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We describe optimization of a cryogenic magnetometer that uses nonlinear kinetic inductance in superconducting nanowires as the sensitive element instead of a superconducting quantum interference device (SQUID). The circuit design consists of a loop geometry with two nanowires in parallel, serving as the inductive section of a lumped LC resonator similar to a kinetic inductance detector (KID). This device takes advantage of the multiplexing capability of the KID, allowing for a natural frequency multiplexed readout. The Kinetic Inductance Magnetometer (KIM) is biased with a DC magnetic flux through the inductive loop. A perturbing signal will cause a flux change through the loop, and thus a change in the induced current, which alters the kinetic inductance of the nanowires, causing the resonant frequency of the KIM to shift. This technology has applications in astrophysics, material science, and the medical field for readout of Metallic Magnetic Calorimeters (MMCs), axion detection, and magnetoencephalography (MEG).

Superconducting Nanowire Single Photon Detectors (SNSPDs) have demonstrated timing and efficiency properties that have placed them at the frontier in single photon sensing applications (e.g. photon counting, deep space optical communications, quantum communication, and quantum encryption). Properties such as timing jitter as low as 3 ps, dark counts under 90 counts per day, high detection efficiency ( $>90\%$ ), short dead time and sensitivity over a wide range of photon energies (UV to IR). SNSPDs have a cutoff energy where the photon does not have enough energy to trigger the detection mechanism. In this paper, we lay out the framework and experimental setup to explore a modified approach in detecting photons below the cut off energy typical in an SNSPD. Leveraging the nanowire's change in kinetic inductance from photon absorption, the resulting voltage pulse is measured through a coupled quantum-noise-limited parametric amplifier. We consider nanowires with widths less than the vortex entry limit of w $ , where $\xi$ is the coherence length, limiting the sources of noise.

The Galaxy Evolution Probe (GEP) is a concept for a mid- and far-infrared space observatory to measure key properties of large samples of galaxies with large and unbiased surveys. GEP will attempt to achieve zodiacal light and Galactic dust emission photon background-limited observations by utilizing a 6-K, 2.0-m primary mirror and sensitive arrays of kinetic inductance detectors (KIDs). It will have two instrument modules: a 10 to 400 μm hyperspectral imager with spectral resolution R = λ / Δλ ≥ 8 (GEP-I) and a 24 to 193 μm, R = 200 grating spectrometer (GEP-S). GEP-I surveys will identify star-forming galaxies via their thermal dust emission and simultaneously measure redshifts using polycyclic aromatic hydrocarbon emission lines. Galaxy luminosities derived from star formation and nuclear supermassive black hole accretion will be measured for each source, enabling the cosmic star formation history to be measured to much greater precision than previously possible. Using optically thin far-infrared fine-structure lines, surveys with GEP-S will measure the growth of metallicity in the hearts of galaxies over cosmic time and extraplanar gas will be mapped in spiral galaxies in the local universe to investigate feedback processes. The science case and mission architecture designed to meet the science requirements is described, and the KID and readout electronics state of the art and needed developments are described. This paper supersedes the GEP concept study report cited in it by providing new content, including: a summary of recent mid-infrared KID development, a discussion of microlens array fabrication for mid-infrared KIDs, and additional context for galaxy surveys. The reader interested in more technical details may want to consult the concept study report.

I will describe our development of a four kilopixel photometric imaging camera paired with a 1.5 meter crossed Dragone telescope. The focal plane is composed of aluminum kinetic inductance detectors (KIDs) fabricated on crystalline silicon tiles. The tiles contain 960 KIDs and are approximately 100 mm x 100 mm in size. KID pairs, each sensitive to an orthogonal linear polarization, are coupled to a waveguide/feedhorn machined from aluminum. A single block, with 480 waveguides/feedhorns arranged in a hexagonal close-pack configuration, is paired with each detector tile. Initial tests with prototype KID tiles show the expected noise and optical performance. Full-scale tiles have now been fabricated with >90% yield, and are currently being characterized. The imager is intended for terrestrial applications, and an initial demonstration with the telescope is planned for early 2020. With relatively minor changes to the KID design, it could also be optimized for astronomical applications.

We develop a simple coordinate transformation that can be employed to compensate for the nonlinearity introduced by a microwave kinetic inductance detector’s (MKID) homodyne readout scheme. This coordinate system is compared to the canonically used polar coordinates and is shown to improve the performance of the filtering method often used to estimate a photon’s energy. For a detector where the coordinate nonlinearity is primarily responsible for limiting its resolving power, this technique leads to increased dynamic range, which we show by applying the transformation to data from a hafnium MKID designed to be sensitive to photons with wavelengths in the 800- to 1300-nm range. The new coordinates allow the detector to resolve photons with wavelengths down to 400 nm, raising the resolving power at that wavelength from 6.8 to 17.

Millimeter-wave imaging provides a promising option for long-range target detection through optical obscurants such as fog, which often occur in marine environments. Given this motivation, we are currently developing a 150 GHz polarization-sensitive imager using a relatively new type of superconducting pair-breaking detector, the kinetic inductance detector (KID). This imager will be paired with a 1.5 m telescope to obtain an angular resolution of 0.09° over a 3.5° field of view using 3,840 KIDs. We have fully characterized a prototype KID array, which shows excellent performance with noise strongly limited by the irreducible fluctuations from the ambient temperature background. Full-scale KID arrays are now being fabricated and characterized for a planned demonstration in a maritime environment later this year.

