Superconducting quantum detectors

Abstract:

We report the investigation of the influence of Josephson pair tunnelling on the sensitivity of an SIS mixer near the superconducting gap of niobium. Hot and cold load measurements were carried out from 600 GHz to 700 GHz, without completely supressing the Josephson effect. We have noticed that when measurements were made at bias points near the first Shapiro step, significantly higher values of Y-factor could be obtained with magnetic field strengths that made the Shapiro step sharper, rather then those that supressed the step. This resulted in a significant improvement in noise temperature of the mixer at those bias points. This observation is important for SIS mixers operating at frequencies near or above gap frequency (ωgap) because the warping of the second negative photon step to the positive display side, narrowing down the bias voltage interval in which measurement of the Y-factor at the first photon step could be done.

Abstract:

Context. The use of large-format focal plane imaging arrays employing multiple feed horns is becoming increasingly important for the next generation of single dish sub-mm telescopes and cosmology experiments. Such receivers are being commissioned on both general purpose, common user telescopes and telescopes specifically designed for mapping intensity and polarisation anisotropies in the cosmic microwave background (CMB). Telescopes are currently being constructed to map the CMB polarisation that employ hundreds of feeds and the cost of manufacturing these feeds has become a significant fraction of the total cost of the telescope. Aims. We have developed and manufactured low-cost easy-to-machine smooth-walled horns that have a performance comparable to the more traditional corrugated feed horns that are often used in focal plane arrays. Our horns are much easier to fabricate than corrugated horns enabling the rapid construction of arrays with a large number of horns at a very low cost. Methods. Our smooth walled horns use multiple changes in flare angle to excite higher order waveguide modes. They are designed using a genetic algorithm to optimise the positions and magnitudes of these flare angle discontinuities. We have developed a fully parallelised software suite for the optimisation of these horns. We have manufactured prototype horns by traditional electroforming and also by a new direct drilling technique and we have measured their beam patterns using a far-field antenna test range at 230 GHz. Results. We present simulated and measured far-field beam patterns for one of our horn designs. They exhibit low sidelobe levels, good beam circularity and low cross-polarisation levels over a fractional bandwidth of 20%. These results offer experimental confirmation of our design technique, allowing us to proceed confidently in the optimisation of horns with a wider operational bandwidth. The results also show that the new manufacturing technique using drilling is successful, enabling the fabrication of large format arrays by repeatedly drilling into a single aluminium plate. This will enable the construction of focal plane arrays at a very low cost per horn. Conclusions. We have developed a new type of high performance feed horn that is fast and easy to fabricate. Having demonstrated the efficacy of our horn designs experimentally, we are building and testing a prototype focal plane array of 37 hexagonally close packed horns. This prototype array will be an important step towards building a complete CMB mapping receiver using these feed horns. © 2011 ESO.

Abstract:

We present a fundamental and accurate approach to compute the attenuation of electromagnetic waves propagating in rectangular waveguides with finite conductivity walls. The wavenumbers kx and ky in the x and y directions respectively, are obtained as roots of a set of transcendental equations derived by matching the tangential component of the electric field (E) and the magnetic field (H) at the surface of the waveguide walls. The electrical properties of the wall material are determined by the complex permittivity ε, permeability μ, and conductivity σ. We have examined the validity of our model by carrying out measurements on the loss arising from the fundamental TE10 mode near the cutoff frequency. We also found good agreement between our results and those obtained by others including Papadopoulos' perturbation method across a wide range of frequencies, in particular in the vicinity of cutoff. In the presence of degenerate modes however, our method gives higher losses, which we attribute to the coupling between modes as a result of dispersion.

Abstract:

We present a novel design and the measured performance for a superconductor-insulator-superconductor (SIS) mixer, operating near the superconducting gap of niobium (Nb), in the frequency range of 600-700 GHz. A key feature of the mixer design is the employment of a unilateral finline taper to provide smooth transition from the high-impedance waveguide mode to the low-impedance of planar circuits suitable for the operation of SIS tunnel junction. This geometry of the transition can be electromagnetically modelled and optimised to give a short mixer chip with wide RF bandwidth. The finline taper and all the superconducting transmission lines are integrated on-chip and deposited on a 60 μm thick quartz substrate. This results in an extremely simple mixer block design, comprising a feed horn and a straight waveguide section; no backshort or any mechanical tuning structure is needed. In this paper, we describe the design of the mixer chip, including full electromagnetic simulations of the passive circuits, combined with the heterodyne mixer gain and noise temperature predictions.We have tested the mixer performance from 595 GHz to 702 GHz and measured best receiver noise temperature of 145 K at 600 GHz, corrected for a beam splitter of 75 μm thickness. The need for a thick beam splitter was caused by the lack of sufficient power from the local oscillator (LO) which was optimised between 630-720 GHz and because our mixer was tuned low as a result of larger than designed tunnel junction. Nevertheless, our investigation has demonstrated that superconducting finline mixers work well at high frequencies and have several advantages over other designs, in particular simple mixer block and large substrate for integration of planar circuits.