arXiv subjects

Jeremy F. Bourhill

Publications and source records attributed to Jeremy F. Bourhill.

7 recordsLinked to original sources

arXiv2026

Foil-Mediated Mutual-Impedance in Microwave Cavities with Enhanced Phase Response

We formulate and validate an equivalent circuit model describing mutual coupling between three microwave cavity resonators interconnected via thin metallic foils. Each cavity is represented as a lumped LCR circuit, while the foils act as interfaces that mediate energy exchange via mutual impedance. This coupling mechanism produces interference effects and a controllable antiresonance when the input resonators are amplitude- and phase-balanced, while enabling coupling across a continuous metallic interface without direct antenna or aperture coupling. All three resonators operated in the TM$_{010}$ mode, where two input resonators each excited the third via a thin copper foil. Analytical expressions are derived for the mutual impedance and coupling coefficient of these foils in this geometry. Under balanced conditions, a sharp antiresonance emerges with a several-fold enhanced phase sensitivity at the resonant frequency of the output cavity, consistent with model predictions. The experimentally extracted normalised mutual coupling coefficients, $Δ_{13}=(10.8\pm0.3)\times10^{-6}$ and $Δ_{23}=(8.6\pm0.2)\times10^{-6}$, fall within the calculated range $Δ_{n3}\approx(7\text{--}17)\times10^{-6}$ derived from the foil's electromagnetic properties, where the spread is dominated by the estimated foil thickness uncertainty of $(8.9\pm0.4)\,μ\mathrm{m}$. These results demonstrate a foil-mediated physical implementation of weak mutual coupling across a metallic interface spanning multiple skin depths, providing a distinct route for engineering controlled interference in three-dimensional multi-resonator systems.

physics.app-ph↗

arXiv2026

UPLOAD-HELIX: High-Helicity Single-Mode Microwave Haloscope with Low-Noise Interferometric Readout for Ultralight Axion Dark Matter

We propose a superconducting single-mode microwave haloscope based on chiral cavity resonators for the detection of ultralight dark matter axions over the mass range $4\times10^{-19}$-$~4\times10^{-14}\,\mathrm{eV}$. Building on the single-mode chiral-cavity concept introduced by Bourhill et al. [Phys. Rev. D 108, 052014 (2023); arXiv:2208.01640], we develop a resonator geometry compatible with subtractive manufacturing from high-purity bulk niobium, taking advantage of the substantially lower surface resistance achievable relative to the additively manufactured Möbius cavity proposed in the earlier work. An inverse-design framework is then used to maximise a figure of merit derived to minimise the measurement time required to achieve a fixed experimental sensitivity. The resulting optimised bulk-niobium design achieves a figure of merit more than three orders of magnitude larger than the additively manufactured Mobius benchmark. An experimentally informed microwave interferometric readout model, incorporating measured electronics noise and active suppression of pump amplitude noise, is used to project the sensitivity of the proposed experiment. For an acquisition time of three months, the haloscope is projected to reach $g_{aγγ}<10^{-11}\,\mathrm{GeV}^{-1}$ across more than four orders of magnitude in axion mass. The projected sensitivity extends approximately one order of magnitude below the current exclusion limits set by CAST, providing a practical pathway towards a high-sensitivity direct search for ultralight dark matter axions.

physics.ins-det↗

arXiv2026

Split-Post Re-entrant Microwave Displacement Transducer with Quadratic Readout

We investigate a microwave-cavity-based displacement readout employing a split-post geometry to measure the motion of a dielectric membrane. Due to symmetry, the cavity response to membrane displacement is inherently quadratic when the membrane is positioned at the centre of the posts. We characterise this behaviour by driving the membrane with a piezoelectric actuator at both central and off-centre positions and we estimate the drive-to-displacement transfer function using the independently calibrated frequency-to-voltage response of the interferometric readout. When the membrane is located at the centre of the cavity and driven, the system exhibits the largest quadratic output, measured at the second harmonic of the membrane acoustic frequency. As the membrane is moved away from the centre, the response transitions from predominantly quadratic to predominantly a linear response at the membrane acoustic frequency. Quadratic optomechanical coupling is a key requirement for displacement - squared readout and, in the quantum regime, for measurements sensitive to mechanical energy or phonon number. The present work therefore establishes the split-post geometry as a promising platform for microwave-mechanical transduction, providing a practical route toward future experiments aimed at probing quantised mechanical motion and energy-sensitive readout schemes.

physics.ins-det↗

arXiv2025

Observation of a phase transition in KTaO$_3$ induced by residual niobium impurities

