Josephson traveling-wave parametric amplifiers (JTWPAs) are key to fast, frequency-multiplexed measurements in superconducting circuits. JTWPAs with periodic modulation are attractive, as they phase match without flux or dc bias, but they often suffer from significant gain ripples. Another common limitation in JTWPAs is intrinsic loss, which has prevented them from reaching the quantum limit of added noise. Here researchers address both issues by implementing an all-aluminum coplanar lumped-element JTWPA, using a qubit-compatible fabrication recipe. Their modulated device suppresses gain ripples for smooth, bias-free amplification, and approaches the quantum limit of added noise.
Josephson traveling-wave parametric amplifiers (JTWPAs) are key to fast, frequency-multiplexed measurements in superconducting circuits. JTWPAs with periodic modulation are attractive, as they phase match without flux or dc bias, but they often suffer from significant gain ripples. Another common limitation in JTWPAs is intrinsic loss, which has prevented them from reaching the quantum limit of added noise. Here researchers address both issues by implementing an all-aluminum coplanar lumped-element JTWPA, using a qubit-compatible fabrication recipe. Their modulated device suppresses gain ripples for smooth, bias-free amplification, and approaches the quantum limit of added noise.
Abstract A Josephson traveling-wave parametric amplifier (JTWPA) is indispensable for the high-fidelity readout of superconducting qubits, owing to its broadband gain and near-quantum-limited noise performance. Conventional JTWPAs implemented with 0-Josephson junctions (0-JTWPAs) avoid magnetic-flux bias but rely on Kerr nonlinearity, which leads to pump-power-dependent phase mismatch, gain ripple, and a limited dynamic range. More recently, three-wave-mixing (3WM) JTWPAs employing flux-biased rf-SQUIDs have been proposed to suppress Kerr effects. However, these designs require a significant external flux bias per unit cell, which complicates cryogenic hardware, introduces sensitivity to flux noise, and limits scalability in large arrays. In this study, we propose and analyze a π -junction-based JTWPA ( π -JTWPA) in which the intrinsic π phase shift of the junction effectively displaces the operating point of the current–phase relation. This approach aims to realize 3WM-type nonlinearity with reduced flux bias while preserving broadband amplification, thereby simplifying flux-bias distribution, enhancing robustness against flux noise, and providing a pathway toward scalable, multiplexed quantum readout architectures. In addition, we examine the potential applicability of the π -JTWPA in four-wave-mixing (4WM) operation.
Recent progress in physics has increased the necessity for ultra-low noise amplifiers capable of operating at the quantum limit performance. These amplifiers are essential to enhance astronomical detection, dark-matter search, and qubit readout in quantum computing, among other applications. Superconducting Josephson junction Travelling Wave Parametric Amplifiers (JTWPAs) operated in the microwave range, have demonstrated high gain over several gigahertz (GHz) bandwidth, with quantum-limited noise performance and minimal heat dissipation. These excellent properties make JTWPAs an ideal solution for ultra-low noise amplification. This thesis delves into the design, fabrication and characterisation of JTWPAs operated in the microwave (1-16 GHz) and the W-band (70-115GHz) regimes. A generalised coupled-mode equations (CME) framework describing JTWPAs based on Josephson junctions and DC superconducting quantum interference devices (SQUIDs) is introduced. This framework can be combined with electromagnetic (EM) simulation softwares to model the JTWPAs, facilitating their design. It is therefore used to model two microwave JTWPA designs, using parallel plate capacitors (PPCs) and interdigitated capacitors (IDCs). The fabrication and characterisation of both designs are presented, and possible fabrication and design issues are carefully discussed. Despite these issues, we observed broadband gain in the JTWPA with PPCs, achieving > 20 dB gain at several frequency points, and a measured P1dB = −95.8dBm when operated at 17.3 dB. The RF properties of a 500-junction Josephson array are also presented in this manuscript. The device creates a Fabry-Perot cavity, leading to non-travelling wave parametric effects, such as narrow-band gain. We also observed indications of efficient phase-sensitive amplification. The thesis is concluded with the first — to the best of our knowledge — experimental attempt to extend the operational frequency of a JTWPA to the W-band. The novel design, fabrication techniques and characterisation of the W-band JTWPA are discussed in detail in this manuscript, leading to promising results suggesting that JTWPA could operate at high frequencies.
The development of large-scale superconducting quantum computers critically depends on the accurate readout of qubit states. To enable this, cryogenic parametric amplifiers have been proposed and developed, and have since found widespread use in small-signal detection applications. Among various amplifier architectures, Josephson traveling-wave parametric amplifiers (JTWPAs) are widely adopted due to their broad bandwidth, high saturation power, and compatibility with superconducting qubits. In this work, we present a design workflow of JTWPAs based on a co-simulation approach combining electromagnetic (EM) and circuit-level simulation. We apply this method to a uniform JTWPA, comparing our simulation results with experimental data. To mitigate the challenge of constructing lumped element circuit models usually used in conventional simulation method, we directly perform full EM analysis of the device to accurately determine its linear response. The extracted S parameters are then fed into a nonlinear harmonic balance simulator with Josephson junctions modeled as circuit elements. By doing so, the linear and gain properties of the JTWPA are extracted. Different factors that impact device performance including gain compression, phase matching, and fabrication imperfections are further investigated and modeled. A good agreement between simulation and measurements is identified, establishing the effectiveness of our approach in designing quantum parametric amplifiers.
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 junction based parametric amplifiers are used to read out the state of superconducting qubits with high fidelity. These amplifiers are themselves superconducting circuits operating at temperatures of tens of millikelvin. A version of this type of amplifier which has exhibited large bandwidth and saturation power is the Josephson traveling wave parametric amplifier (JTWPA). One particular JTWPA topology consists of hundreds to thousands of Josephson junctions in a transmission line configuration along with periodic phase matching resonators to increase gain. Here, we look at using a superconducting qubit fabrication process to realize this circuit To analyze the sensitivity of the JTWPA to fabrication process variation, we discuss a linear model for the circuit which takes into account unit cell to unit cell variation. To extract the measured circuit values, we use in-situ measurements of a JTWPA to compare the designed values to the realized values. Finally we discuss preliminary results realizing an example JTWPA design using a fabrication process designed for superconducting qubits.
Josephson traveling-wave parametric amplifiers (JTWPAs) are wideband, ultralow-noise amplifiers used to enable the readout of superconducting qubits. While individual JTWPAs have achieved high performance, behavior between devices is inconsistent due to wide manufacturing tolerances. Amplifier designs could be modified to improve resilience towards variations in amplifier components; however, existing device models often rely on analytical techniques that typically fail to incorporate component variations. To begin addressing this issue, a 1D numerical method for modeling JTWPAs is introduced in this work. The method treats the Josephson junctions and transmission lines in an amplifier as coupled subsystems and can easily incorporate arbitrary parameter variations. We discretize the transmission line subsystem with a finite element time domain method and the Josephson junction subsystem with a finite difference method, with leap-frog time marching used to evolve the system in time. We validate our method by comparing the computed gain to an analytical model for a traditional JTWPA architecture and one with resonant phase matching. We then use our method to demonstrate the impact of variations in Josephson junctions and phase-matching resonators on amplification. In future work, the method will be adjusted to incorporate additional amplifier architectures and extended to a 3D full-wave approach.