Figure 1

(Color online) Quantum circuit for teleportation-based programmable quantum logic gate. Time proceeds from left to right. The single wires carry quantum bits (qubits) and the double wires carry classical bits. Qubits 2 and 3 form a program register prepared in a particular entangled state ${|\Phi ⟩}_{\text{p}}$. BSM is applied to data qubit 1 in an unknown quantum state ${|\psi ⟩}_{\text{d}}$ and qubit 2 of the program register. For singlet Bell state detected in channels 1 and 2 the measurement yields $x=y=1$ which heralds the successful gate operation leaving the remaining qubit 3 in the target state ${\Sigma }_{\varphi }|\psi ⟩$. The unitary operation ${\Sigma }_{\varphi }$ is determined by the state of the program register.Reuse & Permissions

Figure 2

(Color online) A scheme of the experimental setup. The initial subpicosecond pulses generated by titan-sapphire laser (Ti:Sa) at the fundamental wavelength of 800 nm are frequency doubled to 400 nm by the process of second harmonic generation (SHG) and separated from the fundamental ones by dichroic mirror (DM). The frequency doubled pulses pump a 2-mm-thick nonlinear crystal of $\beta$-barium borate (BBO) cut for non-collinear type-I SPDC. The downconverted correlated photons 2 and 3 at 800 nm are filtered by narrow-band 1.75 nm FWHM interference filters $\left(F\right)$, coupled to single-mode optical fibers (SMF), and mixed by 50/50 fiber beam splitter (S1) which assures the perfect mode matching of the photon modes. Polarization states of the photons are set to be orthogonal before the splitter. The occurrence of one photon in each output port of the splitter yields the entangled polarization singlet state of the program register. It is further set by half-wave plate (HWP) and quarter-wave plate (QWP) to any chosen entangled state. The data qubit 1 is encoded into a polarization state of a single photon from the attenuated (ATT) fundamental laser pulse, frequency filtered, and coupled to single-mode fiber. The data photon interferes with photon 2 from the program register at the second 50/50 fiber beam splitter (S2) and the coincidence events are detected at the output by single-photon avalanche photodiodes (APDs) and multichannel coincidence logic. A polarization state of the remaining photon 3 carrying output data qubit is simultaneously analyzed using wave plates and polarizing beam splitter (PBS). Fixed unitary polarization transformations induced by optical fibers are compensated by fiber polarization controllers (FPCs). The photon arrival times are precisely synchronized by optical delays not shown in the scheme.Reuse & Permissions

Figure 3

(Color online) The complete characterization of the programmable SIGN gate operation by means of full quantum process tomography. The real (left column) and imaginary (right column) parts of the reconstructed process matrix $\chi$ are shown for five different program-register states [Eq. (2)] encoding the sign flip operation in bases $\left\{|{\varphi }_j⟩,|{\varphi }_{j\perp }⟩\right\}$, with $j=1,\dots ,5$.Reuse & Permissions