arXiv subjects

Joshua Montgomery

Publications and source records attributed to Joshua Montgomery.

15 recordsLinked to original sources

arXiv2026

Tunable responsivity and bandwidth in microwave kinetic inductance detectors via readout current nonlinearity

Microwave kinetic inductance detectors (MKIDs) are generally read out with microwave readout tones of high enough amplitude to adequately suppress the noise contribution of the first-stage amplifier. At high readout power, the detector's resonant frequency is altered as a result of the dependence of the kinetic inductance on the internally circulating microwave current. With the tone placed below the resonant frequency, the nonlinear frequency shift results in a positive feedback effect that can significantly enhance the responsivity of the detector, to both optical and microwave power. We report a factor of 10 enhancement in optical response by tuning the readout power and frequency to close to the resonator's bifurcation point. A corresponding decrease in the bandwidth of the resonator is observed under these conditions. We show that the strength of the feedback effect can be easily selected by adjusting the excitation, and provide a map of possible operational states to do so. Operation of MKIDs in this mode could be used to improve sensitivity when non-intrinsic noise sources are significant.

cond-mat.supr-con↗

arXiv2026

Architecture and Validation of the CRS F-Engine for the CHORD Radio Telescope

We introduce the design of the t0.technology Control and Readout System (CRS) F-Engine that will be used for the Canadian Hydrogen Observatory and Radio transient Detector (CHORD), a new radio interferometer currently being commissioned at the Dominion Radio Astrophysical Observatory (DRAO) in Canada. The CRS F-Engine will directly digitize and channelize 1024 individual RF signals from the 512 dual-polarized dishes of the core array using an array of 128 CRS boards, a multi-purpose microwave readout platform using an AMD Zynq Ultrascale+ RF-System-on-Chip (RFSoC) architecture. The CRS supports the required analog and digital signal processing and is appropriately scalable, with rack-mountable crates each supporting up to 16 CRS boards, equipped with a backplane for distribution of power, common clock and time synchronization signals, and a full-mesh network for intra-crate data transmission. Implemented on the CRS boards is the chFPGA firmware which supports the digitization of 8 analog signals at 3.2 GSPS and channelizes them with a CASPER-based PFB/FFT into 8,192 frequency bins with ~195 kHz of resolution, which are then re-quantized into (4 + 4i) bits for data offload to an external X-Engine. chFPGA supports multiple post-channelization signal processing options through separate bitstream files for different applications, such as a 100 GbE packet assembler-transmitter for CHORD to feed channelized data to its external GPU-based X-Engine, as well as FPGA-based N^2 correlators, including a single-board (N = 8) correlator (the ``Pocket Correlator"), and a multi-board corner-turn engine coupled with a half-CRS crate (N = 64) correlator. We demonstrate the performance of chFPGA by injecting a wideband Gaussian noise source into a CRS board running the Pocket Correlator firmware, and find that recovered digitized timestream and channelized data are in excellent agreement with expectations.

astro-ph.IM↗

arXiv2026

A High-Bandwidth Backplane for Wideband Radio Interferometers and Integration with the CHORD Telescope Correlators

Wide-band correlators for radio astronomy interferometers demand accurate, scalable signal processing backends that sustain high aggregate throughput while preserving stable timing alignment across a large number of signal chains. Addressing these coupled constraints is challenging without a co-designed backplane architecture. We present a backplane system developed for the Control Readout System (CRS) Field Programmable Gate Array (FPGA) platform, extending its capabilities to support high channel count wideband radio correlators. The design and validation process are guided by the requirements of the Canadian Hydrogen Observatory and Radio-transient Detector (CHORD) telescope, which provides a representative use case for the system's performance evaluation. Each backplane hosts up to four CRS boards, and a single crate integrates four interconnected backplanes to accommodate sixteen CRS modules (for a total of 128 digitized inputs per crate) interconnected through 25 Gbps per-lane links and an on-backplane data shuffle network. Comprehensive validation demonstrates robust high-speed link performance, sub-sampling-level timing alignment stability, and stable thermal and power behavior across environmental variations. The architecture delivers observatory-scale scalability, providing a low-jitter and phase-consistent foundation for wideband digital correlators. By supporting expansion from single-crate to multi-crate configurations without firmware modification, the system offers a practical and flexible path toward next-generation interferometers requiring tightly synchronized, high-throughput digital backends.

