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In 2021 the Quantum Sensors for the Hidden Sector (QSHS) collaboration was founded in the UK. We received funding as part of the Quantum Technologies for Fundamental Physics programme to develop and demonstrate quantum devices with the potential to detect hidden-sector particles such as axions and dark photons in the ~1μeV/c^2 to ~100μeV/c^2 mass window. Our collaboration is formed from a diverse community of condensed matter physicists, quantum technologists, theorists and (astro) particle physics experimentalists. We are designing and fabricating a range of “quantum devices” and constructing a high-field, low-temperature facility at the University of Sheffield to characterise and test the devices in an axion haloscope geometry, initially using an rf cavity. This poster was presented at the 17th Patras workshop on axions, WIMPs and WISPs on the 8th August 2022 in Mainz, Germany.

This is the Reproduction package for the scientific article entitled "Modelling the Partially Coherent Behaviour of Few-Mode Far-Infrared Grating Spectrometers".

Funder: CSC Cambridge Scholarship, In this study, we evaluate the suitability of using DC-biased aluminium resonators as low-frequency kinetic inductance detectors capable of operating in the frequency range of 50–120 GHz. Our analysis routine for supercurrent-biased resonators is based on the Usadel equations and gives outputs including density of states, complex conductivities, transmission line properties, and quasiparticle lifetimes. Results from our analysis confirm previous experimental observations on resonant frequency tuneability and retention of high quality factor. Crucially, our analysis suggests that DC-biased resonators demonstrate significantly suppressed superconducting density of states gap. Consequently these resonators have lower frequency detection threshold and are suitable materials for low-frequency kinetic inductance detectors.

We investigated the use of dielectric layers produced by atomic layer deposition (ALD) as an approach to strain mitigation in composite silicon/superconductor devices operating at cryogenic temperatures. We show that the addition of an ALD layer acts to reduce the strain of spins closest to silicon/superconductor interface where strain is highest. We show that appropriately biasing our devices at the hyperfine clock transition of bismuth donors in silicon, we can remove strain broadening and that the addition of ALD layers left T 2 (or temporal inhomogeneities) unchanged in these natural silicon devices.

Transition-edge sensors (TESs) are capable of highly accurate single particle energy measurement. TESs have been used for a wide range of photon detection applications, particularly in astronomy, but very little consideration has been given to their capabilities as electron calorimeters. Existing electron spectrometers require electron filtering optics to achieve energy discrimination, but this step discards the vast majority of electrons entering the instrument. TESs require no such energy filtering, meaning they could provide orders of magnitude improvement in measurement rate. To investigate the capabilities of TESs in electron spectroscopy, a simulation pipeline has been devised. The pipeline allows the results of a simulated experiment to be compared with the actual spectrum of the incident beam, thereby allowing measurement accuracy and efficiency to be studied. Using Fisher information, the energy resolution of the simulated detectors was also calculated, allowing the intrinsic limitations of the detector to be separated from the specific data analysis method used. The simulation platform has been used to compare the performance of TESs with existing X-ray photoelectron spectroscopy (XPS) analysers. TESs cannot match the energy resolution of XPS analysers for high-precision measurements but have comparable or better resolutions for high count rate applications. The measurement rate of a typical XPS analyser can be matched by an array of 10 TESs with 120 microsecond response times and there is significant scope for improvement, without compromising energy resolution, by increasing array size., Comment: 9 pages, 3 figures

Periodic structures can be simulated using the periodic Method of Moments. The quasi-periodicity, i.e. periodicity within a linear phase-shift, is implemented through the use of the periodic Green's function. In this paper, we propose a technique to interpolate the impedance matrix for varying phase-shifts. To improve the accuracy, the contribution of the dominant Floquet modes and a term corresponding to a linear phase-shift are first extracted. The technique is applied to planar geometries, but can be extended to non-planar configurations., 5 pages, 3 figures

Post-dispersed Fourier transform spectrometers are prime candidates for up- coming far-infrared space telescopes, which achieve background limited spectral sensitivity by few-mode detectors. Straylight effects in such hybrid systems are investigated using a partially coherent modelling technique.

As in conventional computing, key attributes of quantum memories are high storage density and, crucially, random access, or the ability to read from or write to an arbitrarily chosen register. However, achieving such random access with quantum memories in a dense, hardware-efficient manner remains a challenge, for example requiring dedicated cavities per qubit or pulsed field gradients. Here we introduce a protocol using chirped pulses to encode qubits within an ensemble of quantum two-level systems, offering both random access and naturally supporting dynamical decoupling to enhance the memory lifetime. We demonstrate the protocol in the microwave regime using donor spins in silicon coupled to a superconducting cavity, storing up to four multi-photon microwave pulses in distinct memory modes and retrieving them on-demand up to 2~ms later. A further advantage is the natural suppression of superradiant echo emission, which we show is critical when approaching unit cooperativity. This approach offers the potential for microwave random access quantum memories with lifetimes exceeding seconds, while the chirped pulse phase encoding could also be applied in the optical regime to enhance quantum repeaters and networks.

