# Compensation of polarization dependent loss using noiseless amplification and attenuation

## TL;DR

This paper proposes methods using noiseless amplification and attenuation to compensate polarization dependent loss in quantum networks, reducing vacuum terms and improving qubit fidelity, especially with detector efficiencies above 40%.

## Contribution

It introduces heralded noiseless amplification and attenuation techniques for PDL compensation, outperforming passive methods in fidelity when detector efficiency exceeds 40%.

## Key findings

- Noiseless amplification maintains higher fidelity than attenuation.
- Heralding signals reduce vacuum terms in the quantum state.
- Performance advantage persists with detector inefficiencies above 40%.

## Abstract

Polarization dependent loss (PDL) is a serious problem that hinders the transfer of polarization qubits through quantum networks. Recently it has been shown that the detrimental effects of PDL on qubit fidelity can be compensated for with the introduction of an additional passive PDL element that rebalances the polarization modes of the transmitted qubit. This procedure works extremely well when the output of the system is postselected on photon detection. However, in cases where the qubit might be needed for further analysis this procedure introduces unwanted vacuum terms into the state. Here we present procedures for the compensation of the effects of PDL using noiseless amplification and attenuation. Each of these techniques introduces a heralding signal into the correction procedure that significantly reduces the vacuum terms in the final state. When detector inefficiency and dark counts are included in the analysis noiseless amplification remains superior, in terms of the fidelity of the final state, to both noiseless attenuation and passive PDL compensation for detector efficiencies greater than 40%.

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Source: https://tomesphere.com/paper/1905.07398