Isolator-free, near-quantum-limited qubit readout using the Traveling Wave Parametric Amplifier Isolator

Traveling wave parametric amplifiers (TWPAs) are now widespread for near-quantum-limited broadband amplification of microwave signals. Due to imperfect impedance matching with their environment and unwanted non-linear processes, they can exhibit significant backward emission and reverse gain. Therefore, isolators between the device under test (DUT) and the TWPA are used in order to protect the DUT from radiation from the measurement line. These isolators are bulky, magnetic and add losses before the first amplifier, resulting in the degradation of the noise performance of the amplification chain.

We will present isolator-free qubit measurements utilizing the traveling wave parametric amplifier isolator (TWPAI) which exhibits 20 dB of gain and -35 dB of isolation over 500 MHz. The implementation of the TWPAI in qubit readout experiment without isolators between the qubit and the TWPAI will be shown. In this experiment, the qubit serves as a detector for the backaction of the TWPAI. We probed the evolution of the time coherences of the qubit with respect to the different TWPAI configurations (amplifier, isolator or amplifier and isolator). Finally, quantum efficiency is our metric for the quality of the readout.