A new scheme for photonic quantum computing

A new scheme termed “coherent photon conversion”, could potentially overcome all of the currently unresolved problems for optical implementations of quantum computing.

Quantum technology derives its potential by exploiting uniquely quantum features such as superposition and entanglement. Single photons are excellent quantum information carriers, because they are naturally almost perfectly isolated from their environment. Also, quantum computers based on photons promise to be extremely fast. But current schemes for preparing, processing and measuring photons are inefficient.

The new scheme provides a method of coherent conversion between different photon states and is based on enhancing the nonlinearity of a medium by a strong laser field. The method paves a road to solving all open challenges for optical quantum computation. For example, deterministically doubling single photons solves the preparation and measuring problems, and a novel type of photon-photon interaction gives efficient quantum gates. This new quantum optics toolbox opened up by “coherent photon conversion” promises to lead to a nonlinear optical quantum computer.

Nature – Efficient quantum computing using coherent photon conversion

In a first series of experiments the group uses photons and highly non-linear glass fibers for a proof-of-principle demonstration of a process suitable for implementing the scheme. While deterministic operation has yet to be achieved, the authors’ results suggest a line for development how this might be possible with sophisticated optical technologies, such as using highly nonlinear glasses and stronger lasers. Interestingly, the general idea of “coherent photon conversion” can also be applied to various other physical systems like atoms or nanomechanical devices.

Single photons are excellent quantum information carriers: they were used in the earliest demonstrations of entanglement and in the production of the highest-quality entanglement reported so far. However, current schemes for preparing, processing and measuring them are inefficient. For example, down-conversion provides heralded, but randomly timed, single photons4, and linear optics gates are inherently probabilistic. Here we introduce a deterministic process—coherent photon conversion (CPC)—that provides a new way to generate and process complex, multiquanta states for photonic quantum information applications. The technique uses classically pumped nonlinearities to induce coherent oscillations between orthogonal states of multiple quantum excitations. One example of CPC, based on a pumped four-wave-mixing interaction, is shown to yield a single, versatile process that provides a full set of photonic quantum processing tools. This set satisfies the DiVincenzo criteria for a scalable quantum computing architecture, including deterministic multiqubit entanglement gates (based on a novel form of photon–photon interaction), high-quality heralded single- and multiphoton states free from higher-order imperfections, and robust, high-efficiency detection. It can also be used to produce heralded multiphoton entanglement, create optically switchable quantum circuits and implement an improved form of down-conversion with reduced higher-order effects. Such tools are valuable building blocks for many quantum-enabled technologies. Finally, using photonic crystal fibres we experimentally demonstrate quantum correlations arising from a four-colour nonlinear process suitable for CPC and use these measurements to study the feasibility of reaching the deterministic regime with current technology. Our scheme, which is based on interacting bosonic fields, is not restricted to optical systems but could also be implemented in optomechanical, electromechanical and superconducting systems with extremely strong intrinsic nonlinearities. Furthermore, exploiting higher-order nonlinearities with multiple pump fields yields a mechanism for multiparty mediation of the complex, coherent dynamics.

Method 2: a) Single photon doubler for mode a, producing two photons in the degenerate mode b. Separating the two photons into different modes is achieved via a “reverse Hong-Ou-Mandel”-type interaction at a beam splitter. With the pump field, Ec, energy conservation b) Analogous photon doubler for mode b, but with the roles of modes a and b swapped (a now degenerate). With the different pump field. c) Cascaded concatenation of a) and b) to achieve scaling up to high number of photons.

10 pages of supplemental information

CPC circuits for creating (a) bipartite and (b) tripartite entanglement. (c) A CPC circuit for generating arbitrary, nonlocally prepared, nonmaximally entangled states. (d) A direct implementation of the error-correction encoding step for a simple 9-qubit code.

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