astro-ph.IM↗

arXiv2025

Readout noise of digital frequency multiplexed TES detectors for CUPID

Superconducting transition-edge sensor (TES) detectors have been the standard in Cosmic Microwave Background experiments for almost two decades and are now being adapted for use in nuclear physics, such as neutrinoless double beta decay searches. In this paper we focus on a new high-bandwidth frequency multiplexed TES readout system developed for CUPID, a neutrinoless double beta decay experiment that will replace CUORE. In order to achieve the high energy resolution requirements for CUPID, the readout noise of the system must be kept to a minimum. Low TES operating resistance and long wiring between the readout SQUID and the warm electronics are needed for CUPID, prompting a careful consideration of the design parameters of this application of frequency multiplexing. In this work, we characterize the readout noise of the newly designed frequency multiplexed TES readout system for CUPID and construct a noise model to understand it. We find that current sharing between the SQUID coil impedance and other branches of the circuit, as well as the long output wiring, worsen the readout noise of the system. To meet noise requirements, a SQUID with a low input inductance, high transimpedance and/or low dynamic impedance is needed, and the wiring capacitance should be kept as small as possible. Alternatively, the option of adding a cryogenic low-noise amplifier at the output of the SQUID should be explored.

physics.ins-det↗

arXiv2025

In-situ control of the resonant frequency of kinetic inductance detectors with multiplexed readout

Large multiplexing factors are a primary advantage of kinetic inductance detectors (KIDs), but the implementation of high density arrays still presents significant challenges. Deviations between designed and achieved resonant frequencies are common, and differential loading and responsivity variation across an array may lead to dynamic inter-resonator interactions. It is therefore valuable to be able to both set and maintain the resonant frequency of a KID in situ, using the readout system. We show that it is possible to alter the resonant frequency of the devices by multiple linewidths through the application of readout current, and establish a new stable operational bias point at the driven frequency by making use of the hysteretic bistability commonly seen as bifurcation in frequency-domain measurements. We examine this interaction using a readout tone at fixed frequency positioned near or within the unbiased resonant bandwidth. Development of a control methodology based on this principle remains in an early stage, but a foundational step is understanding the interaction of the readout current with the resonator, in particular its influence on the resonant frequency. In this work, we study conventional KIDs with no physical isolation from the substrate, so we posit that the readout current primarily interacts with the resonator via non-thermal mechanisms, resulting in a predominantly reactive response. This behaviour is reproduced by a simple lumped-element circuit model of the resonance and readout system, providing a straightforward framework for analysis and interpretation. This demonstration is an important early step in the development of techniques which seek to dynamically alter the resonant frequencies of conventional KID arrays, and sets the stage for fast active resonant frequency control under operational conditions.

physics.ins-det↗

arXiv2025

Unified and consistent structure growth measurements from joint ACT, SPT and \textit{Planck} CMB lensing

We present the tightest cosmic microwave background (CMB) lensing constraints to date on the growth of structure by combining CMB lensing measurements from the Atacama Cosmology Telescope (ACT), the South Pole Telescope (SPT) and \textit{Planck}. Each of these surveys individually provides lensing measurements with similarly high statistical power, achieving signal-to-noise ratios of approximately 40. The combined lensing bandpowers represent the most precise CMB lensing power spectrum measurement to date with a signal-to-noise ratio of 61 and an amplitude of $A_\mathrm{lens}^\mathrm{recon} = 1.025 \pm 0.017$ with respect to the theory prediction from the best-fit CMB \textit{Planck}-ACT cosmology. The bandpowers from all three lensing datasets, analyzed jointly, yield a $1.6\%$ measurement of the parameter combination $S_8^\mathrm{CMBL} \equiv \sigma_8\,(\Omega_m/0.3)^{0.25} = 0.825^{+0.015}_{-0.013}$. Including Dark Energy Spectroscopic Instrument (DESI) Baryon Acoustic Oscillation (BAO) data improves the constraint on the amplitude of matter fluctuations to $\sigma_8 = 0.829 \pm 0.009$ (a $1.1\%$ determination). When combining with uncalibrated supernovae from \texttt{Pantheon+}, we present a $4\%$ sound-horizon-independent estimate of $H_0=66.4\pm2.5\,\mathrm{km\,s^{-1}\,Mpc^{-1}} $. The joint lensing constraints on structure growth and present-day Hubble rate are fully consistent with a $\Lambda$CDM model fit to the primary CMB data from \textit{Planck} and ACT. While the precise upper limit is sensitive to the choice of data and underlying model assumptions, when varying the neutrino mass sum within the $\Lambda\mathrm{CDM}$ cosmological model, the combination of primary CMB, BAO and CMB lensing drives the probable upper limit for the mass sum towards lower values, comparable to the minimum mass prior required by neutrino oscillation experiments.

