The Quantum Network Center of Competence aims to build and employ a quantum communication testbed at CERN. One of its goals is to facilitate usage of White Rabbit (WR), a CERN-born technology that provides sub-nanosecond accuracy and picosecond precision for networked distributed devices, for quantum experiments. Specifically, two-node entanglement distribution experiments where the quantum signal co-exists with the WR signal, are performed. The figure on the right illustrates the working principle of such experiments.

Here, the entangled photon source is located at Alice’s node and emits polarisation entangled photon pairs. One photon is kept local, and sent to Alice’s receiver (Polarisation optics, photon detector) directly. The other photon, with a wavelength in the telecom band, is multiplexed on the same fiber as the classical WR signals using DWDM. The combined signal is sent over deployed fiber, and de-multiplexed at Bob’s node, where the quantum signal is sent to Bob’s receiver (Polarisation optics, photon detector). The WR switches ensure the two time taggers run at the same clock for coincidence analysis at a central PC. 

The distribution of entangled photon pairs multiplexed with WR synchronisation signals has been demonstrated over a 29.5 km long fiber (running from the Meyrin site of the CERN campus to the CMS detector and back). The following figure shows the measured coincidence counts as a function of the position of Alice’s half wave plate, for four different positions of Bob’s half wave plate. From the fitted sines to the four curves, the CHSH S-parameter can be calculated, proving entanglement if S > 2. Here, we show a value of S = 2.62 ± 0.02, which is significantly above its classical maximum of 2, proving entanglement distribution with WR classical signals present on the same optical fiber.

Additionally, full characterisation of the distributed entangled state has been performed. The quantum state tomography results yield a fidelity of over 95% with respect to the |Ψ⟩=  (|HH⟩+|VV⟩) / √2 Bell state, while White Rabbit traffic was present on the fiber.