This investigation employed microwave whispering-gallery mode (WGM) analysis to characterize the dielectric properties of a cylindrical, single-crystal sample of calcium tungstate (CaWO4). Through investigation of quasi-transverse-magnetic and quasi-transverse-electric mode families, we can assess loss mechanisms and relative permittivity from room temperature to cryogenic conditions. We report the biaxial permittivity values of epsilon||295 K = 9.029 f 0.009 and epsilon 295 K ture, and epsilon 4 K || = 8.794 f 0.009 and epsilon 4 perpendicular to K = 10.440 f 0.010 at liquid-helium temperature. Components are denoted with respect to the c-axis of the crystal unit cell. The parallel component agrees well with the published literature at megahertz frequencies; however, the measured perpendicular component is 4.8% lower. The WGM technique offers greater precision, with accuracy limited primarily by the uncertainty in the crystal's dimensions. WGMs also serve as sensitive probes of lattice dynamics, enabling the monitoring of temperature-dependent loss mechanisms. At room temperature, the measured loss tangents were || = (4.1 f 1.4) & times; 10-5 and tan 8295 K perpendicular to = (3.64 f 0.92) & times; 10-5. Upon cooling to 4 K, the loss tangents improved by approximately two orders of magnitude, reaching tan 8||4 K = (1.56 f 0.52) & times; 10-7 and perpendicular to = (2.05 f 0.79) & times; 10-7. These cryogenic values are higher than those reported in prior studies, likely due to a magnetic loss channel associated with an unidentified paramagnetic spin ensemble. These findings have implications for the use of CaWO4 in applications such as spin-based quantum systems and cryogenic bolometry.

We propose a superconducting single-mode microwave haloscope based on helical cavity resonators for the detection of ultralight dark matter axions over the mass range 10^-18 to 10^-13 eV. Building on the single-mode helical-cavity concept introduced by Bourhill et al. [Phys. Rev. D 108, 052014 (2023); arXiv:2208.01640], we use an inverse-design framework to develop practical resonator geometries compatible with superconducting niobium fabrication. The optimisation employs a figure of merit derived to minimise the measurement time required to achieve a fixed experimental sensitivity. Relative to the heuristic Möbius-geometry benchmark, the best subtractively manufacturable bulk-niobium design achieves a figure of merit more than three orders of magnitude larger. 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 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.

A recent independent dark photon (DP) focused reanalysis of existing data from the TASEH axion haloscope experiment reported a tentative DP dark matter signal with local significance ∼ 4.7 σ at a frequency f ≈ 4.71 GHz , corresponding to m X ≈ 19.5 μ eV and kinetic mixing ϵ ∼ 6.5 × 10 − 15 . Motivated by this report, we performed a dedicated, narrowband follow-up experiment to confirm or refute the signal with a cryogenic microwave cavity operated a magnetic field, leveraging the ORGAN-Q dilution refrigeration and receiver chain. Scanning a window centered on the reported frequency over a live time of T int ∼ 13 days , we find no excess consistent with a dark photon signal as reported, and set a 95% CL of ϵ ≤ 5.04 × 10 − 15 in a narrow mass range around ∼ 19.5 μ eV , excluding a signal of the strength and frequency reported to 99.92% confidence. We discuss the experiment and present the exclusion limits.

The Axion Dark Matter eXperiment is sensitive to narrow axion flows, given axions comprise a fraction of the dark matter with a non-negligible local density. Detecting these low-velocity dispersion flows requires a high spectral resolution and careful attention to the expected signal modulation due to Earth's motion. We report an exclusion on the local axion dark matter density in narrow flows of rho(a) greater than or similar to 0.03 GeV/cm(3) and rho(a) greater than or similar to 0.004 GeV=cm(3) for Dine-Fischler-Srednicki-Zhitnitski and Kim-Shifman-Vainshtein-Zakharov axion-photon couplings, respectively, over the mass range 3.3 - 4.2 mu eV. Measurements were made at selected resolving powers to allow for a range of possible velocity dispersions.

