Search

The coupling between acoustic vibrations in a lithium niobate bulk acoustic wave resonator and microwave photons of a re-entrant microwave cavity was investigated at a temperature close to 4 K. Coupling was achieved by placing the acoustic resonator in the location of the re-entrant cavity electric field maxima, in a symmetric ``split-post'' configuration, with a large overlap between the microwave field and the acoustic mode, allowing acoustic modulations of the microwave frequency. We show that the acoustic modes in this setup retain large inherent quality factors of greater than $10^6$. A maximum optomechanical coupling rate was determined to be $g_0$ = 0.014 mHz, four orders of magnitude larger than previous results obtained using a quartz BAW at 4 K in a similar experimental setup, but using a single post-re-entrant cavity resonator.

The Axion Dark Matter eXperiment is sensitive to narrow axion flows, given axions compose 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 $\rho_a \gtrsim 0.03\,\mathrm{GeV/cm^3}$ and $\rho_a \gtrsim 0.004\,\mathrm{GeV/cm^3}$ for Dine-Fischler-Srednicki-Zhitnitski and Kim-Shifman-Vainshtein-Zakharov axion-photon couplings, respectively, over the mass range $3.3-4.2\,\mu\text{eV}$. Measurements were made at selected resolving powers to allow for a range of possible velocity dispersions., Comment: 7 pages, 3 figures

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% confidence level) DFSZ axions with masses between 3.27 to 3.34 {\mu}eV, assuming a standard halo model with a local energy density of 0.45 GeV/cc made up 100% of axions.

The impurity paramagnetic ion, $Cu^{2+}$ substitutes $Al$ in the $SrLaAlO_4$ single crystal lattice, this results in a $CuO_6$ elongated octahedron, the resulting measured g-factors shows four-fold axes variation condition. The aggregate frequency width of the electron spin resonance with the required minimum level of impurity concentration has been evaluated in single crystal $SrLaAlO_4$ at $20$ millikelvin. Measured parallel hyperfine constants, $A_{\scriptscriptstyle\parallel Cu}$, were determined to be $-155.7\times10^{-4}~cm^{-1},~ -163.0\times10^{-4}~cm^{-1},~ -178.3\times10^{-4}~cm^{-1} $ and$~-211.1\times10^{-4}~cm^{-1}$ at $9.072~GHz~(WGH_{4,1,1})$ for the nuclear magnetic quantum number $M_I=+\frac{3}{2},+\frac{1}{2},-\frac{1}{2}$,~and$-\frac{3}{2}$ respectively. The anisotropy of the hyperfine structure reveals a characteristics of static Jahn-Teller effect. The second-order-anisotropy-term, $\sim (\frac{spin-orbit~coupling}{10D_q})^2$, is significant and can not be disregarded, with the local strain dominating over the observed Zeeman-anisotropy-energy difference. The Bohr electron magneton, $\beta=9.23\times 10^{-24} JT^{-1}$, (within $-0.43\%$ so-called experimental error) has been found using the measured spin-Hamiltonian parameters. Measured nuclear dipolar hyperfine structure parameter $P_{\scriptscriptstyle\parallel}=12.3\times10^{-4}~cm^{-1}$ shows that the mean inverse third power of the electron distance from the nucleus is $\langle r^{-3}_q\rangle\simeq 5.23$ a.u. for $Cu^{2+}$ ion in the substituted $Al^{3+}$ ion site assuming nuclear electric quadruple moment $Q=-0.211$ barn.

Impurity $Fe^{3+}$ ion electron spin resonance (ESR) spectroscopy using multiple dielectric modes in a $SrTiO_3$ dielectric resonator has been performed with a tuneable DC magnetic field of up to $1.6~T$. The $Ti^{4+}(d^0)$ ion is substituted by $Fe^{3+}$ ion forming $FeO_6$ octahedral complex with an iron-oxygen-vacancy $(Fe^{3+}-V_O)$. In such a metal-ligand complex, a giant g-factor of $g_{\scriptscriptstyle\parallel F} = 5.51$ was observed in the ferroelectric phase at $20~mK$. The the change of $Fe^{3+}$ ion center-symmetry in the $FeO_6$ complex as a soft-mode characteristics of ferroelectric phase transition and the influences of iron-oxygen-vacancy $(Fe^{3+}-V_O)$, interactively sensitive to asymmetry in the octahedral rotational parameter $\Phi$ in $SrTi0_3$.