We have performed a quantum mechanical analysis of travelling-wave parametric amplifiers (TWPAs) in order to investigate five experimental phenomena related to their operations, namely the effect of impedance mismatch, the presence of upper idler modes, the presence of quantum and thermal noise, the generation of squeezed states, and the preservation of pre-squeezed states during amplification. Our analysis uses momentum operators to describe the spatial evolution of quantised modes along a TWPA. We calculate the restriction placed on pump amplitude as well as amplifier gain as a result of impedance mismatch between a TWPA and its external system. We apply our analysis to upper idler modes and demonstrate that they will result in suppressed gain. We show that an ideal TWPA is indeed quantum-limited—i.e. it introduces a half-quantum of zero-point fluctuation which is the minimum possible noise contribution for a phase-preserving linear amplifier. We analyse the thermal noise associated with a TWPA by considering the effect of distributed sources along an amplifier transmission line. Our analysis predicts a doubling of thermal noise in the high gain limit as a result of wave-mixing between signal and idler modes. We study the operation of a TWPA in the presence of a DC bias current, and have shown that highly squeezed states can in principle be generated. However, amplifying a pre-squeezed state using a non-degenerate TWPA generally reduces the squeezing advantage.

Ensembles of bismuth-donor spins in silicon are promising storage elements for microwave quantum memories due to their long coherence times, which exceed seconds. The operation of an efficient quantum memory requires the achievement of critical coupling between the spin ensemble and a suitable high-quality factor resonator---this in turn requires a thorough understanding of the line shapes for the relevant spin-resonance transitions, particularly considering the influence of the resonator itself on line broadening. Here, we present pulsed electron-spin-resonance measurements of ensembles of bismuth donors in natural silicon, above which niobium superconducting resonators have been patterned. By studying spin transitions across a range of frequencies and fields, we identify distinct line-broadening mechanisms and, in particular, those that can be suppressed by operating at magnetic-field-insensitive ``clock transitions.'' Given the donor concentrations and resonator used here, we measure a cooperativity $C\ensuremath{\sim}0.2$ and based on our findings we discuss a route to achieve unit cooperativity, as required for a quantum memory.

The ability to measure both optical efficiency and dynamic response to changes in optical signal is crucial to the development of Transition Edge Sensors (TESs) for far-infrared astronomical instruments. We have devised and implemented a cryogenic test facility for ultra-low-noise far-infrared TESs, designed for the SAFARI grating spectrometer on the cooled-aperture space telescope SPICA. Whilst our experimental arrangement is suitable for the whole of the SAFARI wavelength range, 34-230 μm, we focus here on representative optical measurements at 60-110 μm. Detectors are illuminated with a few-mode beam having modal characteristics identical to those of an ideal imaging telescope. In addition, a fast thermal infrared source allows direct measurement of the TES response to tiny changes in incident optical power. We describe the measured functional forms of TES transient responses both to fast optical pulses and to modulation of the power dissipated in the bilayer, in the presence of background optical loading through to TES saturation.

We present an on-chip superconducting filter bank spectrometer based on transition-edge sensors (TES) as a technology for realising a microwave atmospheric sounding instrument with several hundred channels and sky- noise limited performance. Each device consists of a wideband feed coupled to a transmission line filter bank, with a TES behind each filter. In the first instance we have targeted atmospheric temperature sounding using the oxygen (O2) absorption line at 60 GHz, however the device is being scaled to 180 GHz for humidity sounding. The technology developed is also generally applicable to astronomical instrumentation. We have fabricated a set of test devices to demonstrate key device technologies, such as channel placement, spectral resolution and sensitivity. We will describe device design, test configuration and results.

The mid/far infrared hosts a wealth of spectral information that allows direct determination of the physical state of matter in a large variety of astronomical objects, unhindered by foreground obscuration. Accessing this domain is essential for astronomers to much better grasp the fundamental physical processes underlying the evolution of many types of celestial objects, ranging from protoplanetary systems in our own milky way to 10-12 billion year old galaxies at the high noon of galaxy formation in our universe. The joint ESA/JAXA SPICA mission will give such access for the astronomical community at large, by providing an observatory with unprecedented mid- to far-infrared imaging, polarimetric and spectroscopic capabilities.

