Figure 1

(Color online) Experimental setup. The uv pulses pass through the beamlike cut BBO twice to generate four photons with special polarizations which are coupled by single-mode polarization-maintaining (PM) optical fibers. The initial input state in mode 1 is prepared by a half-wave plate (HWP) and a quarter-wave plate (QWP) to be any required pure state. $P$ is a birefringent phase plate used to introduce a $\pi$ phase shift between horizontal and vertical polarizations. Beam splitter 1 (BS1) with reflection coefficient $R=2∕3$ is used to realize the UQCM and $R=3∕4$ to realize the PQCM. One of the fiber outputs in mode 1 and mode 3 are scanned to achieve spatial and temporal overlapping of photons in modes 1 and 4 at BS1. Beam splitter 2 (BS2) with $R=1∕2$ is used to split mode 6 equally into modes 7 and 8. Polarization analysis (PA) within the measurement apparatus (MA) containing a half-wave plate, a quarter-wave plate, and a polarization beam splitter (PBS) in modes 5, 7, and 8 are inserted into the paths depending on the measurement condition. All four photons are detected by single photon avalanche detectors (SPADs) equipped with $3\mathrm{nm}$ interference filters (IFs).Reuse & Permissions

Figure 2

(Color online) Coincidence probability as a function of the relative delay between (a) photons in modes 2 and 3 arriving at the PBS and (b) photons in modes 1 and 4 arriving at the BS1 with reflection coefficient $2∕3$. The solid lines are the Gauss fittings. The error bars are due to counting statistics.Reuse & Permissions

Figure 3

(Color online) Experimental results for the replicas’ fidelities in the optimal universal cloning machine of the states (a) $\cos \theta \mid H⟩+\sin \theta \mid V⟩$ and (b) $\cos \theta \mid H⟩+i\sin \theta \mid V⟩$. The solid lines represent the theoretical predictions. The error bars due to counting statistics are smaller than the size of the symbols.Reuse & Permissions

Figure 4

(Color online) Experimental results for the replicas’ fidelities in the optimal phase-covariant cloning machine. The solid line is the theoretical prediction. Uncertainties are due to counting statistics.Reuse & Permissions