In my PhD project, I work on cat qubits, a promising type of superconducting qubit where quantum information is encoded in coherent states of a harmonic oscillator with opposite phases. These qubits are particularly interesting because they can operate in a regime where one type of quantum error is autonomously suppressed, making them noise-biased qubits. The remaining errors can be mitigated using a simple one-dimensional repetition code.

To stabilize these coherent states, the standard approach in the community relies on a driven-dissipative scheme, where photons are exchanged in pairs with the environment. However, this method imposes a strict limit on the cat qubit's lifetime due to undesired nonlinearities. To overcome this challenge, we replace the traditional pump with a DC-biased Josephson junction, which leverages the Josephson effect to stabilize the cat states without introducing detrimental nonlinearities.

This strategy opens up exciting possibilities, including the experimental realization of a 4-legged cat state, a superposition of four coherent states. These states can autonomously correct both bit-flip and phase-flip errors, drastically reducing the hardware overhead needed for quantum error correction and paving the way for more efficient logical qubits.

The goal of my PhD is to experimentally implement these stabilized cat qubits using DC-biased Josephson junctions. This work is conducted at ENS de Lyon under the supervision of Benjamin Huard and Ambroise Peugeot, in partnership with the startup Alice & Bob (CIFRE PhD), where I am also supervised by Anil Murani and Jean-Loup Ville.