The galaxy evolution probe (GEP) is a concept for a probe-class space observatory to study the physical processes related to star formation over cosmic time. To do so, the mid- and far-infrared (IR) spectra of galaxies must be studied. These mid- and far-IR observations require large multi-frequency arrays, sensitive detectors. Our goal is to develop low NEP aluminum kinetic inductance detectors (KIDs) for wavelengths of 10–400 $${\upmu }{{\hbox {m}}}$$μm for the GEP and a pathfinder long-duration balloon (GEP-B) that will perform precursor GEP science. KIDs for the lower wavelength range (10–100 $${\upmu }{{\hbox {m}}}$$μm) have not been previously implemented. We present an absorber design for KIDs sensitive to wavelengths of 10 $${\upmu }{{\hbox {m}}}$$μm shown to have around 75–80% absorption efficiency through ANSYS HFSS (high-frequency structure simulator) simulations, challenges that come with optimizing our design to increase the wavelength range, initial tests on our design of fabricated 10 $${\upmu }{{\hbox {m}}}$$μm KIDs, and theoretical NEP calculations.

An on-chip FTS consists of two waveguides coupled to long superconducting transmission lines (STLs) ($$\sim $$ 620 mm) using two coupling probes. The signal propagating on one of the STLs is phase-shifted with respect to the other line with a bias current that affects the nonlinear dependence of kinetic inductance, $$\mathscr {L}_k(I)$$ of the STL material. Here we describe the design and simulation of a superconducting on-chip FTS coupled to a dual polarization W-band (90–110 GHz) waveguide. These devices have applications in ground-based and space-based millimeter-wave spectral surveys.

Self-similar antennas offer extremely broadband functionality and easily scalable designs. Self-similar designs with a four-arm layout are also suited for dual polarization through excitations of opposing arms, although there has only been limited use of them for millimeter-wave detectors. These antennas have been used for measurements of the cosmic microwave background (CMB), which encompass a wide frequency range and are now actively focusing more on polarization anisotropies. We analyze multiple planar self-similar antenna designs with simulations in high-frequency structure simulator and ongoing physical testing. They all exhibit broadband operation between 130 and 230 GHz and can couple to both linear polarizations through the previously mentioned four-arm symmetry. Simulations include each antenna design coupled to an extended hemispherical, AR-coated lenslet. From these, a basic bowtie-like arm design produced minimal polarization wobble with moderate beam efficiency, while a hybrid trapezoidal design provided high beam efficiency with small polarization wobble. Current fabrication versions of each are being tested, coupled to multichroic microwave kinetic inductance detectors. These planar self-similar antennas, when implemented in CMB and other detectors, could improve observations while simultaneously simplifying fabrication and detector layout.

We report on the development of thermal kinetic inductance detectors (TKIDs) suitable to perform X-ray spectroscopy measurements. The aim is to implement MKIDs sensors working in thermal quasi-equilibrium mode to detect X-ray photons as pure calorimeters. The thermal mode is a variation on the MKID classical way of operation that has generated interest in recent years. TKIDs can offer the MKIDs inherent multiplexibility in the frequency domain, a high spatial resolution comparable with CCDs, and an energy resolution theoretically limited only by thermodynamic fluctuations across the thermal weak links. Microresonators are built in Ti/TiN multilayer technology with the inductive part thermally coupled with a metal absorber on a suspended SiN membrane, to avoid escape of phonons from the film to the substrate. The mid-term goal is to optimize the single-pixel design in terms of superconducting critical temperatures, internal quality factors, kinetic inductance and spectral energy resolution. The final goal is to realize a demonstrator array for a next generation thousand pixels X-ray spectrometer. In this contribution, the status of the project after one year of developments is reported, with detailed reference to the microresonators design and simulations and to the fabrication process.

Over the last few years, there has been a growing interest toward the use of superconducting microwave microresonators operated in quasi-thermal equilibrium mode, especially applied to single particle detection. Indeed, previous devices designed and tested by our group with X-ray sources in the keV range evidenced that several issues arise from the attempt of detection through athermal quasiparticles produced within direct strikes of X-rays in the superconductor material of the resonator. In order to prevent issues related to quasiparticles self-recombination and to avoid exchange of athermal phonons with the substrate, our group focused on the development of thermal superconducting microresonators. In this configuration, resonators composed of multilayer films of Ti/TiN sense the temperature of an absorbing material. To maximize the thermal response, low-critical-temperature films are preferable. By lowering the critical temperature, though, the maximum probing power bearable by the resonators decreases abruptly because of the weakening of the electron–phonon coupling. A proper compromise between the value of critical temperature (and hence sensitivity to energy deposition) and readout power bearable by the device has to be found in order to avoid signal-to-noise ratio degradation. In this contribution, we report the latest measurement of the electron–phonon coupling.

Kinetic inductance in thin film superconductors has been used as the basis for low-temperature, low-noise photon detectors. In particular thin films such as NbTiN, TiN, NbN, the kinetic inductance effect is strongly non-linear in the applied current, which can be utilized to realize novel devices. We present results from transmission lines made with these materials, where DC (current) control is used to modulate the phase velocity thereby enabling an on-chip spectrometer. The utility of such compact spectrometers are discussed, along with their natural connection with parametric amplifiers.

The energy resolution of a single photon counting Microwave Kinetic Inductance Detector (MKID) can be degraded by noise coming from the primary low temperature amplifier in the detector's readout system. Until recently, quantum limited amplifiers have been incompatible with these detectors due to dynamic range, power, and bandwidth constraints. However, we show that a kinetic inductance based traveling wave parametric amplifier can be used for this application and reaches the quantum limit. The total system noise for this readout scheme was equal to ~2.1 in units of quanta. For incident photons in the 800 to 1300 nm range, the amplifier increased the average resolving power of the detector from ~6.7 to 9.3 at which point the resolution becomes limited by noise on the pulse height of the signal. Noise measurements suggest that a resolving power of up to 25 is possible if redesigned detectors can remove this additional noise source.

