Preservation of loss in linear-optical processing

TL;DR

This paper introduces a measure of quantum efficiency for multimode light states and proves that linear-optical processing cannot increase this efficiency, preventing the catalytic improvement of photon sources.

Contribution

It defines a quantum efficiency measure and demonstrates its non-increase under linear-optical processing with destructive measurements, establishing fundamental limits.

Findings

Quantum efficiency cannot be increased by linear-optical processing.

Losses in optical states are irrecoverable and cannot be redistributed.

Photon source improvement cannot be catalytically achieved.

Abstract

We propose a measure of quantum efficiency of a multimode state of light that quantifies the amount of optical loss this state has experienced, and prove that this efficiency cannot increase in any linear-optical processing with destructive conditional measurements. Any loss that has affected a state can neither be removed nor redistributed so as to further increase the efficiency in higher-efficiency modes at the expense of lower-efficiency modes. This result eliminates the possibility of catalytically improving photon sources.