Currently there are many qubit types under development. One that’s received relatively little notice is the fluxonium qubit. It’s a less well-studied cousin of the transmon superconducting qubit being developed by IBM, Google, Rigetti and others. A brief article posted today on the AQT/LBNL website looks at an ambitious paper – Blueprint for a High-Performance Fluxonium Quantum Processor – suggesting a path forward for fluxonium-qubit technology.

The list of potential qubit types has been steadily growing with superconducting, trapped ions, photonics, neutral atoms, quantum dots, diamond-vacancy all loosely leading the pack. Each has distinct strengths and weaknesses. The superconducting transmon is perhaps furthest along – IBM plans to introduce a 1000-qubit QPU later this year – but effective error mitigation/correction remains one of the biggest challenges to practical use and scaling system size up. It’s probably worth noting that IBM still hasn’t provided broad access to its 433-qubit Osprey.

“In practice, the requirement to encode logical qubits using a redundant number of physical qubits imposes a large resource overhead that is challenging to achieve. Besides decoherence of physical qubits, crosstalk, frequency crowding, and leakage out of the computational subspace are also the central problems to overcome upon scaling up,” wrote the research team which included members from AQT, UC Berkeley, and Yale.

“One promising superconducting qubit in the quest toward constructing a fault-tolerant quantum computer is fluxonium, due to its long coherence times and high anharmonicity. The circuit consists of three elements in parallel: a capacitor, a Josephson junction, and a superinductor. The inductive shunt eliminates the qubit’s offset charge, and the large inductance value suppresses its sensitivity to flux noise. Fluxonium can be tuned in situ by threading an external magnetic flux through the circuit loop.”

Anharmonicity is a key here. It is, broadly, the difference between transition frequencies in a qubit. One practical result of the low anharmonicity that is characteristic of transmon qubits is something called frequency crowding, which hampers qubit control signaling. Put another way, low anharmonicity can allow overlap of frequencies complicating qubit control efforts. Fluxonium qubits’ high anharmonicity is an advantage along with long coherence times, and simpler control.

There are other groups working on fluxonium qubit approaches. Researchers from National University of Science and Technology, Russia, reported demonstrating high-fidelity two-qubit gates with a tunable coupler in a Nature article last summer (they demonstrated 99.55% and 99.23% fidelities on fSim-type (fermionic simulation) and controlled-Z-gates, respectively). Last November, researchers from IQM reported in Nature developing a “unimon” qubit that also has high anharmonicity and noted “…major progress has been made in the development of fluxonium qubits, one of the most compelling alternatives to transmons thanks to their high anharmonicity and long relaxation and coherence times.”

The race is on to find alternatives to superconducting transmon qubits which currently dominate superconducting-based qubits. The AQT-led work is a good look at progress and proposes a potential path to turning fluxonium qubits into a contender.

Lead author, Long B. Nguyen, a project scientist at AQT, said in the LBNL article, “I focused on fluxonium because it appeared to be a better alternative to the transmon at the time. It has three circuit elements that I could play with to get the type of spectra I wanted. It could be designed to evade decoherence due to material imperfections. I also recently realized that scaling up fluxonium is probably more favorable since the estimated fabrication yield is high, and the interactions between individual qubits can be engineered to have high-fidelity.”

In the paper, the researchers note: “Currently, there is no clear roadmap to construct a fluxonium-based quantum processor with projected performance to surpass state-of-the-art architectures. Combined with the misconception that it is challenging to manipulate and readout low-frequency qubit transitions, and the fear of uncontrollable variability of junction parameters in a multicomponent qubit, this leads to relatively sparse efforts in scaling up the platform.

“In this work, we provide the blueprint for a scalable high-performance quantum architecture based on fluxonium qubits. We show that, in principle, this platform will have suppressed crosstalk, reduced design complexity, improved operational efficiency, high-fidelity gates, and resistance to parameter fluctuations. In contrast to previous works, our analysis focuses on scalability, and thus involves a wide range of practical qubit parameters.”

There’s still a lot of work to be done to bring fluxonium qubits to the forefront, but the same could be said about many qubit modalities. Even among the current crop of qubit types being worked on, it’s far from clear which qubit modalities will become predominant. Those that can be manufactured using existing semiconductor methods are attractive for many obvious reasons. The superconducting transmon qubit has a development lead but it also has shortcomings.

The AQT-Berkeley-Yale team hopes its work spurs more activity. “As experimentalists having to design, characterize, and optimize quantum operations in NISQ devices, we hope that the results and perspectives presented in this paper will, on one hand, hasten further research and development efforts on fluxonium-based quantum architectures, and, on the other hand, motivate similar scalability studies of novel superconducting platforms such as the cos(2φˆ), the bifluxon, and the 0 − π qubits,” they write in their concluding section. This is a detailed paper worth reading.

Stay tuned.

Link to paper: https://journals.aps.org/prxquantum/pdf/10.1103/PRXQuantum.3.037001

Link to AQT article: https://www.hpcwire.com/off-the-wire/berkeley-lab-researchers-develop-blueprint-for-next-gen-quantum-processor-based-on-fluxonium-qubits/