Quantum limits to chiroptical molecular discrimination

Discriminating between enantiomers (mirror-image molecules) is a fundamental challenge in chemistry, often limited by the statistical nature of photon emission and detection. This paper investigates the theoretical limits of identifying the handedness of a chiral quantum emitter by applying quantum hypothesis testing to the emitted light, comparing standard classical detection strategies against optimal collective quantum measurements.

The author models the emission from a randomly oriented chiral source as a quantum state and frames the identification of its handedness as a symmetric binary hypothesis test. The study compares the performance of the conventional 'locally optimal fixed' (LOF) measurement—which essentially counts left- versus right-circularly polarized photons—against the theoretical limit defined by the quantum Chernoff bound. The analysis accounts for the electric dipole, magnetic dipole, and electric quadrupole moments of the molecular transition.

The study demonstrates that while counting photons (a separable measurement) is optimal for a single photon, it is suboptimal for multiple photons. For emitters with large dissymmetry factors, such as certain lanthanide complexes, a collective measurement on a small number of photons can reduce the error probability by orders of magnitude compared to the best possible classical strategy. The author provides a quantitative framework showing that the quantum Chernoff information can be up to twice the classical Chernoff information in relevant physical regimes, leading to a quadratic improvement in the error rate.

This work establishes a new fundamental speed limit for chiroptical discrimination. While current technology for organic molecules with low dissymmetry factors makes this quantum advantage difficult to realize, the findings suggest that for specialized emitters, quantum-enhanced detection could enable near-error-free identification of molecular handedness from very dim sources. This provides a theoretical roadmap for future experiments using quantum memories and advanced photonic platforms to push the boundaries of analytical chemistry.