Enhancing Phase Retrievability of Quantum Channels via Interferometric Coupling

TL;DR

This paper investigates how interferometric coupling can improve the ability to reconstruct quantum phase information, providing structural characterizations and practical methods for enhancing phase retrievability.

Contribution

It introduces an interferometric coupling method that enhances phase retrievability of quantum channels by enlarging the complementary operator system through interference effects.

Findings

Interferometric coupling can significantly improve phase retrieval.

A quantum channel is phase retrievable iff its complementary channel is pure-state informationally complete.

Interference cross terms can enlarge the operator system and enhance phase retrievability.

Abstract

Phase retrievability of a quantum channel asks whether pure states can be reconstructed from suitable measurements. In this paper, we study this problem from three complementary viewpoints: quantum information theory, operator-valued frames, and the physical realization through quantum interferometry. We first show that a quantum channel is phase retrievable if and only if its complementary channel is pure-state informationally complete. This structural characterization leads to several consequences for phase retrievability, including criteria involving the dimension of the complementary operator system, Choi-rank type bounds, and specific results for entanglement breaking channels and twirling channels. We then introduce an interferometric coupling in which two arm channels are coherently recombined through port operators \(M_i(\theta)=A_i+e^{i\theta}B_i\). Unlike classical mixing, this construction produces interference cross terms that can enlarge the complementary operator system and thereby enhance phase retrievability. From the frame theory viewpoint, the interferometer realizes a coherent coupling of operator-valued frames. To quantify this effect, we introduce injectivity indices for completely positive maps. The examples in Section~5 show that coherent interference can significantly improve phase retrieval behavior even when the arm channels are individually not phase retrievable.