Quantum Computation Using Linear Optics

Christoph Adami, Nicolas J. Cerf

Emanuel H. Knill

Emanuel H. Knill, Raymond Laflamme, Gerard J Milburn

We investigate the computational power of passive and active linear optical elements and photo-detectors. We show that single photon sources, passive linear optics and photo-detectors are sufficient for implementing reliable quantum algorithms. Feedback from the detectors to the optical elements is required for this implementation. Without feedback, non-deterministic quantum computation is possible. A single photon source sufficient for quantum computation can be built with an active linear optical element (squeezer) and a photo-detector. The overheads associated with using only linear optics appear to be sufficiently low to make quantum computation based on our proposal a viable alternative.

Jonathan P. Dowling, Hwang Lee, Farrokh Vatan +2 more

In quantum computing a very important problem is that of building a quantum memory. We show that by using new techniques from scalable linear optical quantum computing (LOQC), it is possible to make an all linear-optical quantum memory capable of storing the computational qubit indefinitely. The same approach can be used to make a simple quantum repeater. Recent work by our group has shown how linear optical quantum computing ideas, recently proposed by Knill, Laflamme, and Milburn, and also by Franson and co-workers, can be used to make a scalable quantum repeater [1]. However, this repeater still requires a quantum memory to operate, which apparently requires resources other than those required for linear optical quantum computing. In this presentation, we will explicitly show how to construct a quantum memory using only LOQC resources and employing a simple error correction code for photon loss. The result is a simple, scalable, quantum memory. The device can also be operated as a quantum repeater that is far simpler than our original proposal to implement.

Gerard J Milburn, Timothy C. Ralph, Alexei Gilchrist +3 more

Peter P. Rohde

Linear optics quantum computing (LOQC) is a leading candidate for the implementation of large scale quantum computers. Here quantum information is encoded into the quantum states of light and computation proceeds via a linear optics network. It is well known that in such schemes there are stringent requirements on the spatiotemporal structure of photons---they must be completely indistinguishable and of very high purity. We show that in the boson-sampling model for LOQC these conditions may be significantly relaxed. We present evidence that by increasing the size of the system we can implement a computationally hard algorithm even if our photons have arbitrarily low fidelity and purity. These relaxed conditions may make boson-sampling LOQC within reach of present-day technology.