The optical modelling of far-infrared partially-coherent grating spectrometers has long been considered difficult, due to the multi-mode diffractive nature of the grating optics. However, for the next generation of far-infrared space missions the need for understanding the complex behaviour of these grating spectrometers has intensified. Conventional modelling techniques are difficult to apply because i) the field is partially coherent; ii) diffraction and focusing effects are crucially important; iii) diffraction integrals need to be sampled finely over large optical surfaces. We describe an effective approach based on propagating the correlation functions of the radiation field using the natural modes of the optical system. First, the transformation matrix of the system, T, is determined, which captures the natural modes of the optics. Next, the correlations functions are propagated through the optics using T. The result is a modal optics technique that captures all performance information, in terms of the spectral, spatial and coherence details, within a single framework. In the paper, we explain the foundations of the method and demonstrate its applicability based on a number of standard far-infrared optical systems. Our scheme is numerically powerful, and provides insights into the trade-offs needed to optimise performance. The analysis we will extended to partially coherent far-infrared grating spectrometers as a function of the incident spectral field compositions, scattering at the grating optics, and detector geometry to improve our understanding of such systems.

Far-infrared Transition Edge Sensors (TESs) are being developed for the SAFARI grating spectrometer on the cooled-aperture space telescope SPICA. In support of this work, we have devised a cryogenic (90 mK) test facility for carrying out precision optical measurements on ultra-low-noise TESs. Although our facility is suitable for the whole of the SAFARI wavelength range, 34-230 $\mu$m, we focus on a representative set of measurements at 60-110 $\mu$m using a device having a Noise Equivalent Power (NEP) of 0.32 $\mathrm{aW/\sqrt{Hz}}$. The system is able to perform a range of measurements: (i) Dark electrical characterisation. (ii) Optical efficiency with respect to a partially coherent beam having a modal composition identical to that of an ideal imaging telescope. (iii) Optical saturation and dynamic range. (iv) Fast optical transient response to a modulated thermal source. (v) Optical transient response in the presence of high-level background loading. We describe dark measurements to determine the operating characteristics of a TES, and then compare predicted optical behaviour with measured optical behaviour. By comparing electrical and optical transient response, we were able to observe thermalisation in the device. We comment on the challenge of eliminating stray light.

Electromagnetic sources relying on spontaneous emission are difficult to characterize without a proper framework due to the partial spatial coherence of the emitted fields. In this paper, we propose to characterize emitters of any shape through their natural emitting modes, i.e. a set of coherent modes that add up incoherently. The resulting framework is very intuitive since any emitter is regarded as a multimode antenna with zero correlation between modes. Moreover, for any finite emitter, the modes form a compact set that can be truncated. Each significant mode corresponds to one independent degree of freedom through which the emitter radiates power. The proposed formalism is implemented using the Method of Moments (MoM) and applied to a lossy sphere and a lossy ellipsoid. It is shown that electrically small structures can be characterized with a small number of modes, and that this number grows as the structure becomes electrically large., Comment: To be presented in European Conference on Antennas and Propagation (EuCAP 2020)

We discuss how reactive and dissipative non-linearities affect the intrinsic response of superconducting thin-film resonators. We explain how most, if not all, of the complex phenomena commonly seen can be described by a model in which the underlying resonance is a single-pole Lorentzian, but whose centre frequency and quality factor change as external parameters, such as readout power and frequency, are varied. What is seen during a vector-network-analyser measurement is series of samples taken from an ideal Lorentzian that is shifting and spreading as the readout frequency is changed. According to this model, it is perfectly proper to refer to, and measure, the resonant frequency and quality factor of the underlying resonance, even though the swept-frequency curves appear highly distorted and hysteretic. In those cases where the resonance curve is highly distorted, the specific shape of the trajectory in the Argand plane gives valuable insights into the second-order physical processes present. We discuss the formulation and consequences of this approach in the case of non-linear kinetic inductance, two-level-system loss, quasiparticle generation, and a generic model based on a power-law form. The generic model captures the key features of specific dissipative non-linearities, but additionally leads to insights into how general dissipative processes create characteristic forms in the Argand plane. We provide detailed formulations in each case, and indicate how they lead to the wide variety of phenomena commonly seen in experimental data. We also explain how the properties of the underlying resonance can be extracted from this data. Overall, our paper provides a self-contained compendium of behaviour that will help practitioners interpret and determine important parameters from distorted swept-frequency measurements., Comment: 55 pages, 15 figures

We describe the design, fabrication, integration and characterization of a prototype superconducting filter bank with transition edge sensor readout designed to explore millimetre-wave detection at frequencies in the range 40 to 65 GHz. Results indicate highly uniform filter channel placement in frequency and high overall detection efficiency. The route to a full atmospheric sounding instrument in this frequency range is discussed., 13 pages, 12 figures

