Fig. 2 | Nature Communications

Fig. 2

From: Experimental optical phase measurement approaching the exact Heisenberg limit

Fig. 2

Schematic of the experimental setup. Single photons at 820 nm are generated via a type-I spontaneous parametric downconversion (SPDC) process (blue background) and collected using single-mode fibres and passed into the entangling gate (green background) in order to realise the state \(|\psi _{{\mathrm{exp}}}\rangle\). Input polarisation was adjusted with fibre polarisation controllers (FPC). The non-deterministic universal CNOT gate, composed of 3 partially polarising beam splitters (PPBS) and 2 half-waveplates (HWP), performs the state preparation by post-selecting coincidence events between the control and target output ports with success probability 1/9. The area with grey background corresponds to the implementation of the phase estimation. Photons in mode C pass twice through the HWP (acting as a phase shift element), in order to realise the \(U(\phi )^2\) operation. Photons in mode T experience the phase shift once (performing the \(U(\phi )\) operation). The effect of the feedforward operation, \({\cal R}(\theta )\), is simulated by dialling a HWP (depicted with a white rim), for a fixed time period, with 0 and π/8 corresponding to the ON and OFF settings of the control operation. Finally, photons are independently directed to a polarisation analysis unit consisting of a quarter-wave plate (QWP), HWP and a polarising beam splitter (PBS) followed by a 2 nm spectral filter and a single photon counting module (SPCM). See Methods section for further details on the experimental setup operation

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