Modelling of a Two-Signal SFQ Detection Scheme for the Readout of Superconducting Nanowire Single Photon Detectors

We present a two-signal single flux quantum (SFQ) detection scheme for the purpose of reading out two pixels of a superconducting nanowire single photon detector (SNSPD). The circuit is based on a coincidence buffer element which is able to output a signal when both of its input lines are triggered. The circuit model for the SNSPD element is simulated in SPICE and optimized to match the experimental SNSPD response data. The two-signal detection scheme is simulated using JSIM which allows for the simulation of Josephson junction elements in a circuit. We demonstrate a model of the two-signal circuit operating with two simulated SNSPD pixel inputs and investigate the response of the scheme when a phase shift is applied to one of the inputs. The scheme shows potential as a useful coincidence detector of single photons. We also present preliminary experimental results of nanobridge-based Josephson junctions to be used in the realization of the coincidence detector circuit. Evidence of the nanobridges exhibiting Josephson behavior (SQUID modulation) are presented

Weak link nanobridges as single flux quantum elements

This paper investigates the feasibility of using weak link nanobridges as Josephson junction elements for the purpose of creating Josephson circuits. We demonstrate the development of a single-step electron beam lithography procedure to fabricate niobium nanobridges with dimensions down to $40\,\mathrm{nm}\times 100\,\mathrm{nm}$. The single-step process facilitates fabrication that is scalable to complex circuits that require many junctions. We measure the IV-characteristics (IVC) of the nanobridges between temperatures of $4.2$ and $9\,{\rm{K}}$ and find agreement with numerical simulations and the analytical resistively shunted junction (RSJ) model. Furthermore, we investigate the behaviour of the nanobridges under rf irradiation and observe the characteristic microwave-induced Shapiro steps. Our simulated IVC under rf irradiation using both the RSJ model and circuit simulator JSIM are in agreement with the experimental data. As a potential use of nanobridges in circuits requiring many junctions, we investigate the theoretical performance of a nanobridge-based Josephson comparator circuit using JSIM

Detection and susceptibility measurements of a single Dynal bead

In this work we present detection and susceptibility measurement experiments on a single superparamagnetic Dynal bead with a diameter of 1 mu m and a magnetic moment of approximate to 4 x 10(8) mu(B). Accurate bead positioning was achieved via non-invasive AFM nanomanipulation. The detection and magnetic characterization of the bead were performed using ultra-sensitive InSb Hall devices. Single bead detection was demonstrated using a step-wise change of the dc magnetic field; measurements were performed using only the in-phase component of the total ac Hall voltage. Very clear evidence of the bead presence is demonstrated simultaneously with explicit separation of parasitic inductive signals. Additional experiments performed using a sweeping change of the dc field allowed susceptibility measurements of a single Dynal bead. The numerical outcomes of both sweeping and stepping experiments are in a very good agreement. The method presented here opens up new possibilities for the reliable and accurate detection of small magnetic moments, which is of high importance for metrological applications as well as highly sensitive biological, medical, and environmental detectors. [doi:10.1063/1.3638124