Superconducting microwave microresonators are low temperature detectors (LTDs) suitable for large-scale frequency domain multiplexing readout. A promising approach consists in operating the resonators in quasi-thermal equilibrium mode: the resonator acts as a thermometer sensing the temperature rise of an absorbing material caused by an energy release. Our aim is to develop such detectors to perform spectroscopy in the keV range, with a possible future application to a next generation experiment aimed at directly measuring the neutrino mass . Still, the best configuration in terms of detector design and material must be found. Resonators made of several Ti/TiN multilayer films, which is a high kinetic inductance superconducting material previously developed by our group, were recently produced. The resonators are deposited onto a silicon slab and the sensitive area, thermally coupled to a gold absorber, is kept suspended by means of a SiN x membrane. In this way we plan to exploit the excellent energy resolution of LTDs combined with the simple multiplexing scheme of the resonators. In this contribution we present the devices along with our project, with its status and perspectives.

Assessing the absolute neutrino mass scale is one of the major challenges in particle physics and astrophysics nowadays. A powerful tool to directly estimate the effective electron neutrino mass consists in the calorimetric measurement of the energy released within a nuclear beta decay. The progresses made over the last few decades on low-temperature detector technologies have permitted to design experiments with expected sensitivities on the neutrino mass below 1 eV/c2 with the calorimetric approach. Despite the remarkable performances in both energy (∼ eV at keV) and time resolutions $(\sim\! 1\ \mu\text{s})$ on the single channel, a large number of detectors working in parallel is required to reach a sub-eV sensitivity. Microwave frequency-domain readout provides a powerful technique to read out large arrays of low-temperature detectors, such as transition edge sensors (TESs) or microwave kinetic inductance detectors (MKIDs). In this way, the multiplex factor is only limited by the bandwidth of the available commercial fast digitizers. The microwave multiplexing system will be used to read out the TES array of the HOLMES experiment, which is made of 1000 163Ho-implanted microcalorimeters. HOLMES is a new experiment that aims to measure the electron neutrino mass by means of the electron capture decay of 163Ho with an expected sensitivity of the order of the eV/c2.

Arrays of tens of thousands of sensitive far-infrared detectors coupled to a cryogenic 4–6 m class orbital telescope are needed to trace the assembly of galaxies over cosmic time. The sensitivity of a 4 Kelvin telescope observing in the far-infrared (30–300 $$\upmu $$ m) would be limited by zodiacal light and Galactic interstellar dust emission, and require broadband detector noise equivalent powers (NEPs) in the range of 3 $$\times 10^{-19}$$ W/ $$\sqrt{\mathrm{Hz}}$$ . We are fabricating and testing 96 element arrays of lumped-element kinetic inductance detectors (LEKIDs) designed to reach NEPs near this level in a low-background laboratory environment. The LEKIDs are fabricated with aluminum: the low normal-state resistivity of Al permits the use of very thin wire-grid absorber lines (150 nm) for efficient absorption of radiation, while the small volumes enable high sensitivities because quasiparticle densities are high. Such narrow absorption lines present a fabrication challenge, but we deposit TiN atop the Al to increase the robustness of the detectors and achieve a 95 $$\%$$ yield. We present the design of these Al/TiN bilayer LEKIDs and preliminary sensitivity measurements at 350 $$\upmu $$ m optically loaded by cold blackbody radiation.

We describe a novel class of devices based on the nonlinearity of the kinetic inductance of a superconducting thin film. By placing a current-dependent inductance in a microwave resonator, small currents can be measured through their effect on the resonator’s frequency. By using a high-resistivity material for the film and nanowires as kinetic inductors, we can achieve a large coefficient of nonlinearity to improve device sensitivity. We demonstrate a current sensitivity of 8pA/√Hz, making this device useful for transition-edge sensor (TES) readout and other cutting-edge applications. An advantage of these devices is their natural ability to be multiplexed in the frequency domain, enabling large detector arrays for TES-based instruments. A traveling-wave version of the device, consisting of a thin-film microwave transmission line, is also sensitive to small currents as they change the phase length of the line due to their effect on its inductance. We demonstrate a current sensitivity of 5pA/√Hz for this version of the device, making it also suitable for TES readout as well as other current-detection applications. It has the advantage of multi-GHz bandwidth and greater dynamic range, offering a different approach to the resonator version of the device.

Future generations of far-infrared (FIR) telescopes will need detectors with noise-equivalent powers on the order of 5 x 10^(-20) W/Hz^(1/2) in order to be photon background limited by astrophysical sources. One such mission concept in development is the Galaxy Evolution Probe (GEP), which will characterize galaxy formation and evolution from z=0 to beyond z=4. Kinetic inductance detectors (KIDs) have been baselined for the GEP for spectroscopy and imaging science between 10 μm and 400 μm due to their intrinsic frequency multiplexability and simple readout schemes. We focus on quasiparticle recombination times as a strategy for increasing detector responsivities to move towards the NEP requirements of the GEP. We present a new model for quantifying time constants from the responses of detectors to pulses of light, and test this model on a 40 nm thick ¼ λ Al coplanar waveguide KID. We intend to use this measurement scheme to quantify the dependence of the quasiparticle recombination time on Al thickness.

The Galaxy Evolution Probe (GEP) is a concept for a mid and far-infrared space observatory designed to survey sky for star-forming galaxies from redshifts of z = 0 to beyond z = 4. Furthering our knowledge of galaxy formation requires uniform surveys of star-forming galaxies over a large range of redshifts and environments to accurately describe star formation, supermassive black hole growth, and interactions between these processes in galaxies. The GEP design includes a 2 m diameter SiC telescope actively cooled to 4 K and two instruments: (1) An imager to detect star-forming galaxies and measure their redshifts photometrically using emission features of polycyclic aromatic hydrocarbons. It will cover wavelengths from 10 to 400 μm, with 23 spectral resolution R = 8 filter-defined bands from 10 to 95 μm and five R = 3.5 bands from 95 to 400 μm. (2) A 24 – 193 μm, R = 200 dispersive spectrometer for redshift confirmation, identification of active galactic nuclei, and interstellar astrophysics using atomic fine-structure lines. The GEP will observe from a Sun-Earth L2 orbit, with a design lifetime of four years, devoted first to galaxy surveys with the imager and second to follow-up spectroscopy. The focal planes of the imager and the spectrometer will utilize KIDs, with the spectrometer comprised of four slit-coupled diffraction gratings feeding the KIDs. Cooling for the telescope, optics, and KID amplifiers will be provided by solar-powered cryocoolers, with a multi-stage adiabatic demagnetization refrigerator providing 100 mK cooling for the KIDs.