The Axion Dark Matter eXperiment (ADMX) has previously excluded Dine-Fischler-Srednicki-Zhitnisky (DFSZ) axions between 680-790 MHz under the assumption that the dark matter is described by the isothermal halo model. However, the precise nature of the velocity distribution of dark matter is still unknown, and alternative models have been proposed. We report the results of a non-virialized axion search over the mass range 2.81-3.31 {\mu}eV, corresponding to the frequency range 680-800 MHz. This analysis marks the most sensitive search for non-virialized axions sensitive to Doppler effects in the Milky Way Halo to date. Accounting for frequency shifts due to the detector's motion through the Galaxy, we exclude cold flow relic axions with a velocity dispersion of order 10^-7 c with 95% confidence.

We report the first result of a direct search for a Cosmic ${\it axion}$ Background (C$a$B) - a relativistic background of axions that is not dark matter - performed with the axion haloscope, the Axion Dark Matter eXperiment (ADMX). Conventional haloscope analyses search for a signal with a narrow bandwidth, as predicted for dark matter, whereas the C$a$B will be broad. We introduce a novel analysis strategy, which searches for a C$a$B induced daily modulation in the power measured by the haloscope. Using this, we repurpose data collected to search for dark matter to set a limit on the axion photon coupling of a C$a$B originating from dark matter cascade decay via a mediator in the 800-995 MHz frequency range. We find that the present sensitivity is limited by fluctuations in the cavity readout as the instrument scans across dark matter masses. Nevertheless, we suggest that these challenges can be surmounted using superconducting qubits as single photon counters, and allow ADMX to operate as a telescope searching for axions emerging from the decay of dark matter. The daily modulation analysis technique we introduce can be deployed for various broadband RF signals, such as other forms of a C$a$B or even high-frequency gravitational waves., Comment: 9 pages, 4 figures

We investigate reentrant and dielectric loaded cavities for the purpose of extending the range of axion cavity haloscopes to lower masses, below the range where the Axion Dark Matter eXperiment (ADMX) has already searched. Reentrant and dielectric loaded cavities were simulated numerically to calculate and optimize their form factors and quality factors. A prototype reentrant cavity was built and its measured properties were compared with the simulations. We estimate the sensitivity of axion dark matter searches using reentrant and dielectric loaded cavities inserted in the existing ADMX magnet at the University of Washington and a large magnet being installed at Fermilab., Comment: 33 pages, 24 figures

M\"obius-ring resonators stem from a well-studied and fascinating geometrical structure that features a one-sided topology; the M\"obius strip, and have been shown to exhibit fermion rotational symmetry with respect to a ring resonator with no twist (which exhibits boson rotational symmetry) (see PhysRevLett.101.247701). Here, we present a new type of resonator through the formation of twisted hollow structures using equilateral triangular cross-sections, which leads to the realization of a cavity with anyon rotational symmetry. Unlike all previous cavity resonators, the anyon resonator permits the existence of bulk resonant modes that exhibit non-zero electromagnetic helicity in vacuo, with a non-zero overlap of the electric and magnetic mode eigenvectors, $\int \mathbf{E}_p\cdot\mathbf{B}_p~d\tau$, integrated over the cavity volume. In the upconversion limit, we show that these non-zero helical modes couple naturally to ultra-light dark matter axions within the bandwidth of the resonator by adding amplitude-modulated sidebands through the axion-photon chiral anomaly. Thus, we show a sensitive ultra-light dark matter experiment may be realized by implementing such a resonator in an ultra-stable oscillator configuration and searching for signals in the Fourier spectrum of amplitude fluctuations. This removes the typical requirement for an external magnetic field and therefore permits the use of superconducting materials to reduce surface losses and enhance sensitivity to axions., Comment: New expanded version

We establish new experiments to search for dark matter based on a model of a light scalar field with a dilaton-like coupling to the electromagnetic field, which is strongly motivated by superstring theory. We estimate the power of the photon signal in the process of a non-resonant scalar-photon transition and in a cavity resonator permeated by electric and magnetic fields. We show that existing cavity resonators employed in the experiments like ADMX have a low but non-vanishing sensitivity to the scalar-photon coupling. As a result, by re-purposing the results of the ADMX experiment, we find new limits on the scalar-photon coupling in the range of the scalar field masses from 2.7 to 4.2 $\mu$eV. We discuss possible modifications of this experiment, which enhance the sensitivity to the scalar field dark matter. We also propose a broadband experiment for scalar field dark matter searches based on a high-voltage capacitor. The estimated sensitivity of this experiment exceeds by nearly two orders in magnitude the sensitivity of the experiment based on molecular spectroscopy., Comment: 10 pages, 5 figures; V2: minor changes, references added

