A positive operator-valued measure for two-photon detection via sum-frequency generation

TL;DR

This paper introduces a POVM for two-photon detection using sum-frequency generation, enabling joint measurements crucial for quantum information protocols involving entangled photons.

Contribution

It demonstrates how SFG-based single-photon measurements realize a generalized two-photon POVM, linking it to time-reversed PDC processes for quantum information applications.

Findings

POVM elements are proportional to time-reversed PDC states

Detection acts as a joint measurement on two photons

Analysis relevant for quantum protocols with entangled photons

Abstract

Spontaneous parametric down conversion (PDC), in the perturbative limit, can be considered as a probabilistic splitting of one input photon into two output photons. Conversely, sum-frequency generation (SFG) implements the reverse process of combining two input photons into one. Here we show that a single-photon projective measurement in the temporal-mode basis of the output photon of a two-photon SFG process effects a generalized measurement on the input two-photon state. We describe the positive-operator-valued measure (POVM) associated with such a measurement, and show that its elements are proportional to the two-photon states produced by the time-reversed PDC process. Such a detection acts as a joint measurement on two photons, and is thus an important component of many quantum information processing protocols relying on photonic entanglement. Using the retrodictive approach, we analyze the properties of the two-photon POVM that are relevant for quantum protocols exploiting two-photon states and measurements.