Figure 1

The scheme to obtain a photonic realization of a CNOT gate with two independent qubits. The qubits are encoded in the polarization of the photons. The scheme makes use of linear optical components, polarization entanglement, and postselection. When one and only one photon is detected at the polarization sensitive detectors in the spatial modes $b_3$ and $b_4$ and in the polarization $H$, the scheme works as a CNOT gate.Reuse & Permissions

Figure 2

The experimental setup. A type II spontaneous parametric down-conversion is used both to produce the ancilla pair (in the spatial modes $a_3$ and $a_4$) and to produce the two input qubits (in the spatial modes $a_1$ and $a_2$). In this case initial entanglement polarization is not desired, and it is destroyed by making the photons go through polarization filters that prepare the required input state. Half-wave plates have been placed in the photon paths in order to rotate the polarization; compensators are able to nullify the birefringence effects of the nonlinear crystal and of the polarizing beam splitters. Overlap of the wave packets at the PBSs is assured through spatial and spectral filtering.Reuse & Permissions

Figure 3

This graph shows that the scheme works, indeed, for the linear polarizations $H$,$V$. Fourfold coincidences for all the possible (16) combinations of inputs and outputs are shown. When the control qubit is in the logical value 0 ($HH$ or $HV$), the gate works as the identity gate. In contrast, when the control qubit is in the logical value 1 ($VH$ or $VV$), the gate works as a NOT gate, flipping the second input bit. Noise is due to the nonideal nature of the PBSs.Reuse & Permissions

Figure 4

Demonstration of the ability of the CNOT gate to transform a separable state into an entangled state. In (a) the coincidence ratio between the different terms $HH,\dots ,VV$ is measured, proving the birefringence of the PBS has been sufficiently compensated; in (b) the superposition between $HH$ and $VV$ is proved to be coherent, by showing via the Ou-Hong Mandel dip at $45°$ that the desired ($H+V$) state of the target bit emerges much more often than the spurious state ($H-V$). The fidelity is of $81\%\pm 2\%$ in the first case and $77\%\pm 3\%$ for the second.Reuse & Permissions