Quantum point contact galvanically coupled to planar superconducting resonator: a shot-noise-limited broad-band electrical amplifier

Abstract

Probing single charge dynamics in solids can give insights into various quantum transport phenomena, most of which are fragile and short-time-scaled. Detection of these events in real-time requires a mesoscopic electrical amplifier with unprecedented sensitivity and operational bandwidth. In this work, we explore a hybrid electrical amplifier consisting of a semiconducting quantum point contact galvanically coupled to a superconducting λ/2 transmission-line resonator for ultra-fast and ultra-sensitive charge amplification. The resonator, made of aluminium with a coplanar waveguide geometry, is designed to operate at its first harmonic resonant mode ∼3.4 GHz, where the reflected power from the resonator is amplitude-modulated by the conductance changes in the quantum point contact channel. From the sidebands of the amplitude modulated reflected signal we extract a conductance sensitivity of 2.85×10−7e2/h/Hz 11.05pSHz−1/2 ). This sensitivity translates to a unit signal-to-noise measurement time ∼1.62 ns for a variation of 0.01 ( e2/h in the conductance. From the analysis of the noise characteristics of the device, we find that up to a few MHz of signal frequency the sensitivity is limited by the photon assisted shot-noise associated with the electron tunnelling in the QPC channel. The optimization of various operational parameters of the device reveals a bandwidth of ∼155 MHz which corresponds to a rise-time ∼2.2 ns. Both the sensitivity and bandwidth that we obtain are greater by an order compared to the existing reports. In addition, the device also exhibits very good sensitivities ∼3.58×10−4e2/h/Hz up to a measurable frequency of 240 MHz. The extremely high sensitivity, shot-noise limited detection, ultra-fast operation reaching the nanosecond time-scales, and the circuit QED architecture makes this scheme an attractive choice for single charge detection and counting experiments for spin-qubit readout and quantum electrical metrology.