Fluxonium

A qubit is the fundamental unit of quantum information. Unlike classical bits, which always have to be either 0 or 1, qubits can exist in a quantum superposition with some probability of being 0 and some probability of being 1. They can also become entangled, exhibiting non-classical correlations and anti-correlations. Physically, they can be implemented in a number of different ways.

Superconducting qubits are one of the leading modalities for the development of quantum computers, leveraging humanity’s experience with electronics fabrication, cryogenics, and other technologies. Fluxonium qubits are one of the most promising variations of this modality. Although one qubit might sometimes be referred to as a fluxonium atom, due to shared behaviors with atoms, “fluxonium qubit” is a more precise term.

Like transmon qubits, which are the most commonly implemented superconducting qubits, fluxonium qubits are composed of a capacitor and a Josephson junction. The distinction between the two types of qubits is that the latter includes a superinductor. The superinductor helps shield the qubit from environmental noise, and gives rise to its other unique properties.

For more information, a New Journal of Physics paper titled “Coherent dynamics in long fluxonium qubits” details a variation of the fluxonium qubit called the long fluxonium qubit. The paper is accompanied by a 3-minute 51-second video explainer. And although the video gets rather technical, it starts off with a general introduction to fluxonium qubits.  

What is Fluxonium

Compared to transmons, fluxoniums offer several advantages. The key advantages include:

• Fluxoniums have longer coherence times, which allows more operations to be performed before decoherence becomes an issue.
• Fluxoniums have higher anharmonicity – larger energy differences between quantum states – which makes them less error-prone and easier to control.
• Fluxoniums operate at lower frequencies, which reduces noise and improves stability.

The NewScientist article “'Fluxonium’ is the longest lasting superconducting qubit ever,” dated 25 May 2023, notes that the coherence time is “about 1.48 milliseconds.” Although that’s long for superconducting qubits, it’s worth noting that that’s short compared to some other modalities, such as neutral atoms. It’s also worth noting that fluxoniums have a couple of disadvantages when compared to transmons, specifically in fabrication complexity and execution speeds.

Unique Features and Advantages of Fluxonium

Compared to transmons, fluxoniums offer relatively long coherence times, higher anharmonicity, and lower operating frequencies. And while not truly unique, fluxoniums offer a couple of features that are more common or more versatile than with transmons:

• The transition frequency and anharmonicity can be precisely tuned to optimize performance for specific applications.
• The fluxonium qubit exceptional point might allow greater control over the qubits, which might extend applicability beyond computation to quantum sensing.

Despite the potential of flexmons, it’s worth noting that they are still in a relatively early stage of development. For example, the Phys.org article “Fluxonium qubits bring the creation of a quantum computer closer” notes that 2-qubit operations were performed in 2022. In contrast, in 2023, neutral atoms were used to demonstrate transversal gate operations on logical qubits. Nonetheless, research continues into both of these technologies, as well as into transmons and other modalities.