Quantum dynamics revealed in weakly coupled quantum dot - superconductor turnstiles

Orateur : Soutenance de thèse de David VAN ZANTEN (Institut Néel)

Abstract : Accurate control over the state and motion of single individual electrons would enable a variety of appealing applications reaching from quantized to quantum coherent electron sources. Realizing the accuracy of quantized current sources required for a metrological standard is however extremely challenging and has naturally fuelled fundamental research into single electron transport through mesoscopic structures. A promising candidate, foreseen to meet the demand, combines the concept of quantized charge in single electron transistors (SETs) and the gapped density of states in superconducting metals (hence called hybrid electron turnstile), to produce a quantized current. The time-correlated electron transport (sub-poissonian) between the superconducting leads is conveyed by the continuous density of states of the central normal island. The large amount of available states at the normal island, although favorable in terms of tunnel coupling, has nevertheless two important ramifications i.e. 1) thermal fluctuations and 2) adverse higher-order processes, which limit the performance of hybrid electron turnstiles. Inspired by this ingenious application and the advances in quantum dot transport, we explore the operation of a hybrid electron turnstile embodying a bottom-up quantum dot instead of the usual metallic island. The desired devices are obtained by controlled electromigration of aluminium nano-wires preceded by the deposition of gold nano-particles. This in-situ process (conducted at 4 K) produces pristine tunnel junctions between aluminium leads and gold nano-particles with a yield of about 4%. We characterize the stationary and turnstile operation by direct current measurements at 100 mK, in a heavily filtered, but electromigration compatible, inverse dilution refrigerator. Analysis of the acquired conductance maps under stationary conditions, reveal a large charging energy (> 10 meV) and mean level spacing (> 1 meV). With a detailed study of the coherence peak broadening at the Coulomb blockade (CB) threshold, we show that electron transport through the quantum dot is conveyed by a single quantum level. Although the tunnel coupling is weak, the single level life-time is dominated by the lead - quantum dot hybridization as thermal energy fluctuation and in-elastic scattering are suppressed by the large single level spacing on the quantum dot and the superconducting gap in the leads. The observation of sub-threshold resonances parallel to the CB diamond edges are consistent with earlier predicted higher-order Cooper-pair - electron (CPE) cotunneling processes. Under turnstile operation a periodic modulation signal (sine or square wave) is added to the static gate potential. We demonstrate quantized current up to 200 MHz at which its accuracy starts to worsen due to missed tunnel events. Strong experimental evidence of the single quantum dot level nature of our turnstile device is provided by a sharp onset of backtunneling processes and the temperature-robust operation beyond 300 mK. Finally we observe a systematic current suppression unique to the low frequency sine wave operation. Supported by theoretical work, we show that the underlying missed tunnel events are caused by adiabatic traverses across the avoided crossing of a quantum dot level and superconducting gap edges. These experiments deliver the first experimental observation of the level repulsion between an electronic discrete state and a semi-continuum and demonstrate the quantum coherent evolution of our devices under adiabatic operation conditions.

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