Does entanglement enhance single-molecule pulsed biphoton spectroscopy?

TL;DR

This paper investigates whether entanglement enhances single-molecule biphoton spectroscopy, finding that while unentangled measurements can extract most information, entanglement can still provide tangible quantum advantages in certain conditions.

Contribution

It demonstrates that biphoton spectroscopy with source-engineered PDC probes and unentangled measurements can achieve quantum enhancement, clarifying entanglement's role in this context.

Findings

Unentangled measurements can capture 60-90% of spectroscopic information.

Entanglement improves performance in weakly-pumped PDC probes.

In accessible modes, only a single genuine two-photon contribution exists.

Abstract

It depends. For a single molecule interacting with one mode of a biphoton probe, we show that the spectroscopic information has three contributions, only one of which is a genuine two-photon contribution. When all the scattered light can be measured, solely this contribution exists and can be fully extracted using unentangled measurements. Furthermore, this two-photon contribution can, in principle, be matched by an optimised but unentangled single-photon probe. When the matter system spontaneously emits into inaccessible modes, an advantage due to entanglement can not be ruled out. In practice, time-frequency entanglement does enhance spectroscopic performance of the oft-studied weakly-pumped spontaneous parametric down conversion (PDC) probes. For two-level systems and coupled dimers, more entangled PDC probes yield more spectroscopic information, even in the presence of emission into inaccessible modes. Moreover, simple, unentangled measurements can capture between 60% - 90% of the spectroscopic information. We thus establish that biphoton spectroscopy using source-engineered PDC probes and unentangled measurements can provide tangible quantum enhancement. Our work underscores the intricate role of entanglement in single-molecule spectroscopy using quantum light.