Axions are well-motivated dark matter candidates with simple cosmological production mechanisms. They were originally introduced to solve the strong CP problem, but also arise in a wide range of extensions to the Standard Model. This Snowmass white paper summarizes axion phenomenology and outlines next-generation laboratory experiments proposed to detect axion dark matter. There are vibrant synergies with astrophysical searches and advances in instrumentation including quantum-enabled readout, high-Q resonators and cavities and large high-field magnets. This white paper outlines a clear roadmap to discovery, and shows that the US is well-positioned to be at the forefront of the search for axion dark matter in the coming decade., Comment: restore and expand author list

The last decade has seen unprecedented effort in dark matter model building at all mass scales coupled with the design of numerous new detection strategies. Transformative advances in quantum technologies have led to a plethora of new high-precision quantum sensors and dark matter detection strategies for ultralight ($<10\,$eV) bosonic dark matter that can be described by an oscillating classical, largely coherent field. This white paper focuses on searches for wavelike scalar and vector dark matter candidates., Comment: Snowmass 2021 White Paper

We present a new exclusion bound of axion-like particle dark matter with axion-photon couplings above $\mathrm{10^{-13}}$ $\mathrm{GeV^{-1}}$ over the frequency range 4796.7--4799.5 MHz, corresponding to a narrow range of axion masses centered around 19.84 $\mu$eV. This measurement represents the first implementation of a Josephson Traveling Wave Parametric Amplifier (JTWPA) in a dark matter search. The JTWPA was operated in the insert of the Axion Dark Matter eXperiment (ADMX) as part of an independent receiver chain that was attached to a 0.588-liter cavity. The ability of the JTWPA to deliver high gain over a wide (3 GHz) bandwidth has engendered interest from those aiming to perform broadband axion searches, a longstanding goal in this field.

We report the results from a haloscope search for axion dark matter in the $3.3\text{-}4.2~{\mu}$eV mass range. This search excludes the axion-photon coupling predicted by one of the benchmark models of "invisible" axion dark matter, the KSVZ model. This sensitivity is achieved using a large-volume cavity, a superconducting magnet, an ultra low noise Josephson parametric amplifier, and sub-Kelvin temperatures. The validity of our detection procedure is ensured by injecting and detecting blind synthetic axion signals., Comment: 6 pages, 5 figures

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, with a particular focus on Ultra High-Frequency Gravitational Waves (UHF-GWs), covering the MHz to GHz range. The absence of known astrophysical sources in 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 gravitational sources. We review several detector concepts which 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 the workshop "Challenges and opportunities of high-frequency gravitational wave detection" held at ICTP Trieste, Italy in October 2019, that set up the stage for the recently launched Ultra-High-Frequency Gravitational Wave (UHF-GW) initiative., Comment: Published in Living Reviews in Relativity

Searching for axion dark matter, the ADMX collaboration acquired data from January to October 2018, over the mass range 2.81--3.31 $\mu$eV, corresponding to the frequency range 680--790 MHz. Using an axion haloscope consisting of a microwave cavity in a strong magnetic field, the ADMX experiment excluded Dine-Fischler-Srednicki-Zhitnisky (DFSZ) axions at 100% dark matter density over this entire frequency range, except for a few gaps due to mode crossings. This paper explains the full ADMX analysis for Run 1B, motivating analysis choices informed by details specific to this run., Comment: 17 pages, 17 figures

We report the observation of coupling between a 3D microwave cavity mode and a bulk mechanical resonator mediated by piezoelectric and radiation pressure effects. The system is composed of a quartz bulk acoustic wave resonator placed inside a microwave re-entrant cavity, which is designed to act as both the electrodes for piezoelectric actuation as well as a 3D resonator. The cavity electromagnetic mode is modulated by a 5 MHz bulk acoustic wave shear mode, which is modeled and experimentally verified using the input-output formalism. Through finite element method simulations, we calculate the various contributions to the electromechanical coupling and discuss the potential of the system to reach high cooperativities as well as suitable applications.

Recent progress in observing and manipulating mechanical oscillators at quantum regime provides new opportunities of studying fundamental physics, for example, to search for low energy signatures of quantum gravity. For example, it was recently proposed that such devices can be used to test quantum gravity effects, by detecting the change in the [x,p] commutation relation that could result from quantum gravity corrections. We show that such a correction results in a dependence of a resonant frequency of a mechanical oscillator on its amplitude, which is known as amplitude-frequency effect. By implementing this new method we measure amplitude-frequency effect for 0.3 kg ultra high-Q sapphire split-bar mechanical resonator and for 10 mg quartz bulk acoustic wave resonator. Our experiments with sapphire resonator have established the upper limit on quantum gravity correction constant for $\beta_0<5 \times10^6$ which is a factor of 6 better than previously detected. The reasonable estimates of $\beta_0$ from experiments with quartz resonators yield an even more stringent limit of $4\times10^4$. The data sets of 1936 measurement of physical pendulum period by Atkinson results in significantly stronger limitations on $\beta_0 \ll 1$. Yet, due to the lack of proper pendulum frequency stability measurement in these experiments, the exact upper bound on $\beta_0$ can not be reliably established. Moreover, pendulum based systems only allow testing a specific form of the modified commutator that depends on the mean value of momentum. The electro-mechanical oscillators to the contrary enable testing of any form of generalized uncertainty principle directly due to much higher stability and a higher degree of control., Comment: 3 figures

