Berkeley Lab Achieves Breakthrough in High-Fidelity Qutrit Entanglement

A team at the Advanced Quantum Testbed (AQT) at Lawrence Berkeley National Laboratory and the University of California, Berkeley’s Quantum Nanoelectronics Laboratory have made a breakthrough in quantum computing. They successfully entangled two transmon qutrits, three-level systems, on a superconducting quantum processor, achieving higher gate fidelities than previous attempts. This brings us closer to ternary logic, which can encode more information than binary logic. The research, published in Nature Communications, could improve quantum simulation, error correction, and specific quantum algorithms. The team, led by graduate student Noah Goss, collaborated with Keysight Technologies to measure a process fidelity of up to 97.3% for a two-qutrit entangling gate.

Breakthrough in Quantum Computing Using Qutrits

Qutrits are three-level systems that can encode more information than their binary counterparts, qubits. The team successfully entangled two transmon qutrits with gate fidelities significantly higher than in previously reported works, thus getting closer to enabling ternary logic. This research was published in Nature Communications in November 2022 and was featured as an editor’s highlight.

Using qutrits in quantum information processors offers significant potential advantages in quantum simulation and error correction and the ability to improve specific quantum algorithms and applications. This experimental success pushes AQT’s qutrit research and development, including previous experimental wins published in 2021 in Physical Review X and Physical Review Letters.

Harnessing Ternary Quantum Information Processing

A superconducting qutrit, like a qubit, employs microwave-induced logical gate operations for control. However, ternary quantum logic has a more complex state space and noise environment, making single and two qutrit-logic gates in short timescales difficult to control.

Recent advances in materials science and device design have improved the coherence of superconducting devices, facilitating the control of qutrits, which are generally more susceptible to noise. To fully leverage a qutrit processor’s power, however, it’s necessary to execute operations with high control of individual qutrits, but also entangle neighbouring qutrits with high-fidelity and flexible control.

Overcoming Challenges in Qutrit Operations

Research teams have already demonstrated single qutrit operations with high fidelity. However, entangling-gate speed has been compromised thus far by relying on a slow and static interaction that is always “on.” Speeding up this static interaction without the ability to tune it would increase the undesired noise, crosstalk, and errors in the system.

The team expanded on AQT’s research to implement a faster, flexible, and tunable microwave-activated entanglement between two transmon qutrits with fixed frequency and fixed coupling. This new approach to qutrit entanglement generated two universal two-qutrit gates, the controlled-Z gate (CZ) and the controlled-Z inverse gate (CZ+).

Exploring New Frontiers in Quantum Physics

Noah Goss, a graduate student researcher at AQT and QNL, is the lead paper author. Goss is excited about advancing the understanding of quantum mechanics with qutrit gates.

Building A Quantum-Ready Vision

Generating high-fidelity qutrit gates introduces complexity in all areas of quantum computing. AQT offers an ideal training laboratory for these broad, cutting-edge explorations with increasingly complex superconducting processors. AQT is also training the next generation of scientists and engineers through its research opportunities and open access to the lab’s testbed. In the third year of the testbed user program, the team’s experimental work has sparked further interest in future research collaborations.

“It’s fun and cool to keep building upon prior works while driving a direction of qutrit R&D forward, from a very different angle than many of the rest of academia and industry. AQT is a great place for such exploration. There are still so many details that need to be worked out and so much physics to do in this growing subfield of qutrits,” said Goss.

“A combination of different works in AQT and QNL enabled us to get to this point, where we can characterize and understand the physics with qutrit logic gates well. We synthesized a lot of previous expertise, and took it a step further in the experiment, by introducing an interaction for qutrits with a high degree of control and which had not been studied before,” said Goss.

“We learned how to generate entanglement with two-qutrit gates without sacrificing single qutrit gates. And, if you compare the fidelity achieved in the experimental demonstration with qutrits, it’s competitive with the absolute state-of-the-art three-qubit gates, despite being on an even larger space,” said Goss.

“It’s fun and cool to keep building upon prior works while driving a direction of qutrit R&D forward, from a very different angle than many of the rest of academia and industry. AQT is a great place for such exploration. There are still so many details that need to be worked out and so much physics to do in this growing subfield of qutrits,” said Goss.

Summary

An interdisciplinary team at the Advanced Quantum Testbed and the University of California’s Quantum Nanoelectronics Laboratory has made a significant breakthrough in quantum computing by successfully entangling two transmon qutrits, three-level systems that can encode more information than binary systems. This development offers potential advantages in quantum simulation, error correction, and improving specific quantum algorithms, and represents a significant step forward in ternary quantum information processing.

• A team at the Advanced Quantum Testbed (AQT) at Lawrence Berkeley National Laboratory and the University of California, Berkeley’s Quantum Nanoelectronics Laboratory (QNL) has made a significant breakthrough in quantum computing using qutrits, three-level systems, on a superconducting quantum processor.
• The team successfully entangled two transmon qutrits with gate fidelities higher than previously reported, moving closer to enabling ternary logic that can encode more information than binary counterparts — qubits.
• The research, published in Nature Communications in November 2022, advances AQT’s qutrit research and development, offering potential advantages in quantum simulation, error correction, and improving certain quantum algorithms and applications.
• The team, led by graduate student researcher Noah Goss, implemented a faster, flexible, and tunable microwave-activated entanglement between two transmon qutrits, generating two universal two-qutrit gates and achieving a process fidelity for a two-qutrit entangling gate of up to 97.3%.
• The research also applied and generalized the cross-entropy benchmarking protocol for characterizing gate noise and determining the fidelity of gate operations.
• The findings can be applied to different hardware architectures and superconducting circuits, introducing complexity in all areas of quantum computing.
• AQT, managed by the University of California for the U.S. Department of Energy’s Office of Science, is training the next generation of scientists and engineers through its research opportunities and open access to the lab’s testbed.