Microwave noise and its correlations are important characterization methods in quantum nanophysics. Correlation of current fluctuations can be used to study Hanbury Brown – Twiss (HBT) exchange effect caused by indistinguishability of charge carriers in multiterminal conductors, although this effect has received little experimental attention prior to the two studies included in this thesis. In addition, this thesis presents a correlation measurement system for shot noise and a lumped-element Josephson parametric amplifier (JPA). Shot noise and its correlation were measured using a two-channel system which was designed for 600–900 MHz frequency band and utilized USB-interfaced software-defined radio receivers, digitizing 2 megasamples per second with 8 bit resolution. The sensitivity of the system was close to the theoretical value for the given sample rate. The system was complemented with a cryogenic low-noise amplifier, achieving 21 dB gain with 570–920 MHz bandwidth and 7 K noise temperature. The HBT exchange effect was studied in lithographically patterned disordered graphene samples in cross and box geometries. Both samples showed distinct negative HBT exchange correction whose value changed significantly at low charge carrier density. The experimental results for graphene cross matched with an analytical circuit theory model which combined noise sources of four diffusive arms and a semiballistic central dot. The box was modeled numerically as a two-dimensional diffusive conductor including additional contributions to noise from contact resistances. The experimental results for the box fell between the calculated ones for elastic and hot electron transport. This is attributed to the presence of a crossover regime or features intrinsic to diffusive graphene. JPAs add significantly less noise to the signal than semiconductor amplifiers, and hence noise measurements would benefit from a wideband JPA as a preamplifier. In addition, a simple JPA circuit can be easily modified for the needs of experiments studying the quantum vacuum. The presented JPA was realized as a parallel LC resonator consisting of a superconducting quantum interference device (SQUID) and an interdigital capacitor. The JPA was fabricated with a single electron beam lithography step followed by double-angle evaporation of aluminum. The JPA achieved 20 dB gain with 95 MHz bandwidth around 5.3 GHz, and the noise temperature was close to the one-photon quantum limit.
|Publication status||Published - 2020|
|MoE publication type||G5 Doctoral dissertation (article)|
- graphene, Hanbury Brown – Twiss exchange effect, shot noise, Josephson parametric amplifier, cross-correlation, diffusive electron transport