Parity-Swap State Comparison Amplifier for Schr\"odinger Cat States

We propose a postselecting parity-swap amplifier for Schr"odinger cat states that does not require the amplified state to be known a priori. The device is based on a previously-implemented state comparison amplifier for coherent states. It consumes only Gaussian resource states, which provides an advantage over some cat state amplifiers. It requires simple Geiger-mode photodetectors and works with high fidelity and approximately twofold gain.