Short answer: A kinetic inductance traveling-wave amplifier, or KIT, is a wideband microwave amplifier built from a superconducting transmission line whose inductance changes with signal power. A 2018 Applied Physics Letters paper reported simultaneous readout of two transmon qubits using one with fidelities of 83% and 89%, and a 2021 PRX Quantum paper reported a three-wave-mixing version with noise approaching the quantum limit. A 2024 APL Quantum paper reports a 4 to 8 GHz device, but only its title could be read for this post.
What the amplifier is
The 2018 abstract (Ranzani and coauthors, with affiliations at Raytheon BBN and a national standards laboratory) describes a KIT amplifier suitable for superconducting quantum information measurements. The introduction explains that readout signals are usually amplified with Josephson parametric amplifiers. Lumped-element versions typically have low saturation power and bandwidths of a few megahertz, which can be extended to 700 MHz with impedance matching. A traveling-wave design aims at wider bandwidth for larger circuits.
2018: reading two qubits at once
The authors calibrate the four scattering parameters of the amplifier at the base temperature of a dilution refrigerator, and characterize its noise with a tunable temperature load. They then demonstrate simultaneous dispersive readout of two superconducting transmon qubits. The amplifier gives low-noise amplification of both readout tones, with readout fidelities of 83% and 89%, and negligible effect on qubit lifetime and coherence, according to the authors.
2021: three-wave mixing
The PRX Quantum abstract presents a theoretical model and experimental characterization of a microwave KIT amplifier whose noise, measured with a shot-noise tunnel junction, approaches the quantum limit. Biased with a dc current, it works by three-wave mixing, which the authors say reduces the pump power and parasitic heating by several orders of magnitude compared with conventional four-wave-mixing devices. The device is an artificial transmission line whose dispersion allows a controlled amplification bandwidth. The abstract gives a 2 GHz gain bandwidth and a system-added noise measured between 3.5 and 5.5 GHz; the gain and noise numbers did not render in the retrieved text, so they are not quoted here. The authors conclude the amplifier is suitable for reading large arrays of microwave kinetic inductance detectors and promising for multiplexed superconducting qubit readout, which is their outlook.
2024: a 4 to 8 GHz device
An APL Quantum article from 2024 is titled "A 4-8 GHz kinetic inductance traveling-wave parametric amplifier using four-wave mixing with near quantum-limited noise performance". Only the title and section headings were readable, so this post does not describe its results beyond the title's claim.
Limits
The qubit readout fidelities were achieved in a specific lab setup with two qubits. The three-wave-mixing paper is an amplifier characterization, not a qubit experiment. No source here reports a KIT in a processor of useful size.
FAQ
What does a KIT amplifier do?
It amplifies weak microwave readout signals over a wide bandwidth.
Why use it instead of a Josephson amplifier?
The 2018 text says lumped-element Josephson amplifiers typically have low saturation power and narrow bandwidth.
What did the 2018 experiment show?
Simultaneous readout of two transmons with 83% and 89% fidelity.
What is three-wave mixing here?
A pumping method the 2021 authors say cuts pump power and parasitic heating.
Is this in use in quantum computers?
These sources report laboratory characterizations only.
All field notes