Traveling-wave parametric amplifiers based on superconducting NbTiN films are being developed that provide gain over nearly an octave of bandwidth at microwave frequencies. The amplifiers are non-linear transmission lines, where the nonlinearity comes from the current dependence of the kinetic inductance. Amplification results from three and four-wave mixing processes, and phase matching over a wide frequency range is achieved by engineering the dispersion characteristics of the transmission line. Recent noise measurements over a wide frequency band demonstrate that the added noise of these amplifiers is close to one half photon, consistent with the quantum limit for a phase preserving amplifier. We will discuss applications of this class of amplifier to astronomical instruments.

We report on the status of an ongoing effort to develop arrays of horn-coupled, polarization-sensitive microwave kinetic inductance detectors (MKIDs) that are each sensitive to two spectral bands between 125 and 280 GHz. These multi-chroic MKID arrays are tailored for next-generation, large-detector-count experiments that are being designed to simultaneously characterize the polarization properties of both the cosmic microwave background (CMB) and Galactic dust emission. We present our device design and describe laboratory-based measurement results from two 23-element prototype arrays. From dark measurements of our first engineering array we demonstrated a multiplexing factor of 92, showed the resonators respond to bath temperature changes as expected, and found that the fabrication yield was 100%. From our first optically loaded array we found the MKIDs respond to millimeter-wave pulses, additional optical characterization measurements are ongoing. We end by discussing our plans for scaling up this technology to kilo-pixel arrays over the next two years., LTD-17 paper accepted by Journal of Low Temperature Physics

The determination of the neutrino mass is still an open issue in particle physics. The calorimetric measurement of the energy released in a nuclear beta decay allows measuring the whole energy, except the fraction carried away by the neutrino: due to the energy conservation, a finite neutrino mass m ν causes the energy spectrum to be truncated at Q-m ν , where Q is the transition energy of the decay. The electron capture of 163 Ho (Q ~ 2.5 keV) is an ideal decay, due to the high fraction of events close to the endpoint (i.e., the maximum energy of the relaxation energy spectrum). In order to achieve enough statistics, a large number of detectors (~10 4 ) are required. Superconducting microwave microresonators are detectors suitable for large-scale multiplexed frequency-domain readout, with theoretical energy and time resolution on the order of electronvolts and microseconds, respectively. Our aim is to develop arrays of microresonator detectors applicable to the calorimetric measurement of the energy spectrum of 163 Ho. Currently, a study aimed at the selection of the best design and material for the detectors is in progress. In order to obtain low-Tc detectors, with Tc ranging between ~0.5 and 2 K, different Ti/TiN (titanium nitride) multilayer films were produced. The reduced Tc was obtained by superposing thin layers of stoichiometric TiN to pure Ti layers, and the Tc was tuned by varying the ratio between the thickness of the layers. In this contribution, a comparison between the measurements (critical temperature, gap parameter, and X-ray energy spectra) made with stoichiometric and substoichiometric TiN and Ti/TiN multilayer film microresonators is presented.

HOLMES is a new experiment aiming at directly measuring the neutrino mass with a sensitivity below 2 eV. HOLMES will perform a calorimetric measurement of the energy released in the decay of $^{163}$Ho. The calorimetric measurement eliminates systematic uncertainties arising from the use of external beta sources, as in experiments with spectrometers. This measurement was proposed in 1982 by A. De Rujula and M. Lusignoli, but only recently the detector technological progress has allowed to design a sensitive experiment. HOLMES will deploy a large array of low temperature microcalorimeters with implanted $^{163}$Ho nuclei. HOLMES, besides being an important step forward in the direct neutrino mass measurement with a calorimetric approach, will also establish the potential of this approach to extend the sensitivity down to 0.1 eV and lower. In its final configuration HOLMES will collect about $3\cdot 10^{13}$ decays with 1000 detectors characterized by an instrumental energy resolution of the order of few eV and a time resolution of few microseconds. To embed the $^{163}$Ho into the gold absorbers a custom mass separator ion implanter is being developed. The detectors used for the HOLMES experiment will be Mo/Cu bilayers TESs (Transition Edge Sensors) on SiN$_x$ membrane with gold absorbers. Microwave multiplexed rf-SQUIDs are the best available technique to read out large array of such detectors. An extensive R&D activity is in progress in order to maximize the multiplexing factor while preserving the performances of the individual detectors. The current activities are focused on the the single detector performances optimization and on the $^{163}$Ho isotope production and embedding. A preliminary measurement of a sub-array of $4\times 16$ detectors is planned late in 2017. In this contribution we present the HOLMES project with its technical challenges, its status and perspectives.

SuperSpec is an ultra-compact spectrometer-on-a-chip for mm and submm wavelength astronomy. Its very small size, wide spectral bandwidth, and highly multiplexed detector readout will enable construction of powerful multi-object spectrometers for observations of galaxies at high redshift. SuperSpec is a filter bank with planar, lithographed, superconducting transmission line resonator filters and lumped-element kinetic inductance detectors made from Titanium Nitride. We have built an 81 detector prototype that operates in the 195–310 GHz band. The prototype has a wide-band metal feed horn with a transition to microstrip that feeds the filter bank. The prototype has demonstrated optical filter bank channels with a range of resolving powers from 300 to 700, measured fractional frequency noise of 10^(−17)Hz^(−1) at 1 Hz.