astro-ph.CO↗

arXiv2024

A first demonstration of active feedback control and multi-frequency imaging techniques for kinetic inductance detectors

RF-ICE is a signal processing platform for the readout of large arrays of superconducting resonators. Designed for flexibility, the system's low digital latency and ability to independently and dynamically set the frequency and amplitude of probe tones in real time has enabled previously-inaccessible views of resonator behaviour, and opened the door to novel resonator control schemes. We introduce a multi-frequency imaging technique, developed with RF-ICE, which allows simultaneous observation of the entire resonance bandwidth. We demonstrate the use of this technique in the examination of the response of superconducting resonators to variations in applied readout current and thermal loading. We observe that, used in conjunction with a conventional frequency sweep at sufficiently large amplitude to induce resonance bifurcation, the multi-frequency imaging technique reveals a resonator response which is not captured by the frequency sweep measurement alone. We demonstrate that equivalent resonant frequency shifts can be achieved using either thermal, optical, or readout loading, and use this equivalence to counteract a change in thermal loading by digitally modulating the readout current through a resonator. We develop and implement a proof-of-concept closed-loop negative electro-quasiparticle feedback algorithm which first sets and then maintains the resonant frequency of a lumped element kinetic inductance detector while the loading on it is varied. Although this simple implementation is not yet suitable to deploy at scale, it demonstrates the utility of this feedback technique to improve linearity while addressing amplifier distortion, resonator response non-uniformity, and crosstalk. It can be applied to kinetic inductors in non-bolometric operation, and sets the stage for future developments.

astro-ph.IM↗

arXiv2024

The CRS: a scalable full-stack control system for Microwave Kinetic Inductance Detectors

The t0.technology Control and Readout System (CRS) is a modular microwave control and readout system for mm-wave and radio astronomy, THz imaging, noise radar, and superconducting qubit control. The configuration discussed in this work implements firmware for readout of microwave Kinetic Inductance Detector (KID) arrays. The CRS can operate 4,096 KIDs over 2.5 GHz of complex bandwidth between 0-10 GHz, typically allocated across four independent RF chains at 1,024x multiplexing and 625 MHz of complex bandwidth each. Every CRS can operate as a standalone unit or collectively within one or more backplane-enabled subracks that distribute power, clocking, and synchronization, scaling to an arbitrary number of channels. Each fully populated subrack supports arrays of more than 65,000 KIDs. The signal processing and control software supports recent innovations in multi-probe measurements and dynamic feedback modes, which are described in (Rouble et al. 2024). The CRS has recently been selected as the new baseline readout system for the proposed South Pole Telescope instrument, SPT-3G+. We present the hardware design, firmware capabilities, open-source control and data acquisition software, and the first laboratory characterization measurements.

astro-ph.IM↗

arXiv2022

Development of TRL5 Space Qualified Hardware for Tuning, Biasing, and Readout of Kilopixel TES Bolometer Arrays