We report a novel X-band loop oscillator, whose resonator is formed from two optimally tuned commercial yttrium iron garnet (YIG) filters. We call this a Lambda -resonator, owing to the characteristic Lambda -shape of its amplitude transfer function (ATF). At offset frequencies greater than 70 Hz, oscillator phase noise is limited by the contribution from the amplifier, as described by Leeson's model. The effective Q-factor of the Lambda -resonator (2.6x10(3)) is determined by the slope of its phase transfer function on resonance, which is directly proportional to the group delay of the filters. With off-the-shelf components, we achieve single-sideband (SSB) phase noise of -151 dBc/Hz at 1 MHz offset for an oscillation frequency of 9.2 GHz. This work points to the potential for improvements with custom built filters optimized for high group delay. The high selectivity of the Lambda -resonator (made of two seventh-order YIG-filters) allowed a strong suppression of both phase and amplitude fluctuations of the transmitted signal at Fourier frequencies outside the resonator's bandwidth. We also investigated relevant oscillator noise sources, including power-to-frequency conversion as a function of Fourier frequency, and use a novel, highly sensitive technique to characterize the noise.

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 (∼ 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.

The latest result from the ORGAN experiment, an axion haloscope, is presented. This iteration of the experiment operated at millikelvin temperatures using a flux-driven Josephson parametric amplifier for reduced noise, along with various other improvements over previous iterations. Covering the 25.45-26.27 mu eV (6.15-6.35 GHz) mass (frequency) range, this near-quantum-limited phase of ORGAN employs a conducting rod resonator and a 7-T solenoidal magnet to place the most sensitive exclusion limits on axion-photon coupling in the range to date, with ga gamma gamma greater than or similar to 2.8 x 10-13 at a 95% confidence level.

We report high-resolution measurements of thermal fluctuations in microwave and mechanical resonators using a dual-channel readout system. The latter comprises a low-noise amplifier, an I/Q-mixer, and a cross-correlator. We discovered that, under certain conditions, the intrinsic fluctuations of the low-noise amplifier, which are common to both channels of the readout system, are averaged out when computing the voltage noise cross-spectrum between the mixer's outputs. The suppression of the amplifier's technical fluctuations significantly improves the contrast of the thermal noise peaks exhibited by the resonators. Thus, for the room-temperature-stabilized 9 GHz sapphire-loaded cavity resonator, we observed more than 16 dB improvement in the thermal noise peak contrast relative to the single-channel measurements. The ability of the dual-channel readout system to discriminate between the broad- and narrow-band fluctuations may benefit the search for dark matter, which relies on the use of cryogenic microwave resonators.

A low-noise cryogenic microwave spectroscopy experiment was performed on a high-purity lithium fluoride (LiF) crystal. The spectroscopy data revealed avoided level crossing interactions in whispering gallery modes, indicative of electron spin resonance (ESR) coupling with paramagnetic impurities. Analysis of the interaction spectra identified distinct spin systems corresponding to (S = 3/2, I = 7/2), (S = 1, I = 7/2), and (S = 3/2, I = 0). The number of hyperfine splittings observed, together with the natural abundance of ions possessing the appropriate nuclear spin values, suggest that V^2+ and V^3+ impurities, exhibiting orthorhombic distortion, are the most likely sources of the narrow interaction features. This interpretation is supported by earlier ESR studies and established manufacturing records for LiF crystal growth. Additionally, a separate set of broader interaction points is consistent with an orthorhombic model involving a (S = 3/2, I = 0) spin system, although the specific impurity responsible for this interaction remains unidentified.

Axions are a well-motivated candidate for dark matter. The preeminent method to search for axion dark matter is known as the axion haloscope, which makes use of the conversion of axions to photons in a large magnetic field. Because of the weak coupling of axions to photons, however, the expected signal strength is exceptionally small. To increase signal strength, many haloscopes make use of resonant enhancement and high gain amplifiers, while also taking measures to keep receiver noise as low as possible such as the use of dilution refrigerators and ultra-low-noise electronics. In this paper, we derive the theoretical noise model based on the sources of noise found within a typical axion haloscope receiver chain, using the Axion Dark Matter eXperiment (ADMX) as a case study. We present examples of different noise calibration measurements at 1280 MHz taken during ADMX's most recent data-taking run. These new results shed light on a previously unidentified interaction between the cavity and Josephson Parametric Amplifier as well as provide a better understanding of the systematic uncertainty on the system noise temperature used in the axion search analysis for this data-taking run. Finally, the consistency between the measurements and the detailed model provide suggestions for future improvements within ADMX and other axion haloscopes to reach a lower noise temperature.

