Despite the rapid progress in the field, achieving coherence times that are long enough to be practically useful for a quantum computer remains a paramount challenge. This is primarily due to the susceptibility of quantum electrical circuits to noise. One solution to this issue is to encode the 0 and 1 of the qubit into states with differing parities. This approach has been proposed in seminal theoretical works and is expected to provide potentially infinitely long relaxation times. However, despite several attempts, reaching such a protection regime turned out to request stringent circuit parameters that were beyond the limits of experimental reach.
During the first half of the SuperProtected project, we have taken another look at the Fluxonium qubit, which was invented a decade ago and demonstrated that it possesses such a protection mechanism. When the circuit is connected to a large superinductance (L ~1 μH) and placed in an external magnetic flux of zero, its ground and excited state wavefunctions belong to different flux quanta parity. In our proof-of-principle experiment, the qubit relaxation time exceeded 100 μs, even though we did not use state-of-the-art nanofabrication techniques, showcasing the qubit's built-in protection against energy relaxation. Additionally, the qubit exhibits protection against charge noise and a first-order protection against flux noise. We measure a dephasing time of around 75 μs and provide a complete coherence budget showing how it can be easily improved by a better design.
Moreover our team has made an important discovery regarding the behavior of amorphous superconductors. We have found evidence of a first-order quantum phase transition, marked by a surprising discontinuity in the zero-temperature superfluid stiffness at the point where the material transitions from a superconducting state to an insulating state.
Our research has identified two primary mechanisms behind this transition. Firstly, we have found that strong disorder within the material alters the nature of the superconducting transition itself, transforming it into a phase-driven transition that is primarily governed by fluctuations in the phase of the order parameter. Secondly, we have discovered that the Cooper-pair glass insulator state, which ultimately terminates the superconductivity, competes with the superconducting state, preventing the usual continuous, second-order quantum phase transition that is typically observed. Instead, this competition leads to the emergence of a first-order breakdown of superconductivity.
These findings represent a significant step forward in our understanding of the behavior of amorphous superconductors and may have important implications for the development of new materials and technologies in the future.
