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.

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.