Short answer: Superconducting qubits report their state through very faint microwave signals, so reading them out quickly and accurately needs an amplifier that adds almost no noise. Reviews describe parametric amplifiers as the leading technology for this, because they can add only the minimum noise quantum mechanics allows. Traveling-wave parametric amplifiers (TWPAs) aim to give wide bandwidth as well as gain. This is a research and engineering area with open challenges.
Why readout needs a special amplifier
A review of Josephson parametric amplifiers explains that in superconducting quantum computing, qubit state information is conveyed by low-power microwave fields. Ultralow-noise microwave amplification therefore plays a central role in measuring those fields to infer the qubit state quickly and accurately. The review says parametric amplification, a well-known low-noise concept in the 1960s and 1970s, has become the leading technology for highly efficient microwave measurements of these circuits, and that the new generation of amplifiers differs from the earlier work in important ways.
A perspective on traveling-wave amplifiers makes the noise argument concrete. It says a low-noise amplification chain is a key element of any system detecting low-amplitude signals, with quantum-noise-limited amplification crucial for reading out solid-state qubits such as spin and superconducting qubits, and for other applications including nano-electromechanical systems, electron spin resonance, astronomy instruments and dark matter searches. The noise of a chain is set mainly by the first amplifier, so the first stage should reach quantum-limited added noise and high gain. The perspective states that semiconducting low-noise microwave amplifiers cannot reach quantum-limited added noise because of their dissipative nature, while superconducting parametric amplifiers are well suited to the task.
From early amplifiers to traveling-wave designs
An arXiv paper traces the history. Parametric modulation of reactive elements for amplification dates back to the First World War, with magnetic amplifiers for radio transmitters. The low noise of the first parametric amplifiers, which used passive elements, was on par with the standard of the time. The search for wide-band amplification without the usual bandwidth-gain trade-off led to the proposal of traveling-wave parametric amplifiers, which modulate the inductance or capacitance of a nonlinear transmission line with a traveling, high-amplitude pump, similar to nonlinear optical materials. The paper says Bell Laboratories implemented a first example in 1958 with 10 dB of gain and 200 MHz of bandwidth centred at 700 MHz. Interest in varactor-based amplifiers declined with semiconductor transistors in the 1970s, while interest in superconducting versions grew for faint signals in astronomy and quantum physics. The first superconducting parametric amplification experiments were reported in the mid 1960s, and later work used a single Josephson junction.
Current directions
The perspective focuses on traveling-wave parametric amplifiers, describing the key achievements of recent years and the present open challenges. It also discusses possible directions beyond amplification, such as using these devices for multi-mode entanglement generation and for single-photon detectors. The Josephson parametric amplifier review gives an overview of how the devices work and where there are opportunities for improvement.
What this does not tell you
These sources describe the technology and its history, and they do not give performance figures for current devices or compare specific products. Gain, bandwidth and noise depend on the design and the operating conditions, and the reviews list open challenges, so this is not a solved area.
FAQ
Why do qubits need special amplifiers?
Their readout signals are faint microwaves, and ordinary amplifiers add too much noise.
What is a parametric amplifier?
An amplifier that works by modulating a reactive circuit element, such as a nonlinear inductance, with a pump signal.
What is a traveling-wave parametric amplifier?
A design that uses a nonlinear transmission line to seek wide bandwidth together with gain.
Is this new?
Parametric amplifiers date back decades, but reviews say the new superconducting generation differs in important ways.
Is the problem solved?
Reviews list open challenges for traveling-wave amplifiers.
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