Optics demo does quantum logic

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 practical.

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.