Robust entanglement distribution via telecom fibre

At Alice’s photon pair source, the cw pump light at 780 nm for SPDC is prepared by second-harmonic generation of the light at 1560 nm from an external cavity diode laser with a linewidth of 1.8 kHz. At Bob's ancillary photon source, the pulsed light at 781 nm is filtered by a volume holographic grating (VHG1) with a bandwidth of 0.3 nm and the pulsed light and the cw pump light at 1563 nm are combined by the DM2 and coupled into a PPLN/W. At the data collection stage by TDC, the repetition rate of the electric signal from the Ti:S pulse laser is divided into 800 kHz to reduce the amount of data for avoiding the data overflow.
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Distributing entangled photon pairs over noisy channels is an important task for various quantum information protocols. Encoding an entangled state in a decoherence-free subspace (DFS) formed by multiple photons is a promising way to circumvent the phase fluctuations and polarisation rotations in optical fibres.

Recently, it has been shown that the use of a counter-propagating coherent light as an ancillary photon enables to faithfully distribute entangled photon with success probability proportional to the transmittance of the optical fibres.

Several proof-of-principle experiments have been demonstrated, in which entangled photon pairs from a sender side and the ancillary photon from a receiver side originate from the same laser source. In addition, bulk optics have been used to mimic the noises in optical fibres.

A Japanese team has demonstrated a DFS-based entanglement distribution over 1 km optical fibre using DFS formed by using fully independent light sources at the telecom band, and obtain a high-fidelity entangled state. This shows that the DFS-based scheme protects the entanglement against collective noise in 1 km optical fibre.

In the experiment, they utilised an interference between asynchronous photons from continuous wave pumped spontaneous parametric down conversion (SPDC) and mode-locked coherent light pulse. After performing spectral and temporal filtering, the SPDC photons and light pulse are spectrally indistinguishable.

This property allows to observe high-visibility interference without performing active synchronisation between fully independent sources.

The paper has been published in Nature npj.