For the next generation of quantum devices, we need qubits with good coherence that are less likely to make mistakes. In response to this need, scientists at the AQT at Berkeley Lab, a modern joint research lab, planned for a new quantum processor that uses “fluxonium” qubits. Fluxonium qubits can do better than the most commonly used superconducting qubits. This is a hopeful step toward universal quantum computing that can handle mistakes.
Together with researchers from the University of California, Berkeley, and Yale University, the AQT team was the first to start a systematic theoretical study of how to engineer fluxonium qubits for better performance. They also made practical suggestions for adapting and building cutting-edge hardware that will fully use the power of quantum computing. Their findings were written up in a magazine called PRX Quantum.
At the forefront of superconducting computers
Superconducting quantum processors are made up of many qubits that have different transition rates. This makes it easy to control each qubit and how it interacts with the others. One of the most common types of superconducting computers, the transmon qubit, has low anharmonicity. Anharmonicity is the difference between the frequencies of the critical transitions in a qubit. When qubit frequencies are close to interacting, this is called spectral crowding. This makes it harder to control the processor because the qubit frequencies are relative.
On the other hand, high anharmonicity gives researchers better control over qubits because there is less overlap between the frequencies that control qubits and those that make any given qubit have more energy. The fluxonium qubit has built-in benefits for complex superconducting computers, such as high non-harmonicity, long coherence times, and easy control.
Building on AQT’s long history of research and development on superconducting circuits, the team in charge of the fluxonium-based architecture focused on the scalability and adaptability of the main components of the processor. They created a set of parameters that researchers can change to make quantum circuits run longer and more accurately. Some of these changes make it easier to run the system. For example, researchers suggested that the fluxonium qubits could be controlled at low frequency (1 GHz) using microwave pulses directly made by an electrical random waveform generator. With this simple method, researchers can make computers and set up multiple qubits flexibly.
Large-scale machines can use fluxonium qubits in flexible ways.
Long B. Nguyen is the lead author of the study and works as a project scientist at AQT. Nguyen began looking into other superconducting qubits when he was a graduate student at the University of Maryland and working with Professor Vladimir Manucharyan. Just ten years ago, Manucharyan brought fluxonium qubits to the field. In 2019, Nguyen showed that fluxonium circuits could have longer coherence times. The fluxonium circuit comprises a capacitor, a Josephson Junction, and a superconductor. The superconductor helps block magnetic flux noise, a common source of unwanted interference that affects superconducting qubits and causes decoherence.
“I have always wanted to learn about new science, and at the time, fluxonium seemed like a better choice than the transmon. It has three parts of the circuit that I could change to make the bands I wanted. It could be made to avoid falling apart because of flaws in the material. I also recently learned that scaling up fluxonium is probably better because the expected yield is high, and the interactions between individual qubits can be very accurate,” Nguyen said.
Together with the paper’s researchers, the team at AQT simulated two types of programmable quantum logic gates: the cross-resonance controlled-NOT (CNOT) and the differential ac-Stark controlled-Z (CZ). This was done to predict and test how well the suggested fluxonium blueprint would work. The team’s hopes for the proposed plan were confirmed by the simulations’ high fidelity across the proposed qubit parameters and gates range.
“We showed that it might be possible to build fluxonium processors by giving standard, useful ways to use logic gates with different frequencies. We hope that more research on fluxonium and other alternatives to superconducting qubits will lead to the next generation of devices for handling quantum information,” Nguyen said.
