Resolution to the quantum-classical dilemma in thermal ghost imaging

TL;DR

This paper introduces a density matrix approach in photon orbital angular momentum space to clarify the quantum versus classical nature of thermal ghost imaging, resolving a long-standing debate with a new physical perspective.

Contribution

It formulates a density matrix in OAM space for thermal ghost imaging, revealing quantum correlations beyond entanglement and providing a resolution to the quantum-classical dilemma.

Findings

Provides a mathematically precise description of ghost image formation.

Reveals quantum correlations beyond entanglement in thermal ghost imaging.

Suggests using thermal multi-photon OAM states for quantum information tasks.

Abstract

There has been an intense debate on the quantum versus classical origin of ghost imaging with a thermal light source over the last two decades. A lot of distinguished work has contributed to this topic, both theoretically and experimentally, however, to this day this quantum-classical dilemma still persists. Here we formulate for the first time a density matrix in the photon orbital angular momentum (OAM) Hilbert space to fully characterize the two-arm ghost imaging system with the basic definition of thermal light sources. Our formulation offers a mathematically precise method to describe the formation of a ghost image in a nonlocal fashion. More importantly, it provides a more physically intuitive picture to reveal the quantumness hidden in the thermal ghost imaging, and therefore, presenting a sound resolution to the ongoing quantum-classical dilemma, which distinguishes the quantum correlations beyond entanglement in terms of geometric measure of discord. Our work also suggests further studies of using thermal multi-photon OAM states directly to implement some quantum information tasks.