Increased added noise in traveling-wave parametric amplifiers due to frequency conversion

Traveling-Wave Parametric Amplifiers (TWPAs) have become standard components for readout chains of frequency-multiplexed superconducting qubits. They provide GHz wide amplification with added noise ideally at the quantum limit. Added noise above the quantum limit is usually attributed to the presence of losses in the circuit, or to unwanted parametric processes involving higher frequencies. Here we discuss another contribution, originating not from the TWPA device itself, but from the setup the TWPA is embedded in: spurious waves at frequencies much lower than signal and pump frequencies, unintentionally picked up in the setup and propagating through the TWPA. These waves can drive parametric frequency conversion, transferring energy from the signal, idler, and pump waves to unwanted sidebands. The creation of these sidebands results in detrimental effects including increased transmission loss, reduced gain, increased added noise, and crosstalk of qubit readout channels due to frequency collisions. Particularly severe is the increase of noise: a gain reduction of 2 dB can lead to a noise increase of ca. one photon. Mitigation of this effect is crucial for the practical use of TWPAs, to allow operation at the full potential of the devices.