The ferroelectric phase transition temperature of a single crystal $SrTiO_3$ cylindrical puck inserted into a cylindrical copper cavity has been measured around $51~K$ using quasi $TE_{m,1,1}$ and quasi $TM_{m,1,1}$ electromagnetic modes. The observed results of such dielectric spectroscopy may be explained by implementing a quasiharmonic approximation of phonons in the incipient ferroelectric crystal $SrTiO_3$ lattice. The resonant modes act as very sensitive probe by implementing measurements of $Q$-factor, transmission or frequency shift when cooling down to low temperatures around a few Kelvin. The resonator dielectric losses reveals different crystal unit structure states and ferroelectric phase transition in continuous cooling. The ferroelectric phase transition initiates a spontaneous polarization $(P_s)$ forming a double-well potential in each crystal unit causing a two-level systems, which creates extra loss and changes the transmission characteristics and Q-factors of the modes. This process is able to describe resonator properties over a broad range of frequency. Landau's theory of correlation length supports the reported observations as the implication of phonon wave vector $q_c=0$ of central ion in crystal unit symmetry regarding soft-mode., Comment: Typos fixed

We present the characterisation of the most recent parametric transducers designed to enhance the Mario Schenberg Gravitational Wave Detector sensitivity. The transducer is composed of a microwave re-entrant cavity that attaches to the gravitational wave antenna via a rigid spring. It functions as a three-mode mass-spring system; motion of the spherical antenna couples to a 50 $\mu m$ thick membrane, which converts its mechanical motion into a frequency shift of the cavity resonance. Through the optical spring effect, the microwave transducer frequency-displacement sensitivity was measured to be 726 $MHz/\mu$m at 4 K. The spherical antenna detection sensitivity is determined analytically using the transducer amplification gain and equivalent displacement noise in the test setup, which are 5.5 $\times$ 10$^{11} V/m$ and $1.8 \times 10^{-19} m\sqrt{Hz}^{-1}$, respectively.

Low-loss crystals with defects due to paramagnetic or rare earth impurity ions is a major area of investigation for quantum hybrid systems at both optical and microwave frequencies. In this work we examine the single crystal yttrium aluminium perovskite, YAlO$_3$ using the Whispering Gallery Mode Technique. Multiple resonant microwave modes were measured from room temperature to 20 mK allowing precise characterization of the permittivity tensor at microwave frequencies. We show that it is biaxial and characterize the tensor as a function of temperature with estimated uncertainties below 0.26%. Electron spin resonance spectroscopy was also performed at 20 mK, with new transitions identified with Zero-Field splittings of 16.72 and 9.92 GHz. Spin-photon couplings of order 4.2 and 8.4 MHz were observed for residual levels of concentration, which are stronger than the photon cavity linewidths of 116 kHz but the same order of the linewidths of the discovered spin transitions.

A new method of probing Lorentz invariance in the neutron sector is described. The method is baed on stable quartz bulk acoustic wave oscillators compared on a rotating table. Due to Lorentz-invariance violation, the resonance frequencies of acoustic wave resonators depend on the direction in space via a corresponding dependence of masses of the constituent elements of solids. This dependence is measured via observation of oscillator phase noise built around such devices. The first such experiment now shows sensitivity to violation down to the limit $\tilde{c}^n_Q=(-1.8\pm2.2)\times 10^{-14}$ GeV. Methods to improve the sensitivity are described together with some other applications of the technology in tests of fundamental physics., Comment: Presented at the Seventh Meeting on CPT and Lorentz Symmetry, Bloomington, Indiana, June 20-24, 2016

