Researchers from the University of Science
and Technology of China and the University of Heidelberg in Germany have
demonstrated a method of using four photons to form a logic gate that
can be used for quantum computing.
Quantum computers use attributes of particles like atoms, photons
and electrons to represent information, and manipulate the particles to
carry out computations. Quantum computers have the potential to solve
certain very large problems that are beyond the capabilities of the fastest
possible classical computer.
The researchers' logic gate is a step toward making quantum computers
Logic gates that link pairs of quantum bits, which represent single
bits of binary information, are essential to quantum computing. It has
been difficult to form two-bit logic gates from photons rather than electrons
or atoms because photons interact very weakly, essentially passing through
each other unaltered. At the same time, photons are especially useful
as qubits because they can be transported over communications lines.
The researchers' proof-of-principal experiment demonstrated that
it is possible to make a non-destructive photonic Controlled-NOT, or CNOT,
gate that uses linear optical elements.
A CNOT gate links, or entangles, a chosen pair of qubits. It is
nondestructive in that its output survives measurement and can be forwarded
to the next step in a calculation. Linear optical elements are mirrors,
lenses and been splitters, which are much more practical than the high-power
lasers and special crystals required for nonlinear interactions.
The researchers used the gate in a teleportation experiment. Researchers
are able to teleport single quantum particles; this type of transmission
is akin to sending a fax and in the process destroying the original.
It will be 10 or 20 years before the logic gates could be used
practically, according to the researchers. The work appeared in the January
28, 2005 issue of Physical Review Letters.
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