Abstract
Quantum information science addresses how the processing and transmission of information are affected by uniquely quantum mechanical phenomena. Combination of two-qubit gates has been used to realize quantum circuits, however, scalability is becoming a critical problem. The use of three-qubit gates may simplify the structure of quantum circuits dramatically. Among them, the controlled-SWAP (Fredkin) gates are essential since they can be directly applied to important protocols, e.g., error correction, fingerprinting, and optimal cloning. Here we report a realization of the Fredkin gate for photonic qubits. We achieve a fidelity of 0.85 in the computational basis and an output state fidelity of 0.81 for a 3-photon Greenberger-Horne-Zeilinger state. The estimated process fidelity of 0.77 indicates that our Fredkin gate can be applied to various quantum tasks.
Highlights
Fiurášek’s proposal is based on single-photon interference in a Mach-Zehnder interferometer (Fig. 1A)
A lens was placed in front of the barium borate (BBO) crystals to focus the beam. (D) Absolute value of the reconstructed density matrix of the generated state measured after the circuit using quantum state tomography. (E,F) Experimental results of CNOT gate operation in the computational basis using (E) partially polarizing beam splitters (PPBSs)-A1 and (F) PPBS-A2, where the cC0oNmTOp≡Tut1ga/atitoe2no(aplVebraaTstii+sonofiHnthteTh)ge,ac1toeTmisp≡dlee1fmi/neen2dt(aarsVy0bTaCs−i≡s uHsVinTgC)
In order to demonstrate the proposed simplified CSWAP gate, the interferometer that is integrated with multiple optical quantum gates must be perfectly stabilized
Summary
Fiurášek’s proposal is based on single-photon interference in a Mach-Zehnder interferometer (Fig. 1A). The state of the control qubit is encoded into the state of the two photons incident to the CNOT gates by using an Einstein-Podolsky-Rosen (EPR) source and a quantum parity check: the encoder transforms the ( ) ( ) input state of the control photon α H Cin + β V Cin / 2 into a state α H C1in V C2in + β V C1in H C2in / 2 with a probability of 1/2 In this proof-of-principle demonstration, we adopted a simplified scheme (Fig. 1D), where the control qubit ( ) is directly encoded into the entangled photon pair α H C1in V C2in + β V C1in H C2in / 2 generated via spontaneous parametric down-conversion and local polarization operations. 1 C ≡ H C for the control qubit and for the target qubit. (G,H) Experimental results (G) PPBS-A1 and (H) PPBS-A2, where the of ( ) ( ) computational basis control qubit and 0 of the gate
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