A noise protected superconducting Fluxonium qubit*

Since the early 2000s, there has been a significant improvement in the coherence time of superconducting qubits by five orders of magnitude. Despite this progress, current qubit performances are still a major limitation to the development of a universal quantum processor. Currently, the fluxonium qubit stands as the circuit with the longest observed coherence [1], with recent experiments showing it can achieve milliseconds of coherence.  It is composed of a superconducting loop made of a Josephson junction shunted by a large inductance (often called a super-inductance). An external magnetic field threading the loop can be used to tune the circuit parameters.

Several groups have created Fluxonium devices with a very low frequency of 1-10 MHz. This approach results in a reduced relaxation rate due to the decrease in the charge matrix element at lower frequencies. This is a simple way to enhance the performances of this qubit. Alternatively, a protected qubit can be encoded in multimodal circuit that generalizes the fluxonium to a larger parameter space. In these more complex circuits, protected information can be encoded at the cost of stringent requirements on the circuit parameters [2,3].

In this work, we use the single-loop fluxonium circuit in a new regime. We engineer the circuit parameters so that the qubit wave functions are de-localized over several potential wells to minimize their overlap, hence guaranteeing an extremely small matrix element or equivalently a very long relaxation time. Additionally, the qubit transitions offer a very low sensitivity to flux noise thanks to the very large inductance. We will discuss the depolarization and coherence times, as well as their limitations, for this new type of Fluxonium qubit.

1 Somoroff, Aaron, et al. "Millisecond coherence in a superconducting qubit." Physical Review Letters 130.26 (2023): 267001.

2 Kalashnikov, Konstantin, et al. "Bifluxon: Fluxon-parity-protected superconducting qubit." PRX Quantum 1.1 (2020): 010307.

3 Gyenis, András, et al. "Experimental realization of a protected superconducting circuit derived from the 0–π qubit." PRX Quantum 2.1 (2021): 010339.