New Microwave Amplifier Improves Quantum Readout

Quantum computers are expected to solve complex tasks in a fraction of the time that today’s supercomputers would require. Many challenges remain, however. One of them is reading out the results of a superconducting quantum processor, which requires amplifying extremely weak microwave signals without altering the qubit state. In the TruePA project coordinated by the Karlsruhe Institute of Technology (KIT), researchers have developed an amplifier that makes qubit readout simpler and more reliable. To achieve this, the German-French team has significantly advanced the principle of the traveling-wave parametric amplifier (TWPA). These on-chip devices amplify signals as they propagate along a transmission line.

One-way Signal Flow

“In conventional TWPAs, part of the amplified signal can propagate backward, which may disturb the qubit state and degrade overall gain. To prevent this, TWPAs have typically been surrounded by bulky, ferrite-based cryogenic circulators and isolators,” explains Professor Anja Metelmann from KIT’s Institute for Quantum Materials and Technologies, who coordinates the TruePA project. The researchers have now eliminated the need for these isolators. A second, carefully engineered pump tone enforces forward-only propagation through the device, suppressing backward gain. “We use a nonlinear modulation of circuit parameters – such as the inductance – to transfer energy from a pump tone to the original signal,” says Metelmann. The new amplifier opens the door to more compact, scalable quantum processors and quantum detectors.

ffr, December 16, 2025