Coupling microwave cavity modes with spin qubit transitions is crucial for enabling efficient qubit readout and control, long-distance qubit coupling, quantum memory implementation, and entanglement generation. We experimentally observe the coupling of different spin qubit transitions in Silicon Carbide (SiC) material to a 3D microwave (MW resonator mode around 12.6 GHz at a temperature of 10 mK. Tuning the spin resonances across the cavity resonance via magnetic-field sweeps, we perform MW cavity transmission measurements. We observe spin transitions of different spin defects that are detuned from each other by around 60-70 MHz. By optically exciting the SiC sample placed in the MW cavity with an 810 nm laser, we observe the coupling of an additional spin resonance to the MW cavity, also detuned by around 60-70 MHz from the centre resonance. We perform complementary confocal optical spectroscopy as a function of temperature from 4 K to 200 K. Combining the confocal spectroscopy results and a detailed analysis of the MW-resonator-based experiments, we attribute the spin resonances to three different paramagnetic defects: positively-charged carbon antisite vacancy pair (CAV^+), and the negatively-charged silicon vacancy spins located at two different lattice sites, namely V_1 and V_2 spins. The V_1 and V_2 lines in SiC are interesting qubit transitions since they are known to be robust to decoherence. Additionally, the CAV^+-transition is known to be a bright single-photon source. Consequently, the demonstration of the joint coupling of these spin qubits to a MW cavity mode could lead to interesting new modalities: The microwave cavity could act as an information bus and mediate long-range coupling between the spins, with potential applications in quantum computing and quantum communication, which is an attractive proposition in a CMOS-compatible material such as SiC.

The first direct measurement of gravitational waves by the LIGO and Virgo collaborations has opened up new avenues to explore our Universe. This white paper outlines the challenges and gains expected in gravitational-wave searches at frequencies above the LIGO/Virgo band. The scarcity of possible astrophysical sources in most of this frequency range provides a unique opportunity to discover physics beyond the Standard Model operating both in the early and late Universe, and we highlight some of the most promising of these sources. We review several detector concepts that have been proposed to take up this challenge, and compare their expected sensitivity with the signal strength predicted in various models. This report is the summary of a series of workshops on the topic of high-frequency gravitational wave detection, held in 2019 (ICTP, Trieste, Italy), 2021 (online) and 2023 (CERN, Geneva, Switzerland).

A cylindrical TM0,1,0 mode microwave cavity resonator was excited using a balanced interferometric configuration that allowed manipulation of the electric field and potential within the resonator by adjusting the phase and amplitude of the interferometer arms driving the resonator. With precise tuning of the phase and amplitude, 25-dB suppression of the electric field at the resonance frequency was achieved while simultaneously resonantly enhancing the time-varying electric-scalar potential. Under these conditions, the system demonstrated electromagnetically induced absorption in the cavity response due to the annulment of the electric field at the resonance frequency. This phenomena can be regarded as a form of extreme dispersion, and led to a sharp increase in the cavity phase versus frequency response by an order of magnitude when compared to the cavity Q factor. This work presents an experimental setup that will allow the electric-scalar Aharonov-Bohm effect to be tested under conditions involving a time-varying electricscalar potential, without the presence of an electric field or magnetic vector potential, an experiment that has not yet been realized.

We report the observation of transient nonlinear optical effects in a macroscopic whispering gallery mode resonator made of rutile TiO_2, demonstrating strong optical-microwave transduction under laser irradiation. By comparing the effects of ultraviolet (UV, 385 nm) and near-infrared (NIR, 700 nm) radiation, we find that the UV-induced effects are significantly amplified, consistent with the material's semiconductor bandgap energy. The interaction results in frequency shifts of microwave modes and changes in quality factor, suggesting a localized saturable refractive index tuning. This may be attributed to the saturation of a spin transition of a dopant ion within the crystal lattice. Remarkably, these effects are observed at low optical powers, down to nanowatt levels, indicating high sensitivity and efficient of light-matter interaction in this system. The phenomenon is centered around 15 GHz, yet electron spin resonance measurements reveal no zero-field splitting at this frequency, suggesting an alternative mechanism beyond conventional spin resonance. These findings highlight the potential of low-power optical control of microwave modes in high-Q resonators for applications in quantum technologies, sensing, and reconfigurable photonic-microwave systems.

