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QUANTUM cryptography and quantum teleportation could become easier thanks to
a new way of spotting special photons, say scientists in Austria. The technique
allows researchers to separate ordinary photons from those that are mysteriously
鈥渆ntangled鈥� with a distant twin. 鈥淭his separation is crucial for quantum
communications,鈥� says Richard Hughes, a physicist at the Los Alamos National
Laboratory in New Mexico.

Entangled photons are the Siamese twins of the quantum world. According to
the laws of quantum mechanics, they are linked in such a fundamental way that
carrying out a measurement on one determines the state of the other, no matter
how far apart they might be. This ability to influence a distant photon by
measuring one nearby is important for quantum communication.

But distinguishing entangled photons from ordinary ones is not easy. Until
now, the most promising proposal has been to use the quantum equivalent of a
logic gate, known as a CNOT. The gate flips the state of one photon鈥攆or
example from horizontally to vertically polarised鈥攄epending on the state
of another photon. Because the states of entangled photons are linked, it is
possible to use a CNOT gate to distinguish them from pairs that are not
entangled.

But CNOT gates are difficult to build. 鈥淩ight now, CNOTs don鈥檛 even exist
that work for photons,鈥� says Anton Zeilinger, a physicist at the University of
Vienna in Austria. And others doubt that quantum logic gates will ever be good
enough for high-quality quantum communication.

With a quantum sleight of hand, however, Zeilinger and his colleagues have
proposed using mirrors known as polarising beam splitters to do the same job as
CNOTs. The mirrors work by reflecting photons that are vertically polarised
while transmitting horizontal ones, for example. If two photons enter the mirror
from two directions and emerge from two directions, they are well entangled. But
if they emerge in one direction, they are usually not entangled and so can be
ignored (see Diagram).

The technique has the added bonus of improving the entanglement of pairs that
pass through it. 鈥淭he amazing thing about this is that you can transfer
entanglement from one pair to another,鈥� says Hughes.

He is not expecting the technique to be perfect, however. Half of the
entangled pairs are lost. But Zeilinger says: 鈥淲hen it works, it works
辫别谤蹿别肠迟濒测鈥�.