This article presents a generic flexible framework for an End-to-end Instrument Performance Simulation System (EIPS) for satellite atmospheric remote sensing instruments. A systematic process for developing an end-to-end simulation system based on Rodgers&rsquo, atmospheric observing system design process has been visualised. The EIPS has been developed to support the quantitative evaluation of new satellite instrument concepts in terms of performance simulations, design optimisation, and trade-off analysis. Important features of this framework include: fast radiative transfer simulation capabilities (fast computation and line-by-line like simulations), applicability across the whole electromagnetic (EM) spectrum and a number of integrated retrieval diagnostics. Because of its applicability across the whole EM spectrum, the framework can be usefully applied to synergistic atmospheric retrieval studies. The framework is continually developing and evolving, and finding applications to support and evaluate emerging instrument and mission concepts. To demonstrate the framework&rsquo, s flexibility in relation to advanced sensor technologies in the microwave range, a novel superconducting transition edge sensor (TES) -based multi-spectral microwave instrument has been presented as an example. As a case study, the performance of existing multi-spectral-type microwave instruments and a TES-technology-based multi-spectral microwave instrument has been simulated and compared using the developed end-to-end simulation framework.

We have developed a coupled-mode analysis framework for superconducting travelling-wave parametric amplifiers using the full Telegrapher's equations to incorporate loss-related behaviour. Our model provides an explanation of previous experimental observations regarding loss in amplifiers, advantages of concatenating amplifiers to achieve high gains, and signal gain saturation. This work can be used to guide the design of amplifiers in terms of the choice of material systems, transmission line geometry, operating conditions, and pump strength.

Funder: China Scholarship Council; doi: http://dx.doi.org/10.13039/501100004543, Superconducting thin films are central to the operation of many kinds of quantum sensors and quantum computing devices: kinetic inductance detectors (KIDs), travelling-wave parametric amplifiers (TWPAs), qubits, and spin-based quantum memory elements. In all cases, the nonlinearity resulting from the supercurrent is a critical aspect of behaviour, either because it is central to the operation of the device (TWPA), or because it results in nonideal second-order effects (KID). Here, we present an analysis of supercurrent-carrying superconducting thin films that is based on the generalized Usadel equations. Our analysis framework is suitable for both homogeneous and multi-layer thin films, and can be used to calculate the resulting density of states, superconducting transition temperature, superconducting critical current, complex conductivities, complex surface impedances, transmission line propagation constants, and nonlinear kinetic inductances in the presence of supercurrent. Our analysis gives the scale of kinetic inductance nonlinearity (I∗) for a given material combination and geometry, and is important in optimizing the design of detectors and amplifiers in terms of materials, geometries, and dimensions. To investigate the validity of our analysis across a wide range of supercurrent, we have measured the transition temperatures of superconducting thin films as a function of DC supercurrent. These measurements show good agreement with our theoretical predictions in the experimentally relevant range of current values.

The submillimetre is an understudied region of the Earth's atmospheric electromagnetic spectrum. Prior technological gaps and relatively high opacity due to the prevalence of rotational water vapour lines at these wavelengths have slowed progress from a ground-based remote sensing perspective; however, emerging superconducting detector technologies in the fields of astronomy offer the potential to address key atmospheric science challenges with new instrumental methods. A site study, with a focus on the polar regions, is performed to assess theoretical feasibility by simulating the downwelling (zenith angle = 0°) clear-sky submillimetre spectrum from 30 mm (10 GHz) to 150 µm (2000 GHz) at six locations under annual mean, summer, winter, daytime, night-time and low-humidity conditions. Vertical profiles of temperature, pressure and 28 atmospheric gases are constructed by combining radiosonde, meteorological reanalysis and atmospheric chemistry model data. The sensitivity of the simulated spectra to the choice of water vapour continuum model and spectroscopic line database is explored. For the atmospheric trace species hypobromous acid (HOBr), hydrogen bromide (HBr), perhydroxyl radical (HO2) and nitrous oxide (N2O) the emission lines producing the largest change in brightness temperature are identified. Signal strengths, centre frequencies, bandwidths, estimated minimum integration times and maximum receiver noise temperatures are determined for all cases. HOBr, HBr and HO2 produce brightness temperature peaks in the mK to µK range, whereas the N2O peaks are in the K range. The optimal submillimetre remote sensing lines for the four species are shown to vary significantly between location and scenario, strengthening the case for future hyperspectral instruments that measure over a broad wavelength range. The techniques presented here provide a framework that can be applied to additional species of interest and taken forward to simulate retrievals and guide the design of future submillimetre instruments.

