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arXiv:2503.03268 · Physical review. B./Physical review. B

Temporal CW polarization-tomography of photon pairs from the biexciton radiative cascade: Theory and experiment

Noam Tur, Ismail M. Nassar, Ido Schwartz, J. E. Avron, Dan Dalacu, Philip J. Poole, D. Gershoni

Published Jun 2, 2025• 1 citation
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Abstract

We study, experimentally and theoretically, temporal correlations between the polarization of photon pairs emitted during the biexciton-exciton radiative cascade from a single semiconductor quantum dot, optically excited by a continuous-wave light source. The system is modeled by a Lindbladian coupled to two Markovian baths: One bath represents the continuous light source, and a second represents the emitted radiation. Very good agreement is obtained between the theoretical model that we constructed and a set of 36 different time-resolved, polarization correlation measurements between cascading photon pairs.

DOI
10.1103/4z82-v82w

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We study, experimentally and theoretically, temporal correlations between the polarization of photon pairs emitted during the biexciton-exciton radiative cascade from a single semiconductor quantum dot, optically excited by a continuous-wave light source. The system is modeled by a Lindbladian coupled to two Markovian baths: One bath represents the continuous light source, and a second represents the emitted radiation. Very good agreement is obtained between the theoretical model that we constructed and a set of 36 different time-resolved, polarization correlation measurements between cascading photon pairs.
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We perform full time-resolved tomographic measurements of the polarization state of pairs of photons emitted during the radiative cascade of the confined biexciton in a semiconductor quantum dot. The biexciton was deterministically initiated using a $\ensuremath{\pi}$-area pulse into the biexciton two-photon absorption resonance. Our measurements demonstrate that the polarization states of the emitted photon pair are maximally entangled. We show that the measured degree of entanglement depends solely on the temporal resolution by which the time difference between the emissions of the photon pair is determined. A route for fabricating an on-demand source of maximally polarization entangled photon pairs is thereby provided.
The generation of entangled photons from radiative cascades has enabled milestone experiments in quantum information science with several applications in photonic quantum technologies. Significant efforts are being devoted to pushing the performances of near-deterministic entangled-photon sources based on single quantum emitters often embedded in photonic cavities, so to boost the flux of photon pairs. The general postulate is that the emitter generates photons in a nearly maximally entangled state of polarization, ready for application purposes. Here, we demonstrate that this assumption is unjustified. We show that in radiative cascades there exists an interplay between photon polarization and emission wavevector, which can be further amplified by embedding the emitters in micro-cavities. We discuss how the polarization entanglement of photon pairs from a biexciton-exciton cascade in quantum dots strongly depends on their propagation wavevector and we even observe entanglement vanishing for large emission angles. Our experimental results, backed by theoretical modeling, yield a brand-new understanding of cascaded emission for various quantum emitters. In addition, our model provides quantitative guidelines for designing optical microcavities that retain both a high degree of entanglement and collection efficiency, moving the community one step further towards an ideal source of entangled photons for quantum technologies. The assumption that light sources based on radiative cascades provide maximally polarization-entangled photons is not always correct. Here the authors show experimentally in cavity-embedded quantum dots that photon polarization correlates with its emission mode, impacting the degree of entanglement for emitted photon pairs.
We develop a theoretical model to study the intensity–intensity correlation of polarization-entangled photons emitted in a biexciton–exciton cascade. We calculate the degree of correlation and show how polarization correlations are affected by the presence of dephasing and energy-level splitting of the excitonic states. Our theoretical calculations are in agreement with the recent observation of polarization-dependent intensity–intensity correlations from a single semiconductor quantum dot (Stevenson et al 2006 Nature 439 179). Our model can be extended to study polarization-entangled photon emission in coupled quantum dot systems.
We report on polarisation correlation from the cascaded recombination of biexcitons in a quantum dot emitting at a telecommunication wavelength. The fine structure splitting of the exciton state in this InAs/GaAs quantum dot is of the order of 100 μeV and polarisation correlation is expected. Strong polarisation correlation between the biexciton and exciton emission lines is observed under both continuous wave (CW) and pulsed laser excitation so telecom wavelength quantum dots with lower energy splittings could be suitable for entangled photon pair generation. Measurements were performed using nanowire superconducting single photon detectors (SSPDs). SSPDs offer low time-jitter and improve the resolution of features in the correlation spectra, including the asymmetric dip and peak resulting from the cascaded emission with the peak extending more than an order of magnitude above the Poissonian level.
We detect a radiative cascade which initiates from a metastable biexciton state in a neutral semiconductor quantum dot. In this biexciton, the heavy holes form a spin-triplet configuration, Pauli blockaded from relaxation to the spin-singlet ground state. The triplet biexciton has two photon-phonon-photon decay paths. Unlike in the singlet-ground-state biexciton radiative cascade, in which the two photons are colinearly polarized, in the triplet-biexciton cascade they are cross-linearly polarized. We measured the two-photon polarization density matrix and show that the phonon emitted when the intermediate exciton relaxes from excited to ground state, preserves the exciton's spin. The phonon, thus, does not carry with it any which-path information other than its energy. Nevertheless, entanglement distillation by spectral filtering was found to be rather ineffective for this cascade. This deficiency results from the opposite sign of the anisotropic electron-hole exchange interaction in the excited exciton relative to that in the ground exciton.