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arXiv:2406.19476

A Traveling-Wave Parametric Amplifier and Converter

M. Malnou, Miller, B. T., J. A. Estrada, Kristen L. Genter, Katarina Cicak, John D. Teufel, José Aumentado, Florent Lecocq

Published Jun 27, 2024• 1 citation
arXivHTMLPDF

Abstract

High-fidelity qubit measurement is a critical element of all quantum computing architectures. In superconducting systems, qubits are typically measured by probing a readout resonator with a weak microwave tone that must be amplified before reaching the room temperature electronics. Superconducting parametric amplifiers have been widely adopted as the first amplifier in the chain, primarily because of their low noise performance, approaching the quantum limit. However, they require isolators and circulators to route signals up the measurement chain and to protect qubits from amplified noise. While these commercial components are wideband and simple to use, their intrinsic loss, size, and magnetic shielding requirements impact overall measurement efficiency and scalability. Here we report a parametric amplifier that achieves both broadband forward amplification and backward isolation in a single, compact, non-magnetic circuit that could be integrated on chip with superconducting qubits. The approach relies on a nonlinear transmission line that supports traveling-wave parametric amplification of forward propagating signals, and isolation via frequency conversion of backward propagating signals. This traveling-wave parametric amplifier and converter has the potential to reduce the readout hardware overhead when scaling up the size of superconducting quantum computers.

DOI
10.48550/arxiv.2406.19476

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High-fidelity qubit measurement is a critical element of all quantum computing architectures. In superconducting systems, qubits are typically measured by probing a readout resonator with a weak microwave tone that must be amplified before reaching the room temperature electronics. Superconducting parametric amplifiers have been widely adopted as the first amplifier in the chain, primarily because of their low noise performance, approaching the quantum limit. However, they require isolators and circulators to route signals up the measurement chain and to protect qubits from amplified noise. While these commercial components are wideband and simple to use, their intrinsic loss, size, and magnetic shielding requirements impact overall measurement efficiency and scalability. Here we report a parametric amplifier that achieves both broadband forward amplification and backward isolation in a single, compact, non-magnetic circuit that could be integrated on chip with superconducting qubits. The approach relies on a nonlinear transmission line that supports traveling-wave parametric amplification of forward propagating signals, and isolation via frequency conversion of backward propagating signals. This traveling-wave parametric amplifier and converter has the potential to reduce the readout hardware overhead when scaling up the size of superconducting quantum computers.
Superconducting traveling-wave parametric amplifiers have emerged as highly promising devices for near-quantum-limited broadband amplification of microwave signals and are essential for high quantum-efficiency microwave readout lines. Built-in isolation, as well as gain, would address their primary limitation: lack of true directionality due to potential backward travel of electromagnetic radiation to their input port. Here, we demonstrate a Josephson-junction-based traveling-wave parametric amplifier isolator. It utilizes third-order nonlinearity for amplification and second-order nonlinearity for frequency upconversion of backward propagating modes to provide reverse isolation. These parametric processes, enhanced by a novel phase matching mechanism, exhibit gain of up to 20~dB and reverse isolation of up to 30~dB over a static 3~dB bandwidth greater than 500~MHz, while keeping near-quantum limited added noise. This demonstration of a broadband truly directional amplifier ultimately paves the way towards broadband quantum-limited microwave amplification lines without bulky magnetic isolators and with inhibited back-action.
Traveling-Wave Parametric Amplifiers (TWPAs) are ubiquitous in superconducting circuit readout, providing high gain with near-quantum-limited noise performance across a wide bandwidth. Their operation, however, relies on a strong microwave pump tone that is typically delivered using off-chip passive components, such as directional couplers or diplexers. These external elements add loss, increase system complexity, and limit scalability. Here, we present a traveling-wave parametric amplifier that incorporates on-chip input and output diplexers for pump routing. This co-fabricated architecture offers a compact and scalable solution for superconducting-circuit readout.
Traveling-wave parametric amplifiers (TWPAs) have attracted much attention for their broadband amplification and near-quantum-limited noise performance. TWPAs are nonreciprocal by nature providing gain for forward-propagating signals and transmission line losses for backward traveling waves. This intrinsic nonreciprocity is insufficient to protect sensitive quantum devices from backaction due to noise from warmer amplification stages in practical systems, and requires the use of bulky cryogenic isolators. We present a multistage TWPA (mTWPA) that addresses this limitation by achieving passive in-band reverse isolation alongside near-quantum-limited noise performance. The multistage architecture consists of two mode conversion stages and a reflectionless high-pass filter that provides the passive isolation. Experimental measurements of a prototype mTWPA demonstrated 20 dB of forward gain across a 1.6 GHz bandwidth and greater than 35 dB of reverse isolation. Noise measurements indicate performance at 1.7 times the quantum limit. This demonstrates that the increased complexity of a multistage TWPA design does not lead to significant added noise. The designed distribution of gain across the stages is engineered to minimize internal amplifier noise at the input, and we propose further optimization strategies in redistribution of the gain between the stages. This level of isolation mitigates noise from warmer amplification stages, matching the performance of conventional isolators. The passive reverse isolation of the mTWPA, allows for the amplifier to operate in pulsed mode where the amplifier is off during qubit gate operations and is turned on during readout allowing for an isolator free scalable readout architecture.
Detecting single-photon level signals—carriers of both classical and quantum information—is particularly challenging for low-energy microwave frequency excitations. Here we introduce a superconducting amplifier based on a Josephson junction transmission line. Unlike current standing-wave parametric amplifiers, this traveling wave architecture robustly achieves high gain over a bandwidth of several gigahertz with sufficient dynamic range to read out 20 superconducting qubits. To achieve this performance, we introduce a subwavelength resonant phase-matching technique that enables the creation of nonlinear microwave devices with unique dispersion relations. We benchmark the amplifier with weak measurements, obtaining a high quantum efficiency of 75% (70% including noise added by amplifiers following the Josephson amplifier). With a flexible design based on compact lumped elements, this Josephson amplifier has broad applicability to microwave metrology and quantum optics.
When making a quantum measurement, one must be careful not to disturb the system. Nonreciprocal elements called isolators allow microwave signals to propagate in one direction but not the reverse, and so can mitigate disturbance, but commercial isolators require strong permanent magnets that cannot be placed close to superconducting amplifiers or qubits. To this end, the authors demonstrate a microwave isolator with directionality provided by a strong, traveling ``pump'' wave. Its simple construction and large bandwidth afford this isolator integrability with existing superconducting quantum hardware.
Superconducting qubits are a leading platform for scalable quantum computing and quantum error correction. One feature of this platform is the ability to perform projective measurements orders of magnitude more quickly than qubit decoherence times. Such measurements are enabled by the use of quantum-limited parametric amplifiers in conjunction with ferrite circulators-magnetic devices which provide isolation from noise and decoherence due to amplifier backaction. Because these nonreciprocal elements have limited performance and are not easily integrated on chip, it has been a long-standing goal to replace them with a scalable alternative. Here, we demonstrate a solution to this problem by using a superconducting switch to control the coupling between a qubit and amplifier. Doing so, we measure a transmon qubit using a single, chip-scale device to provide both parametric amplification and isolation from the bulk of amplifier backaction. This measurement is also fast, high fidelity, and has 70% efficiency, comparable to the best that has been reported in any superconducting qubit measurement. As such, this work constitutes a high-quality platform for the scalable measurement of superconducting qubits.