Lumped impedance matchers and GHz noise investigation of quantum dots

Abstract

Via on-chip superconducting coils, highly resistive quantum devices are impedance-matched in cryogenic microwave setups. Thus, electron transport in quantum dots (QDs) can be studied by means of gigahertz shot noise in addition to direct-current measurements. QDs exhibit varying dot-lead tunnel couplings, that depend on the inter-state transition produced by an electron tunneling event. Those states followed only by weakly coupled transitions are called blocking states, as characterized by a strong suppression of the electrical current. The blocking states revealed here in carbon nanotube QDs are spotted with super-Poissonian noise and, less generally, with negative differential conductance. Indeed, the enhanced noise is the signature of electron bunching, which originates in random switches between the strongly and weakly conducting regimes of the QD. Using a master equation approach, numerical simulations finally reproduce both the shot noise and the conductance patterns observed in measurements.