We propose and analyse a quantum electromechanical system composed of a monolithic quartz bulk acoustic wave (BAW) oscillator coupled to a superconducting transmon qubit via an intermediate LC electrical circuit. Monolithic quartz oscillators offer unprecedentedly high effective masses and quality factors for the investigation of mechanical oscillators in the quantum regime. Ground-state cooling of such mechanical modes via resonant piezoelectric coupling to an LC circuit, which is itself sideband cooled via coupling to a transmon qubit, is shown to be feasible. The fluorescence spectrum of the qubit, containing motional sideband contributions due to the couplings to the oscillator modes, is obtained and the imprint of the electromechanical steady-state on the spectrum is determined. This allows the qubit to function both as a cooling resource for, and transducer of, the mechanical oscillator. The results described are relevant to any hybrid quantum system composed of a qubit coupled to two (coupled or uncoupled) thermal oscillator modes., Comment: 19 pages, 3 figures

We investigate the microwave magnetic field confinement in several microwave 3D-cavities, using 3D finite-element analysis to determine the best design and achieve strong coupling between microwave resonant cavity photons and solid state spins. Specifically, we design cavities for achieving strong coupling of electromagnetic modes with an ensemble of nitrogen vacancy (NV) defects in diamond. We report here a novel and practical cavity design with a magnetic filling factor of up to 4 times (2 times higher collective coupling) than previously achieved using 1D superconducting cavities with small mode volume. In addition, we show that by using a double-split resonator cavity, it is possible to achieve up to 200 times better cooperative factor than the currently demonstrated with NV in diamond. These designs open up further opportunities for studying strong and ultra-strong coupling effects on spins in solids using alternative systems with a wider range of design parameters., Comment: 20 pages, 9 figures

In the context of engineered quantum systems, there is a demand for superconducting tunable devices able to operate with high Q-factors at power levels equivalent to only a few photons. In this work, we developed a 3D microwave reentrant cavity with such characteristics ready to provide a very fine-tuning of a high-Q resonant mode over a large dynamic range. This system has an electronic tuning mechanism based on a mechanically amplified piezoelectric actuator, which controls the resonator dominant mode frequency by changing the cavity narrow gap by very small displacements. Experiments were conducted at room and dilution refrigerator temperatures showing a large dynamic range up to 4 GHz and 1 GHz, respectively, and were compared to a FEM model simulated data. At elevated microwave power input, nonlinear thermal effects were observed to destroy the superconductivity of the cavity due to the large electric fields generated in the small gap of the reentrant cavity.

The Y2SiO5 (YSO) crystal is a dielectric material with biaxial anisotropy with known values of refractive index at optical frequencies. It is a well-known rare-earth (RE) host material for optical research and more recently has shown promising performance for quantum-engineered devices. In this paper, we report the first microwave characterization of the real permittivity tensor of a bulk YSO sample, as well as an investigation of the temperature dependence of the tensor components from 296 K down to 6 K. Estimated uncertainties were below 0.26%, limited by the precision of machining the cylindrical dielectric. Also, the electrical Q-factors of a few electromagnetic modes were recorded as a way to provide some information about the crystal losses over the temperature range. To solve the tensor components necessary for a biaxial crystal, we developed the multi-mode technique, which uses simultaneous measurement of low order Whispering Gallery Modes. Knowledge of the permittivity tensor offers important data, essential for the design of technologies involving YSO, such as microwave coupling to electron and hyperfine transitions in RE doped samples at low temperatures., Comment: 4 pages, 3 figures

We present a technique for addressing single NV$^{-}$ center spins in diamond over macroscopic distances using a tunable dielectric microwave cavity. We demonstrate optically detected magnetic resonance (ODMR) for a single NV$^{-}$ center in a nanodiamond (ND) located directly under the macroscopic microwave cavity. By moving the cavity relative to the ND, we record the ODMR signal as a function of position, mapping out the distribution of the cavity magnetic field along one axis. In addition, we argue that our system could be used to determine the orientation of the NV$^{-}$ major axis in a straightforward manner.

Lorentz symmetry is a foundational property of modern physics, underlying the standard model of particles and general relativity. It is anticipated that these two theories are low energy approximations of a single theory that is unified and consistent at the Planck scale. Many unifying proposals allow Lorentz symmetry to be broken, with observable effects appearing at Planck-suppressed levels; thus precision tests of Lorentz invariance are needed to assess and guide theoretical efforts. Here, we use ultra-stable oscillator frequency sources to perform a modern Michelson-Morley experiment and make the most precise direct terrestrial test to date of Lorentz symmetry for the photon, constraining Lorentz violating orientation-dependent relative frequency changes $\Delta\nu$/$\nu$ to 9.2$\pm$10.7$\times10^{-19}$ (95$\%$ confidence interval). This order of magnitude improvement over previous Michelson-Morley experiments allows us to set comprehensive simultaneous bounds on nine boost and rotation anisotropies of the speed of light, finding no significant violations of Lorentz symmetry., Comment: 20 pages, 13 figures

