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Journal of Applied Physics

Normal-mode splitting in coupled high-Q microwave cavities

Alessandro R. Castelli, Luis A. Martinez, Jacob M. Pate, R. Y. Chiao, Jay E. Sharping

Published Nov 5, 2019• 2 citations
Publisher

Abstract

Three-dimensional radio frequency cavities demonstrate excellent frequency selectivity and, as such, are known for their use in RF filters. These cavities have potential applications in quantum information science, precision displacement metrology, and quantum electrodynamics. Additionally, coupled cavities that form a spectral doublet allow for parametric gain when incorporating mechanical elements. Here, we investigate normal-mode splitting in a pair of quarter-wave stub microwave cavities at room temperature and cryogenic environments in order to identify coupling mechanics for normal and superconducting systems. Superconducting quarter-wave stub cavities with a resonant frequency of 10 GHz are made from reactor-grade niobium and exhibit Q ranging from 105 to 109. We varied cavity-to-cavity coupling to observe several normal-mode splittings of increasing peak separation until we observed a mode crossing. The minimum observed peak separation was 7 MHz for room temperature tests and 200 kHz for cryogenic tests. We also report on values of an intrinsic quality factor for the tuning cavity as a dielectric rod is translated along its symmetry axis. The realization of coupled superconducting radio frequency (SRF) cavities of this type is a necessary step toward implementation of parametric SRF-mechanical gain.