We investigated gram-scale macroscopic bulk acoustic wave (BAW) resonators manufactured from plates of piezoelectric lithium niobate. The intrinsic competing loss mechanisms were studied at cryogenic temperature through precision measurements of various BAW modes. Exceptional quality factors were measured for the longitudinal BAW modes in the 1-100 MHz range, with a maximum quality factor of 8.9 million, corresponding to a quality factor x frequency product of 3.8 x 1014 Hz. Through measurements of the acoustic response to a strong drive tone, anomalous self-induced absorption and transparency effects are observed. We show that such observations can be explained by microscopic impurities and defect sites in the crystal bulk by the use of a nonlinear model of acoustic dissipation. The losses associated with these defects provide the ultimate limit of resonator performance, which could be improved in the future if more pure samples were available.

We propose a multi-mode bar consisting of mass elements of decreasing size for the implementation of a gravitational version of the photo-electric effect through the stimulated absorption of up to kHz gravitons from a binary neutron star merger and post-merger. We find that the multi-mode detector has normal modes that retain the coupling strength to the gravitational wave of the largest mass-element, while only having an effective mass comparable to the mass of the smallest element. This allows the normal modes to have graviton absorption rates due to the tonne-scale largest mass, while the single graviton absorption process in the normal mode could be resolved through energy measurements of a mass-element in-principle smaller than pico-gram scale. We argue the feasibility of directly counting gravito-phonons in the bar through energy measurements of the end mass. This improves the transduction of the single-graviton signal, enhancing the feasibility of detecting single gravitons.

To derive the best oscillator phase noise when implementing a high-Q Q resonator, the resonant spectral line shape must have high contrast and symmetry. Ideally, this line shape is second order and Lorentzian; however, in a high-mode-density spectral region, low-Q Q background spurious modes interact and distort the resonance. For a sapphire-loaded cavity resonator operating with whispering-gallery modes confined within the sapphire crystal, we show that this high contrast and symmetry can be achieved by meticulously changing the dimensions of the surrounding metallic cavity shield to tune the background low-Q Q structures into antiresonance. This works because the high-Q Q resonances are primarily defined by the sapphire, while the background modes are defined by the cavity shield. Alternatively, it has been shown that a similar result can be achieved by exciting the high-Q Q resonator with a balanced microwave dipole probe in a MachZehnder interferometric configuration. The probe has been 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 to which the probe couples. The probe couples to the high-Q Q resonant modes as well as low-Q Q background modes, which can be made resonant or antiresonant with respect to the high-Q Q modes by changing this external phase. When the background modes are in antiresonance, the line shape of the high-Q Q mode can be made symmetric and with higher contrast. This technique has been applied to both whispering-gallery sapphire modes, as well as hollow- cavity resonators, without changing the dimensions of the cavity.

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.

We report the results of a QCD axion dark matter search with discovery ability for Dine Fischler Srednicki Zhitnitsky (DFSZ) axions using an axion haloscope. Sub-Kelvin noise temperatures are reached with an ultra low-noise Josephson parametric amplifier cooled by a dilution refrigerator. This work excludes (with a 90 3.34 μeV, assuming a standard halo model with a local energy density of 0.45 GeV/cm^3 made up 100

Axions are a compelling dark matter candidate, and one of the primary techniques employed to search for them is the axion haloscope, in which a resonant cavity is deployed inside a strong magnetic field so that some of the surrounding axions may convert into photons via the inverse Primakoff effect and become trapped inside the resonator. Resonant cavity design is critical to the sensitivity of a haloscope, and several geometries have been utilised and proposed. Here we consider a relatively simple concept - a rectangular resonant cavity with a tunable wall - and compare it to the standard tuning rod-type resonators employed in the field. We find that the rectangular cavities support similar modes to cylindrical tuning rod cavities, and have some advantages in terms of axion sensitivity and practicality, particularly when moving to higher frequencies which are of great and growing interest in the international axion dark matter community.