A superconducting qubit is a tiny electrical circuit that behaves like an artificial atom. In its normal state, current flows through it without resistance, and the energy levels are evenly spaced, like steps on a staircase. But if a single electron gets stuck in the circuit, everything changes. That extra electron is like a passenger who squeezes into an overcrowded car: it doesn't just take up space: it forces everyone else to shift and rearrange.
The authors calculated what the energy levels look like in such a qubit with one stuck electron. It turns out they line up in a completely different order than in a normal qubit without the extra electron. Moreover, this order depends on which is stronger: the electric field that holds the electrons, or the ability of the current to flow without resistance. It's as if passengers in a crowded car reseat themselves differently depending on how much the train shakes.
This behavior could be tested in experiments with hybrid superconductors, which combine a superconductor with a regular semiconductor. In recent years, researchers have learned to make these structures very carefully — both from thin wires and from flat layers where electrons move as if on a highway. If the prediction holds up, it would offer a new way to control quantum bits — simply by adding or removing one electron.
as written by the authors arXiv:2601.23100
The quantum mechanics of the Josephson effect is the core ingredient for quantum technologies with superconducting circuits. A new avenue was recently opened in this field by predicting that the Josephson quantum mechanics in the odd parity sector, when a quasiparticle in trapped in an Andreev bound state, is fundamentally different from the conventional one in the even sector. The focus was then on a Josephson junction surrounded by an electromagnetic environment formed of a collection of bosonic modes, including the case of an ohmic environment. Here we consider the distinct case of a superconducting qubit made of a single Josephson junction whose environment reduces to a capacitance. We find a novel structure for the low-lying discrete states in the odd sector, which is altogether different from the one that appears in the even sector. Our study of the bound-state spectrum ranges from the Coulomb-dominated (Cooper pair box) to the Josephson-dominated (transmon) regime. Our prediction could be tested in forthcoming experiments with superconductor/semiconductor/superconductor junctions, which have been studied intensively in recent years, both using nanowires as well as two-dimensional electron gases.
In a superconducting qubit, one extra electron can change the system's energy as noticeably as one wrong note changes a melody.