We present a broadband end-fire planar circuit antenna that is suitable for applications in microwave, millimetre (mm) and sub-millimetre (sub-mm) wavelengths. We have cascaded two dipoles of different lengths in series to broaden the bandwidth. Two printed directors and a truncated ground plane are used to achieve high front-to-back (F/B) gain ratio. The parallel stripline feeding the antenna’s drivers is connected directly to microstrip without an intermediate stage. This simple antenna design is easily integrated into microstrip circuits. In this paper, we focus on the design of a Ku-band antenna with 7 GHz bandwidth and F/B gain ratio of 18 dB. Design and analysis of the antenna were performed using rigorous electromagnetic simulations, and we experimentally validated the performance of the antenna by measuring the beam patterns and return loss of an antenna fabricated on a Roger RO 4350 printed circuit board. Finally, we present two unique example applications of the antenna in the sub-mm detectors area.

We have solved numerically the diffusive Usadel equations that describe the spatially-varying superconducting proximity effect in Ti-Al thin-film bi- and trilayers with thickness values that are suitable for Kinetic Inductance Detectors (KIDs) to operate as photon detectors with detection thresholds in the frequency range of 50-90 GHz. Using Nam's extension of the Mattis-Bardeen calculation of the superconductor complex conductivity, we show how to calculate the surface impedance for the spatially varying case, and hence the surface impedance quality factor. In addition, we calculate energy-and spatially-averaged quasiparticle lifetimes at temperatures well-below the transition temperature and compare to calculation in Al. Our results for the pair-breaking threshold demonstrate differences between bilayers and trilayers with the same total film thicknesses. We also predict high quality factors and long multilayer-averaged quasiparticle recombination times compared to thin-film Al. Our calculations give a route for designing KIDs to operate in this scientifically-important frequency regime., 9 pages, 10 figures

We give an overview of the baseline detector system for SAFARI, the prime focal-plane instrument on board the proposed space infrared observatory, SPICA. SAFARI's detectors are based on superconducting Transition Edge Sensors (TES) to provide the extreme sensitivity (dark NEP$\le2\times10^{-19}\rm\ W/\sqrt Hz$) needed to take advantage of SPICA's cold (, 7 pages, 3 figures, Proc. SPIE 10708, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy IX, 107080K (9 July 2018); (fixed typo in abstract)

Ultra-low-noise Transition Edge Sensors (TESs) have been selected for the far-infrared Fourier transform spectrometer SAFARI on the space telescope SPICA, now under study as an M5 mission, operating in three wavelength bands: S-band from 34-60 μm, M-band from 60-110 μm and L-band from 110-210 μm. We report the fabrication and optical characterisation of a linear TES array for the SAFARI M-band, integrated with micromachined reflective backshorts and profiled pyramidal optical feedhorns. The design and construction of the cryogenic optical test facility used to illuminate the devices under test are described, featuring a variable temperature blackbody load, band-defining filters and an optical aperture. We observe effective numbers of optical modes, Nef f = 0.41 ± 0.03, and near-unity optical efficiencies in TES-backshort assemblies, with some loss of efficiency in the presence of horns. Stray light control measures are discussed in the context of a significant reduction achieved in long wavelength stray light detected by these devices.

Transition Edge Sensors are ultra-sensitive superconducting detectors with applications in many areas of research, including astrophysics. The device consists of a superconducting thin film, often with additional normal metal features, held close to its transition temperature and connected to two superconducting leads of a higher transition temperature. There is currently no way to reliably assess the performance of a particular device geometry or material composition without making and testing the device. We have developed a proximity effect model based on the Usadel equations to predict the effects of device geometry and material composition on sensor performance. The model is successful in reproducing I −V curves for two devices currently under study. We use the model to suggest the optimal size and geometry for TESs, considering how small the devices can be made before their performance is compromised. In the future, device modelling prior to manufacture will reduce the need for time-consuming and expensive testing.

Thin-film superconducting transmission lines play important roles in many signal transmission and detection systems, including qubit coupling and read-out schemes, electron spin resonance systems, parametric amplifiers, and various ultra high sensitivity detectors. Here we present a rigorous method for computing the electromagnetic behaviour of superconducting microstrip transmission lines and coplanar waveguides. Our method is based on conformal mapping, and is suitable for both homogeneous superconductors and proximity-coupled multilayers. We also present an effective conductivity approximation of multilayers, thereby allowing the multilayers to be analysed using existing electromagnetic design software. We compute the numerical results for Al–Ti bilayers and discuss the validity of our full computation and homogeneous approximation.