SuperSpec is a novel on-chip spectrometer we are developing for (sub)millimeter wavelength astronomy. Our approach utilizes a filterbank of moderate resolution (R∼500) channels, coupled to lumped element kinetic inductance detectors (KIDs), all integrated onto a single silicon chip. The channels are half-wave resonators formed by lithographically depositing segments of superconducting transmission line, and the KIDs are titanium nitride resonators. Here we present optical measurements of a first generation prototype, operating in the 180–280 GHz frequency range. We have used a coherent source to measure the spectral profiles of 17 channels, which achieve linewidths corresponding to quality factors as high as Q__(filt)=700, consistent with the designed values plus additional dissipation characterized by Q_i≈1440. We have also used a Fourier Transform Spectrometer to characterize the spectral purity of all 72 channels on the chip, and measure typical out of band responses ∼30 dB below the peak response.

We are developing athermal-phonon mediated particle detectors that utilize microwave kinetic inductance detectors (MKIDs) as phonon sensors. MKIDs afford natural frequency domain multiplexing, which allows for massive substrates to be patterned with hundreds of sensors while keeping readout complexity to a minimum. Previously, our 2 cm $$\times $$ 2 cm $$\times $$ 1 mm proof-of-principle device utilized 20 MKIDs and, from the magnitude and timing of their response, we were able to reconstruct the position of a particle interaction to $$

We present our current progress on the design and test of Ti/TiN multilayer for use in kinetic inductance detectors. Sensors based on sub-stoichiometric TiN film are commonly used in several applications. However, it is difficult to control the targeted critical temperature $$T_c$$ , to maintain precise control of the nitrogen incorporation process and to obtain a production uniformity. To avoid these problems we investigated multilayer Ti/TiN films that show a high uniformity coupled with high quality factor, kinetic inductance and inertness of TiN. These features are ideal to realize superconductive microresonator detectors for astronomical instruments application but also for the field of neutrino physics. Using pure Ti and stoichiometric TiN, we developed and tested different multilayer configurations, in terms of number of Ti/TiN layers and in terms of different interlayer thicknesses. The target was to reach a critical temperature $$T_c$$ around $$(1{\div } 1.5)$$ K in order to have a low energy gap and slower recombination time (i.e. low generation–recombination noise). The results prove that the superconductive transition can be tuned in the $$(0.5{\div } 4.6)$$ K temperature range by properly choosing the Ti thickness in the $$(0{\div } 15)$$ nm range, and the TiN thickness in the $$(5{\div } 100)$$ nm range.

We present a technique for increasing the internal quality factor of kinetic inductance detectors (KIDs) by nulling ambient magnetic fields with a properly applied magnetic field. The KIDs used in this study are made from thin-film aluminum, they are mounted inside a light-tight package made from bulk aluminum, and they are operated near $150 \, \mathrm{mK}$. Since the thin-film aluminum has a slightly elevated critical temperature ($T_\mathrm{c} = 1.4 \, \mathrm{K}$), it therefore transitions before the package ($T_\mathrm{c} = 1.2 \, \mathrm{K}$), which also serves as a magnetic shield. On cooldown, ambient magnetic fields as small as approximately $30 \, \mathrm{\mu T}$ can produce vortices in the thin-film aluminum as it transitions because the bulk aluminum package has not yet transitioned and therefore is not yet shielding. These vortices become trapped inside the aluminum package below $1.2 \, \mathrm{K}$ and ultimately produce low internal quality factors in the thin-film superconducting resonators. We show that by controlling the strength of the magnetic field present when the thin film transitions, we can control the internal quality factor of the resonators. We also compare the noise performance with and without vortices present, and find no evidence for excess noise beyond the increase in amplifier noise, which is expected with increasing loss., Comment: 5 pages, 4 figures

To characterize further the cosmic star formation history at high redshifts, a large-area survey by a cryogenic 4-6 meter class telescope with a focal plane populated by tens of thousands of far-infrared (FIR, 30-300 μm) detectors with broadband detector noise equivalent powers (NEPs) on the order of 3×10^(-9) W/√ Hz is needed. Ideal detectors for such a surveyor do not yet exist. As a demonstration of one technique for approaching the ultra-low NEPs required by this surveyor, we present the design of an array of 96 350 µm KIDs that utilize phonon recycling to boost responsivity. Our KID array is fabricated with TiN deposited on a silicon-on-insulator (SOI) wafer, which is a 2 μm thick layer of silicon bonded to a thicker slab of silicon by a thin oxide layer. The backside thick slab is etched away underneath the absorbers so that the inductors are suspended on just the 2 μm membrane. The intent is that quasiparticle recombination phonons are trapped in the thin membrane, thereby increasing their likelihood of being re-absorbed by the KID to break additional Cooper pairs and boost responsivity. We also present a Monte-Carlo simulation that predicts the amount of signal boost expected from phonon recycling given different detector geometries and illumination strategies. For our current array geometry, the simulation predicts a measurable 50% boost in responsivity.

We present the design and characterization of low-volume, lumped-element aluminum kinetic inductance de- tectors for sensitive far-infrared astronomy observations. The lumped-element kinetic inductance detectors are comprised of meandered inductors that serve as radiation absorbers in parallel with interdigitated capacitors, forming high quality factor resonators. Low inductor volumes lead to low noise equivalent powers by raising quasiparticles densities, and hence responsivities, with respect to larger volumes. Low volumes are achieved with thin (20 nm), narrow (150 nm) inductors. The interdigitated capacitor architecture is designed to mitigate two-level system noise by lowering electric fields in the silicon substrate. Resonance frequencies are in the range of 190 to 500 MHz, with measured internal quality factors in excess of 1 x 10 5 . In a prior incarnation, a titanium nitride layer on top of the aluminum served as a protective layer, but complicated the superconducting proper- ties. These results were reported previously. In the current incarnation, the aluminum layer is left bare with no titanium nitride over-layer. The results for these bare aluminum devices include a yield of 88%, frequency responsivity of 10 9 W -1 , and noise equivalent power of 1 x 10 -17 W Hz -1/2 for a 350μm array. There is no evidence for 1=f noise down to at least 200 mHz. The sensitivity is currently limited by white noise, very likely from stray light in the testbed; for this detector design, sensitivities limited by generation-recombination noise in a lower-background environment should be several orders of magnitude lower.

