Fixed Frequency and Coupled Microwave Activated Dual Qutrit Entangled Gate Technical Description. Image Credit: Noah Goss/Berkeley Labs
An interdisciplinary team at Lawrence Berkeley National Laboratory (Berkeley Lab) at the Advanced Quantum Testbed (AQT) and the UC Berkeley Laboratory for Quantum Nanoelectronics (QNL) have made technological breakthroughs using qutrits (three-level systems) on superconducting quantum processors.
The team managed to entangle two transmon ququits with much higher gate fidelity than previously reported work, bringing closer to implementing ternary logic that can encode more information than binary opposites (qubits).
The results were published in the journal Nature Communications in December 2022 and served as an editorial highlight to advance AQT's qutrit development, including previous experiments published in Physical Review X and Physical Review Letters in 2021. Ternary quantum information processors offer significant potential advantages in quantum simulation and error correction, as well as in improving certain quantum algorithms and applications.
Utilize ternary quantum information processing
Superconducting qubits, like qubits, are controlled using microwave-induced logic gate operations. However, ternary quantum logic has a more complex state-space and noisy environment that makes single-quantum logic gates and two-quantum logic gates difficult to control on short timescales.
Recent advances in materials science and device design have improved the coherence of superconducting devices and facilitated the control of quantum dots that are often more susceptible to noise. However, in order to fully utilize the capabilities of the Qutrit processor, it is necessary to perform operations with a high degree of control over a single Qutrit, along with high-fidelity and flexible control of adjacent Qutrits.
The research team has demonstrated single-quantum operations with high fidelity. Until now, however, the speed of entanglement gates has been compromised by relying on slow and static interactions that have been in an "open" state. Accelerating this static interaction without adjusting it will increase undesirable noise, crosstalk, and errors in the system.
Noah Goss, a graduate researcher at AQT and QNL, is the lead author of the Nature paper. Photo by Trevor Chistolini/Berkeley Lab
The team leading the demonstration expanded AQT's state-of-the-art research to enable faster, more flexible and tunable microwave-activated entanglement between two transmon quququits with fixed frequency and fixed coupling. This new quantum entanglement method yields two universal dual quantum gates, the control z-gate (CZ) and the control z-inverse gate (CZ+).
Using AQT's previous qutrit loop benchmarking methodology with industry partner Keysight Technologies, the team measured process fidelity of double qutrit entanglement doors up to 97.3 percent, reducing infidelity from previous work by about 4 times. In addition, in the qutrits study, AQT researchers for the first time applied and generalized another established protocol—the cross-entropy benchmark—to characterize gate noise and determine the fidelity of gate operations.
Explore new frontiers in quantum physics
Noah Goss, a graduate researcher at AQT and QNL, is the paper's lead author. Gauss is excited about using quantum gates to advance his understanding of quantum mechanics.
"The combination of different jobs in AQT and QNL allowed us to get to this point, and we can describe and understand the physics of qutrit logic gates very well." We synthesized a lot of previous expertise and went a step further in our experiments by introducing a highly controlled interaction that had not been studied before. Goss said.
AQT's team demonstrated in 2021 how to deploy microwave-activated tunable coupling between fixed-frequency qubits. To do this, Goss and his team applied differential AC Stark shifts to two fixed-frequency transmon ququits and characterized them. AC Stark shifts use microwave light to make small changes to the transition frequency and energy level structure of a coupled quantum well system to tune entanglement between two quantum wells.
"We learned how to create entanglement with a double quantum gate without sacrificing a single quantum gate. And, if you compare the fidelity obtained in the experimental demonstration to qutrits, it competes with state-of-the-art three-qubit gates, despite being on a larger space," Goss said.
Noah Goss, a graduate researcher at AQT and QNL, is the lead author of the Nature paper. Photo by Trevor Chistolini/Berkeley Lab
Build a quantum-ready vision
Generative high-fidelity qutrit gates introduce complexity in various areas of quantum computing. AQT provides an ideal training lab for these types of extensive, cutting-edge exploration with increasingly complex superconducting processors. AQT also trains the next generation of scientists and engineers through its research opportunities and open access to laboratory test platforms. In the third year of the testbed user program, the team's experimental work sparked further interest in future research collaborations.
"It's fun and cool to build on the work of our predecessors and continue to push Qutrit R&D, from a very different perspective than many others in academia and industry. AQT is a great place to do this type of exploration. There's still a lot of detail to be worked out and a lot of physics to be done in this evolving Qutrits subfield," Goss said.
The physics of creating qutrit entanglement between two fixed-frequency transmissions investigated in this work can be applied to different hardware architectures, including those with tunable coupling, or different superconducting circuits, such as fluxonium.
Further information: Noah Goss et al., High-fidelity quantum entanglement gates for superconducting circuits, Nature Communications (2022). DOI: 10.1038 / s41467 - 022 - 34851 - z。 www.nature.com/articles/s41467 - 022 - 34851 - z
Journal Information: Nature Communications