A Joint Research Team has Achieved New Progress in the Field of Quantum Algorithm Implementation and Quantum Architecture

Date: Nov 24, 2022
A joint research team from Institute of Computing Technology, CAS and Shenzhen Institute for Quantum Science and Engineering, SUSTech has achieved new progress in the field of quantum algorithm implementation and quantum architecture. The team proposed and implemented a novel quantum version of AND logic gate, reducing the resource overhead of implementing AND logic on quantum hardware and enabling the execution of key quantum circuits. The result has been published on November 14, 2022 under the title of "Scalable algorithm simplification using quantum AND logic" in Nature Physics.
Quantum computing is predicted to provide a computational speed-up over classical computing on some practical problems such as database search and integer factorization. In order to implement a quantum algorithm on a specific quantum device, the quantum circuit which describes the quantum algorithm, needs to be decomposed into the naturally available elementary gates. Given the resource constraints of near-term quantum devices, optimizing the quantum circuits for describing quantum algorithms is of great importance for the realization of near-term applications, and is as an important aspect as the enhancement of the qubit quality.
   a. Decomposition, truth table of QuAND and its reversal b. Decomposition of multi-qubit-controlled Z gate
In this work, the research team proposed a quantum version of AND gate (QuAND). AND logic is a basic ingredient for design algorithms in traditional computer science. QuAND gate enriches the toolbox of quantum instruction set and can significantly reduce the cost of decomposing some important large-scale quantum circuits such as multi-qubit Toffoli gates and quantum arithmetic circuits. The research team developed a superconducting quantum processor consisting of 8 qubits. The processor utilizes the latest tunable coupler combined with fixed frequency bits, for high scalability and simplified control. In the experiment, the high-fidelity QuAND gate is implemented by applying a parametric flux drive to the coupler.

The QuAND-based decomposition and measured truth table of 4-qubit, 6-qubit and 8-qubit Toffoli.
The research team successfully implemented a multi-qubit Toffoli gate of up to 8 qubits. The QuAND gate-based optimization method requires only a linear number of two-bit gates, while traditional methods require a quadratic number two-bit gate. It is the dramatic reduction in gate operation overhead that enabled the experimental team to successfully implement the largest multi-qubit Toffoli gate to date and demonstrate the Grover search algorithm up to a 64-entries search space which is a much larger experimental scale than before. This work demonstrates how non-traditional but hardware-native quantum logic gate operations can be constructed on scalable quantum computing hardware to optimize the compiled results of quantum algorithms to quantum circuits, illustrating the importance of tapping the manipulation potential of quantum hardware and enriching the set of quantum instruction sets, laying the foundation for more meaningful future applications of larger scale.

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