arXiv subjects

Michael E. Tobar

Publications and source records attributed to Michael E. Tobar.

At least 91 records · Page 5Linked to original sources

arXiv2016

Axion Dark Matter Coupling to Resonant Photons via Magnetic Field

We show that the magnetic component of the photon field produced by dark matter axions via the two-photon coupling mechanism in a Sikivie Haloscope is an important parameter passed over in previous analysis and experiments. The interaction of the produced photons will be resonantly enhanced as long as they couple to the electric or magnetic mode structure of the Haloscope cavity. For typical Haloscope experiments the electric and magnetic coupling is the same and implicitly assumed in past sensitivity calculations. However, for future planned searches such as those at high frequency, which synchronize multiple cavities, the sensitivity will be altered due to different magnetic and electric couplings. We define the complete electromagnetic form factor and discuss its implications for current and future high and low mass axion searches, including some effects which have been overlooked, due to the assumption that the two couplings are the same.

hep-ph↗

arXiv2016

A 3D Printed Superconducting Aluminium Microwave Cavity

3D printing of plastics, ceramics, and metals has existed for several decades and has revolutionized many areas of manufacturing and science. Printing of metals in particular has found a number of applications in fields as diverse as customized medical implants, jet engine bearings, and rapid prototyping in the automotive industry. Whilst many techniques can be used for 3D printing metals, they commonly rely on computer controlled melting or sintering of a metal alloy powder using a laser or electron beam. The mechanical properties of parts produced in such a way have been well studied, but little attention has been paid to their electrical properties. Here we show that a microwave cavity (resonant frequencies 9.9 and 11.2 GHz) 3D printed using an Al-12Si alloy exhibits superconductivity when cooled below the critical temperature of aluminium (1.2 K), with a performance comparable to the common 6061 alloy of aluminium. Superconducting cavities find application in numerous areas of physics, from particle accelerators to cavity quantum electrodynamics experiments. The result is achieved even with a very large concentration of non-superconducting silicon in the alloy of 12.18%, compared to Al-6061, which has between 0.4 to 0.8%. Our results may pave the way for the possibility of 3D printing superconducting cavity configurations that are otherwise impossible to machine.

physics.ins-det↗

arXiv2016

A new method of probing mechanical losses of coatings at cryogenic temperatures

A new method of probing mechanical losses and comparing the corresponding deposition processes of metallic and dielectric coatings in 1-100 MHz frequency range and cryogenic temperatures is presented. The method is based on the use of extremely high-quality quartz acoustic cavities whose internal losses are orders of magnitude lower than any available coatings nowadays. The approach is demonstrated for Chromium, Chromium/Gold and a multilayer tantala/silica coatings. The ${\rm Ta}_2{\rm O}_5/{\rm Si}{\rm O}_2$ coating has been found to exhibit a loss angle lower than $1.6\times10^{-5}$ near 30 {\rm MHz} at 4 {\rm K}. The results are compared to the previous measurements.

physics.ins-det↗

arXiv2016

Superstrong Coupling of a Microwave Cavity to YIG Magnons

Multiple-post reentrant 3D lumped cavity modes have been realized to design the concept of discrete Whispering Gallery and Fabry-Perot-like Modes for multimode microwave Quantum Electrodynamics experiments. Using a magnon spin-wave resonance of a submillimeter-sized Yttrium-Iron-Garnet sphere at milliKelvin temperatures and a four-post cavity, we demonstrate the ultra-strong coupling regime between discrete Whispering Gallery Modes and a magnon resonance with strength of 1.84 GHz. By increasing the number of posts to eight and arranging them in a D$_4$ symmetry pattern, we expand the mode structure to that of a discrete Fabry-Perot cavity and modify the Free Spectral Range (FSR). We reach the superstrong coupling regime, where spin-photon coupling strength is larger than FSR, with coupling strength in the 1.1 to 1.5 GHz range.

quant-ph↗

arXiv2015

Creating Tuneable Microwave Media from a Two-Dimensional Lattice of Re-entrant Posts