This work demonstrates strong coupling regime between an Erbium ion spin ensemble and microwave Hybrid Cavity-Whispering Gallery Modes in a Yttrium Aluminium Garnet dielectric crystal. Coupling strengths of $220$~MHz and mode quality factors in excess of $10^6$ are demonstrated. Moreover, the magnetic response of high-Q modes demonstrates behaviour which is unusual for paramagnetic systems. This behaviour includes hysteresis and memory effects. Such qualitative change of the system's magnetic field response is interpreted as a phase transition of Rare Earth ion impurities. This phenomenon is similar to the phenomenon of dilute ferromagnetism in semiconductors. The clear temperature dependence of the phenomenon is demonstrated.

We present cavity QED experiments with an Er:YSO crystal magnetically coupled to a 3D cylindrical sapphire loaded copper resonator. Such waveguide cavities are promising for the realization of a superconducting quantum processor. Here, we demonstrate the coherent integration of a rare-earth spin ensemble with the 3D architecture. The collective coupling strength of the Er$^{3+}$ spins to the 3D cavity is 21 MHz. The cylindrical sapphire loaded resonator allowed us to explore the anisotropic collective coupling between the rare-earth doped crystal and the cavity. This work shows the potential of spin doped solids in 3D quantum circuits for application as microwave quantum memories as well as for prospective microwave to optical interfaces., Comment: 4 pages, 4 figures

We report the observation of a mechanism of maser generation in an ensemble of inter-coupled, inhomogeneously broadened two-level systems, enhanced by high quality factor electromagnetic cavity modes. In this previously unobserved form of population inversion, an inseparable quantum system leads to cavity-enhanced stimulated emission arising from interactions within an ensemble of two-level systems, as opposed to a traditional ensemble of noninteracting identical three level systems. The effect is observed in a cryogenically cooled whispering gallery mode sapphire resonator containing dilute Fe$^{3+}$ impurity ions. These ions exhibit strong spin-lattice interaction, leading to both electron spin resonance broadening and phonon mediated spin-spin coupling. The maser effect is due to a $\left|1/2\right\rangle \rightarrow \left|3/2\right\rangle$ energy transition in electron spin angular momentum observed at zero external magnetic field. Both continuous and oscillating regimes are observed with corresponding thresholds both in detuning frequency and incident power., Comment: Phys. Rev. B, 2013

Dark sectors, consisting of new, light, weakly-coupled particles that do not interact with the known strong, weak, or electromagnetic forces, are a particularly compelling possibility for new physics. Nature may contain numerous dark sectors, each with their own beautiful structure, distinct particles, and forces. This review summarizes the physics motivation for dark sectors and the exciting opportunities for experimental exploration. It is the summary of the Intensity Frontier subgroup "New, Light, Weakly-coupled Particles" of the Community Summer Study 2013 (Snowmass). We discuss axions, which solve the strong CP problem and are an excellent dark matter candidate, and their generalization to axion-like particles. We also review dark photons and other dark-sector particles, including sub-GeV dark matter, which are theoretically natural, provide for dark matter candidates or new dark matter interactions, and could resolve outstanding puzzles in particle and astro-particle physics. In many cases, the exploration of dark sectors can proceed with existing facilities and comparatively modest experiments. A rich, diverse, and low-cost experimental program has been identified that has the potential for one or more game-changing discoveries. These physics opportunities should be vigorously pursued in the US and elsewhere., Comment: Report of the Community Summer Study 2013 (Snowmass) Intensity Frontier New, Light, Weakly-Coupled Particles subgroup

Analysis of the properties of resonant modes in a reentrant cavity structure comprising of a post and a ring was undertaken and verified experimentally. In particular we show the existence of higher order reentrant cavity modes in such a structure. Results show that the new cavity has two re-entrant modes, one which has a better displacement sensitivity than the single post resonator and the other with a reduced sensitivity. The more sensitive mode is better than the single post resonator by a factor of 2 to 1.5 when the gap spacing is below 100 $ \mu$m. This type of cavity has the potential to operate as a highly sensitive transducer for a variety of precision measurement applications, in particular applications which require coupling to more than one sensitive transducer mode.

Low loss Bulk Acoustic Wave devices are considered from the point of view of the solid state approach as phonon-confining cavities. We demonstrate effective design of such acoustic cavities with phonon-trapping techniques exhibiting extremely high quality factors for trapped longitudinally-polarized phonons of various wavelengths. Quality factors of observed modes exceed 1 billion, with a maximum $Q$-factor of 8 billion and $Q\times f$ product of $1.6\cdot10^{18}$ at liquid helium temperatures. Such high sensitivities allow analysis of intrinsic material losses in resonant phonon systems. Various mechanisms of phonon losses are discussed and estimated.

