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We present magnetic field sensitivity measurements of microwave SQUID multiplexers designed for bolometric applications. These devices are often used in environments with changing external magnetic fields, due to either motion through Earth's field or radiation from other pieces of instrumentation. To minimize the pickup of magnetic fields, these multiplexers utilize gradiometric SQUIDs, but residual pickup remains and must be characterized in order to design adequate magnetic shielding for a given application. To study the perpendicular field response, we utilize a single-axis cryogenic Helmholtz coil mounted to the mixing chamber of a dilution refrigerator, calibrated with a cryogenic fluxgate magnetometer. Measurements were made on multiplexer devices across varying microwave readout frequencies for AC magnetic fields from 0.01 to 100 Hz. We measure the magnetic field sensitivity of the multiplexer devices to be 0.1–0.3 ϕ 0 /Gauss, equivalent to an effective area of gradiometric failure of 2–6 μm 2 .Please check if the corresponding author affiliation is correctly identified.This looks correct to me. [ABSTRACT FROM AUTHOR]

The microwave SQUID multiplexer (μmux) has enabled higher bandwidth or higher channel counts across a wide range of experiments in particle physics, astronomy, and spectroscopy. The large multiplexing factor coupled with recent commercial availability of microwave components and warm electronics readout systems make it an attractive candidate for systems requiring large cryogenic detector counts. Since the multiplexer is considered for both bolometric and calorimetric applications across several orders of magnitude of signal frequencies, understanding the bandwidth of the device and its interaction with readout electronics is key to appropriately designing and engineering systems. Here, we discuss several important factors contributing to the bandwidth properties of μ mux systems, including the intrinsic device bandwidth, interactions with warm electronics readout systems, and aliasing. We present simulations and measurements of μ mux devices coupled with SLAC Microresonator RF (SMuRF) tone-tracking electronics and discuss several implications for future experimental design. [ABSTRACT FROM AUTHOR]

We describe the design and measurement of feedhorn-coupled, transition-edge sensor (TES) polarimeters with two passbands centered at 220 GHz and 280 GHz, intended for observations of the cosmic microwave background. Each pixel couples polarized light in two linear polarizations by use of a planar orthomode transducer and senses power via four TES bolometers, one for each band in each linear polarization. Previous designs of this detector architecture incorporated passbands from 27 to 220 GHz; we now demonstrate this technology at frequencies up to 315 GHz. Observational passbands are defined with an on-chip diplexer, and Fourier-transform-spectrometer measurements are in excellent agreement with simulations. We find coupling from feedhorn to TES bolometer using a cryogenic, temperature-controlled thermal source. We determine the optical efficiency of our device is η = 77 % ± 6 % ( 75 % ± 5 % ) for 220 (280) GHz, relative to the designed passband shapes. Lastly, we compare two power-termination schemes commonly used in wide-bandwidth millimeter-wave polarimeters and find equal performance in terms of optical efficiency and passband shape. [ABSTRACT FROM AUTHOR]

We describe an on-sky demonstration of a microwave-multiplexing readout system in one of the receivers of the Keck Array, a polarimetry experiment observing the cosmic microwave background at the South Pole. During the austral summer of 2018–2019, we replaced the time-division multiplexing readout system with microwave-multiplexing components including superconducting microwave resonators coupled to radio frequency superconducting quantum interference devices at the sub-Kelvin focal plane, coaxial-cable plumbing and amplification between room temperature and the cold stages, and a SLAC Microresonator Radio Frequency system for the warm electronics. In the range 5–6 GHz, a single coaxial cable reads out 528 channels. The readout system is coupled to transition-edge sensors, which are in turn coupled to 150-GHz slot-dipole phased-array antennas. Observations began in April 2019, and we report here on an initial characterization of the system performance. [ABSTRACT FROM AUTHOR]

