Fidelity of 98.26%, the first realization of a high-fidelity three-qubit gate

High-fidelity quantum logic gates applied to qubits are the fundamental building blocks of programmable quantum circuits. Recently, researchers at the Advanced Quantum Testbed (AQT) at Lawrence Berkeley National Laboratory performed the first experimental demonstration of three-qubit high-fidelity iToffoli native gates in a superconducting quantum information processor.

Experimental schematic of the high-fidelity iToffoli gate of the Advanced Quantum Testbed (AQT). Source: Yosep Kim/Berkeley Lab

Noisy Intermediate Scale (NISQ) quantum processors typically support native gates of one or two qubits - this is the type of gate that can be implemented directly in hardware; more complex gate operations are performed by decomposing them into sequences of native gates to be realized. The AQT team's demonstration adds a new, powerful native three-bit iToffoli gate to general-purpose quantum computing. In addition, the team demonstrated that the fidelity of the gate is very high: 98.26%.

The team's experimental breakthrough is titled "High-Fidelity Three-Qubit iToffoli Gate for Fixed-Frequency Superconducting Qubits" and will be published in "Nature Physics" in May 2022 [1].

The Toffoli gate, also known as the Controlled-Controlled-Not (CCNOT) gate, is a key logic gate in classical computing because it is universal, so all logic circuits can be built to compute any binary operation. Furthermore, it is reversible, and the binary input (bits) can be determined and recovered from the output, so no information is lost.

Common quantum logic gates

In a quantum circuit, the input qubits can be in a superposition of 0s and 1s. Since qubits are physically connected to other qubits in the circuit, this makes it more difficult to implement high-fidelity quantum gates as the number of qubits increases. The fewer quantum gates required to compute an operation, the shorter the quantum circuit, necessitating improved algorithm implementation before qubit decoherence, reducing errors in the final result. Therefore, reducing the complexity and running time of quantum gates is crucial.

Like Hadamard gates, Toffoli gates form a general set of quantum gates that allow researchers to run any quantum algorithm. Experiments implementing multi-qubit gates in existing computing techniques: superconducting circuits, ion traps, and Rydberg atoms demonstrate Toffoli gates on three-qubit gates with fidelity averaging between 87% and 90%. Such a demonstration, however, required the researchers to decompose the Toffoli gate into a single-bit gate and a two-bit gate, which made the gate longer to operate and reduced its fidelity.

To create an easy-to-implement three-qubit gate, AQT designed an iToffoli gate. Unlike conventional Toffoli gates, the new logic gate simultaneously applies microwave pulses fixed at the same frequency to three superconducting qubits in a linear chain.

Experiments show that, similar to the Toffoli gate, this three-qubit iToffoli gate can be used for high-fidelity general-purpose quantum computing. In addition, the researchers showed that the gate principle on a superconducting quantum processor can generate additional three-qubit gates, which provides more efficient gate synthesis -- quantum gates can be broken down into shorter gates to improve circuit runtime the process of.

Yosep Kim, one of the principal investigators of the experiment, was a postdoc at AQT and is currently a senior scientist at the Korea Institute of Science and Technology. According to Kim [2], "As a result of decoherence, we know that longer and more complex gate sequences can hurt the fidelity of the results, so the total gate operation time to execute a certain algorithm is important. Demonstration shows that we can implements a three-qubit gate in one step and reduces the circuit depth (the length of the gate sequence) for gate synthesis. Furthermore, unlike previous approaches, our gate scheme does not include highly excited states of qubits that are prone to decoherence, The result is a high-fidelity door."

"I'm still impressed by the simplicity and fidelity of this iToffoli gate. Now, the use of three-qubit operations like in this work can greatly accelerate the development of quantum applications and quantum error correction." Former AQT postdoc, current said Alexis Morvan, a research scientist at Google.

Researcher Yosep Kim verifies the operation of a high-fidelity iToffoli gate at the Advanced Quantum Testbed (AQT). Source: Yosep Kim/Berkeley Lab

AQT is one of the most advanced quantum information science collaborative research laboratories funded by the Advanced Scientific Computing Research Program of the U.S. Department of Energy's Office of Science. The lab operates an open experimental testbed designed to collaborate in depth with Berkeley Lab researchers and external users from academia, national labs, and industry. These interactive collaborations allow experts to extensively explore cutting-edge science on AQT's superconducting platform, which relies on high-quality qubits, gates, and error mitigation operations.

"I used photonic systems to study quantum information science during my Ph.D., so I didn't have the good knowledge and experience to conduct experiments in superconducting processors," recalls Kim, "but because the experimental testbed was so well established and there were many interdisciplinary Colleagues know the internal structure of the device and have collaborated in the experiments, so I was able to quickly carry out experiments without much experience. If it weren't for the platform and team of AQT, I don't think my ideas would be at such a high level realized.”

AQT offers researchers and users an excellent opportunity to collaborate with people from different backgrounds and interests, and this iToffoli project is an example of such a fusion of ideas. In the future, the researchers hope that this experimental approach to high-fidelity, easy-to-implement multi-qubit gates will lead to further research to design different multi-qubit gates for novel quantum information processing.