We discuss the design considerations and initial measurements from arrays of dual-polarization, lumped element kinetic inductance detectors (LEKIDs) nominally designed for cosmic microwave background (CMB) studies. The detectors are horn-coupled, and each array element contains two single-polarization LEKIDs, which are made from thin-film aluminum and optimized for a single spectral band centered on 150 GHz. We are developing two array architectures, one based on 160 micron thick silicon wafers and the other based on silicon-on-insulator (SOI) wafers with a 30 micron thick device layer. The 20-element test arrays (40 LEKIDs) are characterized with both a linearly-polarized electronic millimeter wave source and a thermal source. We present initial measurements including the noise spectra, noise-equivalent temperature, and responsivity. We discuss future testing and further design optimizations to be implemented., Comment: 7 pages, 5 figures, Proc. SPIE Astronomical Telescopes + Instrumentation 2016, Paper 9914-24

We report on the development of scalable prototype microwave kinetic inductance detector (MKID) arrays tailored for future multi-kilo-pixel experiments that are designed to simultaneously characterize the polarization properties of both the cosmic microwave background (CMB) and Galactic dust emission. These modular arrays are composed of horn-coupled, polarization-sensitive MKIDs, and each pixel has four detectors: two polarizations in two spectral bands between 125 and 280 GHz. A horn is used to feed each array element, and a planar orthomode transducer, composed of two waveguide probe pairs, separates the incoming light into two linear polarizations. Diplexers composed of resonant-stub band-pass filters separate the radiation into 125 to 170 GHz and 190 to 280 GHz pass bands. The millimeter-wave power is ultimately coupled to a hybrid co-planar waveguide microwave kinetic inductance detector using a novel, broadband circuit developed by our collaboration. Electromagnetic simulations show the expected absorption efficiency of the detector is approximately 90%. Array fabrication will begin in the summer of 2016., To appear in the Proc. SPIE

The European Research Council has recently funded HOLMES, a project with the aim of performing a calorimetric measurement of the electron neutrino mass measuring the energy released in the electron capture decay of 163Ho. The baseline for HOLMES are microcalorimeters coupled to transition edge sensors read-out with rf-SQUIDs, for microwave multiplexing purposes. A promising alternative solution is based on superconducting microwave resonators that have undergone rapid development in the last decade. These detectors, called Microwave Kinetic Inductance Detectors (MKIDs), are inherently multiplexed in the frequency domain and suitable for even larger-scale pixel arrays, with theoretical high energy resolution and fast response. The aim of our activity is to develop arrays of microresonator detectors for X-ray spectroscopy and suitable for the calorimetric measurement of the energy spectra of 163Ho. Superconductive multilayer films composed by a sequence of pure Titanium and stoichiometric TiN layers show many ideal properties for MKIDs, such as low loss, large sheet resistance, large kinetic inductance, and tunable critical temperature $$T_\mathrm{c}$$ . We developed Ti/TiN multilayer microresonators with $$T_\mathrm{c}$$ within the range from 70 mK to 4.5 K and with good uniformity. In this contribution, we present the design solutions adopted, the fabrication processes, and the characterization results.

We report on the recent progress of the Multicolor Submillimeter (kinetic) Inductance Camera, or MUSIC. MUSIC will use antenna-coupled Microwave Kinetic Inductance Detectors to observe in four colors (150 GHz, 230 GHz, 290 GHz and 350 GHz) with 2304 detectors, 576 per band, at the Caltech Submillimeter Observatory. It will deploy in 2012. Here we provide an overview of the instrument, focusing on the array design. We have also used a pathfinder demonstration instrument, DemoCam, to identify problems in advance of the deployment of MUSIC. In particular, we identified two major limiters of our sensitivity: out-of-band light directly coupling to the detectors (i.e. not through the antenna), effectively an excess load, and a large 1/f contribution from our amplifiers and electronics. We discuss the steps taken to mitigate these effects to reach background-limited performance (BLIP) in observation.

Superconducting microwave microresonators are low temperature detectors which are compatible with large-scale multiplexed frequency domain readout. We aim to adapt and further advance the technology of microresonator detectors to develop new devices applied to the problem of measuring the neutrino mass. More specifically, we aim to develop detector arrays for calorimetric measurement of the energy spectra of 163Ho EC decay (Q ∼ 2-3 keV) for a direct measurement of the neutrino mass. In order to achieve these goal, we need to find the best design and materials for the detectors. A recent advance in microwave microresonator technology was the discovery that some metal nitrides, such as TiN, possess properties consistent with very high detector sensitivity.We plan to investigate nitrides of higher-Z materials, for example TaN and HfN, that are appropriate for containing the energy of keV decay events, exploring the properties relevant to our detectors, such as quality factor, penetration depth and recombination time. © Springer Science+Business Media, LLC 2012.

We are developing athermal-phonon mediated particle detectors using microwave kinetic inductance detectors (MKIDs) as the phonon sensing elements. Since MKIDs are easily multiplexed, hundreds of sensors patterned on a single dielectric substrate can be read out simultaneously, leading to a precise, time-resolved measurement of the phonon flux at each point on the detector surface. In addition to providing a high-resolution measurement of the location of the interaction, the energy deposited by the particle can be reconstructed with an expected baseline resolution of tens of eV. The complexity of the cryogenic readout electronics is significantly reduced relative to designs based on multiplexed transition edge sensors (TES). Initial proof-of-principle devices demonstrate energy resolutions as good as 0.7 keV at 30 keV, dominated by the position dependence of the phonon signal. New designs are aimed at improving this resolution by more than an order of magnitude. Such high-resolution phonon mediated detectors would have applications including direct detection of dark matter, hard X-ray/soft gamma-ray astrophysics, neutrinoless double beta decay, and coherent neutrino-nucleus scattering.