The next generation of space-based mm-wave telescopes, such as JAXA's LiteBIRD mission, require focal planes with thousands of detectors in order to achieve their science goals. Digital frequency-domain multiplexing (dfmux) techniques allow detector counts to scale without a linear growth in wire harnessing, sub-Kelvin refrigerator loads, and other scaling problems. In this paper, we introduce Technology Readiness Level 5 (TRL5) electronics suitable for biasing and readout of LiteBIRD's Transition Edge Sensor (TES) bolometers using dfmux techniques. These electronics sit between the spacecraft's payload computer and the cryogenic focal plane, and provide detector biasing, tuning, and readout interfaces between these detectors and the spacecraft's on-board storage. We describe the overall architecture of the electronics, including functional decomposition into modules, the numerology and interconnection of these modules, and their internal and external interfaces. We describe performance measurements to date, including power consumption, thermal performance, and mass, volume, and reliability estimates. This paper is a companion piece to a description of the electronics' on-board Field-Programmable Gate Array (FPGA) firmware.

astro-ph.IM↗

arXiv2022

Development of TRL5 Firmware for Tuning, Biasing, and Readout of Kilopixel TES Bolometer Arrays

The next generation of space-based mm-wave telescopes, such as JAXA's LiteBIRD mission, require focal planes with thousands of detectors in order to achieve their science goals. Digital frequency-domain multiplexing (dfmux) techniques allow detector counts to scale without a linear growth in wire harnessing, sub-Kelvin refrigerator loads, and other scaling problems. In this paper, we describe the Digital Signal Processing (DSP) firmware executed in the design's FPGA. This firmware is responsible for synthesizing bias tones, performing dynamic feedback control of the bolometer voltage bias and/or Superconducting Quantum Interference Device (SQUID) nuller currents, demodulating and decimating bolometer channels into science data, and streaming the results for storage and eventual downlink. We describe how this firmware has been tailored for LiteBIRD, including the control path, improvements to power- and resource-efficiency, the addition of radiation-mitigation functions, and the integration of new bolometer biasing schemes that may help mitigate mission-specific design challenges. This paper is a companion piece to the description of the electronics platform in which the firmware operates.

astro-ph.IM↗

arXiv2022

Digital Active Nulling for Frequency-Multiplexed Bolometer Readout: Performance and Latency

We consider the stability and performance of a discrete-time control loop used as a dynamic nuller in the presence of a relatively large time delay in its feedback path. Controllers of this form occur in mm-wave telescopes using frequency-multiplexed Transition Edge Sensor (TES) bolometers. In this application, negative feedback is needed to linearize a Superconducting Quantum Interference Device (SQUID) used as an amplifier. $M$ such feedback loops are frequency-multiplexed through a SQUID at distinct narrowband frequencies in the MHz region. Loop latencies stem from the use of polyphase filter bank (PFB) up- and down-converters and have grown significantly as the detector count in these experiments increases. As expected, latency places constraints on the overall gain $K$ for which the loop is stable. However, latency also creates spectral peaks at stable gains in the spectral response of the closed loop. Near these peaks, the feedback loop amplifies (rather than suppresses) input signals at its summing junction, rendering it unsuitable for nulling over a range of stable gains. We establish a critical gain $K_C$ above which this amplifying or "anti-nulling" behaviour emerges, and find that $K_C$ is approximately a factor of 3.8 below the gain at which the system becomes unstable. Finally, we describe an alteration to the loop tuning algorithm that selects an appropriate (stable, effective for nulling) loop gain without sensitivity to variations in analog gains due to component tolerances.

astro-ph.IM↗

arXiv2021

Low Noise Frequency Domain Multiplexing of TES Bolometers using Sub-kelvin SQUIDs

Digital Frequency-Domain Multiplexing (DfMux) is a technique that uses MHz superconducting resonators and Superconducting Quantum Interference Device (SQUID) arrays to read out sets of Transition Edge Sensors. DfMux has been used by several Cosmic Microwave Background experiments, including most recently POLARBEAR-2 and SPT-3G with multiplexing factors as high as 68, and is the baseline readout technology for the planned satellite mission LiteBIRD. Here, we present recent work focused on improving DfMux readout noise, reducing parasitic impedance, and improving sensor operation. We have achieved a substantial reduction in stray impedance by integrating the sensors, resonators, and SQUID array onto a single carrier board operated at 250 mK. This also drastically simplifies the packaging of the cryogenic components and leads to better-controlled crosstalk. We demonstrate a low readout noise level of 8.6 pA/Hz$^{-1/2}$, which was made possible by operating the SQUID array at a reduced temperature and with a low dynamic impedance. This is a factor of two improvement compared to the achieved readout noise level in currently operating Cosmic Microwave Background experiments using DfMux and represents a critical step toward maturation of the technology for the next generation of instruments.

