Figure 1
Experimental setup for production of a GHZ state and its conversion to an approximate
state. A double-pass pulsed parametric down-conversion crystal (BBO) is used to create two pairs of polarization-entangled photons both in the state
. Extra crystals (COMP) compensate for walk-off effects. A polarizing beam splitter (PBS) performs a parity check on two of the photons, one from each pair; given that the parity check succeeds, signaled by the two photons taking different output modes, the emerging photons are in a 4-photon GHZ state. Projection of photon 4 onto the state
leaves the remaining three photons in the desired 3-photon GHZ state. Each photon in the GHZ state was rotated locally—photons
and
were rotated using additional half-wave plates (HWP) and photon 1 was rotated using the existing half-wave plate, rotating the polarization only 45° instead of 90°. Our 3-qubit local POVM is performed using 3 partial polarizers (PP). Each partial polarizer consists of two microscope slides (MS) mounted such that the light is incident at 56° from normal, Brewster’s angle for
. This configuration had a measured transmission of 88% for
polarization and 33% for
polarization. Each POVM was oriented to reduce the horizontal component of the light relative to the vertical component. The 3-photon polarization measurements for tomography were taken using quarter-wave plates (QWP) and rotatable polarizers (POL) for photons
and
, and a half- or quarter-wave plate and a fixed polarizer for photon
before photon-counting detectors (DET and TRIG). A fixed QWP in mode
was used to compensate birefringence in the PBS.
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