The potential capabilities of resonators based on two dimensional arrays of re-entrant posts is demonstrated. Such posts may be regarded as magnetically-coupled lumped element microwave harmonic oscillators, arranged in a 2D lattices structure, which is enclosed in a 3D cavity. By arranging these elements in certain 2D patterns, we demonstrate how to achieve certain requirements with respect to field localisation and device spectra. Special attention is paid to symmetries of the lattices, mechanical tuning, design of areas of high localisation of magnetic energy, which in turn creates unique discrete mode spectra. We demonstrate analogies between systems designed on the proposed platform and well known physical phenomena such as polarisation, frustration and Whispering Gallery Modes. The mechanical tunability of the cavity with multiple posts is analysed and its consequences to optomechanical applications is calculated. One particular application to quantum memory is demonstrated with a cavity design consisting of separate resonators analogous to discrete Fabry-Pérot resonators. Finally, we propose a generalised approach to a microwave system design based on the concept of Programmable Cavity Arrays.

quant-ph↗

arXiv2015

Determination of the Anisotropy of Permittivity of Quantum Paraelectric Strontium Titanate

The dielectric properties of strontium titanate (SrTiO$_3$) have previously been reported from room temperature to low temperatures with conflicting results. In this work, precision measurement of the permittivity is undertaken by simultaneously measuring transverse electric and transverse magnetic resonant modes within a single crystal. It is unequivocally shown that the permittivity is isotropic at room temperature with a permittivity of order $316.3\pm2.2$ by measuring multiple modes of different electric field polarisations. As the crystal is cooled to 5 K and undergoes well known phase transitions, we show the material becomes uniaxial anisotropic with the ratio of the parallel to perpendicular permittivity to the cylinder z-axis of the sample as high as 2.4 below 6 K.

cond-mat.mtrl-sci↗

arXiv2015

Bounds on higher-order Lorentz-violating photon sector coefficients from an asymmetric optical ring resonator experiment

Optical resonators provide a powerful tool for testing aspects of Lorentz invariance. Here, we present a reanalysis of an experiment where a path asymmetry was created in an optical ring resonator by introducing a dielectric prism in one arm. The frequency difference of the two fundamental counter-propagating modes was then recorded as the apparatus was orientation-modulated in the laboratory. By assuming that the minimal Standard-Model Extension coefficients vanish we are able to place bounds on higher-order parity-odd Lorentz-violating coefficients of the Standard-Model Extension. The results presented in this work set the first constraints on two previously unbounded linear combinations of d=8 parity-odd nonbirefringent nondispersive coefficients of the photon sector.

hep-ph↗

arXiv2015

Collective Behaviour of Cr$^{3+}$ ions in Ruby Revealed by Whispering Gallery Modes

We present evidence for collective action of Cr$^{3+}$ ion impurities in a highly doped ruby crystal at microwave frequencies. The cylindrical geometry of the crystal allows for the creation of a superradiant, or "spin-mode" doublet, with spatial structure similar to that of microwave whispering gallery modes (WGMs). This results in a strict criteria of selection rules regarding the interaction of resonant WGMs and spin-modes; namely that only modes with the same wavenumber and azimuthal phase may interact. What results is an avoided level crossing between the two, in which both WGM doublet constituents are seen to interact with the spin resonance. We demonstrate that a four harmonic oscillator model is necessary to accurately describe this result.

cond-mat.mtrl-sci↗

arXiv2015

Strong Coupling Between Whispering Gallery Photons and Spin States of Iron Group Impurity Ions

Interaction of Whispering Gallery Modes (WGM) with dilute spin ensembles in solids is an interesting paradigm of Hybrid Quantum Systems potentially beneficial for Quantum Signal Processing applications. In the present work, WGM spectroscopy of Rare-Earth doped YSO crystal reveals the existence of Iron Group Ion (IGI) co-dopings with concentration levels of order 100 ppb. The IGI spin ensembles demonstrate large Zero Field Splittings at 14.7GHz, 18.4GHz and 25.4GHz, considerable anisotropy of the \textrm{g}-tensors, as well as two inequivalent lattice sites. Strong coupling regimes between an ensemble of IGI spins and WGM photons have been demonstrated at $18.4$ GHz and near zero field. This approach together with useful optical properties of these ions opens a new avenue for 'spins-in-solids' Quantum Electrodynamics.

cond-mat.mes-hall↗

arXiv2015

Single Photon Level Study of Microwave Properties of Lithium Niobate at milli-Kelvin Temperatures

