Ultra-sensitive detection schemes at microwave frequencies play a central role in quantum computing and quantum sensing.
In many applications, the need to read a large array of devices (e.g. qubits, detectors, and cavities) calls for large-bandwidth amplifiers with the lowest possible noise.
A leading proposal for achieving broadband bandwidth and noise at the standard quantum limit is through the use of a traveling wave parametric amplifier (TWPA) such as the Josephson JTWPA or a kinetic inductance KI-TWPA.
KI-TWPAs, the focus of this project, offer several key advantages, including high dynamic range, resilience to high magnetic fields, the possibility of operation over a broad temperature range (from millikelvin to 4 K), and simple microfabrication, requiring only a few lithography and etching steps, without overlapping structures.
KI-TWPAs achieve amplification through wave-mixing processes induced by the film’s intrinsic nonlinear superconducting kinetic inductance.
When a strong pump current propagates with a weak signal current along a line, energy is transferred from the pump to the signal, achieving parametric amplification.
Despite these promising results, the developed device still requires a relatively high pump power to reach maximum gain.
The pump must be isolated from the device under test by components that unavoidably insert loss, thereby degrading the noise performance of the chain.
An amplifier functioning with lower pump power may necessitate fewer of these isolating components and may improve read-out performance.
One solution to overcome this problem is to use thinner superconducting films. In fact, decreasing the thickness of the film increases both the kinetic inductance and the inductive non-linearity.
In the first two years of the project, we developed the KI-TWPA amplifier, utilizing high kinetic inductance and featuring an innovative geometry based on an inverted microstrip transmission line.
In the final year of the project we refined the KI-TWPA amplifier design developed in the firt two years. The amplifier exhibited performance improvements over the amplifiers developed during the first two years, achieving higher gain, broader bandwidth, and noise levels at the Standard Quantum Limit (SQL).
The developed amplifier was utilized to read out an array of eight qubits, created under a separate project, achieving a maximum improvement in the qubit state measurement signal-to-noise ratio (SNR) and in readout fidelity.
Such advancements are crucial for the development of reliable quantum computing systems, which have the potential to revolutionize society by enabling breakthroughs in fields like cryptography, material science, drug discovery, and complex system optimization.
