Implementation of polarization-encoded quantum Toffoli gate

Qianke Wang, Dawei Lyu, Jun Liu +1 more

Controlled quantum gates play a crucial role in enabling quantum universal operations by facilitating interactions between qubits. Direct implementation of three-qubit gates simplifies the design of quantum circuits, thereby being conducive to performing complex quantum algorithms. Here, we propose and present an experimental demonstration of a quantum Toffoli gate fully exploiting the polarization and orbital angular momentum of a single photon. The Toffoli gate is implemented using the polarized diffractive neural networks scheme, achieving a mean truth table visibility of 97.27±0.20%. We characterize the gate's performance through quantum state tomography on 216 different input states and quantum process tomography, which yields a process fidelity of 94.05±0.02%. Our method offers a novel approach for realizing the Toffoli gate without requiring exponential optical elements while maintaining extensibility to the implementation of other three-qubit gates.

Samy C. Policarpo, João Batista Rosa Silva

He-Liang Huang, Wan‐Su Bao, Tan Li +5 more

Md. Mazder Rahman, Mathias Soeken, Gerhard W. Dueck

The Toffoli gate, as originally proposed, had only positive controls. It has been shown that mixed polarity controlled Toffoli gates can be efficiently implemented. In fact, their quantum cost is the same as for positive controlled gates in most cases. Thus it is advantageous to consider circuits with mixed polarity Toffoli gates. Template matching has been successfully used to reduce the number of Toffoli gates in reversible circuits. Little work on templates with mixed polarity gates has been reported. Unfortunately, the number of potential templates increases dramatically, if mixed polarity is introduced. Here we propose a dynamic template matching algorithm that takes templates with few lines and dynamically extends the lines to find matches. Experimental results show that the proposed approach has a significant impact on reducing the total number of gates (57% in the best case) in circuits.

Michal Mičuda, Martina Miková, Ivo Straka +4 more

We report on a detailed characterization of a three-qubit linear optical quantum Toffoli gate. Our experiment utilizes correlated photon pairs generated in the process of spontaneous parametric down-conversion. Two qubits are encoded into polarization and spatial degrees of freedom of a signal photon, and the third qubit is represented by polarization of an idler photon. The linear optical Toffoli gate is implemented by interference of photons on a partially polarizing beam splitter inserted inside a Mach Zehnder interferometer formed by two calcite beam displacers. We have measured 4032 different two-photon coincidences, which allows us to estimate the fidelity of the gate to be 90%. Although these data are not tomographically complete, we show that they are sufficient for a reliable reconstruction of the quantum process matrix of the gate via the recently proposed maximum likelihood--maximum entropy estimation procedure. To probe the entangling capability of the gate, we have investigated generation of three-qubit GHZ states from fully and partially separable input states and we have performed a full tomography of the output states. We compare the reconstructed states with theoretical predictions obtained with the use of the estimated quantum process matrix and obtain a very good agreement.

Xiao‐Qiang Shao, Hong‐Fu Wang, Li Chen +2 more