As the need for scaling up quantum computers increases there is also an increased need for more compact adjacent circuitry surrounding the quantum processor. In the amplification of a qubit's readout-signal there is a need for quantum-limited amplifiers achieving a high signal-to-noise ratio. Today, it is common to use traveling-wave parametric amplifiers (TWPAs), which are currently powered by an external microwave pump. To further reduce the circuitry an option is to use an on-chip pump only requiring DC-wires to operate. This is possible due to the AC Josephson effect which can convert a DC voltage to an AC current. By connecting multiple Josephson junctions in series it is possible to increase the output power by utilizing their coherent oscillations. The purpose of this project was to simulate and characterize Josephson voltage controlled oscillators in terms of power and frequency-output, with the goal of providing the pump to a TWPA. This also included bandpass filters utilizing the Josephson inductance, to remove higher order harmonics of the oscillations. These bandpass filters were also characterized both in simulations and in measurements. To evaluate these components, a cryogenic testbed was implemented, and benchmarked, with a capacity of 12 parallel experiments. Simulations showed that it was possible to use Josephson junctions to produce stable oscillations which were filtered to give a pure 5 GHz signal with a -86.78 dBm power. When the corresponding device was measured it was not possible to observe the harmonic spectrum, likely due to noise in the DC-biasing system. The measured bandpass filters exhibited either high insertion loss or limited bandwidth as compared to design-specifications. A power-threshold was identified at approximately -90 dBm at which the bandpass filters showed severe non-linear effects, both in simulations and in measurements.
As the need for scaling up quantum computers increases there is also an increased need for more compact adjacent circuitry surrounding the quantum processor. In the amplification of a qubit's readout-signal there is a need for quantum-limited amplifiers achieving a high signal-to-noise ratio. Today, it is common to use traveling-wave parametric amplifiers (TWPAs), which are currently powered by an external microwave pump. To further reduce the circuitry an option is to use an on-chip pump only requiring DC-wires to operate. This is possible due to the AC Josephson effect which can convert a DC voltage to an AC current. By connecting multiple Josephson junctions in series it is possible to increase the output power by utilizing their coherent oscillations. The purpose of this project was to simulate and characterize Josephson voltage controlled oscillators in terms of power and frequency-output, with the goal of providing the pump to a TWPA. This also included bandpass filters utilizing the Josephson inductance, to remove higher order harmonics of the oscillations. These bandpass filters were also characterized both in simulations and in measurements. To evaluate these components, a cryogenic testbed was implemented, and benchmarked, with a capacity of 12 parallel experiments. Simulations showed that it was possible to use Josephson junctions to produce stable oscillations which were filtered to give a pure 5 GHz signal with a -86.78 dBm power. When the corresponding device was measured it was not possible to observe the harmonic spectrum, likely due to noise in the DC-biasing system. The measured bandpass filters exhibited either high insertion loss or limited bandwidth as compared to design-specifications. A power-threshold was identified at approximately -90 dBm at which the bandpass filters showed severe non-linear effects, both in simulations and in measurements.
Bringing signals from quantum to classical, from few-photon to many-photon levels for further processing, is a common challenge in rising quantum technologies. These include quantum computing, quantum communication, quantum sensing, radio astronomy, and dark matter searches, typically operating at microwave frequencies and millikelvin temperatures. Superconducting parametric amplifiers, as the first piece in the readout chain, have become a cornerstone of these technologies, since they offer unrivaled noise performance near the quantum-mechanical limit and high gain to surpass the noise of subsequent amplifiers. A notable implementation is the Josephson traveling-wave parametric amplifier (TWPA), which stands out due to its bandwidth of several Gigahertz and high saturation power, often larger than -100 dBm. These key properties come at the cost of complexity in design and fabrication: TWPA metamaterials often contain several thousands of Josephson junctions. Designing high-gain TWPAs involves three major challenges: ensuring impedance-matching, phase-matching of parametric amplification, and suppressing unwanted processes, all achievable through sub-wavelength dispersion engineering. This thesis covers the circuit design and analysis, implementation, and experimental characterization of a dispersion-engineered TWPA based on radio-frequency superconducting interference devices (rf-SQUIDs). It focuses on a detailed investigation of dispersion-engineering techniques and their effects on amplifier performance, particularly through the use of time-domain circuit simulations to analyze the multimode dynamics of the circuit. The experimentally realized device features 18 dB of gain, an instantaneous bandwidth spanning from 2.2 to 6.7 GHz, a system noise of 2.5 quanta, and a saturation power of (−86.5 ± 4.0) dBm. With these characteristics, it is suitable for frequency-multiplexed readout of detector arrays or qubits. Its high saturation power, about 10 dB higher than other reported Josephson-junction-based TWPAs, allows increasing the number of qubits per readout line, or decreasing the intermodulation between frequency-multiplexed qubits, both crucial for scaling up quantum computation.
The recent developments in quantum technologies, as well as advanced detection experiments, have raised the need to detect extremely weak signals in the microwave frequency spectrum. To this aim, the Josephson travelling wave parametric amplifier, a device capable of reaching the quantum noise limit while providing a wide bandwidth, has been proposed as a suitable cryogenic front-end amplifier. This work deals with the numerical study of a Josephson travelling wave parametric amplifier, without approximations regarding the nonlinearity of the key elements. In particular, we focus on the investigation of the system of coupled nonlinear differential equations representing all the cells of the Josephson travelling wave parametric amplifier, with proper input and output signals at the boundaries. The investigation of the output signals generated by the parametric amplification process explores the phase-space and the Fourier spectral analysis of the output voltage, as a function of the parameters describing the pump and signal tones that excite the device. Beside the expected behavior, i.e., the signal amplification, we show that, depending on the system operation, unwanted effects (such as pump tone harmonics, incommensurate frequency generation, and noise rise), which are not accounted for in simple linearized approaches, can be generated in the whole nonlinear system.
