[Paper Review] Signatures of the Optical Stark Effect on Entangled Photon Pairs from Resonantly-Pumped Quantum Dots

Francesco Basso Basset, Michele B. Rota+13 more|arXiv (Cornell University)|Dec 14, 2022

Semiconductor Quantum Structures and Devices4 citations

TL;DR

This study investigates the impact of the optical Stark effect on polarization-entangled photon pairs generated from resonantly pumped quantum dots. By tuning laser pulse duration and power, the authors observe a reduction in concurrence due to AC-Stark shifts, revealing that additional decoherence mechanisms beyond the Stark effect are needed to explain the sub-unity entanglement fidelity.

ABSTRACT

Two-photon resonant excitation of the biexciton-exciton cascade in a quantum dot generates highly polarization-entangled photon pairs in a near-deterministic way. However, the ultimate level of achievable entanglement is still debated. Here, we observe the impact of the laser-induced AC-Stark effect on the quantum dot emission spectra and on entanglement. For increasing pulse-duration/lifetime ratios and pump powers, decreasing values of concurrence are recorded. Nonetheless, additional contributions are still required to fully account for the observed below-unity concurrence.

Motivation & Objective

• To understand how the optical Stark effect influences the entanglement quality of photon pairs from resonantly pumped quantum dots.
• To identify the role of laser-induced AC-Stark shifts in degrading entanglement fidelity in quantum dot-based sources.
• To determine whether the observed sub-unity concurrence in entangled photon pairs can be fully explained by the Stark effect alone.
• To characterize the spectral and coherence dynamics of the biexciton-exciton cascade under varying excitation conditions.
• To assess the limitations of current quantum dot sources in achieving near-deterministic, high-fidelity entanglement.

Proposed method

• Resonant two-photon excitation of the biexciton-exciton cascade in self-assembled InGaAs quantum dots.
• Application of variable-duration laser pulses to control the pulse-duration-to-lifetime ratio and excitation power.
• Measurement of emission spectra to detect signatures of the AC-Stark effect on the quantum dot energy levels.
• Quantification of entanglement fidelity via concurrence, derived from polarization correlation measurements.
• Systematic variation of pulse duration and power to isolate the influence of the Stark shift on entanglement degradation.
• Comparison of experimental concordance values with theoretical predictions to identify unaccounted decoherence channels.

Experimental results

Research questions

• RQ1 How does the optical Stark effect, induced by laser excitation, affect the spectral characteristics of entangled photon emission from quantum dots?
• RQ2 To what extent does the AC-Stark shift contribute to the observed reduction in concordance below unity in polarization-entangled photon pairs?
• RQ3 Are there additional decoherence mechanisms beyond the Stark effect that must be considered to explain the measured entanglement fidelity?
• RQ4 How do pulse duration and excitation power influence the interplay between Stark shifts and entanglement quality?
• RQ5 Can the spectral broadening and shift observed in the emission spectra be quantitatively linked to the reduction in concordance?

Key findings

• Increasing laser pulse duration and excitation power lead to a measurable decrease in concordance, indicating degradation of entanglement fidelity.
• The observed reduction in concordance correlates with spectral shifts and broadening in the emission spectra, consistent with the AC-Stark effect.
• Despite accounting for Stark shifts, the measured concordance remains below unity, indicating the presence of additional decoherence mechanisms.
• The spectral signatures of the Stark effect are clearly resolved in the emission spectra, confirming its role in modifying the quantum dot energy levels.
• The pulse-duration-to-lifetime ratio is a key parameter in controlling the strength of the Stark shift and its impact on entanglement.
• The results suggest that the optical Stark effect alone cannot fully explain the entanglement degradation, necessitating further investigation into other decoherence pathways.