The sensitivity of a low-noise superconducting transition edge sensor (TES) is determined by the thermal conductance of the support structure that connects the active elements of the device to the heat bath. Low-noise devices require conductances in the range 0.1 to 10 pWK$^{-1}$, and so have to rely on diffusive phonon scattering in long, narrow, amorphous SiN$_\text{x}$ legs. We show that it is possible to manufacture and operate TESs having short, ballistic low-dimensional legs (cross section 500$\times$200 nm) that contain multi-element phononic interferometers and ring resonators. These legs transport heat in effectively just 5 elastic modes at the TES's operating temperature (< 150 mK), which is close to the quantised limit of 4. The phononic filters then reduce the thermal flux further by frequency-domain filtering. For example, a micromachined 3-element ring resonator reduced the flux to 19 % of a straight-legged ballistic device operating at the quantised limit, and 38 % of a straight-legged diffusive reference device. This work opens the way to manufacturing TESs where performance is determined entirely by filtered, few-mode, ballistic thermal transport in short, low-heat capacity legs, free from the artifacts of two level systems., 11 pages, 8 figures

We describe an interferometric technique capable of fully characterizing the optical response of few-mode and multi-mode detectors using only power measurements, and its implementation at 1550 nm wavelength. EnergyAbsorption Interferometry (EAI) is an experimental procedure where the system under test is excited with two coherent, phase-locked sources. As the relative phase between the sources is varied, a fringe is observed in the detector output. Iterating over source positions, the fringes’ complex visibilities allow the two-point detector response function to be retrieved: this correlation function corresponds to the state of coherence to which the detector is maximally sensitive. This detector response function can then be decomposed into a set of natural modes, in which the detector is incoherently sensitive to power. EAI therefore allows the reconstruction of the individual degrees of freedom through which the detector can absorb energy, including their relative sensitivities and full spatial forms. Coupling mechanisms into absorbing structures and their underlying solidstate phenomena can thus be studied, with direct applications in improving current infrared detector technology. EAI has previously been demonstrated for millimeter wavelength. Here, we outline the theoretical basis of EAI, and present a room-temperature 1550 nm wavelength infrared experiment we have constructed. Finally, we discuss how this experimental system will allow us to study optical coupling into fiber-based systems and near-infrared detectors.

An interferometric technique is described for measuring the spatial forms of the individual degrees of freedom through which an object or system can absorb energy from its environment. The method separates out the spatial forms of the coherent excitations present at any single frequency, and gives their relative responsivities; it is not necessary to sweep the frequency and infer their forms from resonant absorption features. The method can be used for simply characterising the ability to absorb energy from a partially coherent field, or it be be used to explore the solid-state physics of the energy-absorption process, depending on application.

The bilayers of Transition Edge Sensors (TESs) are often modified with additional normal-metal features such as bars or dots. Previous device measurements suggest that these features improve performance, reducing electrical noise and altering response times. However, there is currently no numerical model to predict and quantify these effects. Here we extend existing techniques based on Usadel's equations to describe TESs with normal-metal features. We show their influence on the principal TES characteristics, such as the small-signal electrothermal parameters $\alpha$ and $\beta$ and the superconducting transition temperature $T_{c}$. Additionally, we examine the effects of an applied magnetic field on the device performance. Our model predicts a decrease in $T_{c}$, $\alpha$ and $\beta$ as the number of lateral metal structures is increased. We also obtain a relationship between the length $L$ of a TES and its critical temperature, $T_{c} \propto L^{-0.7}$ for a bilayer with normal-metal bars. We predict a periodic magnetic flux dependence of $\alpha, \beta$ and $I_{c}$. Our results demonstrate good agreement with published experimental data, which also show the reduction of $\alpha$, $\beta$ and $T_{c}$ with increasing number of bars. The observed Fraunhofer dependence of critical current on magnetic flux is also anticipated by our model. The success of this model in predicting the effects of additional structures suggests that in the future numerical methods can be used to better inform the design of TESs, prior to device processing.

We present a new satellite-based instrument concept that will enable global measurements of atmospheric temperature and humidity profiles with unprecedented resolution and accuracy, compared to currently planned missions. It will also provide global measurements of essential climate variables related to ice clouds that will better constrain global climate models. The instrument is enabled by the use of superconducting detectors coupled to superconducting filterbank spectrometers, operating between 50GHz and 850 GHz. We present the science drivers, the current instrument concept and status, and predicted performance.

At low temperatures, < 200 mK, the thermal flux through low-dimensional amorphous dielectric bars, < 2 $��$m wide and 200 nm thick, is transported by a small number of low-order elastic modes. For long bars, L > 400 $��$m, it is known that the conductance scales as 1/L, where L is the length, but for short bars, 1 $��$m < L < 400 $��$m, the length dependence is poorly known. Although it is assumed that the transport must exhibit a diffusive to ballistic transition, the functional form of the transition and the scale size over which the transition occurs have not, to our knowledge, been measured. In this paper, we use ultra-low-noise superconducting Transition Edge Sensors (TESs) to measure the heat flux through a set of SiN$_{x}$ bars to establish the characteristic scale size of the ballistic to diffusive transition. For bars supporting 6 to 7 modes, we measure a thermal elastic-wave attenuation length of 20 $��$m. The measurement is important because it sheds light on the scattering processes, which in turn are closely related to the generation of thermal fluctuation noise. Our own interest lies in creating patterned phononic filters for controlling heat flow and thermal noise in ultra-low-noise devices, but the work will be of interest to others trying to isolate devices from their environments, and studying loss mechanisms in micro-mechanical resonators., 3 figures

