Superconducting quantum detectors

Abstract:

The forthcoming generation of cosmic microwave background polarization observatories is developing large format detector arrays which will operate at 100 mK. Given the volume of detector wafers that will be required, fast-cooldown 100 mK test cryostats are increasingly needed. A miniature dilution refrigerator (MDR) has been developed for this purpose and is reported. The MDR is precooled by a doublestage ³He–⁴He Chase Research Cryogenics sorption refrigerator. The test cryostat based on this MDR will enable fast cooldown to 100 mK to support rapid feedback testing of detector wafers fabricated for the Simons Observatory. The MDR has been designed to provide a 100  mK stage to be retrocompatible with existing CRC10 sorption coolers, reducing the base temperature from 250 mK for the new generation of detectors. Other 250 mK cryostats can be retroftted in the same way. This confguration will meet the cryogenic requirements for single-wafer testing, providing 5–10 μW of cooling power at 100 mk for over 8 h. The system operates in a closed cycle, thereby avoiding external gas connections and cold o-rings. No moving parts are required, with the system operated entirely by heaters.

Abstract:

We present a new software package, called QMix, for simulating the behavior of superconductor/insulator/ superconductor (SIS) mixers. The software uses a harmonic balance procedure to calculate the ac voltage across the SIS junction and multitone spectral domain analysis to calculate the quasiparticle tunneling currents. This approach has two major advantages over other simulation techniques: 1) it can include an arbitrary number of higher order harmonics, and 2) it can simulate the effect of multiple strong nonharmonic frequencies. This allows the QMix software to simulate a wide range of SIS mixer operation, including the effects of harmonics in the local-oscillator (LO) signal and gain saturation with respect to the RF signal power. In this article, we compare the simulated results from QMix to the measured performance of a 230-GHz SIS device, both to validate the software and to investigate the experimental data. To begin, we simulated the conversion gain of the mixer and we found excellent agreement with the experimental results. We were also able to recreate the broken photon step effect by adding higher and lower order harmonics to the LO signal. We then simulated a range of incident RF signal powers in order to calculate the gain saturation point. This is an important metric for SIS mixers because we require linear gain for reliable measurements and calibration. In the simulated results, we found both gain compression and gain expansion, which is consistent with other studies. Overall, these examples demonstrate that QMix is a powerful software package that will allow researchers to simulate the performance of SIS mixers, investigate experimental results, and optimize mixer operation. We have made all of the QMix software open-source and we invite others to contribute to the project.

Abstract:

DC blocks are used frequently in planar circuits to enable separate DC voltage/current biasing of active components inserted along the transmission lines. In this Letter, we present a slotline DC block design where the conductors of the transmission line can be physically broken, while allowing the propagation of the RF signal across the discontinuity with negligible insertion loss. The DC block comprises two break-lines with narrow gaps, patterned on the two ground planes of a slotline with each breakline connected to an RF choke. The RF chokes present open circuit nodes that prevent the RF power from leaking into the break-lines gaps. We have fabricated and tested the DC block, and demonstrated that the measured performance agrees very well with simulated results. The insertion loss was close to – 0.5 dB in the designated range of 12–16 GHz, demonstrating that the RF leakage through the DC block is indeed negligible.

Abstract:

In this letter, we present a new E-plane rectangular waveguide bend that can be micromachined using split-block fabrication. The 90° bend is created by machining two linear channels with a notch left in the outside corner. This notch significantly improves the return loss and the insertion loss of the bend. The primary advantage of this technique is that it produces a compact waveguide bend, minimizing both the size and the conduction loss of the waveguide circuit. Another advantage is that this technique only requires linear channels to be machined, thus it can be fabricated without requiring a computer numerically controlled (CNC) milling machine. The performance of this waveguide bend was optimized using electromagnetic simulation software, and the optimal reflection coefficient was found to be below -28 dB across the entire operational bandwidth of the waveguide. The simulated design was then validated using two experimental prototypes that were tested at microwave (12-18 GHz) and millimeter frequencies (140-220 GHz).