Direct terrestrial test of Lorentz symmetry in electrodynamics to 10(-18)

Acoustic, Phononic, Brillouin Light Scattering and Faraday Wave-Based Frequency Combs: Physical Foundations and Applications

Frequency combs (FCs)-spectra containing equidistant coherent peaks-have enabled researchers and engineers to measure the frequencies of complex signals with high precision, thereby revolutionising the areas of sensing, metrology and communications and also benefiting the fundamental science. Although mostly optical FCs have found widespread applications thus far, in general FCs can be generated using waves other than light. Here, we review and summarise recent achievements in the emergent field of acoustic frequency combs (AFCs), including phononic FCs and relevant acousto-optical, Brillouin light scattering and Faraday wave-based techniques that have enabled the development of phonon lasers, quantum computers and advanced vibration sensors. In particular, our discussion is centred around potential applications of AFCs in precision measurements in various physical, chemical and biological systems in conditions where using light, and hence optical FCs, faces technical and fundamental limitations, which is, for example, the case in underwater distance measurements and biomedical imaging applications. This review article will also be of interest to readers seeking a discussion of specific theoretical aspects of different classes of AFCs. To that end, we support the mainstream discussion by the results of our original analysis and numerical simulations that can be used to design the spectra of AFCs generated using oscillations of gas bubbles in liquids, vibrations of liquid drops and plasmonic enhancement of Brillouin light scattering in metal nanostructures. We also discuss the application of non-toxic room-temperature liquid-metal alloys in the field of AFC generation.

Upconversion Loop Oscillator Axion Detection Experiment: A Precision Frequency Interferometric Axion Dark Matter Search with a Cylindrical Microwave Cavity

First experimental results from a room-temperature tabletop phase-sensitive axion haloscope experiment are presented. The technique exploits the axion-photon coupling between two photonic resonator oscillators excited in a single cavity, allowing low-mass axions to be upconverted to microwave frequencies, acting as a source of frequency modulation on the microwave carriers. This new pathway to axion detection has certain advantages over the traditional haloscope method, particularly in targeting axions below 1  μeV (240 MHz) in energy. At the heart of the dual-mode oscillator, a tunable cylindrical microwave cavity supports a pair of orthogonally polarized modes (TM_{0,2,0} and TE_{0,1,1}), which, in general, enables simultaneous sensitivity to axions with masses corresponding to the sum and difference of the microwave frequencies. However, in the reported experiment, the configuration was such that the sum frequency sensitivity was suppressed, while the difference frequency sensitivity was enhanced. The results place axion exclusion limits between 7.44-19.38 neV, excluding a minimal coupling strength above 5×10^{-7}  1/GeV, after a measurement period of two and a half hours. We show that a state-of-the-art frequency-stabilized cryogenic implementation of this technique, ambitious but realizable, may achieve the best limits in a vast range of axion space.

Editorial Commentary: Radiographic Measurements of Knee Joint Space Are Inadequate for Demonstrating Chondral Restoration

Orthopaedic advancements into the 21st century will increasingly focus on chondral restoration to either halt or reverse degenerative processes. Researchers and clinicians will need tools beyond patient-reported outcomes to measure the effectiveness of these treatment efforts. The use of joint space width (JSW) as a surrogate for chondral restoration is inadequate. At a minimum, such observations must standardize load transmission across the joint to be useful. Simple, readily available, standardized, and clinically useful measures of knee chondral restoration would facilitate and accelerate advances in the field. For now, it may be that improvement in JSW after chondral restoration could be attributable to changes in mechanical alignment of the knee and not the chondral restoration. JSW is an inadequate surrogate for chondral restoration, and anyone doing a stress radiograph of a unicompartmental degenerative knee recognizes this point.

