Realizing Photonic Quantum Entanglement from a Two-Level Emitter in a Nanophotonic Waveguide

We show that a single two-level emitter embedded in a nanophotonic waveguide can actively induce entanglement between two scattered photons, opening new avenues for on-chip generation of photonic quantum entangled states.
Published in Materials and Physics
Realizing Photonic Quantum Entanglement from a Two-Level Emitter in a Nanophotonic Waveguide

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Challenges in Generating Quantum Entanglement

The quest for efficient optical nonlinearity is critical for enhancing interactions among single photons in quantum information processing. Different approaches have been explored for decades to generate photonic quantum entanglement. One commonly-used approach is to utilize bulky χ(2) and χ(3) nonlinear media, which usually exhibits feeble nonlinearity and requires intense excitation. In contrast, spin-based systems offer greater versatility in generating entangled states but require elaborate excitation or active spin control, with spin decoherence processes playing a critical role.

Enhanced Nonlinearity via Quantum Dots in Nanophotonics

The journey to this research began with a fundamental question: How can high-fidelity entangled states be generated in a simple and energy-efficient manner on-chip? This question is crucial because entanglement is a key resource for quantum technologies, including quantum communication, computation, and metrology. As shown in Fig. 1, we explore a solid-state quantum dot (QD) coupled to a photonic crystal waveguide (PhC WG) as our two-level system, facilitating enhanced interactions between photons at the single-photon level. The waveguide is engineered for strong coupling between the QD and the guided modes, ensuring high-efficiency photon-photon interactions. This single-photon nonlinearity is enabled by waveguide interference, ideally reflecting single photons and transmitting photon-bound states responsible for two-photon time-energy entanglement. In this regime, with perfect photon-emitter coupling efficiency and no decoherence processes, only two uncorrelated photons suffice as the minimum resource to access optical nonliearities, required for generating an energy-time entangled pair in experiment.

Our QD-WG device exhibits bunching statistics in resonance transmission. The second-order autocorrelation function g2(0) is measured in a Hanbury-Brown-Twiss (HBT) experiment, reaching values above 200. This indicates that the incoming Poissonian photon distribution is significantly altered by strong nonlinear interaction with the QD. 


Fig. 1: Two-photon energy-time entanglement induced by coherent interaction of two photons with a QD integrated into a PhC WG.

Validation of Scattering-Enabled Entanglement

A continuous-wave laser excites the QD through the PhC WG, and the transmitted light is guided out by one of the shallow etched gratings (SEGs), with an external cavity filter used to separate the residual laser from the QD emission. To validate entanglement, we use two unbalanced Mach-Zehnder interferometers with a predesigned path length difference for energy-time measurements. One notable achievement of our research  was the observation of energy-time entanglement from two scattered photons by violating a Bell inequality. This was a non-trivial task that required long-time precise control and lock of two unbalanced Mach-Zehnder interferemeters and one cavity filter simultaneously during the measurement. 

Conclusion and Outlook

Our study has demonstrated energy-time entanglement using photon scattering off a two-level emitter in a nanophotonic waveguide. Looking ahead, it would be more interesting to investigate the scattering mechanism by involving more photons. The potential applications range from quantum simulators and metrology to quantum communication and computing. Future endeavors will focus on scaling up and integrating these systems into larger quantum networks, exploring higher-dimensional entanglement and advancing material fabrication for enhanced quantum capabilities on chip-scale devices. 

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