Properties of doped and natural impurities in Lithium Niobate single crystals are studied using the Whispering Gallery Mode method at low temperatures as a function of magnetic field. The study reveals considerable coupling of microwave photon modes to the Fe$^{3+}$ spin ensemble in iron-doped and non-doped crystals. The $S=5/2$ structure of the Fe$^{3+}$ impurities demonstrate Zero Field Splittings of $11.21$ and $20.96$ GHz, significant asymmetry of the Zeeman lines and additional lines with anomalous $\text{g}$-factors of $1.37$ and $3.95$. Also, interactions between different transitions of the Fe$^{3+}$ ion is observed. An additional ion impurity ensemble with a splitting of about $1.7$ GHz is shown to couple to the dominating Fe$^{3+}$ spins and the effect on $Q$-factors of microwave photon modes due to the Fe$^{3+}$ ion ensemble is also demonstrated. Measurements down to less than one photon level are made with a loss tangent of order $10^{-5}$ determined.

cond-mat.mtrl-sci↗

arXiv2014

Strong Coupling Between P1 Diamond Impurity Centres and 3D Lumped Photonic Microwave Cavity

We report strong coupling between an ensemble of N impurity (P1) centres in diamond and microwave photons using a unique double post re-entrant cavity. The cavity is designed so that the magnetic component of the cavity field is spatially separated from the electric component and focused into the small volume in which the diamond sample is mounted. The novelty of the structure simultaneously allows high magnetic filling factor (38.4\%) and low frequencies necessary to interact, at low magnetic field, with transitions in diamond such as those in NV$^{-}$ and P1 centres. Coupling strength (or normal-mode splitting) of 51.42 MHz, was achieved with P1 centres at 6.18 GHz and 220 mT in a centimetre-scale cavity, with a corresponding cooperativity factor of 4.7. This technique offers an alternative way, with some significant advantages, to couple 3D cavities to transitions in diamond and achieve the strong coupling necessary for applications to quantum information processing.

cond-mat.mes-hall↗

arXiv2014

The 3D Split-Ring Cavity Lattice: A New Metastructure for Engineering Arrays of Coupled Microwave Harmonic Oscillators

A new electromagnetic cavity structure, a lattice of 3D cavities consisting of an array of posts and gaps is presented. The individual cavity elements are based on the cylindrical re-entrant (or Klystron) cavity. We show that these cavities can also can be thought of as 3D split-ring resonators, which is confirmed by applying symmetry transformations, each of which is an electromagnetic resonator with spatially separated magnetic and electric field. The characteristics of the cavity is used to mimic phonon behaviour of a one dimensional chain of atoms. It is demonstrated how magnetic field coupling can lead to phonon-like dispersion curves with acoustical and optical branches. The system is able to reproduce a number of effects typical to one-dimensional lattices exhibiting acoustic vibration, such as band gaps, phonon trapping, and effects of impurities. In addition, quasicrystal emulations predict the results expected from this class of ordered structures. The system is easily scalable to simulate 2D and 3D lattices and shows a new way to engineer arrays of coupled microwave resonators with a variety of possible applications to hybrid quantum systems proposed.

physics.ins-det↗

arXiv2014

Spectroscopy and laser cooling on the $^{1}S_{0}-\,^{3}P_{1}$ line in Yb via an injection-locked diode laser at 1111.6nm

We generate 555.8nm light with sub-MHz linewidth through the use of laser injection-locking of a semiconductor diode at 1111.6nm, followed by frequency doubling in a resonant cavity. The integrity of the injection lock is investigated by studying an offset-beat signal between slave and master lasers, by performing spectroscopy on the $(6s)^{2}$ $^{1}S_{0}-\,(6s6p)$ $^{3}P_{1}$ transition in magneto-optically trapped ytterbium, and by demonstrating additional laser cooling of $^{171}$Yb with the 555.8nm light. For the $^{1}S_{0}-$ $^{3}P_{1}$ spectroscopy, we confirm the linear dependence between ground state linewidth and the intensity of an off-resonant laser, namely, that used to cool Yb atoms in a $^{1}S_{0}-$ $^{1}P_{1}$ magneto-optical trap. Our results demonstrate the suitability of injection locked 1100-1130nm laser diodes as a source for sub-MHz linewidth radiation in the yellow-green spectrum.

physics.optics↗

arXiv2014

Hyperparametric effects in a whispering-gallery mode rutile dielectric resonator at liquid helium temperatures

