Figure 3
Optical realization of a post-selected Fredkin gate. Lowercase letters and numbers label the beams. Photons in beams
,
, and
are the control and two target qubits. Polarizing beam splitters (PBSs) transmit horizontally polarized
photons and reflect vertically polarized
photons. A heralded controlled-NOT (CNOT) gate proposed in Ref. [
7] lies in the dotted box. EPR is an ancillary Bell state source. RPBS, transmitting 45° polarized
photons and reflecting
polarized
photons, can be realized by inserting one half-wave plate (HWP) oriented at 22.5° in each of the two inputs and two outputs of a PBS. D1 and D2 are photon detectors detecting
basis and
basis, respectively. A post-selected CNOT gate suggested in Ref. [
12] is enclosed by the dashed box. Beam splitters are represented as black lines with their reflectivity indicated aside and dotted line indicates the surface from which a sign change occurs upon reflection. HWP1, HWP3, and HWP5 are oriented at 67.5°. HWP2, HWP4, and HWP6 are set to 22.5°. HWP7 is set to 45°. This gate succeeds conditioned on the coincidence of the successful detection at D1 and D2 and exactly one photon in each of the output beams
,
, and
. Symbols “
” denote the discarded outputs.
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