We review advances in low-temperature detector (LTD) arrays for cosmic microwave background (CMB) polarization experiments, with a particular emphasis on imaging arrays. We briefly motivate the science case, which has spurred a large number of independent experimental efforts. We describe the challenges associated with CMB polarization measurements, and how these challenges impact LTD design. Key aspects of an ideal CMB polarization imaging array are developed and compared to the current state of the art. These aspects include dual-polarization sensitivity, background-limited detection over a 10:1 bandwidth ratio, and frequency-independent angular responses. Although existing technology lacks all of this capability, today’s CMB imaging arrays achieve many of these ideals and are highly advanced superconducting integrated circuits. Deployed arrays map the sky with pixels that contain elements for beam formation, polarization diplexing, passband definition in multiple frequency channels, and bolometric sensing. Several detector architectures are presented. We comment on the implementation of both transition-edge sensor bolometers and microwave kinetic inductance detectors for CMB applications. Lastly, we discuss fabrication capability in the context of next-generation instruments that call for ∼106 sensors. [ABSTRACT FROM AUTHOR]

Large arrays of cryogenic sensors for various imaging applications ranging across x-ray, gamma-ray, cosmic microwave background, mm/sub-mm, as well as particle detection increasingly rely on superconducting microresonators for high multiplexing factors. These microresonators take the form of microwave SQUIDs that couple to transition-edge sensors or microwave kinetic inductance detectors. In principle, such arrays can be read out with vastly scalable software-defined radio using suitable FPGAs, ADCs and DACs. In this work, we share plans and show initial results for SLAC Microresonator Radio Frequency (SMuRF) electronics, a next-generation control and readout system for superconducting microresonators. SMuRF electronics are unique in their implementation of specialized algorithms for closed-loop tone tracking, which consists of fast feedback and feedforward to each resonator’s excitation parameters based on transmission measurements. Closed-loop tone tracking enables improved system linearity, a significant increase in sensor count per readout line, and the possibility of overcoupled resonator designs for enhanced dynamic range. Low-bandwidth prototype electronics were used to demonstrate closed-loop tone tracking on twelve 300-kHz-wide microwave SQUID resonators, spaced at ∼ 6 MHz with center frequencies ∼ 5-6 GHz. We achieve multi-kHz tracking bandwidth and demonstrate that the noise floor of the electronics is subdominant to the noise intrinsic in the multiplexer. [ABSTRACT FROM AUTHOR]

Microwave kinetic inductance detectors (MKIDs) provide a compelling path forward to the large-format polarimeter, imaging, and spectrometer arrays needed for next-generation experiments in millimeter-wave cosmology and astronomy. We describe the development of feedhorn-coupled MKID detectors for the TolTEC millimeter-wave imaging polarimeter being constructed for the 50-m Large Millimeter Telescope (LMT). Observations with TolTEC are planned to begin in early 2019. TolTEC will comprise ∼7000 polarization-sensitive MKIDs and will represent the first MKID arrays fabricated and deployed on monolithic 150 mm diameter silicon wafers—a critical step toward future large-scale experiments with over 105 detectors. TolTEC will operate in observational bands at 1.1, 1.4, and 2.0 mm and will use dichroic filters to define a physically independent focal plane for each passband, thus allowing the polarimeters to use simple, direct-absorption inductive structures that are impedance matched to incident radiation. This work is part of a larger program at NIST-Boulder to develop MKID-based detector technologies for use over a wide range of photon energies spanning millimeter-waves to X-rays. We present the detailed pixel layout and describe the methods, tools, and flexible design parameters that allow this solution to be optimized for use anywhere in the millimeter and sub-millimeter bands. We also present measurements of prototype devices operating in the 1.1 mm band and compare the observed optical performance to that predicted from models and simulations. [ABSTRACT FROM AUTHOR]

