Research ArticlePHYSICS

Beating the channel capacity limit for superdense coding with entangled ququarts

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Science Advances  20 Jul 2018:
Vol. 4, no. 7, eaat9304
DOI: 10.1126/sciadv.aat9304
  • Fig. 1 Experimental setup.

    A continuous-wave violet laser (power, 4 mW; wavelength, 404 nm) is focused by two lenses, and the waist radius is approximately 0.25 mm. Then, the light beam is separated into two paths by a BD. These two beams are injected into a Sagnac interferometer to pump a type II cut periodically poled potassium titanyl phosphate (ppKTP) crystal (1 mm × 7 mm × 10 mm) and generate two-photon polarization entanglement (|H|H+|V|V)/2 in each path (33). The ppKTP is temperature-controlled by a homemade temperature controller, and the temperature stability is K to ensure the phase stability between the two paths. Then, we encode horizontally polarized (H) photons in path a1 (a3) as |0〉, vertically polarized (V) photons in path a1 (a3) as |1〉, H photons in path a2 (a4) as |2〉, and V photons in path a2 (a4) as |3〉 and carefully adjust the relative phase between the two paths; the state is prepared in a four-dimensional maximally entangled two-photon state Ψ11 = (|00〉 + |11〉 + |22〉 + |33〉)/2. Finally, Alice encodes her information using four computer-controlled LCs and sends her photon to Bob. Bob performs a measurement on the two photons and decodes the information that Alice encoded (details can be found in the Supplementary Materials). PBS, polarizing beam splitter; HWP, half-wave plate.

  • Fig. 2 Measured probabilities.

    Alice sends the five four-dimensional Bell states to Bob, and Bob performs a measurement on the two photons and obtains the probabilities for each state. In our experiment, the photon-pair count rate is 1000/s, and the integration time is 20 s for each input state. Error bars are due to the statistical error.

  • Fig. 3 Transmit a real five-color image using encoded information.

    Red spots are encoded to Ψ11, yellow spots are encoded to Ψ12, blue spots are encoded to Ψ13s, green spots are encoded to Ψ14, and white spots are encoded to Ψ23. (A) Original five-color 53 × 188 pixel image. (B) Image received using SDC. The calculated fidelity is 0.952.

  • Fig. 4 Realization of a single-photon operation on Alice’s side.

    The optical axes of the LCs are set at different angles, as shown in the figure. By applying different voltages, the LCs will introduce different phases between the fast axis and the slow axis.

Supplementary Materials

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Measurement on four-dimensional two-photon Bell state.
    • Fig. S2. Realization of single-photon four-dimensional identical operation and U gate.

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