Recent advancements in quantum technologies and advanced detection experiments have underscored the pressing need for the detection of exceedingly weak signals within the microwave frequency spectrum. Addressing this challenge, the Josephson Traveling Wave Parametric Amplifier (JTWPA) has been proposed as a cryogenic front-end amplifier capable of approaching the quantum noise limit while providing a relevant bandwidth. This research is centered on a comprehensive numerical investigation of the JTWPA, without resorting to simplifications regarding the nonlinearity of the essential components. Specifically, this study focuses on a thorough examination of the system, characterized by coupled nonlinear differential equations representing all components of the device. Proper input and output signals at the device's boundaries are considered. The analysis of the output signals undergoing the parametric amplification process involves a detailed exploration of phase-space dynamics and Fourier spectral analysis of the output voltage. This study is conducted while considering the parameters ruling the response of the device under pump and signal excitations. In addition to the expected signal amplification, the findings reveal that the nonlinear nature of the system can give rise to unforeseen phenomena, depending on the system's operational conditions, which include: the generation of pump tone harmonics, modulation of the signal gain, and incommensurate frequency generation—effects that are not easily accommodated by simplistic linearized approaches.
Ultralow-noise microwave amplification and detection play a central role in different applications, going from fundamental physics experiments to the deployment of quantum technologies. In many applications the necessity of reading multiple detectors, or cavities or qubits, calls for large bandwidth amplifiers with the lowest possible noise. Current technologies are based on High Electron Mobility Transistors and Josephson Parametric Amplifiers. Both have limitations, the former in terms of the minimum noise, the latter in terms of bandwidth. Superconducting Traveling Wave Parametric Amplifiers (TWPAs) have the potential of offering quantum limited noise and large bandwidth. These amplifiers are based on the parametric amplification of microwaves traveling along a transmission line with embedded nonlinear elements. We are developing superconducting TWPAs based both on Josephson junction arrays (Traveling Wave Josephson Parametric Amplifiers) and on nonlinear kinetic inductance (Dispersion Engineered Traveling Wave Kinetic Inductance Amplifiers). Our goal is to achieve large bandwidth (in the 5 to 10 GHz range), large gain (more than 20 dB), large saturation power (more than −50 dBm), and near quantum limited noise (noise temperature less than 600 mK). Current achievements in the design and development of the high performance TWPAs are here reported and discussed, together with current limitations and possible future developments.
Advancing error-corrected quantum computing and fundamental science necessitates quantum-limited amplifiers with near-ideal quantum efficiency and multiplexing capability. However, existing solutions achieve one at the expense of the other; for example, Josephson traveling wave parametric amplifiers (JTWPAs) are highgain, broadband, and chip-based quantum amplifiers that conventionally incur a bandwidth-noise tradeoff. When operated at 20-dB gain and instantaneous bandwidths of a few GHz, JTWPAs typically reach near-quantum limited intrinsic efficiencies of 70% - 85% relative to that of an ideal phase-preserving quantum amplifier. This is due to information leakage to the sidebands of the JTWPA, which can be recovered by adiabatically transforming the input modes to Floquet modes of the system within the device. In this thesis, we experimentally demonstrate the first Floquet-mode travelingwave parametric amplifier (Floquet TWPA). Fabricated in a superconducting qubit process, this Floquet TWPA achieves minimal dissipation, quantum-limited noise performance, and broadband operation. Our device exhibits > 20-dB amplification over a 3-GHz instantaneous bandwidth, <0.5 -dB average in-band insertion loss, and the highest-reported intrinsic quantum efficiency for a TWPA of 92.1±7.6%, relative to an ideal phase-preserving amplifier. When measuring a superconducting qubit, our Floquet TWPA enables a system measurement efficiency of 65.1 ± 5.8%, the highest-reported in a superconducting qubit readout experiment utilizing phase-preserving amplifiers to the best of our knowledge. Finally, we discuss the noise limitations of our current experimental setup, as well as impedance matching strategies that will enable us to push towards ideal JTWPA performance. These general-purpose Floquet TWPAs are suitable for fast, high-fidelity multiplexed readout in large-scale quantum systems and future monolithic integration with quantum processors.
Josephson traveling-wave parametric amplifiers (TWPAs) enable the amplification of weak microwave signals close to the quantum limit with large bandwidth, which has a broad range of applications in superconducting quantum computing and in the operation of single-photon detectors. While the large bandwidth allows for their use in frequency-multiplexed detection architectures, an increased number of readout tones per amplifier puts more stringent requirements on the dynamic range to avoid saturation. Here, we characterize the undesired mixing processes between the different frequency-multiplexed tones applied to a Josephson TWPA, a phenomenon also known as intermodulation distortion. The effect becomes particularly significant when the amplifier is operated close to its saturation power. Furthermore, we demonstrate that intermodulation distortion can lead to significant crosstalk and reduction of fidelity for multiplexed readout of superconducting qubits. We suggest using large detunings between the pump and signal frequencies to mitigate crosstalk. Our work provides insights into the limitations of current Josephson TWPAs and highlights the importance of performing further research on these devices.