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Two-photon correlations in nonlinear optical systems

Mikołaj K. Schmidt, Rubén Esteban, G. Giedke, Javier Aizpurua, Alejandro González-Tudela

Published Dec 30, 2019• 0 citations
Publisher

Abstract

Accessing complete information about the emission from any quantum-optical systems requires tools capable of a frequency-resolved characterization of multi-photon transitions that would go beyond the standard g(2) correlation measurements. This is particularly important when analyzing systems which exhibit strong nonlinear response with multiple emission pathways. In this work, we calculate and analyze in details correlations of light emitted from two such canonical nonlinear optical systems, one incorporating Kerr nonlinear medium, and another exhibiting optomechanical coupling between light and quantized motion of a generic mechanical oscillator. We compare their single- and two-photon emission characteristics by employing a recently developed framework that allows us to calculate the frequency-resolved g(2) correlations between emitted photons with two arbitrary frequencies. Our analysis shows a rich landscape of bunching and antibunching associated with multi-photon emission events, and reveals the distinctive temporal characteristics of such processes. This new understanding provides a new asset to characterize and exploit nonlinear phenomena in Kerr and optomechanical systems in future experiments.

DOI
10.1117/12.2541116

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We discuss two-photon physics, taking for illustration the particular but topical case of resonance fluorescence. We show that the basic concepts of interferences and correlations provide at the two-photon level an independent and drastically different picture than at the one-photon level, with landscapes of correlations that reveal various processes by spanning over all the possible frequencies at which the system can emit. Such landscapes typically present lines of photon bunching and circles of antibunching. The theoretical edifice to account for these features rests on two pillars: (i) a theory of frequency-resolved photon correlations and (ii) admixing classical and quantum fields. While experimental efforts have been to date concentrated on correlations between spectral peaks, strong correlations exist between photons emitted away from the peaks, which are accessible only through multi-photon observables. These could be exploited for both fundamental understanding of quantum-optical processes as well as applications by harnessing these unsuspected resources. This article is part of the theme issue 'Celebrating the 15th anniversary of the Royal Society Newton International Fellowship'.
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Frequency-resolved photon emission is central to applications from quantum information encoding to high-resolution spectroscopy, and then studying their correlations is therefore essential for revealing the underlying emission pathways and multiphoton statistics. Here, we investigate frequency-resolved N-photon correlations in an ultrastrongly coupled cavity QED system where a qubit interacts with a single-mode cavity. Owing to counter-rotating interactions, the eigenstates and energy spectrum are strongly modified, giving rise to rich spectral and statistical properties in the emitted frequency-resolved photons. Through frequency-selective detection, we reveal pronounced multiphoton antibunching, as well as multiphoton bunching originating from cascade transitions among dressed eigenstates. In particular, we show that parity symmetry plays a decisive role in shaping these correlations. The symmetry-breaking opens additional transition channels and dramatically enhances the generation of correlated photon pairs and even photon triplets of different frequencies. Our work extends frequency-resolved correlations to the ultra-strong coupling regime and demonstrates their potential as a sensitive probe of symmetry in light-matter interaction systems.
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The interaction between quantum emitters leads to the formation of superradiant and subradiant states with possible applications in quantum technologies. To improve the characterization of light emission from these systems, we present here a systematic theoretical analysis of the intensity correlation from two strongly interacting quantum emitters at cryogenic temperatures as a function of the frequency and intensity of the excitation laser. This analysis effectively accounts for the effect of vibrational modes of the emitters and of phonons of the environment through the combined Debye-Waller/Franck-Condon factor. First, we analyze the color-blind intensity correlation and show that it can be tailored from strong antibunching to strong bunching by tuning the laser from the two-photon resonance to the transition frequency of the superradiant state. We also find a particularly complex behavior of the intensity correlation when the laser frequency is tuned to that of the transition of the subradiant state, giving raise to the possibility of emitting bunched and antibunched light depending on the laser intensity and the detuning between the two emitters. The numerical results are supported by analytical equations that can be used for the experimental characterization of the interacting emitters. Additionally, by selecting photons of particular frequencies, we analyze the rich landscape of frequency-resolved intensity correlations, which also depend on the laser detuning and intensity. The analysis of the frequency-resolved correlations provides further information about the different relaxation processes underlying the photon emission, unveiling one-photon and two-photon emission processes that cannot be resolved neither in the emission spectrum nor in the color-blind intensity correlation. These results show that two interacting emitters are a versatile and practical source of quantum light and highlight the usefulness of the intensity correlation to unveil complex dynamics in this system. Published by the American Physical Society 2024