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Efficient quantum computing using coherent photon conversion


Single photons are excellent quantum information carriers: they were used in the earliest demonstrations of entanglement1 and in the production of the highest-quality entanglement reported so far2,3. 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 probabilistic5. 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 architecture6, 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 technology4,7. 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 systems8,9,10,11,12 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.

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Figure 1: Satisfying the DiVincenzo criteria with CPC.
Figure 2: Heralded single-photon source.
Figure 3: Experimental set-up.
Figure 4: Experimental results.


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The authors would like to acknowledge discussions with T. Jennewein, A. Fedrizzi, D. R. Austin, T. Paterek, B. J. Smith, W. J. Wadsworth, M. Halder, J. G. Rarity, F. Verstraete and A. G. White. This work was supported by the ERC (Advanced Grant QIT4QAD), the Austrian Science Fund (grant F4007 and an Erwin Schroedinger Fellowship), the EC (QU-ESSENCE and QAP), the Vienna Doctoral Program on Complex Quantum Systems, the John Templeton Foundation and in part by the Japanese FIRST programme and the Ontario Ministry of Research and Innovation.

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N.K.L. and S.R. conceived the original theory and developed it with A.Z., G.J.M. and W.J.M. N.K.L., S.R., R.P. and A.Z. designed the experiment and N.K.L., S.R. and R.P. performed the experiment and carried out the data analysis. All authors contributed to writing the manuscript.

Corresponding authors

Correspondence to N. K. Langford or A. Zeilinger.

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The authors declare no competing financial interests.

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The file contains Supplementary Text and Data, Supplementary Figures 1-4 with legends, Supplementary Table 1 and additional references. (PDF 361 kb)

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Langford, N., Ramelow, S., Prevedel, R. et al. Efficient quantum computing using coherent photon conversion. Nature 478, 360–363 (2011).

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