Kinetic Inductance Magnetometer

Ultrasensitive magnetic field detection is utilized in the fields of science, medicine and industry. We report on a novel magnetometer relying on the kinetic inductance of superconducting material. The kinetic inductance exhibits a non-linear response with respect to DC current, a fact that is exploited by applying magnetic flux through a superconducting loop to generate a shielding current and a change in the inductance of the loop. The magnetometer is arranged into a resonator, allowing readout through a transmission measurement that makes the device compatible with radio frequency multiplexing techniques. The device is fabricated using a single thin-film layer of NbN, simplifying the fabrication process compared to existing magnetometer technologies considerably. Our experimental data, supported by theory, demonstrates a magnetometer having potential to replace established technology in applications requiring ultra-high sensitivity.Comment: 16 pages, 6 figure

Flux-driven Josephson parametric amplifier for sub-GHz frequencies fabricated with side-wall passivated spacer junction technology

We present experimental results on a Josephson parametric amplifier tailored for readout of ultra-sensitive thermal microwave detectors. In particular, we discuss the impact of fabrication details on the performance. We show that the small volume of deposited dielectric materials enabled by the side-wall passivated spacer niobium junction technology leads to robust operation across a wide range of operating temperatures up to 1.5 K. The flux-pumped amplifier has gain in excess of 20 dB in three-wave mixing and its center frequency is tunable between 540 MHz and 640 MHz. At 600 MHz, the amplifier adds 105 mK $\pm$ 9 mK of noise, as determined with the hot/cold source method. Phase-sensitive amplification is demonstrated with the device

Advanced Concepts in Josephson Junction Reflection Amplifiers

Low-noise amplification atmicrowave frequencies has become increasingly important for the research related to superconducting qubits and nanoelectromechanical systems. The fundamental limit of added noise by a phase-preserving amplifier is the standard quantum limit, often expressed as noise temperature $T_{q} = \hbar {\omega}/2k_{B}$. Towards the goal of the quantum limit, we have developed an amplifier based on intrinsic negative resistance of a selectively damped Josephson junction. Here we present measurement results on previously proposed wide-band microwave amplification and discuss the challenges for improvements on the existing designs. We have also studied flux-pumped metamaterial-based parametric amplifiers, whose operating frequency can be widely tuned by external DC-flux, and demonstrate operation at $2\omega$ pumping, in contrast to the typical metamaterial amplifiers pumped via signal lines at $\omega$.Comment: 9 pages, 6 figure

Characterizing cryogenic amplifiers with a matched temperature-variable noise source

We present a cryogenic microwave noise source with a characteristic impedance of 50 $\Omega$, which can be installed in a coaxial line of a cryostat. The bath temperature of the noise source is continuously variable between 0.1 K and 5 K without causing significant back-action heating on the sample space. As a proof-of-concept experiment, we perform Y-factor measurements of an amplifier cascade that includes a traveling wave parametric amplifier and a commercial high electron mobility transistor amplifier. We observe system noise temperatures as low as $680^{+20}_{-200}$ mK at 5.7 GHz corresponding to $1.5^{+0.1}_{-0.7}$ excess photons. The system we present has immediate applications in the validation of solid-state qubit readout lines.Comment: The following article has been accepted by Review of Scientific Instruments. After it is published, it will be found at https://doi.org/10.1063/5.002895

Metallisen termoakustisen äänilähteen optimointi

Propagation of sound in fluids is regarded only as an isentropic process in traditional applications, but the underlying theory of fluid dynamics and thermodynamics gives rise to interesting phenomena in specialized conditions. Thermoacoustic coupling is a weak, localized process which can be used to drive thermal engines with acoustic power. In the inverse situation, thermal energy can be converted into sound and even ultrasound. In this work, theoretical foundations of thermoacoustic loudspeakers are reviewed. Coupling of temperature and pressure in fluids can be explained with two partial differential equations. By solving thermoacoustic fields analytically and numerically, the performance of various loudspeaker designs can be studied. Theory is compared to experiments with the aid of suspended metal wire array loudspeakers manufactured by VTT Technical Research Centre of Finland. It is essential to understand the interplay between frequency-dependent factors that limit acoustic performance. In the search for the ultimate limit of loudspeaker efficiency, it can be justified that the upper bounds are determined by loudspeaker size and the parameters of the fluid which control thermoacoustic coupling strength. Several optimization steps are proposed for improving the metal wire array loudspeakers.Äänen etenemistä kaasussa, kuten esimerkiksi ilmassa, käsitellään perinteisesti isentrooppisena ilmiönä. Teoria virtaus- ja termodynamiikasta on kuitenkin monipuolinen ja sen avulla voidaan ymmärtää mielenkiintoisia ilmiöitä, jotka näkyvät vain erikoistilanteissa. Näistä ilmiöistä yksi on termoakustinen kytkeytyminen, joka sitoo paine- ja lämpötilakentät heikosti yhteen. Lämpövoimakoneita voidaan ajaa termoakustisesti käyttämällä ääniaaltoja teholähteenä. Kääntäen on mahdollista tuottaa lämpöenergiasta ääntä sekä ihmisten kuulemilla taajuuksilla että ultraäänitaajuuksilla. Tässä diplomityössä selitetään teoreettisesti, miten termoakustiset kaiuttimet toimivat. Äänen ja lämpötilan oskillaatioita kuvataan kahdella toisiinsa kytkeytyneellä osittaisdifferentiaaliyhtälöllä. Kaiuttimien suorituskykyä tutkitaan ratkaisemalla ilman paine- ja lämpötilakentät analyyttisesti ja numeerisesti. Tuloksia vertaillaan akustisiin mittauksiin, joissa termoakustisina äänilähteinä toimivat ripustettujen metallilankojen muodostamat hilat. Lankahilat on suunniteltu ja valmistettu Valtion teknillisessä tutkimuskeskuksessa (VTT). Kaiutinanalyysissä selvitetään, miten suorituskykyä eli hyötysuhdetta vähentävät, taajuudesta riippuvat tekijät suhtautuvat toisiinsa. Työssä löydetään termoakustisen kaiuttimen hyötysuhteen yläraja, johon vaikuttavat ainoastaan kaiuttimen koko ja kaiutinta ympäröivän ilman termofysikaaliset ominaisuudet. Lisäksi pohditaan lankahilakaiuttimien optimoimista monelta kantilta