Using a noise thermometry technique, we measured the temperature dependence of the heat capacity C(T) of suspended SixNy membranes for two different geometries from 30 mK to 300 mK. We determined a volumetric specific heat of the SixNy on the order of 10 aJ/(mum3K) at 0.1 K for both absorber geometries. This value for the specific heat is comparable to a typical metal and is more than a factor of 10 larger than typical glassy materials. This result affects the design of highly efficient optical absorbers for membrane-isolated transition-edge sensors (TESs) for the Background-Limited far-IR/Submillimeter Spectrograph (BLISS), a proposed grating spectrometer that could fly on the next generation of large, cryogenic space-borne telescopes. We show that it is possible to reduce the thermal mass of the absorber of TESs so the effective response time taue is less than 100 ms-a requirement for all the wavelength bands for BLISS.

The opacity due to water vapor in the Earth's atmosphere obscures portions of the sub-THz spectrum (mm/sub-mm wavelengths) to ground based astronomical observation. For maximum sensitivity, instruments operating at these wavelengths must be designed to have spectral responses that match the available windows in the atmospheric transmission that occur in between the strong water absorption lines. Traditionally, the spectral response of mm/sub-mm instruments has been set using optical, metal-mesh bandpass filters [1]. An alternative method for defining the passbands, available when using superconducting detectors coupled with planar antennas, is to use on-chip, superconducting filters [2]. This paper presents the design and testing of superconducting, lumped element, on-chip bandpass filters (BPFs), placed inline with the microstrip connecting the antenna and the detector, covering the frequency range from 209–416 GHz. Four filters were designed with pass bands 209–274 GHz, 265–315 GHz, 335–361 GHz and 397–416 GHz corresponding to the atmospheric transmission windows. Fourier transform spectroscopy was used to verify that the spectral response of the BPFs is well predicted by the computer simulations. Two-color operation of the pixels was demonstrated by connecting two detectors to a single broadband antenna through two BPFs. Scalability of the design to multiple (four) colors is discussed.

We are pursuing the development of a phonon- and ionization-mediated WIMP dark matter detector employing microwave kinetic inductance detectors (MKIDs) in the phonon-sensing channel. Prospective advantages over existing detectors include: improved reconstruction of the phonon signal and event position; simplified readout wiring and cold electronics; and simplified and more reliable fabrication. We have modeled a simple design using available MKID sensitivity data and anticipate energy resolution as good as existing phonon-mediated detectors and improved position reconstruction. We are doing preparatory experimental work by fabricating strip absorber architectures. Measurements of diffusion length, trapping efficiency, and MKID sensitivity with these devices will enable us to design a 1 cm^2×2 mm prototype device to demonstrate phonon energy resolution and position reconstruction.

We present results from a demonstration camera using Microwave Kinetic Inductance Detectors (MKIDs) (Day et al. in Nature 425, 817-821, 2003 )a t the Cal- tech Submillimeter Observatory. The focal plane consists of 16 two-color (240 and 350 GHz) pixels. Each pixel is a phased-array of slot dipole antenna whose output power is coupled to MKIDs via in-line color-defining bandpass filters. A prototype software-defined radio system was used to read out up to four MKIDs simultane- ously. We obtained maps of Jupiter, Saturn, and G34.3 and demonstrated sensitivities of approximately 1 Jy s 1/2 and 10 Jy s 1/2 in the two bands, respectively, limited by detector noise due to a low-efficiency optical train. We anticipate that a second engi- neering run in 2008 with a 36-element, 4-color array and an optimized optical train will be background limited at 240, 270, 350, and 400 GHz. We are undertaking the construction of a full-size MKID camera with 576 four-color spatial pixels and using 2304 MKIDs readout by an expanded software-defined radio system.

We built and measured the electrical properties of membrane-isolation transition-edge sensing bolometers (TESs) suitable for background-limited far-IR/submillimeter direct-detection spectroscopy. Each TES consists of a Mo/Au bilayer patterned onto a suspended, thermally isolated absorber that is connected to the substrate through four SixNy beams deposited by low pressure chemical vapor deposition (LPCVD). We fabricated TESs with straight and meander support beams. The dimensions of the meander (straight) support beams are 700 μm (700 μm) long by 0.25 μm (0.5 μm) thick by 0.35 μm (0.5 μm) wide. We measured I–V characteristics for these TESs and determined that the thermal conductance G equals 72 fW/K (straight) and 19 fW/K (meander) for our best devices. The thermal conductance exhibits a T1/2 dependence with temperature which is evidence of effective elastic scattering of the acoustic phonon modes. The transition temperatures Tc for the same TESs are Tc=137 mK (straight) and Tc=71 mK (meander). If we assume the TESs are temperature-fluctuation noise limited, then the derived noise equivalent power (NEP) equals 1.9×10−19 W/Hz1/2 (straight) and 6.1×10−20 W/Hz1/2 (meander), using our measured values for G and Tc. The meander-beam TES has a derived NEP that is close to two orders of magnitude lower than the state-of-the-art. Finally, we measured an effective time constant τ of about 300 ms (straight) and 400 ms (meander) using electrical and optical pulses. These values for the NEP and τ for the meander-beam TES meet the requirements for the Background-Limited far-IR/Submillimeter Spectrograph (BLISS), a proposed NASA instrument.

We report progress toward large arrays of sensitive TES bolometers for submillimeter and far-infrared wavelengths with noise equivalent power (NEP) suitable for either imaging from a cooled space telescope or ground based spectroscopy. The arrays are based on a pixel design that makes use of a distributed transition edge sensor (TES) coupled to a slot antenna array. We have electrically characterized prototype detectors consisting of 256 TiN hot-electron TES microbolometers biased in parallel with T c =50 mK. The measured electron-phonon thermal conductance of prototype devices is as low as 1.1 pW/K at 50 mK corresponding to an electrical NEP of 4×10−19 W/Hz1/2. The time constant of two detectors with different geometries and transition widths was measured under a range of bias conditions. We have measured time constants ≳10−3 seconds, which is long enough for straightforward multiplexing with existing multiplexer technology.