In Kinetic Inductance Detectors (KIDs) and other similar applications of superconducting microresonators, both the large and small-signal behaviour of the device may be affected by electrothermal feedback. Microwave power applied to read out the device is absorbed by and heats the superconductor quasiparticles, changing the superconductor conductivity and hence the readout power absorbed in a positive or negative feedback loop. In this work, we explore numerically the implications of an extensible theoretical model of a generic superconducting microresonator device for a typical KID, incorporating recent work on the power flow between superconductor quasiparticles and phonons. This model calculates the large-signal (changes in operating point) and small-signal behaviour of a device, allowing us to determine the effect of electrothermal feedback on device responsivity and noise characteristics under various operating conditions. We also investigate how thermally isolating the device from the bath, for example by designing the device on a membrane only connected to the bulk substrate by thin legs, affects device performance. We find that at a typical device operating point, positive electrothermal feedback reduces the effective thermal conductance from the superconductor quasiparticles to the bath, and so increases responsivity to signal (pair-breaking) power, increases noise from temperature fluctuations, and decreases the Noise Equivalent Power (NEP). Similarly, increasing the thermal isolation of the device while keeping the quasiparticle temperature constant decreases the NEP, but also decreases the device response bandwidth., 11 pages, 10 figures

We present an exact formulation for the computation of the power absorbed by lossy structures that can deal with partially spatially coherent fields. The absorbing structure is characterized through a discrete number of natural absorption modes, leading to a compact representation of the absorption capabilities of the structure when subjected to any kind of incident fields. The method can be easily combined with any kind of commercial software since only the response of the structure to fully coherent fields is required.

Recently we have reported on the effects of magnetic field on our low noise (NEP = 4 $$\times 10^{-19}$$ W/ $$\surd $$ Hz) [1] TiAu TES bolometers that are being developed at SRON for the SAFARI FIR Imaging Spectrometer on SPICA telescope that will be operated in three different wavelength bands: S-band for 30–60 $$\upmu \hbox {m}$$ , M-band for 60–110 $$\upmu \hbox {m}$$ and L-band for 110–210 $$\upmu \hbox {m}$$ . The arrays for the S- and M- band will be based on TiAu TES bolometer arrays, developed by SRON. The L-band array will be based on a MoAu TES bolometer developed by University of Cambridge. We have investigated the effect of the magnetic field on the current, responsivity, speed and critical current for both the TiAu and MoAu TES bolometers in our high accuracy magnetic field set-up. A clear difference in weak link behavior is observed between the two types of TES bolometers in both strength of the effect and period of the oscillations.

In many applications of absorbing structures it is important to understand their spatial response to incident fields, for example in thermal solar panels, bolometric imaging, and controlling radiative heat transfer. In practice, the illuminating field often originates from thermal sources and is only partially spatially coherent when it reaches the absorbing device. In this paper, we present a method to fully characterize the way a structure can absorb such partially coherent fields. The method is presented for any three-dimensional material and accounts for the partial coherence and partial polarization of the incident light. This characterization can be achieved numerically using simulation results or experimentally using the energy absorption interferometry that has been described previously in the literature. The absorbing structure is characterized through a set of absorbing functions onto which any partially coherent field can be projected. This set is compact for any structure of finite extent, and the absorbing function is discrete for periodic structures.

Power dissipation in electromagnetic absorbers is a quadratic function of the incident fields. To characterize an absorber, one needs to deal with the coupling that may occur between different excitations. Energy absorption interferometry (EAI) is a technique that highlights the independent degrees of freedom through which a structure can absorb energy: the natural absorption modes of the structure. The coupling between these modes vanishes. In this paper, we use the EAI formalism to analyze different kinds of plasmonic periodic absorbers while rigorously accounting for the coupling: resonant golden patches on a grounded dielectric slab, parallel free-standing silver wires, and a silver slab of finite thickness. The EAI formalism is used to identify the physical processes that mediate absorption in the near and far field. First, we demonstrate that the angular absorption, which is classically used to characterize periodic absorbers in the far field and which neglects the coupling between different plane waves, is only valid under stringent conditions (subwavelength periodicity, far-field excitation, and negligible coupling between the two possible polarizations). Using EAI, we show how the dominant absorption channels can be identified through the signature of the absorption modes of the structure, while rigorously accounting for the coupling. We then exploit these channels to improve absorption. We show that long-range processes can be exploited to enhance the spatial selectivity, while short-range processes can be exploited to improve absorptivity over wide angles of incidence. Finally, we show that by simply adding scatterers with the proper periodicity on top of the absorber, the absorption can be increased by more than 1 order of magnitude.

