Comment on "Efficient high-fidelity quantum computation using matter qubits and linear optics" (3 pages)

Simon C. Benjamin

Recently Barrett and Kok proposed an elegant method for entangling separated matter qubits. They outlined a strategy for using their entangling operation (EO) to build graph states, the resource for one-way quantum computing. Here I argue that their strategy considerably underrates the power and utility of their EO. By viewing their EO as a graph fusion event, one perceives that each successful event introduces an ideal redundant graph edge, which growth strategies should exploit. For example, if each EO succeeds with probability $p\ensuremath{\gtrsim}0.4$ then a highly connected graph can be formed with an overhead of only about ten EO attempts per graph edge. The Barrett and Kok (BK) scheme then becomes competitive with the more elaborate entanglement procedures designed to permit $p$ to approach unity [Phys. Rev. Lett. 95, 030505 (2005)].

E. Knill, Raymond Laflamme, Gerard J Milburn

S. D. Barrett, Pieter Kok

We propose a practical, scalable, and efficient scheme for quantum computation using spatially separated matter qubits and single-photon interference effects. The qubit systems can be nitrogen-vacancy centers in diamond, Pauli-blockade quantum dots with an excess electron, or trapped ions with optical transitions, which are each placed in a cavity and subsequently entangled using a double-heralded single-photon detection scheme. The fidelity of the resulting entanglement is extremely robust against the most important errors such as detector loss, spontaneous emission, and mismatch of cavity parameters. We demonstrate how this entangling operation can be used to efficiently generate cluster states of many qubits, which, together with single-qubit operations and readout, can be used to implement universal quantum computation. Existing experimental parameters indicate that high-fidelity clusters can be generated with a moderate constant overhead.

Kumar Sharma Amit, Ashish Kumar Ghunawat

Federico M. Spedalieri

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