physics.ins-det↗

arXiv2021

Anomalous Frequency Noise from the Megahertz Channelizing Resonators in Frequency-Division Multiplexed Transition Edge Sensor Readout

Superconducting lithographed resonators have a broad range of current and potential applications in the multiplexed readout of cryogenic detectors. Here, we focus on LC bandpass filters with resonances in the 1-5 MHz range used in the transition edge sensor (TES) bolometer readout of the Simons Array cosmic microwave background (CMB) experiment. In this readout scheme, each detector signal amplitude-modulates a sinusoidal carrier tone at the resonance frequency of the detector's accompanying LC filter. Many modulated signals are transmitted over the same wire pair, and quadrature demodulation recovers the complex detector signal. We observe a noise in the resonant frequencies of the LC filters, which presents primarily as a current-dependent noise in the quadrature component after demodulation. This noise has a rich phenomenology, bearing many similarities to that of two-level system (TLS) noise observed in similar resonators in the GHz regime. These similarities suggest a common physical origin, thereby offering a new regime in which the underlying physics might be probed. We further describe an observed non-orthogonality between this noise and the detector responsivities, and present laboratory measurements that bound the resulting sensitivity penalty expected in the Simons Array. From these results, we do not anticipate this noise to appreciably affect the overall Simons Array sensitivity, nor do we expect it to limit future implementations.

astro-ph.IM↗

arXiv2018

Optical Characterization of the SPT-3G Focal Plane

The third-generation South Pole Telescope camera is designed to measure the cosmic microwave background across three frequency bands (95, 150 and 220 GHz) with ~16,000 transition-edge sensor (TES) bolometers. Each multichroic pixel on a detector wafer has a broadband sinuous antenna that couples power to six TESs, one for each of the three observing bands and both polarization directions, via lumped element filters. Ten detector wafers populate the focal plane, which is coupled to the sky via a large-aperture optical system. Here we present the frequency band characterization with Fourier transform spectroscopy, measurements of optical time constants, beam properties, and optical and polarization efficiencies of the focal plane. The detectors have frequency bands consistent with our simulations, and have high average optical efficiency which is 86%, 77% and 66% for the 95, 150 and 220 GHz detectors. The time constants of the detectors are mostly between 0.5 ms and 5 ms. The beam is round with the correct size, and the polarization efficiency is more than 90% for most of the bolometers

astro-ph.IM↗

arXiv2014

Digital frequency domain multiplexing readout electronics for the next generation of millimeter telescopes

Frequency domain multiplexing (fMux) is an established technique for the readout of transition-edge sensor (TES) bolometers in millimeter-wavelength astrophysical instrumentation. In fMux, the signals from multiple detectors are read out on a single pair of wires reducing the total cryogenic thermal loading as well as the cold component complexity and cost of a system. The current digital fMux system, in use by POLARBEAR, EBEX, and the South Pole Telescope, is limited to a multiplexing factor of 16 by the dynamic range of the Superconducting Quantum Interference Device pre-amplifier and the total system bandwidth. Increased multiplexing is key for the next generation of large format TES cameras, such as SPT-3G and POLARBEAR2, which plan to have on the of order 15,000 detectors. Here, we present the next generation fMux readout, focusing on the warm electronics. In this system, the multiplexing factor increases to 64 channels per module (2 wires) while maintaining low noise levels and detector stability. This is achieved by increasing the system bandwidth, reducing the dynamic range requirements though active feedback, and digital synthesis of voltage biases with a novel polyphase filter algorithm. In addition, a version of the new fMux readout includes features such as low power consumption and radiation-hard components making it viable for future space-based millimeter telescopes such as the LiteBIRD satellite.

astro-ph.IM↗