Optical Quantum Gates

From: Andrew Swallow (am.swallow_at_eatspam.btinternet.com)
Date: 11/22/03 

Date: Sat, 22 Nov 2003 06:05:14 +0000 (UTC)

A possible new way of performing quantum calculations

Extract from Nature, vol 426 p 264.

Demonstration of an all-optical quantum controlled-NOT gate
http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v426/n6964/a
bs/nature02054_fs.html

The promise of tremendous computational power, coupled with the development
of robust error-correcting schemes, has fuelled extensive efforts to build a
quantum computer. The requirements for realizing such a device are
confounding: scalable quantum bits (two-level quantum systems, or qubits)
that can be well isolated from the environment, but also initialized,
measured and made to undergo controllable interactions to implement a
universal set of quantum logic gates. The usual set consists of single qubit
rotations and a controlled-NOT (CNOT) gate, which flips the state of a
target qubit conditional on the control qubit being in the state 1. Here we
report an unambiguous experimental demonstration and comprehensive
characterization of quantum CNOT operation in an optical system. We produce
all four entangled Bell states as a function of only the input qubits'
logical values, for a single operating condition of the gate. The gate is
probabilistic (the qubits are destroyed upon failure), but with the addition
of linear optical quantum non-demolition measurements, it is equivalent to
the CNOT gate required for scalable all-optical quantum computation.

Quantum cryptography takes to the skies
http://www.newscientist.com/hottopics/quantum/quantum.jsp?id=ns99992875

Andrew Swallow

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