Figure 4: QST in an integrated chip.

(a) Scheme of the integrated device (QST chip) for quantum tomography of a two-photon state: it is composed of two specular circuits, one for photon A and one for photon B; these circuits, by means of integrated waveplates of suitable length and tilt, prepare the two-photon state to have it projected on the horizontal (H)/vertical (V), diagonal (D)/antidiagonal (A) and circular left (L)/circular right (R) bases by a single PBS placed afterwards (see set-up in b). (b) Scheme of the experimental set-up for the tomography of entangled states (for details see Methods). (c) Fidelities for quantum tomography of different single-photon polarization states; blue and red bars represent the results obtained by input of the same polarization state in A and B modes, respectively. Error bars are omitted as they were too small compared with the axis scale. (d) Real (left) and imaginary (right) part of the reconstructed density matrix for a polarization-entangled Bell state (Ψ⁻); comparison is shown between the measurement made with the integrated chip and that performed by a standard QST set-up with bulk optical elements.