Quantum mechanics imposes stringent constraints on the amplification of a quantum signal. Deterministic amplification of an unknown quantum state always implies the addition of a minimal amount of noise. 1 Linear and noiseless amplification is allowed in principle provided that it only works probabilistically 23. Here we present a probabilistic amplifier that combines two quantum state comparison amplifiers (SCAMP) 4 together with a feed-forward state correction strategy. Our system outperforms the unambiguous state discrimination (USD) 5 measure-and-resend based amplifier in terms of the success probability-fidelity product and requires a no more complex experimental setting.

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We propose a postselecting parity-swap amplifier for Schrödinger 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.

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