One-way quantum computation
Quantum computers, by processing information via the states of quantum systems such as
atoms and photons, promise to outperform classical computers for specific tasks. One
approach to making a practical version of such a device is the so-called one-way quantum
computer. Proposed in 2001, it relies on entanglement. In a photonic realization, the output
from a pump laser passes through a nonlinear beta barium borate (BBO) crystal twice in order
to generate two entangled pairs of photons, which means that the four spatial modes contain
four photons altogether. Coherent superposition at the two polarizing beamsplitters ensures
that the final four detected photons are in a “cluster state”, given appropriate placing of
half-wave plates and polarizers. Since 2005, proof-of-concept experiments (above) with this
basic quantum-computer set-up allowed computations of Grover’s search algorithm, a
quantum prisoner’s dilemma and the demonstration of decoherence-free subspaces (as
required for fault-tolerant quantum computation) to be carried out.