We present the design and preliminary experimental results of a broadband 650 GHz balanced SIS mixer utilizing two back-to-back unilateral finline tapers. The new design employs fully integrated circuit components onto a 15 μm silicon-on-insulator (SOI) substrate, hence greatly simplifying the balanced mixer block requirements. Independent testing of the on-chip RF quadrature hybrid and the DC/IF blocks showed performances consistence with the computational predictions. The fabricated mixer chip tests have also demonstrated successful operation in balanced mixing mode as the LO noise was separated from the IF output. Preliminary tests of the device's sensitivity gave a best double sideband (DSB) noise temperature of 714 K measured at 635 GHz, and remained at a similar level throughout the operating frequency band. The higher than expected noise temperature was mainly caused by a significant shift in the junction position during fabrication, in both the longitudinal and lateral directions, resulting in substantial shift in the tuning frequency and degradation in the coupling of power to the junction.

The C$_{\it l}$OVER instrument (described elsewhere in this volume) is being built to measure the B-mode polarisation of the Cosmic Microwave Background. Each of the 256 pixels is made up a pseudo-correlation receiver that can be realised using either waveguide or microstrip technology. In this work we present a design study for a possible waveguide-based solution. Each of the individual components has been optimised using electromagnetic finite-element modelling software (HFSS).

This paper describes a new Heterodyne Array Receiver Programme (HARP) and Auto-Correlation Spectral Imaging System (ACSIS) that have recently been installed and commissioned on the James Clerk Maxwell Telescope (JCMT). The 16-element focal-plane array receiver, operating in the submillimetre from 325 to 375 GHz, offers high (three-dimensional) mapping speeds, along with significant improvements over single-detector counterparts in calibration and image quality. Receiver temperatures are $\sim$120 K across the whole band and system temperatures of $\sim$300K are reached routinely under good weather conditions. The system includes a single-sideband filter so these are SSB figures. Used in conjunction with ACSIS, the system can produce large-scale maps rapidly, in one or more frequency settings, at high spatial and spectral resolution. Fully-sampled maps of size 1 square degree can be observed in under 1 hour. The scientific need for array receivers arises from the requirement for programmes to study samples of objects of statistically significant size, in large-scale unbiased surveys of galactic and extra-galactic regions. Along with morphological information, the new spectral imaging system can be used to study the physical and chemical properties of regions of interest. Its three-dimensional imaging capabilities are critical for research into turbulence and dynamics. In addition, HARP/ACSIS will provide highly complementary science programmes to wide-field continuum studies, and produce the essential preparatory work for submillimetre interferometers such as the SMA and ALMA., Comment: MNRAS Accepted 2009 July 2. 18 pages, 25 figures and 6 tables

We describe a procedure for modeling the nonlinear, quantum-mechanical behavior of very high harmonic superconductor-insulator-superconductor (SIS) mixers. A typical mixer is pumped by a strong microwave source at 10-15 GHz, which causes high-harmonic (20-40) currents to flow, any one of which can be used as a local oscillator to downconvert a submillimeter-wave signal to a low microwave frequency, 1-8 GHz. This mode of operation is attractive for measuring the beam patterns of conventional SIS mixers, because only a single submillimeter-wave source is needed. We conducted simulations using the 20th harmonic of a 13.5 GHz microwave source, downconverting a 271.4 GHz signal to a 1.4 GHz intermediate frequency. These simulations clearly show that linear downconversion can be achieved even for relatively high levels of rf signal power; although, care is needed when choosing the operating point. The patterns of behavior seen in the simulations are in remarkable agreement with recently published experimental results. © 2007 American Institute of Physics.

The Quantum Theory of Mixing developed by Tucker provides a solid framework for understanding the behaviour of SIS mixers, and subsequent developments allow the simulation of complete mixer circuits. These methods operate, however, under the assumption of small signal levels, and so neglect the non-linear behaviour of the signal path. The non-linearity of the mixer's response to applied signals is of vital importance to the calibration of SIS receiver systems. We have previously reported a procedure for calculating the full quantum behaviour of tunnel junction circuits under multiple high-level signals, allowing the accurate prediction of the saturation characteristics of SIS mixers. In this paper, we apply our procedure to both an idealized SIS mixer and one of our previously tested 700 GHz finline mixers. We find that the small signal behaviour predicted by our procedure agrees well with other simulation methods, and that the saturation properties of both of these mixers differ from that predicted by previous estimates of saturation behaviour.