Heralded noiseless amplification of a photon polarization qubit


Photons are the best long-range carriers of quantum information, but the unavoidable absorption and scattering in a transmission channel places a serious limitation on viable communication distances. Signal amplification will therefore be an essential feature of quantum technologies, with direct applications to quantum communication, metrology and fundamental tests of quantum theory. Non-deterministic noiseless amplification of a single mode1,2,3,4,5 can circumvent the challenges related to amplifying a quantum signal, such as the no-cloning theorem6 and the minimum noise cost for deterministic quantum state amplification7. However, existing devices are not suitable for amplifying the fundamental optical quantum information carrier: a qubit coherently encoded across two optical modes. Here, we construct a coherent two-mode amplifier to demonstrate the first heralded noiseless linear amplification of a qubit encoded in the polarization state of a single photon. In doing so, we increase the transmission fidelity of a realistic qubit channel by up to a factor of five. Qubit amplifiers promise to extend the range of secure quantum communication8,9 and other quantum information science and technology protocols.

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Figure 1: Conceptual and experimental representations of the qubit amplifier circuit.
Figure 2: States of the qubit subspace before and after the amplifier.
Figure 3: Absolute values of the ρin and ρout density matrices.


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This research was conducted by the Australian Research Council Centre of Excellence for Quantum Computation and Communication Technology (Project number CE110001027). S.K. thanks D. J. Saunders and M. J. W. Hall for useful discussions.

Author information

G.J.P. and T.C.R. conceived and managed the project. S.K. built and conducted the experiment with assistance from G.Y.X. and G.J.P. S.K. analysed the data with assistance from G.J.P. All authors contributed to the theory and writing of the manuscript.

Correspondence to G. J. Pryde.

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The authors declare no competing financial interests.

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Kocsis, S., Xiang, G., Ralph, T. et al. Heralded noiseless amplification of a photon polarization qubit. Nature Phys 9, 23–28 (2013). https://doi.org/10.1038/nphys2469

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