We report on the realization of a single-electron source, where current is transported through a single-level quantum dot (Q), tunnel-coupled to two superconducting leads (S). When driven with an ac gate voltage, the experiment demonstrates electron turnstile operation. Compared to the more conventional superconductor -normal metal -superconductor turnstile, our SQS device presents a number of novel properties, including higher immunity to the unavoidable presence of non-equilibrium quasiparticles in superconducting leads. In addition, we demonstrate its ability to deliver electrons with a very narrow energy distribution.

Hybrid nanowires with proximity-induced superconductivity in the topological regime host Majorana zero modes (MZMs) at their ends, and networks of such structures can produce topologically protected qubits. In a double-island geometry where each segment hosts a pair of MZMs, inter-pair coupling mixes the charge parity of the islands and opens an energy gap between the even and odd charge states at the inter-island charge degeneracy. Here, we report on the spectroscopic measurement of such an energy gap in an InAs/Al double-island device by tracking the position of the microwave-induced quasiparticle (qp) transitions using a radio-frequency (rf ) charge sensor. In zero magnetic field, photon assisted tunneling (PAT) of Cooper pairs gives rise to resonant lines in the 2e-2e periodic charge stability diagram.In the presence of a magnetic field aligned along the nanowire, resonance lines are observed parallel to the inter-island charge degeneracy of the 1e-1e periodic charge stability diagram, where the 1e periodicity results from a zero-energy subgap state that emerges in magnetic field. Resonant lines in the charge stability diagram indicate coherent photon assisted tunneling of singleelectron states, changing the parity of the two islands. The dependence of resonant frequency on detuning indicates a sizable (GHz-scale) hybridization of zero modes across the junction separating islands.In a mesoscopic superconducting island, the presence of a single unpaired electron comes at an energy cost δ determined by the lowest quasiparticle state. As a consequence, the odd-parity charge states are elevated in energy. The total energy of a charge state with N electrons is E N = E C (N − n g ) 2 + δ mod(N, 2), where E C is the single-electron charging energy and n g is the gateinduced charge. For δ > E C (Fig. 1a), the island ground states are of even charge parity only, and the island occupation changes in steps of 2e as n g is changed [1]. When * These authors contributed equally to this work † email: marcus@nbi.ku.dk the island is weakly coupled to a superconducting reservoir the coherent exchange of Cooper pairs, characterized by Josephson energy E J , mixes ground states of equal parity, resulting in an anti-crossing (blue lines in Fig. 1a). This superposition of even-parity charge states is the basis of conventional superconducting qubits [2]. In a topological superconducting island, nonoverlapping Majorana zero modes comprise a zero-energy qp state denoted a Majorana bound state (MBS). Since δ = 0 in this case, the island charge occupation changes with regular steps of width 1e. Overlap between MZMs within the island leads to δ = 0. Consequently, the width of Coulomb valleys with even parity will either increase or decrease depending on the sign of δ [3], as recently demonstrated experimentally [4][5][6]. In contrast to the intra-island coupling of MZMs, the wave function overlap with an exterior MZM, characterized by E M , leaves δ unchanged but mixes states of different parity (see Fig. 1c). The ground states of the i...

Radio-frequency (RF) reflectometry is implemented in hybrid semiconductor-superconductor nanowire systems designed to probe Majorana zero modes. Two approaches are presented. In the first, hybrid nanowire-based devices are part of a resonant circuit, allowing conductance to be measured as a function of several gate voltages ∼40 times faster than using conventional lowfrequency lock-in methods. In the second, nanowire devices are capacitively coupled to a nearby RF single-electron transistor made from a separate nanowire, allowing RF detection of charge, including charge-only measurement of the crossover from 2e inter-island charge transitions at zero magnetic field to 1e transitions at axial magnetic fields above 0.6 T, where a topological state is expected. Single-electron sensing yields signal-to-noise exceeding 3 and visibility 99.8% for a measurement time of 1 µs. arXiv:1902.00789v1 [cond-mat.mes-hall]