DOI
10.1063/1.5110611

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Three-dimensional radio frequency cavities demonstrate excellent frequency selectivity and, as such, are known for their use in RF filters. These cavities have potential applications in quantum information science, precision displacement metrology, and quantum electrodynamics. Additionally, coupled cavities that form a spectral doublet allow for parametric gain when incorporating mechanical elements. Here, we investigate normal-mode splitting in a pair of quarter-wave stub microwave cavities at room temperature and cryogenic environments in order to identify coupling mechanics for normal and superconducting systems. Superconducting quarter-wave stub cavities with a resonant frequency of 10 GHz are made from reactor-grade niobium and exhibit Q ranging from 105 to 109. We varied cavity-to-cavity coupling to observe several normal-mode splittings of increasing peak separation until we observed a mode crossing. The minimum observed peak separation was 7 MHz for room temperature tests and 200 kHz for cryogenic tests. We also report on values of an intrinsic quality factor for the tuning cavity as a dielectric rod is translated along its symmetry axis. The realization of coupled superconducting radio frequency (SRF) cavities of this type is a necessary step toward implementation of parametric SRF-mechanical gain.
Superconducting radio frequency (SRF) cavities present a unique opportunity in their potential for extremely large quality factors (Q) which allow for low-loss microwave systems which extend photon lifetimes. They have subsequently been used and improved upon as individual cells of particle accelerators where large accelerating voltages are required. More recently, they have been used as double SRF cavity systems for coupling electromagnetic fields via qubits inserted into the cavity bodies. Capitalizing on this Q enhancement, we propose an SRF cavity system for parametric amplification of these microwave fields from vacuum. This triple SRF cavity consists of a pump cavity with a silicon nitride (SiN) membrane acting as one end-wall and two signal/idler cavities that are separated by an iris plane. Parametric amplification occurs when some parameter of the system varies sinusoidally in time and this subsequently drives the system into oscillation. Here, we vary the length of the cavities by driving the SiN membrane end-wall into motion using the radiation pressure from the pump cavity. Sufficient energy transfer between cavities will allow amplification and subsequent oscillation of the vacuum fields. We will introduce and explain theoretical background, SRF cavity fabrication and testing, SiN membrane mechanics, and lastly double cavity mechanics. We will identify which facets of the project seem feasible at this time.
A wide variety of applications of microwave cavities, such as measurement and control of superconducting qubits, magnonic resonators, and phase noise filters, would be well served by having a highly tunable microwave resonance. Often this tunability is desired in situ at low temperatures, where one can take advantage of superconducting cavities. To date, such cryogenic tuning while maintaining a high quality factor has been limited to ∼500 MHz. Here we demonstrate a three-dimensional superconducting microwave cavity that shares one wall with a pressurized volume of helium. Upon pressurization of the helium chamber, the microwave cavity is deformed, which results in in situ tuning of its resonant frequency by more than 5 GHz, greater than 60% of the original 8 GHz resonant frequency. The quality factor of the cavity remains approximately constant at ≈7 × 103 over the entire range of tuning. As a demonstration of its usefulness, we implement a tunable cryogenic phase noise filter, which reduces the phase noise of our source by approximately 10 dB above 400 kHz.
Superconducting radio-frequency (SRF) cavities coupled to transmon circuits have proven to be a promising platform for building high-coherence quantum information processors. An essential aspect of this realization involves designing high quality factor three-dimensional superconducting cavities to extend the lifetime of quantum systems. To increase the computational capability of this architecture, we are exploring a multi-mode approach. This paper presents the design optimization process of a multi-cell SRF cavity to perform quantum computation based on an existing design developed in the scope of particle accelerator technology. We perform parametric electromagnetic simulations to evaluate and optimize the design. In particular, we focus on the analysis of the interaction between a nonlinear superconducting circuit known as the transmon and the cavity. This parametric design optimization is structured to serve as a blueprint for future studies on similar systems.
Superconducting radio-frequency (SRF) cavities coupled to transmon circuits have proven to be a promising platform for building high-coherence quantum information processors. An essential aspect of this realization involves designing high quality factor three-dimensional superconducting cavities to extend the lifetime of quantum systems. To increase the computational capability of this architecture, we are exploring a multimode approach. This paper presents the design optimization process of a multi-cell SRF cavity to perform quantum computation based on an existing design developed in the scope of particle accelerator technology. We perform parametric electromagnetic simulations to evaluate and optimize the design. In particular, we focus on the analysis of the interaction between a nonlinear superconducting circuit known as the transmon and the cavity. This parametric design optimization is structured to serve as a blueprint for future studies on similar systems.
The design of a hi-Q resonant RF cavity with coupled detection scheme and associated instrumentation is reported for studying quantum vacuum within the framework of Quantum Electrodynamics (QED), the quantum theory of interaction of matter with radiation. Although this system could possibly serve in any general QED, cavity QED and RF applications in the range of a few GHz, the design was specifically devised for an experiment in studying intrinsic noise in quantum vacuum owing to vacuum fluctuations, which we call “Eavesdropping on Vacuum”. The center frequency of the cavity is 2.75GHz (which may be modified slightly to around 2.5GHz band for use in contemporary WiLAN and WiFi RF applications), with a high Q value. Initial design is made using copper but in the next stage it will be replaced with Niobium for use in low-temperature (around 0.1-0.3K) studies, making it an SRF (Superconducting Radio Frequency) cavity. A carefully devised scheme of sensitive RF detectors and ultra-low-noise cryogenic Low-Noise Amplifiers (cLNA) is an integral part of the cavity system. Detection element is an array of cavity-backed Au or Au-NbN spiral antennas in association with GaAs Schottky Barrier Diodes (SBD's), in conjunction with GaAs/InP HEMT's working in the GHz to THz regime. External fields are achieved with the help of time-dependent RF fields delivered to the cavity with the help of an antenna coupled with a GaAs Resonant Tunneling Diode (RTD). In addition to the cavity design, this report presents an overview of the detection scheme as well as necessary instrumentation, including RF detection, amplification, and the DAQ (Data Acquisition) required for digital read-out.
Frequency tunability of three dimensional (3D) microwave cavities opens up numerous possibilities for their use in hybrid quantum systems and related technologies. For many applications, it is desirable to tune the resonance at cryogenic temperatures without mechanical actuation. We show that a superconducting 3D microwave cavity can be tuned at the percent level by taking advantage of the dielectric properties of superfluid 4He at millikelvin temperatures, without affecting its intrinsic quality factor—reaching 3 × 105 in the present experiment.