The responsivity of microwave kinetic inductance detector (MKID) is related to the derivative of the complex conductivity σ with respect to the quasiparticle density n _(qp). For thermal quasiparticles, this derivative dσ/dn_(qp) can be evaluated from σ(T) and n_(qp)(T) given by the Marttis-Bardeen theory (Phys. Rev. 14:412, [1958]). For excess quasiparticles due to external pair breaking, d σ/dn _(qp) can be evaluated by introducing an effective chemical potential μ ^* into the Marttis-Bardeens theory, as did by Owen and Scalapino (Phys. Rev. Lett. 28:1559, [1972]). In this paper, we derive the expressions of dσ/dn_(qp) for both thermal and excess quasiparticles and show that they are approximately equal within the temperature and frequency range MKIDs operate. This equivalence validates the method of using bath temperature sweep to calibrate the responsivity of MKIDs. Response curves of an antenna-coupled submm MKID to bath temperature sweep and blackbody source temperature sweep are compared which demonstrates this equivalence.

D-Wave's quantum processors require a high-fidelity readout architecture for fast and accurate operation. The need for cold operation precludes readout methods with significant dissipation. The readout architecture must be scalable and compatible with an inherently dense qubit circuit layout. Superconducting microresonators have been successfully utilized as detection elements for a wide variety of applications, enabling superior multiplexing factors. In existing designs, resonators undergo a non-uniform shift in frequency and sensitivity with fabrication imperfections, ultimately limiting effective bandwidth utilization. We overcome this drawback by implementing independent control of both frequency and sensitivity. We describe the design of the detector and report experimental results demonstrating its resilience to fabrication tolerances.

SuperSpec is a novel on-chip spectrometer we are developing for multi-object, moderate resolution (R = 100 - 500), large bandwidth (~1.65:1) submillimeter and millimeter survey spectroscopy of high-redshift galaxies. The spectrometer employs a filter bank architecture, and consists of a series of half-wave resonators formed by lithographically-patterned superconducting transmission lines. The signal power admitted by each resonator is detected by a lumped element titanium nitride (TiN) kinetic inductance detector (KID) operating at 100-200 MHz. We have tested a new prototype device that is more sensitive than previous devices, and easier to fabricate. We present a characterization of a representative R=282 channel at f = 236 GHz, including measurements of the spectrometer detection efficiency, the detector responsivity over a large range of optical loading, and the full system optical efficiency. We outline future improvements to the current system that we expect will enable construction of a photon-noise-limited R=100 filter bank, appropriate for a line intensity mapping experiment targeting the [CII] 158 micron transition during the Epoch of Reionization, Comment: 16 pages, 10 figures, Proceedings of the SPIE Astronomical Telescopes + Instrumentation 2014 Conference, Vol 9153, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VII

We have developed antenna-coupled transition-edge sensor (TES) bolometers for a wide range of cosmic microwave background (CMB) polarimetry experiments, including BICEP2, Keck Array, and the balloon borne SPIDER. These detectors have reached maturity and this paper reports on their design principles, overall performance, and key challenges associated with design and production. Our detector arrays repeatedly produce spectral bands with 20%-30% bandwidth at 95, 150, or 220~GHz. The integrated antenna arrays synthesize symmetric co-aligned beams with controlled side-lobe levels. Cross-polarized response on boresight is typically ~0.5%, consistent with cross-talk in our multiplexed readout system. End-to-end optical efficiencies in our cameras are routinely 35% or higher, with per detector sensitivities of NET~300 uKrts. Thanks to the scalability of this design, we have deployed 2560 detectors as 1280 matched pairs in Keck Array with a combined instantaneous sensitivity of ~9 uKrts, as measured directly from CMB maps in the 2013 season. Similar arrays have recently flown in the SPIDER instrument, and development of this technology is ongoing., Comment: 16 pgs, 20 figs

We report photon-noise limited performance of horn-coupled, aluminum lumped-element kinetic inductance detectors at millimeter wavelengths. The detectors are illuminated by a millimeter-wave source that uses an active multiplier chain to produce radiation between 140 and 160 GHz. We feed the multiplier with either amplified broadband noise or a continuous-wave tone from a microwave signal generator. We demonstrate that the detector response over a 40 dB range of source power is well-described by a simple model that considers the number of quasiparticles. The detector noise-equivalent power (NEP) is dominated by photon noise when the absorbed power is greater than approximately 1 pW, which corresponds to $\mathrm{NEP} \approx 2 \times 10^{-17} \, \mathrm{W} \, \mathrm{Hz}^{-1/2}$, referenced to absorbed power. At higher source power levels we observe the relationships between noise and power expected from the photon statistics of the source signal: $\mathrm{NEP} \propto P$ for broadband (chaotic) illumination and $\mathrm{NEP} \propto P^{1/2}$ for continuous-wave (coherent) illumination., Comment: Matches published version (which is open access) and includes supplemental material

We discuss the design and measured performance of a titanium nitride (TiN) mesh absorber we are developing for controlling optical crosstalk in horn-coupled lumped-element kinetic inductance detector arrays for millimeter-wavelengths. This absorber was added to the fused silica anti-reflection coating attached to previously-characterized, 20-element prototype arrays of LEKIDs fabricated from thin-film aluminum on silicon substrates. To test the TiN crosstalk absorber, we compared the measured response and noise properties of LEKID arrays with and without the TiN mesh. For this test, the LEKIDs were illuminated with an adjustable, incoherent electronic millimeter-wave source. Our measurements show that the optical crosstalk in the LEKID array with the TiN absorber is reduced by 66\% on average, so the approach is effective and a viable candidate for future kilo-pixel arrays., Comment: 7 pages, 5 figures, accepted for publication in the Journal of Low Temperature Physics