Following the demonstration of semiconductor-based Josephson junctions, which are fully tunable by electrical means, new routes have been opened for the study of hybrid semiconductor–superconductor qubits. These include semiconductor-based transmon qubits, single-spin Andreev qubits, and fault-tolerant topological qubits based on Majorana zero modes. In this perspective, we review recent progress in the path toward such hybrid qubit designs. After a short introduction and a brief digression about the historical roadmap that has led to the experimental state-of-the-art, the emphasis is placed on superconducting qubits based on semiconductor nanowires.
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These fluctuations mostly appear due to fluctuations on the on-chip current bias that is applied to induce the flux and are typically in the range , with Wb being the superconducting magnetic flux quantum (see Ref. 5 for a detailed discussion about the different noise sources and their spectra in superconducting qubits).
Here, by charge fluctuations, we mean noise generated by charge traps of various origins (tunnel barrier, substrate dielectrics, etc.). In standard superconducting qubits, it is usually modelled as a combination of noise at low frequencies and quantum (Nyquist) noise that progressively takes over at high frequencies in the GHz regime.133 The authors of Ref. 37 speculate that this lack of correlation between charge dispersion and linewidth could be attributed to quasiparticles populating the induced (soft) superconducting gap as opposed to better epitaxial samples with hard gaps like the ones in Ref. 36.
In Ref. 37, the non-sinusoidal CPRs in the NW JJs induce strong deviations from transmon behavior in the split geometry: while near the system behaves as a flux qubit, it exhibits transmon-like behavior near zero applied flux.