high-Q Microwave Resonator Research Articles

Superconducting surface trap chips for microwave-driven trapped ions

Microwave-driven trapped ion logic gates offer a promising avenue for advancing beyond laser-based logic operations. In future microwave-based operations, however, the joule heat produced by large microwave currents flowing through narrow microwave electrodes would potentially hinder improvements in gate speed and fidelity. Moreover, scalability, particularly in cryogenic trapped ion systems, is impeded by the excessive joule heat. To address these challenges, we present a novel approach: superconducting surface trap chips that integrate high-Q microwave resonators with large current capacities. Utilizing sub-ampere microwave currents in superconducting Nb resonators, we generate substantial magnetic field gradients with significantly reduced losses compared to conventional metal chips. By harnessing the high Q factors of superconducting resonators, we propose a power-efficient two-qubit gate scheme capable of achieving a sub-milliwatt external microwave input power at a gate Rabi frequency of 1 kHz.

Superradiance of Spin Defects in Silicon Carbide for Maser Applications

Masers as telecommunication amplifiers have been known for decades, yet their application is strongly limited due to extreme operating conditions requiring vacuum techniques and cryogenic temperatures. Recently, a new generation of masers has been invented based on optically pumped spin states in pentacene and diamond. In this study, we pave the way for masers based on spin S = 3/2 silicon vacancy (VSi) defects in silicon carbide (SiC) to overcome the microwave generation threshold and discuss the advantages of this highly developed spin hosting material. To achieve population inversion, we optically pump the VSi into their mS = ±1/2 spin sub-states and additionally tune the Zeeman energy splitting by applying an external magnetic field. In this way, the prerequisites for stimulated emission by means of resonant microwaves in the 10 GHz range are fulfilled. On the way to realising a maser, we were able to systematically solve a series of subtasks that improved the underlying relevant physical parameters of the SiC samples. Among others, we investigated the pump efficiency as a function of the optical excitation wavelength and the angle between the magnetic field and the defect symmetry axis in order to boost the population inversion factor, a key figure of merit for the targeted microwave oscillator. Furthermore, we developed a high-Q sapphire microwave resonator (Q ≈ 104–105) with which we find superradiant stimulated microwave emission. In summary, SiC with optimized spin defect density and thus spin relaxation rates is well on its way of becoming a suitable maser gain material with wide-ranging applications.

Entangling two high-Q microwave resonators assisted by a resonator terminated with SQUIDs

We propose a superconducting circuit for quantum information processing (QIP) on high-quality (high-Q) superconducting resonators (SRs). In the circuit, two high-Q SRs are coupled to a high-frequency SR (acts as a quantum bus) assisted by superconducting quantum interference devices (SQUIDs) terminate in both ends of the high-frequency resonator. Each coupling strength between each high-Q resonator and the high-frequency resonator can be tuned independently from zero to the strong-coupling regime via the external flux threading through the SQUID. In the circuit, the frequencies of the two high-Q resonators are far detuned from the high-frequency resonator. That is, quantum information stored in high-Q resonators cannot be populated in the high-frequency resonator, which lets the bus can be designed to link lots of high-Q resonators for the large-scale QIP. To show the circuit can be used to achieve the QIP, we present a high-fidelity scheme to generate Bell state on the two high-Q resonators. The scheme shows that, to achieve the entanglement operation on high-Q resonators, fast tuning on the coupling is no longer mandatory and the coupling strengths are not required to be turned on or off simultaneously.

High-Cooperativity Coupling of a Rare-Earth Spin Ensemble to a Superconducting Resonator Using Yttrium Orthosilicate as a Substrate

Yttrium orthosilicate (Y$_2$SiO$_5$, or YSO) has proved to be a convenient host for rare-earth ions used in demonstrations of microwave quantum memories and optical memories with microwave interfaces, and shows promise for coherent microwave--optical conversion owing to its favourable optical and spin properties. The strong coupling required by such microwave applications could be achieved using superconducting resonators patterned directly on Y$_2$SiO$_5$, and hence we investigate here the use of Y$_2$SiO$_5$ as an alternative to sapphire or silicon substrates for superconducting hybrid device fabrication. A NbN resonator with frequency 6.008 GHz and low power quality factor $Q \approx 400000$ was fabricated on a Y$_2$SiO$_5$ substrate doped with isotopically enriched Nd$^{145}$. Measurements of dielectric loss yield a loss-tangent $\tan\delta = 4 \times 10^{-6}$, comparable to sapphire. Electron spin resonance (ESR) measurements performed using the resonator show the characteristic angular dependence expected from the anisotropic Nd$^{145}$ spin, and the coupling strength between resonator and electron spins is in the high cooperativity regime ($C = 30$). These results demonstrate Y$_2$SiO$_5$ as an excellent substrate for low-loss, high-Q microwave resonators, especially in applications for coupling to optically-accessible rare earth spins.