We report the observation of a phase transition in a KTaO$_3$ crystal, corresponding to a paraelectric-to-ferroelectric transition. The crystal was placed inside a copper cavity to form a dielectric-loaded microwave cavity, and the transition was observed to occur near 134 K. As the cavity was cooled, the frequencies of both transverse electric and transverse magnetic resonant modes decreased (corresponding to an increase in permittivity). The mode frequencies converge at the transition temperature (near 134 K) and, below this point, reverse their tuning direction, increasing their frequency with decreasing temperature. This behaviour corresponds to a decrease in dielectric permittivity and is atypical for pure KTaO$_3$. To investigate further, we conducted impurity analysis using Laser Ablation inductively coupled mass spectrometry (LA-ICPMS), revealing a significant concentration ($\sim$ 7\%) of niobium (Nb) in the crystal. This suggests that the observed phase transition is driven by residual Nb impurities, which induce ferroelectricity in an otherwise paraelectric host. Similar crystals with a lower concentration ($<$ 2\%) did not undergo a phase transition but exhibited a loss peak at this temperature. These findings have practical implications for the design of tunable devices, for example, resonator-based dark matter detectors, where low-loss material phase stability and tunability are crucial.

cond-mat.mtrl-sci↗

arXiv2024

Microwave Cavity Mode Optimisation by Background Anti-Resonance Tuning

To derive the best oscillator phase noise when implementing a high-Q resonator, the spectral line-shape must have high contrast and symmetry. Ideally, this line-shape is Lorentzian, however, in a high mode density spectral region, low-Q background spurious modes interact and distort the resonance. For a sapphire-loaded cavity resonator operating with whispering gallery modes we show that this high contrast and symmetry can be achieved by changing the dimensions of the surrounding cavity shield to tune the background low-Q structures into anti-resonance. This works because the high-Q resonances are primarily defined by the sapphire while the background modes are defined by the cavity shield. Alternatively, it was shown that a similar result can be achieved by exciting the high-Q resonator with a balanced microwave dipole probe in a Mach Zehnder interferometric configuration. The probe was constructed from two separate coaxial electric field probes symmetrically inserted into a cylindrical cavity resonator, from opposite sides with a small gap between them, so they can behave like an active wire dipole antenna. The power into the two separate probes may be matched with an external variable attenuator in one of the arms of the interferometer. Conversely, the phase between the two electric field probes may be changed with an external variable phase shifter, which changes the nature of the field components the probe couples to. The probe couples to the high-Q resonant modes as well as low-Q background modes, which can be made resonant or anti-resonant for the high-Q modes by changing this external phase. When the background modes are in anti-resonance the line shape of the high-Q mode can be made symmetric and with higher contrast. This technique was applied to both whispering gallery sapphire modes, as well as hollow cavity resonators, without changing the dimensions of the cavity.

physics.app-ph↗

arXiv2024

Conductivity Freeze-Out in Isotopically Pure Si-28 at milli-Kelvin Temperatures

Silicon is a key semiconducting material for electrical devices and hybrid quantum systems where low temperatures and zero-spin isotopic purity can enhance quantum coherence. Electrical conductivity in Si is characterised by carrier freeze out at around 40 K allowing microwave transmission which is a key component for addressing spins efficiently in silicon quantum technologies. In this work, we report an additional sharp transition of the electrical conductivity in a Si-28 cylindrical cavity at around 1 Kelvin. This is observed by measuring microwave resonator Whispering Gallery Mode frequencies and Q factors with changing temperature and comparing these results with finite element models. We attribute this change to a transition from a relaxation mechanism-dominated to a resonant phonon-less absorption-dominated hopping conduction regime. Characterising this regime change represents a deeper understanding of a physical phenomenon in a material of high interest to the quantum technology and semiconductor device community and the impact of these results is discussed.

cond-mat.mtrl-sci↗

arXiv2022

Comparing Instrument Spectral Sensitivity of Dissimilar Electromagnetic Haloscopes to Axion Dark Matter and High Frequency Gravitational Waves

It is known that haloscopes that search for dark matter axions via the axion-photon anomaly are also sensitive to gravitational radiation through the inverse Gertsenshtein effect. Recently this way of searching for high frequency gravitational waves has gained momentum as it has been shown that the strain sensitivities of such detectors are of the same order of sensitivity to the axion-photon theta angle. Thus, after calculating the sensitivity of a haloscope to an axion signal, we also have calculated the order of magnitude sensitivity to a gravitational wave signal of the same spectral and temporal form. However, it is unlikely that a gravitational wave and an axion signal will be of the same form, since physically the way the signals are generated are completely different. For gravitational wave detection, the spectral strain sensitivity is in units strain per square root Hz, is the natural way to compare the sensitivity of gravitational wave detectors due to its independence on the gravitational wave signal. In this work, we introduce a systematic way to calculate the spectral sensitivity of an axion haloscope, so instrument comparison may be achieved independent of signal assumptions and only depends on the axion to signal transduction sensitivity and noise in the instrument. Thus, the calculation of the spectral sensitivity not only allows the comparison of dissimilar axion detectors independent of signal, but also allows us to compare the order of magnitude gravitational wave sensitivity in terms of spectral strain sensitivity, allowing comparisons to standard gravitational wave detectors based on optical interferometers and resonant-mass technology.

physics.ins-det↗