Amplifying hybrid entangled states and superpositions of coherent states

TL;DR

This paper compares two amplification schemes applied to hybrid entangled states and superpositions of coherent states, analyzing their fidelity, gain, and quantum Fisher information to determine optimal conditions for state amplification.

Contribution

It provides a detailed comparison of photon addition and subtraction schemes on hybrid and superposed states, revealing their different efficiencies depending on state parameters.

Findings

Fidelity and gain are similar for HESs and coherent states.

SCSs exhibit complex behaviors depending on amplitudes and phases.

The $ ext{a} ext{a}^ ext{dagger}$ scheme performs better at small amplitudes, while $ ext{a}^ ext{dagger}^2$ is better at larger amplitudes.

Abstract

We compare two amplification schemes, photon addition and then subtraction ($\hat{a}\hat{a}^\dagger$) and successive photon addition ($\hat{a}^\dagger{}^2$), applied to hybrid entangled states (HESs) and superpositions of coherent states (SCSs). We show that the amplification schemes' fidelity and gain for HESs are the same as those of coherent states. On the other hand, SCSs show quite nontrivial behaviors by the amplification schemes, depending on the amplitudes of coherent states, number of coherent-state components, and relative phases between the components. This implies that appropriate amplification schemes for SCSs should be chosen depending on the tasks and specific forms of the states. To investigate the quality of amplified states, we calculate the quantum Fisher information, a measure of quantum phase estimation. In terms of the quantum Fisher information, the $\hat{a}\hat{a}^\dagger$ scheme tends to show better performance for relatively small amplitudes while the $\hat{a}^\dagger{}^2$ scheme is better in larger amplitude regime. The performance of the two schemes becomes indistinguishable as the amplitude grows sufficiently large.