The confinement of high frequency phonons approaching 1 GHz is demonstrated in phonon-trapping acoustic cavities at cryogenic temperatures using a low-coupled network approach. The frequency range is extended by nearly an order of magnitude, with excitation at greater than the 200$^{\text{th}}$ overtone achieved for the first time. Such high frequency operation reveals Rayleigh-type phonon scattering losses due to highly diluted lattice impurities and corresponding glass-like behaviour, with a maximum $Q_L\times f$ product of $8.6\times 10^{17}$ at 3.8K and $4\times10^{17}$ at 15mK. This suggests a limit on the $Q\times f$ product due to unavoidable crystal disorder. Operation at 15 mK is high enough in frequency that the average phonon occupation number is less than unity, with a loaded quality factor above half a billion. This work represents significant progress towards the utilisation of such acoustic cavities for hybrid quantum systems.

Cylindrical re-entrant cavities are unique three-dimensional structures that resonate with their electric and magnetic fields in separate parts of the cavity. To further understand these devices, we undertake rigorous analysis of the properties of the resonance using in-house developed Finite Element Method (FEM) software capable of dealing with small gap structures of extreme aspect ratio. Comparisons between the FEM method and experiments are consistent and we illustrate where predictions using established lumped element models work well and where they are limited. With the aid of the modeling we design a highly tunable cavity that can be tuned from 2 GHz to 22 GHz just by inserting a post into a fixed dimensioned cylindrical cavity. We show this is possible as the mode structure transforms from a re-entrant mode during the tuning process to a standard cylindrical Transverse Magnetic (TM) mode.

The frequencies of three separate Cs fountain clocks and one Rb fountain clock have been compared to various hydrogen masers to search for periodic changes correlated with the changing solar gravitational potential at the Earth and boost with respect to the Cosmic Microwave Background (CMB) rest frame. The data sets span over more than eight years. The main sources of long-term noise in such experiments are the offsets and linear drifts associated with the various H-masers. The drift can vary from nearly immeasurable to as high as 1.3*10^-15 per day. To circumvent these effects we apply a numerical derivative to the data, which significantly reduces the standard error when searching for periodic signals. We determine a standard error for the putative Local Position Invariance (LPI) coefficient with respect to gravity for a Cs-Fountain H-maser comparison of 4.8*10^-6 and 10^-5 for a Rb-Fountain H-maser comparison. From the same data the putative boost LPI coefficients were measured to a precision of up to parts in 10^11 with respect to the CMB rest frame. By combining these boost invariance experiments to a Cryogenic Sapphire Oscillator versus H-maser comparison, independent limits on all nine coefficients of the boost violation vector with respect to fundamental constant invariance (fine structure constant, electron mass and quark mass respectively), were determined to a precision of parts up to 10^10., Comment: accepted for publication in Phys Rev D

Progress in realizing the SI second had multiple technological impacts and enabled to further constraint theoretical models in fundamental physics. Caesium microwave fountains, realizing best the second according to its current definition with a relative uncertainty of 2-4x10^(-16), have already been superseded by atomic clocks referenced to an optical transition, both more stable and more accurate. Are we ready for a new definition of the second? Here we present an important step in this direction: our system of five clocks connects with an unprecedented consistency the optical and the microwave worlds. For the first time, two state-of-the-art strontium optical lattice clocks are proven to agree within their accuracy budget, with a total uncertainty of 1.6x10^(-16). Their comparison with three independent caesium fountains shows a degree of reproducibility henceforth solely limited at the level of 3.1x10^(-16) by the best realizations of the microwave-defined second., Comment: 9 pages, 4 figures, 2 tables

We report the operation of a dual Rb/Cs atomic fountain clock. 133Cs and 87Rb atoms are cooled, launched, and detected simultaneously in LNE-SYRTE's FO2 double fountain. The dual clock operation occurs with no degradation of either the stability or the accuracy. We describe the key features for achieving such a simultaneous operation. We also report on the results of the first Rb/Cs frequency measurement campaign performed with FO2 in this dual atom clock configuration, including a new determination of the absolute 87Rb hyperfine frequency., Comment: 6 pages, 10 figures, 23 references

Whispering Gallery modes in bulk cylindrical Gallium Arsenide and Gallium Phosphide samples have been examined both in darkness and under white light at cryogenics temperatures < 50 K. In both cases persistent photoconductivity was observed after initially exposing semiconductors to white light from a halogen lamp. Photoconductance decay time constants for GaP and GaAs were determined to be 0.900 +/- 0.081 ns and 1.098 +/- 0.063 ns, respectively, using this method., Comment: 13 pages, 5 figures

