A team has experimentally demonstrated a balanced architecture for superconducting travelling-wave parametric amplifiers (TWPAs) that doubles the instantaneous useful bandwidth while rejecting the image frequency. TWPAs offer broadband gain, low added noise and high dynamic range, qualities that make them attractive for amplifying multiplexed signals spread across a wide frequency range, such as qubit resonator arrays, or inherently wideband signals, such as those from spectrometers and radiometers. In many cases, however, the useful bandwidth of a TWPA is limited by the production of an idler tone at an image frequency.

In TWPAs based on three-wave mixing (3WM), an input signal at frequency fₛ is indistinguishable from one at fₚ − fₛ, where fₚ is the pump frequency. This sideband degeneracy introduces spectral ambiguity when the input spectrum is broadband or not known in advance, forcing the use of complex filtering to clean the output. Resolving this ambiguity without sacrificing bandwidth has been one of the practical brakes on using these amplifiers in wideband applications.

The proposed architecture spatially separates the amplified signal from the output pump and the generated idler, resolving the degeneracy. It consists of two amplifiers connected between a pair of 180° hybrid couplers, with the signal and pump injected and output through the sum and difference ports, respectively. Using two kinetic-inductance TWPAs, the authors measured approximately 20 dB of signal gain across nearly an octave of bandwidth, with pump rejection below −20 dB and an average image rejection of about −15 dB over the operating band.

The results establish the balanced TWPA as a practical route to broadband parametric amplification, with a larger fraction of contamination-free bandwidth and no need for complex filtering. The improvement is especially relevant for reading out superconducting qubits and for sensitive microwave instrumentation, where spectral cleanliness directly at the amplification stage reduces downstream hardware and simplifies the detection chain.