A simplified cryogenic optical resonator apparatus providing ultra-low frequency drift

A system providing an optical frequency with instability comparable to that of a hydrogen maser is presented. It consists of a 5 cm long, vertically oriented silicon optical resonator operated at temperatures between 1.5 K and 3.6 K in a closed-cycle cryostat with a low-temperature Joule-Thomson stage. We show that with a standard cryostat, a simple cryogenic optomechanical setup, and no active or passive vibration isolation, a minimum frequency instability of 2.5 × 10^-15 at τ = 1500 s integration time can be reached. The influence of pulse-tube vibrations was minimized by using a resonator designed for low acceleration sensitivity. With reduced optical laser power and interrogation duty cycle, an ultra-low fractional frequency drift of -2.6 × 10^-19/s is reached. At 3.5 K, the resonator frequency exhibits a vanishing thermal sensitivity and an ultra-small temperature derivative 8.5 × 10^-12/K². These are favorable properties that should lead to high performance also in simpler cryostats not equipped with a Joule-Thomson stage.

Spectral Purification of Microwave Signals with Disciplined Dissipative Kerr Solitons

Continuous-wave-driven Kerr nonlinear microresonators give rise to self-organization in terms of dissipative Kerr solitons, which constitute optical frequency combs that can be used to generate low-noise microwave signals. Here, by applying either amplitude or phase modulation to the driving laser we create an intracavity potential trap to discipline the repetition rate of the solitons. We demonstrate that this effect gives rise to a novel spectral purification mechanism of the external microwave signal frequency, leading to reduced phase noise of the output signal. We experimentally observe that the microwave signal generated from disciplined solitons is injection locked by the external drive at long timescales, but exhibits an unexpected suppression of the fast timing jitter. Counterintuitively, this filtering takes place for frequencies that are substantially lower than the cavity decay rate. As a result, while the long timescale stability of the Kerr frequency comb's repetition rate is improved by more than 4 orders of magnitude, the purified microwave signal shows a reduction of the phase noise by 30 dB at offset frequencies above 10 kHz.

Next Generation of Phonon Tests of Lorentz Invariance Using Quartz BAW Resonators

We demonstrate technological improvements in phonon sector tests of the Lorentz invariance that implement quartz bulk acoustic wave oscillators. In this experiment, room temperature oscillators with state-of-the-art phase noise are continuously compared on a platform that rotates at a rate of order of a cycle per second. The discussion is focused on improvements in noise measurement techniques, data acquisition, and data processing. Preliminary results of the second generation of such tests are given, and indicate that standard model extension coefficients in the matter sector can be measured at a precision of order 10^-16 GeV after taking a year's worth of data. This is equivalent to an improvement of two orders of magnitude over the prior acoustic phonon sector experiment.

Determination of Low Loss in Isotopically Pure Single Crystal 28Si at Low Temperatures and Single Microwave Photon Energy

The low dielectric losses of an isotopically pure single crystal ²⁸Si sample were determined at a temperature of 20 mK and at powers equivalent to that of a single photon. Whispering Gallery Mode (WGM) analysis revealed large Quality Factors of order 2 × 10⁶ (dielectric loss ~5 × 10⁻⁷) at high powers, degrading to 7 × 10⁻⁵ (dielectric loss ~1.4 × 10⁻⁶ at single photon energy. A very low-loss narrow line width paramagnetic spin flip transition was detected with extreme sensitivity in ²⁸Si, with very small concentration below 10¹⁰ cm⁻³ (less than 10 parts per trillion) and g-factor of 1.995 ± 0.008. Such determination was only possible due to the low dielectric photonic losses combined with the long lifetime of the spin transition (low magnetic loss), which enhances the magnetic AC susceptibility. Such low photonic loss at single photon energy combined with the narrow line width of the spin ensemble, indicate that single crystal ²⁸Si could be an important crystal for future cavity QED experiments.

Naked eye picometer resolution in a Michelson interferometer using conjugated twisted beams

Michelson interferometry is one of the most widely used techniques for accuracy measurements. Its main characteristic feature is to infer a displacement in one of the arms of the interferometer from a phase measurement. Two different twisted beams, also called vortex beams, with opposite twisted rotations in each arm of the interferometer interfere in a daisy flower-like pattern. The number of petals is twice the topological charge. Their position depends on the relative phase of the beams. Naked eye detection of 44 pm displacements is achieved. The sensitivity of such an interferometer together with possible further improvements, and applications are then discussed.