Fig. 2: Experimental implementation of the triangle network.
From: Experimental nonclassicality in a causal network without assuming freedom of choice
The source ρAB generates polarization-entangled photon pairs in the singlet state \(\left|{\Psi }^{-}\right\rangle\), by pumping with a continuous wave UV laser a periodically poled potassium titanyl phosphate (ppKTP) crystal. Conversely, ΛAC (ΛBC) produces classically correlated states \((\left|00\right\rangle \left\langle 00\right |+\left|11\right\rangle \left\langle 11\right|)/2\) obtained by splitting the output signal of two single-photon avalanche photodiodes subjected to environmental light noise. In nodes A and B, to implement the measurements needed to reconstruct the probability distribution p(a, b, c), the photons from the source ρAB are collected by the input single-mode fiber (SMF) of a 5ns rise-time optical switch. In Fritz-like distributions, the measurement result a0 (b0) on part of the source ΛAC (ΛBC) determines the observable to be measured on the photon coming from ρAB, leading to outcomes a1 (b1). In our implementation, this is achieved by appropriately driving the optical switches through a specially designed electronic driver, which receives signals coming from ΛAC (ΛBC) and drives the output port of the optical switch based on the results a0 (b0). The bit a1 (b1) is obtained by performing a polarization measurement on the photons produced by the ppKTP source through a half-waveplate (HWP) and a polarizing beam splitter (PBS), implemented in fiber. In node C, c0 and c1 are measured independently by directly feeding the electrical signals produced by ΛAC and ΛBC into a time to digital converter (TDC).