We demonstrate the use of a fiber-based femtosecond laser locked onto an ultra-stable optical cavity to generate a low-noise microwave reference signal. Comparison with both a liquid Helium cryogenic sapphire oscillator (CSO) and a Ti:Sapphire-based optical frequency comb system exhibit a stability about $3\times10^{-15}$ between 1 s and 10 s. The microwave signal from the fiber system is used to perform Ramsey spectroscopy in a state-of-the-art Cesium fountain clock. The resulting clock system is compared to the CSO and exhibits a stability of $3.5\times10^{-14}\tau^{-1/2}$. Our continuously operated fiber-based system therefore demonstrates its potential to replace the CSO for atomic clocks with high stability in both the optical and microwave domain, most particularly for operational primary frequency standards., Comment: 3 pages, 3 figures

We summarise the scientific and technological aspects of the SAGAS (Search for Anomalous Gravitation using Atomic Sensors) project, submitted to ESA in June 2007 in response to the Cosmic Vision 2015-2025 call for proposals. The proposed mission aims at flying highly sensitive atomic sensors (optical clock, cold atom accelerometer, optical link) on a Solar System escape trajectory in the 2020 to 2030 time-frame. SAGAS has numerous science objectives in fundamental physics and Solar System science, for example numerous tests of general relativity and the exploration of the Kuiper belt. The combination of highly sensitive atomic sensors and of the laser link well adapted for large distances will allow measurements with unprecedented accuracy and on scales never reached before. We present the proposed mission in some detail, with particular emphasis on the science goals and associated measurements., Comment: 39 pages. Submitted in abridged version to Experimental Astronomy

Cryogenic Sapphire Oscillators (CSO) developed at UWA have now been in operation around the world continuously for many years. Such oscillators, due to their excellent spectral purity are essential for interrogating atomic frequency standards at the limit of quantum projection noise; otherwise aliasing effects will dominate the frequency stability due to the periodic sampling between successive interrogations of the atomic transition. For this reason, UWA oscillators are now operational at NMI (Sydney), LNE-SYRTE (Paris), the French Space Agency (CNES, Toulouse) and at UWA (Perth). Other applications, which have attracted attention in recent years, include tests on fundamental principles of physics, such as tests of Lorentz invariance. This paper reports on the long-term operation and performance of such oscillators. We compare the long-term drift of some different CSOs. The drift rates turn out to be linear over many years and in the same direction. However, the magnitude seems to vary by more than one order of magnitude between the oscillators, ranging from 10^14 per day to a few parts in 10^13 per day., Comment: Accepted for publication in IEEE Trans. UFFC 21st June 2006

We report on the development of a sapphire cryogenic microwave resonator oscillator long-term fractional frequency stability of 2x10^-17Sqrt[\tau] for integration times \tau>10^3 s and negative drift of about 2.2x10^-15/day. The short-term frequency instability of the oscillator is highly reproducible and also state-of-the-art: 5.6x10^-16 for an integration time of \tau ~ 20 s., Comment: Accepted for publication in Applied Physics Letters

We report the first observation of above-threshold maser oscillation in a whispering-gallery(WG)-mode resonator, whose quasi-transverse-magnetic, 17th azimuthal-order WG mode, at a frequency of approx. 12.038 GHz, with a loaded Q of several hundred million, is supported on a cylinder of mono-crystalline sapphire. An electron spin resonance (ESR) associated with Fe3+ ions, that are substitutively included within the sapphire at a concentration of a few parts per billion, coincides in frequency with that of the (considerably narrower) WG mode. By applying a c.w. `pump' to the resonator at a frequency of approx. 31.34 GHz, with no applied d.c. magnetic field, the WG (`signal') mode is energized through a three-level maser scheme. Preliminary measurements demonstrate a frequency stability (Allan deviation) of a few times 1e-14 for sampling intervals up to 100 s., Comment: REVTeX v.4, 3 pages, with a separate .bbl file and 3 .eps figures

We describe two experimental tests of the Equivalence Principle that are based on frequency measurements between precision oscillators and/or highly accurate atomic frequency standards. Based on comparisons between the hyperfine frequencies of 87Rb and 133Cs in atomic fountains, the first experiment constrains the stability of fundamental constants. The second experiment is based on a comparison between a cryogenic sapphire oscillator and a hydrogen maser. It tests Local Lorentz Invariance. In both cases, we report recent results which improve significantly over previous experiments., Comment: 13 pages, 5 figures