Andreev Molecule in Parallel InAs Nanowires

Abstract

Coupling individual atoms fundamentally changes the state of matter: electrons bound to atomic cores become delocalized turning an insulating state to a metallic one. A chain of atoms could lead to more exotic states if the tunneling takes place via the superconducting vacuum and can induce topologically protected excitations like Majorana or parafermions. Although coupling a single atom to a superconductor is well studied, the hybridization of two sites with individual tunability was not reported yet. The peculiar vacuum of the Bardeen–Cooper–Schrieffer (BCS) condensate opens the way to annihilate or generate two electrons from the bulk resulting in a so-called Andreev molecular state. By employing parallel nanowires with an Al shell, two artificial atoms were created at a minimal distance with an epitaxial superconducting link between. Hybridization via the BCS vacuum was observed and the spectrum of an Andreev molecule as a function of level positions was explored for the first time.

Double nanowires for hybrid quantum devices

“…The demonstration of Coulomb blockade via different leads to the same superconducting island is promising for investigations of the topological Kondo effect, Majorana box qubits 11 and specific proposals based on the hybrid double nanowire geometry. [8][9][10]36,37,[39][40][41][42][43][44][45][46][47][48] Yet other experiments addressing the transport of the same double nanowire materials demonstrate coupled Andreev states 24 and Yu-Shiba-Rusinov physics 25 emerging in the double nanowire Josephson junction geometry. Furthermore, the full-shell double nanowire devices such as the one represented by Figs.…”

“…We now turn to electrical transport characterization of the hybrid double nanowire system, focusing here on low temperature transport measurements of two different types of devices in the quantum regime. The objective is to demonstrate that decoupled double nanowires and multi-terminal device functionalities can be achieved (more advanced functionalities are demonstrated in forthcoming dedicated publications [24][25][26] ). Prior transport experiments on parallel nanowires have been based on individual nanowires that were accidentally merged during deposition.…”

“…[33][34][35] The superconductor-DQD-normal metal device geometry is easily extended to S-parallel DQD-S Josephson junctions, where supercurrent and bound states behavior can be addressed. 24,25 The second example is a superconducting island defined across the two nanowires, a geometry relevant for several device proposals in the topological and non-topological regimes. 8,9,36,37 The SEM picture in Fig.…”

“…1d-f). We note that these materials have recently been exploited for studies of Andreev molecular states, 24 double quantum dot Josephson junctions 25 (Fig. 1d) and Little-Parks oscillations using full-shell DNWs 26 (Fig.…”

Asymmetric Little–Parks oscillations in full shell double nanowires

“…As the single-cylinder model predicts the presence of the asymmetry for any misalignment, the degree of asymmetry can be used as an accurate measurement of the degree of misalignment of the field with the long axis of the sample. For completeness, we note that similar double nanowires, however, with only half-shell superconductor coverage are addressed in several parallel works [13][14][15] . arXiv:2106.01181v1 [cond-mat.supr-con] 2 Jun 2021…”