Curator's Take
This article introduces "fraxonium," a clever superconducting circuit design that could unlock quantum computing beyond traditional two-level qubits by creating stable multi-level quantum systems called qudits. The researchers have engineered a way to trap fractional fluxon states in carefully designed Josephson potentials, creating natural protection against one of quantum computing's biggest challenges - leakage errors where quantum information escapes to unwanted energy levels. What makes this particularly exciting is their demonstration of how to control these higher-dimensional quantum states using sophisticated laser protocols, potentially allowing each quantum element to store and process more information than conventional qubits. This represents a significant step toward more efficient quantum architectures that could dramatically reduce the number of physical components needed for complex quantum algorithms.
— Mark Eatherly
Summary
We propose a superconducting circuit hosting $d$ low-lying states, well separated from the rest of the spectrum, that naturally realizes a qudit system protected from leakage errors. The system represents a generalization of the fluxonium and the low-energy states are constituted by fractional fluxon states, that we call {\it fraxons}, localized in the minima of a suitably designed Josephson potential. The latter is tailored through a Fourier engineering approach, that employs multi-harmonic Josephson building block elements composed by a Josephson junction and an inductance connected in series. We present the spectrum of a $d=4$ and a $d=5$ qudit system and study in detail the qutrit case. We analyze the dipole matrix elements for coupling to radiation and propose a non-Abelian, stimulated Raman adiabatic passage (STIRAP) protocol for single-qutrit gates, that is particularly suited for the present system. The proposed platform opens novel perspectives in circuit engineering and quantum computing beyond the qubit paradigm.