# Theory and limits of on-demand single photon sources using plasmonic resonators: a quantized quasinormal mode approach

## TL;DR

This paper develops a rigorous quantized quasinormal mode approach to analyze the efficiency and indistinguishability of plasmonic resonator-based single photon sources, considering realistic loss and excitation conditions.

## Contribution

It extends the quantized quasinormal mode theory to accurately quantify figures of merit for plasmonic single photon sources, including efficiency and indistinguishability, under realistic conditions.

## Key findings

- Ultrashort pulses are necessary for optimal single photon emission.
- Large Purcell and beta factors achieved in gold nanorod dimers.
- Nonradiative beta factor significantly impacts photon emission quality.

## Abstract

Quantum emitters coupled to plasmonic resonators are known to allow enhanced broadband Purcell factors, and such systems have been recently suggested as possible candidates for on-demand single photon sources, with fast operation speeds. However, a true single photon source has strict requirements of high efficiency (brightness) and quantum indistinguishability of the emitted photons, which can be quantified through two-photon interference experiments. To help address this problem, we employ and extend a recently developed quantized quasinormal mode approach, which rigorously quantizes arbitrarily lossy open system modes, to compute the key parameters that accurately quantify the figures of merit for plasmon-based single photon sources. We also present a quantized input-output theory to quantify the radiative and nonradiative quantum efficiencies. We exemplify the theory using a nanoplasmonic dimer resonator made up of two gold nanorods, which yields large Purcell factors and good radiative output beta factors. Considering an optically pulsed excitation scheme, we explore the key roles of pulse duration and pure dephasing on the single photon properties, and show that ultrashort pulses (sub-ps) are generally required for such structures, even for low temperature operation. We also quantify the role of the nonradiative beta factor both for single photon and two-photon emission processes. Our general approach can be applied to a wide variety of plasmon systems, including metal-dielectrics, and cavity-waveguide systems, without recourse to phenomenological quantization schemes.