Cluster state quantum computers or one-way quantum computer are a scheme of quantum computation that consists entirely of one-qubit measurements on a particular class of entangled states, the cluster states. (“A One-Way Quantum Computer”, Oct 2000, Robert Raussendorf and Hans J. Briegel Theoretische Physik, Ludwig-Maximilians-Universität München, Germany). The measurements are used to imprint a quantum logic circuit on the state, thereby destroying its entanglement at the same time. Cluster states are thus one-way quantum computers and the measurements form the program.
The Irish and Austrian group’s quantum computer makes use of four entangled photons in a cluster state. Tame explains how it works:
“Our setup is completely based on light, where quantum information is encoded on each photon. The information is in the polarization of each photon, horizontal or vertical, and superpositions in between. An ultra-violet laser pumps a crystal and produces an entangled pair of photons in one direction. The laser beam then hits a mirror and bounces back to form another pair of entangled photons on its second passage through the crystal. These four photons are then made to interact at beamsplitters to form the entangled cluster state resource on which we perform the quantum computation.”
Next, Tame says, come the calculations. “We perform Deutsch’s Algorithm as a sequence of the measurements. When you measure in a specific basis, you can manipulate the quantum information in the photons using their shared entanglement.” He continues with an illustration related to classical computing: “You can think of the cluster state as the ‘hardware’, and the measurements as the ‘software’.”