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Active Feed-Forward Improves Linear Optics Quantum Computing

Photonics Spectra
Mar 2007
Electro-optical modulation enables feed-forward steps as small as 150 ns.

Lauren I. Rugani

In one-way quantum computing, applying a feed-forward technique — in which each measurement is dependent on previous results — helps to surmount quantum measurement randomness. Researchers from the University of Vienna in Austria, Harvard University in Cambridge, Mass., and the Austrian Academy of Sciences, also in Vienna, have employed a four-qubit cluster state and three electro-optical modulators to demonstrate deterministic single- and two-qubit gate operations along with Grover’s search algorithm.

Two polarization-entangled photon pairs are generated as an ultraviolet laser pulse passes forward and backward through a nonlinear crystal and is transformed into a four-photon cluster state through an extended optical setup. A measurement made in the computational basis of the cluster state will remove a qubit, leaving a smaller cluster state, whereas measurements in a different basis process quantum information. The outcomes of the measurements define the computation, which will proceed without error only if the right result is obtained. Otherwise, a well-defined Pauli error is introduced that can be corrected by applying a feed-forward technique.

The measurement apparatus for a given basis comprises a quarter-wave plate, a half-wave plate and a polarizing beamsplitter. Qubits 1 and 2 are measured without delay, whereas measurements for qubits 3 and 4 are delayed 150 and 300 ns, respectively, in optical single-mode fibers measuring 30 and 60 m long. Three electro-optic modulators implement the active feed-forward on these two qubits, with one adapting the measurement basis of qubit 3 and two correcting Pauli errors on qubit 4.

The researchers found the output of a single-qubit computation with active feed-forward to be 0.84 ±0.08, compared with 0.55 ±0.06 for the same computation when no feed-forward is applied. Feed-forward is more complex for two-qubit gates, which are realized using horseshoe or box cluster states and implemented by measuring qubits 2 and 3 and thus transferring the two-qubit quantum state to qubits 1 and 4.

Grover’s search algorithm also can be performed on such a four-qubit box cluster. A quantum device called an oracle labels one of the four computational basis states by changing its sign. This tagged element is then found with certainty after an inversion-about-the-mean operation. Pauli errors introduced by undesired results at qubits 2 and 3 can cause the wrong element to be tagged; however, feed-forward compensates for these errors and identifies the correct element with a probability of 85 ±3%.

Nature, Jan. 4, 2007, pp. 65-69.

The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
Communicationselectro-optical modulatorsfiber opticsphotonicsquantum computingResearch & TechnologyUniversity of Vienna

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