Figure 1

Configuration of a multiparty quantum network and GHZ entanglement swapping. Initially, users $A$, $B$, and $C$ share entangled qubit pairs with the central exchange Ex. If Ex projects the three particles, 1, 3, and 5, into a GHZ state, the other three particles, 2, 4, and 6 belonging to $A$, $B$, and $C$ respectively, will be entangled into a GHZ state by entanglement swapping.Reuse & Permissions

Figure 2

Experimental setup for entanglement swapping of a three-photon GHZ state. Ultraviolet laser pulses (with a central wavelength of $\sim 394\mathrm{nm}$, a pulse duration of $\sim 120\mathrm{fs}$, and a repetition rate of $\sim 76\mathrm{MHz}$) are focused on three BBO crystals, producing entangled photon pairs emitted into spatial modes 1–2, 3–4, and 5–6. Photons 1, 3, and 5 are projected into a GHZ state (dashed box, see text and Ref. 18), and the photons 2, 4, and 6 are analyzed by a combination of a quarter-wave plate (QWP), a half-wave plate (HWP) and a PBS. The photons are spectrally filtered by narrow-band filters ($\Delta {\lambda }_{\mathrm{FWHM}}=3.2\mathrm{nm}$) and monitored by fiber-coupled silicon avalanche single-photon detectors ($\mathrm{D}1,\mathrm{D}2\mathrm{T},\cdots ,\mathrm{D}6\mathrm{R}$). The multiphoton events are registered by a laser clocked multichannel coincidence unit.Reuse & Permissions

Figure 3

Step-by-step observation of the Hong-Ou-Mandel-Shih-Alley [20] and Rarity-Zeilinger [21] type multiphoton interferences as a function of temporal delay. The right insets show briefly the setup employed for each step. (a) The photon pairs 1–2 and 3–4 are prepared in the $|\varphi ⟩$ state. After combining the photons 1 and 3 on the PBS1, the four photons are analyzed in the $+/-$ polarization setting as a function of the delay ($\Delta d1$). (b) The photon 2 is used as a trigger. The photon 1 is directed through the PBS1, prepared in the $|+⟩$ state using a HWP, and then superposed on the PBS2 with the photon 5. (c). The photons 1, 3, and 5 are superposed on the two PBSs as required by the GHZ-state analyzer (see text) and the six-photon coincidence in the $|\pm ⟩$ basis is registered, showing constructive and destructive interference. The accumulation time for the multiphoton coincidence counts in (a),(b), and (c) is 60 s, 40 s, and 18 h, respectively. Gaussian curves are to guide the eyes.Reuse & Permissions

Figure 4

Sixfold coincidence in the measurement basis of: (a) $H/V$, (b) $A/B$, (c) $+/-$, and (d) $C/D$ for witnessing the genuine entanglement of the three emerging photons 2, 4, and 6. The accumulation time for each data set is 24 h in (a) and 18 h in (b),(c), and (d). The error bars represent 1 standard deviation deduced from Poissonian counting statistics of the raw detection events.Reuse & Permissions