Radiatively Cooled Superconducting Parametric Amplifier with Near-Quantum-Limited Noise

As a key component in modern quantum technology for high-sensitivity readout, state-of-the-art superconducting microwave amplifiers are usually installed at the milli-Kelvin temperatures for two reasons - to maintain superconductivity for the aluminum junctions and to ensure quantum-limited added noise performance. In this work, we demonstrate quantum-limited microwave readout with a nanobridge kinetic-inductance superconducting parametric amplifier (NKPA) installed at the 4-K plate of a dilution refrigerator. Since NKPA is made from a high-$T_mathrm{c}$ NbN thin film, the reflective amplifier is able to maintain an over-coupled condition at higher temperatures. Therefore, the equivalent temperature of the amplification added noise is cooled to below the material temperature of the amplifier through radiative cooling. This demonstration shows the possibility of moving microwave amplifiers out of the mixing chamber of a dilution refrigerator, without compromising the readout efficiency, to allow more space and cooling power in mixing chamber for more delicate quantum devices.