In the next decade, new ground-based cosmic microwave background (CMB) experiments such as Simons Observatory, CCAT-prime, and CMB-S4 will increase the number of detectors observing the CMB by an order of magnitude or more, dramatically improving our understanding of cosmology and astrophysics. These projects will deploy receivers with as many as hundreds of thousands of transition edge sensor (TES) bolometers coupled to superconducting quantum interference device (SQUID)-based readout systems. It is well known that superconducting devices such as TESes and SQUIDs are sensitive to magnetic fields. However, the effects of magnetic fields on TESes are not easily predicted due to the complex behavior of the superconducting transition, which motivates direct measurements of the magnetic sensitivity of these devices. We present comparative four-lead measurements of the critical temperature versus applied magnetic field of AlMn TESes varying in geometry, doping, and leg length, including Advanced ACT and POLARBEAR-2/Simons Array bolometers. MoCu ACTPol TESes are also tested and are found to be more sensitive to magnetic fields than the AlMn devices. We present an observation of weak-link-like behavior in AlMn TESes at low critical currents. We also compare measurements of magnetic sensitivity for time division multiplexing SQUIDs and frequency division multiplexing microwave (μMUX) rf-SQUIDs. We discuss the implications of our measurements on the magnetic shielding required for future experiments that aim to map the CMB to near-fundamental limits. [ABSTRACT FROM AUTHOR]

Polarization sensitive, microwave kinetic inductance detectors (MKIDs) are under development for the next generation BLAST instrument (BLAST-TNG). BLAST-TNG is a balloon-borne submillimeter polarimeter designed to study magnetic fields in diffuse dust regions and molecular clouds. We present the design and performance of feedhorn-coupled, dual-polarization sensitive MKIDs fabricated from TiN/Ti multilayer films, which have been optimized for the 250 $$\upmu $$ m band. Measurements show effective selection of linear polarization and good electrical isolation between the orthogonally crossed X and Y detectors within a single spatial pixel. The detector cross-polar coupling is $$<$$ 3 %. Passband measurements are presented, which demonstrate that the desired band-edges (1.0-1.4 THz) have been achieved. We find a near linear response to the optical load from a blackbody source, which has been observed in previous devices fabricated from TiN. Blackbody-coupled noise measurements demonstrate that the sensitivity of the detectors is limited by photon noise when the optical load is greater than 1 pW. [ABSTRACT FROM AUTHOR]

MUSTANG-2 is a 90 GHz feedhorn-coupled, microwave SQUID-multiplexed TES bolometer array in the final stages of development for operation on the 100-m Robert C. Byrd Green Bank Telescope. We present the camera design and report the performance during the first season of observation, in which 64 of the available 215 pixels in the focal plane were populated. We highlight the microwave multiplexing readout technology, which is envisioned as a path to read out the next generation of large pixel-count cryogenic focal planes. In this regard, MUSTANG2 is a pathfinder for this multiplexing technology. We present noise spectra which show no detector noise degradation when read out with microwave SQUID multiplexing, and we present first light images of Jupiter and M87, which demonstrate the end-to-end system performance. [ABSTRACT FROM AUTHOR]

This paper describes MUSTANG 2, a 338 element focal plane array that is being built for the Green Bank Telescope. Each element consists of a profiled feedhorn coupled to two transition edge sensor bolometers, one for each polarization. Initial deployment will be with 32 detectors, but once fully populated, MUSTANG 2 will be capable of mapping a $$8'\times 8'$$ area to $$23~\upmu $$ Jy in 1 h with good image fidelity on angular scales from $$9''$$ to $$6'$$ . As well as an instrument overview, the choice of bandpass and the design of the feeds, detectors and readout are given. [ABSTRACT FROM AUTHOR]

We are developing arrays of kinetic inductance detectors for sub-millimeter polarimetry that will be deployed on the BLAST balloon-borne instrument. The array is feedhorn-coupled, and each pixel contains two lumped-element kinetic inductance detectors (LEKIDs) made of TiN. The absorbing, inductive sections of the LEKID-pair are orthogonal, which allows simultaneous measurement of both horizontal and vertical polarizations within one spatial pixel. In this paper, we show efficient absorption in TiN films when coupled to waveguide at room temperature and present dark measurements of single polarization devices with varying capacitor geometries. We show that it will be difficult to achieve background-limited performance in BLAST with stoichiometric TiN films with T $$_{c}=4.5$$ K, and that non-stoichiometric films with lower T $$_{c}$$ will be required. [ABSTRACT FROM AUTHOR]