First experimental demonstration of high-fidelity three-qubit native quantum gate
Science and Technology Daily, Beijing, May 25 (Reporter Zhang Mengran) High-fidelity quantum logic gates applied to qubits are the basic building blocks of programmable quantum circuits.Researchers at Lawrence Berkeley National Laboratory's Advanced Quantum Testbed (AQT) have performed the first experimental demonstration of a three-qubit high-fidelity iToffoli native gate in a superconducting quantum information processor.The research results were recently published in Nature Physics.
Noisy mid-scale quantum processors typically support native gates of one or two qubits, and these gate types can be implemented directly in hardware.More complex gates are implemented by decomposing them into sequences of native gates.The team's demonstration adds a novel and powerful native three-qubit iToffoli gate to general quantum computing with a very high fidelity of 98.26%.
Toffoli (also known as Control-Control-Not Gate, CCNOT) is a key logic gate in classical computing because it is universal, so it can build all logic circuits to compute any desired binary operation.Furthermore, it is reversible, allowing binary inputs (bits) to be determined and recovered from the output, so no information is lost.
In a quantum circuit, an input qubit can be in a superposition of 0 and 1 states.Qubits are physically connected to other qubits in a circuit, and as the number of qubits increases, it becomes more difficult to implement high-fidelity quantum gates.Fewer quantum gates and shorter quantum circuits are required for computational operations, allowing algorithms to be improved before the final result goes wrong.Therefore, reducing the complexity and runtime of quantum gates is critical.
Together with Hadamard gates, Toffoli gates form a universal set of quantum gates that allow researchers to run any quantum algorithm.Experiments implementing multi-qubit gates in major computing techniques (superconducting circuits, trapped ions, and Rydberg atoms) have successfully demonstrated Toffoli gates on three-qubit gates with fidelity averaging between 87% and 90%.Such demonstrations, however, required researchers to decompose Toffoli gates into one- and two-qubit gates, making the gates operate longer and reducing their fidelity.
To create an easy-to-implement three-qubit gate, AQT designed an unconventional iToffoli gate by applying synchronized microwave pulses fixed at the same frequency to three superconducting qubits in a linear chain.
Similar to the Toffoli gate, this three-qubit iToffoli gate can be used to perform general-purpose quantum computations with high fidelity.In addition, the researchers found that gate schematics on superconducting quantum processors can generate additional three-qubit gates, providing more efficient gate synthesis to break down quantum gates into shorter gates to improve circuit runtime.
"Due to decoherence, we know that longer and more complex gate sequences compromise fidelity, so the total gate operation time to execute an algorithm is important," the researchers said. "This demonstration demonstrates that we can implement a three-qubit gate in one step. , and reduce the circuit depth (the length of the gate sequence) for gate synthesis. Furthermore, unlike previous approaches, our gate scheme does not include higher excited states where qubits are prone to decoherence, thus yielding high-fidelity gates."
