Approaching near-perfect state discrimination of photonic Bell states through the use of unentangled ancilla photons

TL;DR

This paper explores how adding multiple unentangled ancilla photons to a linear optical setup can nearly achieve perfect discrimination of Bell states, challenging previous limitations and guiding design principles.

Contribution

It introduces a numerical trend indicating near-perfect Bell state measurement with eight or more ancilla photons and establishes conditions preventing non-ideal photon bunching in linear optical circuits.

Findings

Adding eight or more unentangled ancilla photons improves measurement fidelity.

Certain photon bunching configurations are proven to prevent ideal Bell state discrimination.

A set of design conditions for linear optical circuits to avoid non-ideal outcomes.

Abstract

Despite well-established no-go theorems on a perfect linear optical Bell state analyzer, we find a numerical trend that appears to approach a near-perfect measurement if we incorporate eight or more un-entangled ancilla photons into our device. Following this trend, we begin a promising inductive approach to building an ideal optical Bell measurement device. In the process, we determine that any Bell state analyzer that (even occasionally) bunches all photons into only two of the output modes cannot perform an ideal measurement and we find a set of conditions on our linear optical circuit that prevent this outcome.