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IEEE MTT-S International Microwave Symposium digest

Characterization of a Josephson Junction Comb Generator

Akim A. Babenko, Alírio Boaventura, Nathan E. Flowers-Jacobs, Justus A. Brevik, Anna E. Fox, Dylan Forrest Williams, Zoya B. Popovic, Paul D. Dresselhaus, Samuel P. Benz

Published Aug 1, 2020• 4 citations
Publisher

Abstract

We present a new type of microwave frequency combs with a potentially calculable pulse shape. The device is an array of 1500 Josephson junctions (JJs) connected in series along a transmission line. The pulse generation is based on the nonlinearity of the JJs. A large-signal network analyzer and a cryogenic probe station are used to characterize the pulses in the frequency domain up to 50 GHz. We compare the measured data to simulations that use the resistively and capacitively shunted JJ model. The amplitude stability of the demonstrated comb generator is better than 0.5 dB per 0.1 dB input drive variation within the operating range. Finally, we observe qualitative agreement between the measured and simulated power spectrum dependence on the input power, and discuss possible improvements to the system model.

DOI
10.1109/ims30576.2020.9223811

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We present a new type of microwave frequency combs with a potentially calculable pulse shape. The device is an array of 1500 Josephson junctions (JJs) connected in series along a transmission line. The pulse generation is based on the nonlinearity of the JJs. A large-signal network analyzer and a cryogenic probe station are used to characterize the pulses in the frequency domain up to 50 GHz. We compare the measured data to simulations that use the resistively and capacitively shunted JJ model. The amplitude stability of the demonstrated comb generator is better than 0.5 dB per 0.1 dB input drive variation within the operating range. Finally, we observe qualitative agreement between the measured and simulated power spectrum dependence on the input power, and discuss possible improvements to the system model.
Frequency combs represent exceptionally precise measurement tools due to the coherence of their spectral lines. While optical frequency comb sources constitute a well-established technology, superconducting circuits provide a relatively unexplored on-chip platform for low-dissipation comb emitters able to span from gigahertz to terahertz frequencies. We demonstrate coherent microwave frequency comb generation by leveraging the ac Josephson effect in a superconducting quantum interference device. A time-dependent magnetic drive periodically generates voltage pulses, which in the frequency domain correspond to a comb with dozens of spectral modes here reported up to mode 46. The emitted power at the device level ranges from -170 dBm to -130 dBm per harmonic, corresponding to 40 dB dynamic range in the 4-8 GHz bandwidth. The micrometer-scale footprint and minimal dissipation inherent to superconducting systems foster the integration of our comb generator with advanced cryogenic electronics. Transferring optical techniques to the solid-state domain may enable new applications in quantum technologies.
Microwave frequency combs constitute promising resources for quantum technologies, cryogenic electronics, and multiplexed sensing architectures. In this work, we propose a frequency-comb generator based on a Josephson field-effect transistor operated in a relaxation-oscillation regime. The device comprises a gate-tunable ballistic superconductor-semiconductor-superconductor junction embedded in a resistively shunted circuit, in which electrostatic control of the carrier density enables in situ tuning of both the critical current and the Josephson inductance. Time-domain circuit simulations indicate that the resulting oscillator produces coherent voltage pulses whose Fourier spectrum forms a microwave frequency comb. In contrast to conventional Josephson-based comb architectures, the proposed platform provides direct electrical control of the comb spacing, emission frequencies, and modal power distribution via a gate electrode. For a representative Al/InAs implementation, we demonstrate continuous frequency coverage in the technologically relevant 1-10 GHz range. Furthermore, the concept is shown to be compatible with higher-TcT_cTc​ superconductors, underscoring its potential as a compact and scalable microwave source for cryogenic quantum information and sensing applications.
We demonstrate a sub-GHz resolution, fully programmable Fourier-domain pulse shaper capable of generating arbitrary optical pulse patterns for superconducting circuit platforms. This high resolution allows line-by-line pulse shaping of a 1 GHz-spaced comb, and the pulse shaper can accommodate an optical bandwidth as large as 1 THz, which represents the highest resolution programmable line-by-line pulse shaping to our knowledge. Linear optical sampling with a dual-comb system confirms independent control of 1 GHz-spaced optical lines, and the low phase noise of the pulse shaper is characterized. We apply the pulse shaper as an optical drive for an array of Josephson junctions operating at a temperature of 4 K, where cryogenic photodetection of pulse doublets with user-defined separation characterizes the Josephson junction response. Furthermore, we demonstrate a pulse-density modulation pattern of 4 ps duration optical pulses that can serve as the high bandwidth drive of a quantum-based Josephson arbitrary waveform synthesizer. By leveraging the exquisite control, large bandwidth, and low noise of photonics, this represents an important advance toward the realization of high power and high spectral purity AC voltage standards at gigahertz frequencies without requiring 100 GHz bandwidth driving electronics.
Work with large two-dimensional arrays of Josephson junctions for submillimeter power generation is discussed. The basic design of the quasioptical Josephson oscillator (QJO) is presented. The reasons for each design decision are discussed. Superconducting devices have not yet been fabricated, but scale models and computer simulations have been done. A novel method for characterizing array RF coupling structures is described, and initial results with this model are presented. Microwave scale models of the radiation structure were built, and a series of measurements was made with a network analyzer. These measurements were summed in a computer to find properties of the structure when all elements in it are operating in phase. The goal of these measurements was to develop structures for oscillators which are tunable over a very broad band. Computer programs simulating a Josephson junction in any frequency-dependent coupling structure have been developed. An iterative harmonic balance technique finds the time-domain current and voltage waveforms across the junction for an arbitrary frequency-dependent RF load impedance. DC bias, and therefore oscillator frequency, can be held fixed in this technique, in contrast to time-domain techniques where the oscillation frequency is found after the waveform is found. With these programs design goals for maximum oscillator power and design limits to achieve tolerable harmonic distortion were found.