
These operations, which are specified by Bennett and Wiesner, affect
the quantum correlations in one of four ways. These are not distinguishable
by measurements on the transmitted bit and the reference bit separately.
But by measuring both bits jointly, the receiver can determine which of
the four operations the sender performed, and so receive one of four messages.
Because of the pre-transmission, the total number of bit journeys is
the same as it would be classically, so in that respect Bennett and Wiesner’s
communications system gains us nothing. But the fact that half the journeys
are in the opposite direction could be useful in situations where transmission
in the desired direction is more expensive than in the opposite direction.
Most importantly, the pre-transmission can take place before the sender
has even thought of the message. So it may be possible to devise a communications
system that hoard bits during off-peak periods and then used them during
peak periods, so doubling its peak capacity.
As a bonus, the transmitted message is automatically ‘quantum encrypted’
– the Heisenberg uncertainty principle prevents anyone who does not possess
the reference bit from reading the message (see ‘Quantum keys for keeping
secret’, New Scientist, 16 January). If they try to measure the polarisation
of the transmitted bit, they will get a purely random answer, regardless
of the values of the two bits the polarisation effectively carries. However,
this method of quantum cryptography is vulnerable to an eavesdropper who
intercepts and replaces the pre-transmission as well as the actual transmission.
After the pre-transmission, the parties can, alternatively, choose to
send a message in the opposite direction to that shown in the diagram. To
do this, the ‘receiver’ performs one of four operations on the reference
bit and transmits it to the ‘sender’.
Like most practical applications of the quantum theory of computation,
this one is not yet technologically feasible. Bennett and Wiesner, and Anton
Zeilinger, suggest that this scheme could be implemented using nonlinear
optical crystals. But whether or not it turns out to be of value in transmitting
messages, such an unexpected upset to ideas about communications is of undoubted