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Dephasing of entangled atoms as an improved test of quantized space time

Cavity quantum electrodynamics (QED) has been highly successful in the demonstration of many features of quantum mechanics and in this paper we introduce a potential future application of cavity QED to the detection of quantized space time. Wang et al (2006 Class. Quantum Grav. 23 L59), demonstrated that the phase of a particle fluctuated due to interactions with random deviations of a conformal gravitational field and therefore that atom interferometers may be sensitive to these fluctuations. Hence, it is possible that sensitivity to Planck scale effects could be achieved with a sufficiently sensitive interferometer. In this paper we demonstrate that a class of entangled states, the N-atom Greenberger–Horne–Zeilinger states, provide a better scaling than atom interferometry and that current experiments are capable of making a significant impact in this field. We outline an experiment which uses atomic beams of rubidium atoms excited to Rydberg states that undergo controlled collisions in high-Q microwave resonators in a sequence that makes the resulting state highly sensitive to conformal field fluctuations. We show that a significant advance in sensitivity is possible.

Stabilizer quantum error correction toolbox for superconducting qubits.

We present a general protocol for stabilizer operator measurements in a system of N superconducting qubits. Using the dispersive coupling between the qubits and the field of a resonator as well as single qubit rotations, we show how to encode the parity of an arbitrary subset of M ≤ N qubits, onto two quasiorthogonal coherent states of the resonator. Together with a fast cavity readout, this enables the efficient measurement of arbitrary stabilizer operators without locality constraints.

Effect of inhomogeneous microwave field on the threshold of multipactor discharge on a dielectric

Multipactor discharge on a dielectric (quartz) surface has been studied in a high-Q microwave resonator excited by TE01n mode in the 3-cm wavelength band. It is established that the discharge threshold significantly depends on the magnitude and direction of a ponderomotive force, which appears at the dielectric surface due to a gradient of the high-frequency potential. The force that repulses electrons from the surface leads to a significant increase in the threshold, whereas the attractive force decreases the threshold. Experiments confirm that a multipactor can arise in inhomogeneous electromagnetic fields in the absence of an external static field that returns emitted electrons to the dielectric surface.

Parametric excitation of temperature oscillations in a high-Q dielectric microwave resonator in liquid helium vapor

We study the threshold conditions of the parametric excitation of temperature oscillations using pulsed microwave pumping in three-mode regime for high-Q ferroelectric cryogenic resonators made of SrTiO3 and KTaO3. Comparative analysis is performed for the threshold power of the excitation of temperature oscillations and the threshold power of the ponderomotive parametric excitation of acoustic oscillations in the resonator. It is shown that in three-mode regime, temperature oscillations can develop under a rather moderate pump power of about 0.1–8 μW, depending on the combination of interacting temperature and electromagnetic modes. The calculated low threshold powers allow one to apply resonators in practice as high-sensitivity infrared sensors, resonant bolometers, and parametric amplifiers.

Accurate characterization of low-Q microwave resonator using critical-points method

The critical-points method for direct characterization of high-Q microwave resonators is adapted to low-Q resonators. Based on a new approximation, the crossover points are removed and the critical points are used in the computation of the equivalent-circuit elements for the low-Q resonator. Due to the large separation of the crossover frequencies from the resonant frequency, the modification is necessary in order to have a good estimate of the element values in the equivalent circuit. The high accuracy of this one-port measurement method is verified using a microstrip patch antenna.