We report the first observation of low power drive level sensitivity, hyperparametric amplification, and single-mode hyperparametric oscillations in a dielectric rutile whispering-gallery mode resonator at 4.2 K. The latter gives rise to a comb of sidebands at 19.756 GHz. Whereas, most frequency combs in the literature have been observed in optical systems using an ensemble of equally spaced modes in microresonators or fibers, the present work represents generation of a frequency comb using only a single-mode. The experimental observations are explained by an additional 1/2 degree-of-freedom originating from an intrinsic material nonlinearity at optical frequencies, which affects the microwave properties due to the extremely low loss of rutile. Using a model based on lumped circuits, we demonstrate that the resonance between the photonic and material 1/2 degree-of-freedom, is responsible for the hyperparametric energy transfer in the system.

quant-ph↗

arXiv2014

Cryogenic resonant microwave cavity searches for hidden sector photons

The hidden sector photon is a weakly interacting hypothetical particle with sub-eV mass that kinetically mixes with the photon. We describe a microwave frequency light shining through a wall experiment where a cryogenic resonant microwave cavity is used to try and detect photons that have passed through an impenetrable barrier, a process only possible via mixing with hidden sector photons. For a hidden sector photon mass of 53 $μ$eV we limit the hidden photon kinetic mixing parameter $χ< 1.7\times10^{-7}$, which is an order of magnitude lower than previous bounds derived from cavity experiments in the same mass range. In addition, we use the cryogenic detector cavity to place new limits on the kinetic mixing parameter for hidden sector photons as a form of cold dark matter.

hep-ex↗

arXiv2014

Observation of the Fundamental Nyquist Noise Limit in an Ultra-High $Q$-Factor Cryogenic Bulk Acoustic Wave Cavity

Thermal Nyquist noise fluctuations of high-$Q$ Bulk Acoustic Wave (BAW) cavities have been observed at cryogenic temperatures with a DC Superconducting Quantum Interference Device (SQUID) amplifier. High $Q$ modes with bandwidths of few tens of milliHz produce thermal fluctuations with a Signal-To-Noise ratio of up to 23dB. The estimated effective temperature from the Nyquist noise is in good agreement with the physical temperature of the device, confirming the validity of the equivalent circuit model and the non-existence of any excess resonator self-noise. The measurements also confirm that the quality factor remains extremely high ($Q>10^8$ at low order overtones) for very weak (thermal) system motion at low temperatures, when compared to values measured with relatively strong external excitation. This result represents an enabling step towards operating such a high-Q acoustic device at the standard quantum limit.

physics.ins-det↗

arXiv2014

High Cooperativity Cavity QED with Magnons at Microwave Frequencies

Using a sub-millimetre sized YIG (Yttrium Iron Garnet) sphere mounted in a magnetic field-focusing cavity, we demonstrate an ultra-high cooperativity of $10^5$ between magnon and photon modes at millikelvin temperatures and microwave frequencies. The cavity is designed to act as a magnetic dipole by using a novel multiple-post approach, effectively focusing the cavity magnetic field within the YIG crystal with a filling factor of 3%. Coupling strength (normal-mode splitting) of 2 GHz, (equivalent to 76 cavity linewidths or $0.3$ Hz per spin), is achieved for a bright cavity mode that constitutes about 10% of the photon energy and shows that ultra-strong coupling is possible in spin systems at microwave frequencies. With straight forward optimisations we demonstrate that with that this system has the potential to reach cooperativities of $10^7$, corresponding to a normal mode splitting of 5.2 GHz and a coupling per spin approaching 1 Hz. We also observe a three-mode strong coupling regime between a dark cavity mode and a magnon mode doublet pair, where the photon-magnon and magnon-magnon couplings (normal-mode splittings) are 143 MHz and 12.5 MHz respectively, with HWHM bandwidth of about 0.5 MHz.

quant-ph↗

arXiv2014

Strong Coupling between Whispering Gallery Modes and Chromium Ions in Ruby

We report the study of interactions between cavity photons and paramagnetic Cr$^{3+}$ spins in a ruby (Cr$^{3+}$:Al$_2$O$_3$) Whispering Gallery mode (WGM) resonator. Examining the system at microwave frequencies and millikelvin temperatures, spin-photon couplings up to 610 MHz or about 5% of photon energy are observed between the impurity spins and high quality factor ($Q > 10^5$) WGM. Large tunability and spin-spin interaction allows operation in the strong coupling regime. The system exhibits behaviour not predicted by the usual Tavis-Cummings model because of interactions within the two-level spin bath, and the existence of numerous photonic modes.

cond-mat.mtrl-sci↗