Physics bans cloning
It is impossible to make an exact copy of any object, living or not.
21 May 2002
PHILIP BALL
Attack of the Clones is classically implausible.
© Lucasfilm
Perfect clones can't exist, say physicists. They're not doing down
the hottest topic in biology, merely pointing out that the laws of
classical physics forbid making an exact copy of an object, living or
inanimate, just as the laws of quantum physics have been known to do
for 20 years.
Scientists have created replicas of individual quantum particles such
as atoms and photons with properties almost identical to those of the
original. Last month, a team at the University of Oxford, UK, even
showed that they could clone a photon - a particle of light1.
But quantum cloning is only feasible if one accepts a little bit of
copying error - as two US physicists proved in 1982 with their
'no-cloning' theorem.
This theorem means that quantum teleportation - recreating a quantum
particle in a different location from the original, but in exactly
the same state - is only possible if the original is destroyed, so
that it does not coexist with its copy. Quantum teleportation of
photons was demonstrated experimentally in 19972.
But the laws that govern quantum particles are different from those
that apply to everyday systems, which are made up of countless
billions of such particles. No one had given much thought to ideas
such as exact cloning and teleportation on macroscopic scales, where
Newton's laws of classical physics apply.
Until now. Daniel Collins and Sandu Popescu at the University of
Bristol, UK, have shown that the quantum property of entanglement has
a classical analogue3. Entangled quantum particles have mutually
dependent states; they seem to interact by way of an instantaneous
action at a distance.
Entanglement is central to quantum teleportation and cloning. Collins
and Popescu show that systems that are governed by classical physics
can exhibit secret correlations in their behaviour that can give rise
to effects rather like teleportation.
And Andreas Daffertshofer of the Free University in Amsterdam and
colleagues have proved that there is a kind of no-cloning theorem for
classical systems4.
The key is that classical, many-particle systems are characterized by
statistical distributions. The particles are distributed between a
huge number of possible states, like grains of wind-blown sand
distributed over a hillside.
Daffertshofer's team shows that it is impossible to copy any
arbitrary statistical distribution of one many-particle system onto
another to make an exact clone. Attempts to do so will disturb the
original distribution. There is a sole possible exception: classical
systems in which all particles occupy the same state, like sand
grains stacked in a single tiny well.
Biological clones are nothing like these idealized versions. The
statistical distributions of particles in an organism and its clone
are inevitably very different - so physicists would not accept them
as true clones at all.
References
Lamas-Linares, A., Simon, C., Howell, J. C. & Bouwmeester, D.
Experimental quantum cloning of single photons. Science, 296, 712 -
714, (2002).
Bouwmeester et al. Experimental quantum teleportation. Nature, 390,
575, (1997).
Collins, D. & Popescu, S. A classical analogue of entanglement.
Preprint, arXiv:quant-ph/0107082 (2001).
Daffertshofer, A., Plastino, A. R. & Plastino, A. Classical
no-cloning theorem. Physical Review Letters, 88, 210601, (2002).
© Nature News Service / Macmillan Magazines Ltd 2002
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