Accurate and direct characterization of high-Q microwave resonators using one-port measurement

An accurate and direct characterization of high-Q microwave resonators using one-port vector network measurement technique is presented. In this characterization, the delay due to the connector between the resonator and network analyzer is first deembedded. The elements of the equivalent-circuit of the resonator are then readily extracted from the experimental data in a straightforward manner without any numerical optimization. The extraction method works very accurately for high-Q resonators coupled to external circuits electrically or magnetically, and can deal with universal cases including external lossy couplings and weak under-coupled resonators. Parameters such as coupling coefficient, loaded quality factor, and loaded resonant frequency can also be directly computed. For the loaded resonant frequency, an original expression is derived in this paper. This paper also provides an approximate relation between the loaded resonant frequency and measured frequency at which the minimum magnitude of the reflection coefficient occurs. The high accuracy of the method is demonstrated with two examples: a hollow cavity resonator and a dielectric loaded cavity resonator, which is also supported by the well-established one-port multipoint technique.

Pulsed microwave discharge at atmospheric pressure for NOx decomposition

A 3.0 GHz pulsed microwave source operated at atmospheric pressure with apulse power of 1.4 MW, a maximum repetition rate of 40 Hz, and a pulse lengthof 3.5 µs is experimentally studied with respect to the ability toremove NOx from synthetic exhaust gases. Experiments in gas mixturescontaining N2/O2/NO with typically 500 ppm NO are carried out. Thedischarge is embedded in a high-Q microwave resonator, which provides areliable plasma ignition. Vortex flow is applied to the exhaust gas toimprove gas treatment. Concentration measurements by Fourier transform infraredspectroscopy confirm an NOx reduction of more than 90% in the case of N2/NO mixtures. The admixture of oxygen lowers the reductive potential ofthe reactor, but NOx reduction can still be observed up to 9% O2 concentration. Coherent anti-Stokes Raman scattering technique isapplied to measure the vibrational and rotational temperature of N2.Gas temperatures of about 400 K are found, whilst the vibrationaltemperature is 3000-3500 K in pure N2. The vibrational temperaturedrops to 1500 K when O2 and/or NO are present. The randomlydistributed relative frequency of occurrence of selected breakdownfield intensities is measuredby a calibrated, short linear-antenna. The breakdown field strengthin pure N2amounts to 2.2×106 V m-1, a value that is reproduciblewithin 2%. In the case of O2 and/or NO admixture, the frequencydistribution of the breakdown field strength scatters more and extends overa range from 3 to 8×106 V m-1.

Cryogenic high-Q microwave resonators for stable oscillators

Cryogenic microwave resonators have a strong potential as frequency stabilising elements for oscillators to be used in advanced radar systems and high-bitrate microwave communication links. Depending on frequency, either 2D planar HTS-resonators, HTS-shielded sapphire TE/sub 011/ resonators or cryogenic sapphire whispering-gallery mode resonators represent the best compromise between resonator quality factor and size. We have built and tested an all-cryogenic oscillator based on a WG-resonator at f=23 GHz. Phase noise measurements indicate values superior to quartz stabilized oscillators. A two-step electric frequency tuning consisting of an integrated varactor phase shifter and a dielectric plunger moved by a piezomechanical transducer is introduced to compensate frequency drifting with temperature. For further improvement of long-time frequency stability we have developed rutile-sapphire composite dielectric resonators. Due to the opposite sign of the temperature slope of the dielectric constant of sapphire and rutile a turning point appears in the temperature dependence of the resonance frequency. Employing a moderate temperature stabilization as good as a few millikelvin around the turning point at T=78 K, we have demonstrated a long time frequency stability at least as good as for oven controlled quartz oscillators.

Precise measurements of the density and critical phenomena near the phase transitions in helium using high-Q niobium microwave cavities

The experimental approach of using high-Q niobium microwave resonator (Q≈1010) and high resolution thermometry to achieve precise measurements of the density and critical phenomena in liquid helium near phase transitions is described. The numerical verifications of the experimental feasibility as well as the applications of the precision density measurement techniques are discussed.

Microstrip circuit applications of high-Q open microwave resonators

An open microwave resonator can be formed above a planar microstrip substrate by suspending a spherical reflector above it. A theory is developed to account for the coupling between such an open resonator mode and a microstrip line. The open resonator is shown to have useful circuit properties similar to a dielectric resonator, but with the potential of efficient operation well into the millimeter-wave range. Experimental confirmation of the theory is demonstrated by a scale model of a microstrip-based single-pole bandpass filter, which shows a loaded Q of 860 and a minimum loss of 0.8 dB+or-0.4 dB at 10 GHz. >