Phase slips in voltage-biased superconducting rings: a qubit proposal?

The quantization of magnetic flux in superconductors lies at the heart of realizing qubits using superconducting circuits. To construct a flux qubit, one must coherently couple the flux states of a superconducting ring, which requires breaking the rotational symmetry of the ring to introduce a phase-slip center. Established implementations guarantee an inherently broken symmetry by interrupting the ring with an insulating barrier; hence, forming a Josephson junction, giving rise to the flux qubit. However, these implementations are plagued by the variablitly of the fabrication process of the junction. Here, we theoretically propose a junctionless flux qubit consisting of a voltage-biased superconducting ring. The in-plane electric field, arising from the bias voltage, locally suppresses the density of superconducting electrons; hence, imitating the effect of interrupting the superconductor with an insulator. Furthermore, the proposed qubit could allow for electric-tunability of the transition frequency, a desired feature for multi-qubit systems since it renders the circuit less sensitive to magnetic noise. Realizing electrically-tunable flux qubits with long coherence times paves the road towards scaling up superconducting quantum computers.