Abstract
Recent progress in quantum computing and networking has enabled high-performance, large-scale quantum processors by connecting different quantum modules. Optical quantum systems show advantages in both computing and communications, and integrated quantum photonics further increases the level of scaling and complexity. Here we demonstrate an efficient SWAP gate that deterministically swaps a photon’s polarization qubit with its spatial-momentum qubit on a nanofabricated two-level silicon photonics chip containing three cascaded gates. The on-chip SWAP gate is comprehensively characterized by tomographic measurements with high fidelity for both single-qubit and two-qubit operation. The coherence preservation of the SWAP gate process is verified by single-photon and two-photon quantum interference. The coherent reversible conversion of our SWAP gate facilitates examinations of a quantum interconnect between two chip-scale photonic subsystems with different degrees of freedom, now demonstrated by distributing four Bell states between the two chips. We also elucidate the source of decoherence in the SWAP operation in pursuit of near-unity fidelity. Our deterministic SWAP gate in the silicon platform provides a pathway towards integrated quantum information processing for interconnected modular systems.
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Data availability
All the data and methods are presented in the main text and the Supplementary Information. The figures are also available on Figshare at https://doi.org/10.6084/m9.figshare.22590367. The raw datasets generated and/or analysed during the current study are available from the corresponding author upon request. Source data are provided with this paper.
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Acknowledgements
We acknowledge discussions with H. Liu, K. Yu, Y. Cho, A. Veitia, T. Zhong and F. Sun and SEM assistance from J. F. McMillan. This work is supported by the National Science Foundation (EFRI-ACQUIRE 1741707, QII-TAQS 1936375, 1919355 and 2008728) and the University of California National Laboratory research programme (LFRP-17-477237).
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X.C., K.C.C., Z.X. and Y.S.L. performed the measurements. Y.L., S.K. and X.X. performed the design layout. M.Y., P.G.Q.L. and D.L.K. performed the device nanofabrication. X.C., M.C.S., X.X., A.K.V., J.H.S. and F.N.C.W. contributed to the theory and numerical modelling. X.C., Z.X., X.X., J.H.S., F.N.C.W. and C.W.W. wrote the manuscript, with contributions from all authors.
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Nature Photonics thanks Marco Fiorentino and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Supplementary Information
Supplementary Figs. 1–20 and discussion.
Source data
Source Data Fig. 2
Data for SPDC photons’ wavelength tenability and truth table for PC-NOT, MC-NOT and SWAP gate.
Source Data Fig. 3
Data for Bloch spheres, single-qubit process tomography and two-qubit process tomography.
Source Data Fig. 4
Data for single-photon self-interferences, HOM interferences, and density matrices of the four polarization Bell states.
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Cheng, X., Chang, KC., Xie, Z. et al. A chip-scale polarization-spatial-momentum quantum SWAP gate in silicon nanophotonics. Nat. Photon. 17, 656–665 (2023). https://doi.org/10.1038/s41566-023-01224-x
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DOI: https://doi.org/10.1038/s41566-023-01224-x