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arXiv:2605.26466 · ACS Nano

Selective Biexciton Generation under Energy-Time Entangled Quantum Light in Quantum Dots

Kaiyue Peng, Chieh Tsao, Hendrik Utzat, Eran Rabani

Published Oct 8, 2026• 0 citations
arXivHTMLPDF

Abstract

Quantum light provides opportunities for controlling multiphoton absorption beyond the classical limits. Here, we investigate biexciton generation in nanocrystal quantum dots driven by energy-time-entangled photon pairs generated via spontaneous parametric downconversion. We show that frequency-time correlations between pairs of photons increase the population of biexcitons over excitons, thereby lending specificity to the excitation of many-body states. By employing a three-level model, we demonstrate that biexciton generation depends nontrivially on the photon entanglement time and pump bandwidth. We find that maximizing efficiency requires an optimally shaped entangled photon field rather than simply scaling parameters for a monotonic improvement. Extending to a realistic CdSe/CdS core-shell quantum dot containing many excitonic states coupled to the quantum field, we demonstrate that increasing the biphoton arrival-time entanglement (closer arrival time) enhances constructive pathway interference and expands accessible excitation channels while preserving better energy-conservation in excitation than classical light when generating biexcitons. Furthermore, tuning the time-correlation properties enables selective excitation of closely spaced biexciton states. These results establish entangled photons as tools for selective excitation and control of higher-order excited states in quantum-confined systems.

DOI
10.1021/acsnano.6c10075

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Quantum light provides opportunities for controlling multiphoton absorption beyond the classical limits. Here, we investigate biexciton generation in nanocrystal quantum dots driven by energy-time-entangled photon pairs generated via spontaneous parametric downconversion. We show that frequency-time correlations between pairs of photons increase the population of biexcitons over excitons, thereby lending specificity to the excitation of many-body states. By employing a three-level model, we demonstrate that biexciton generation depends nontrivially on the photon entanglement time and pump bandwidth. We find that maximizing efficiency requires an optimally shaped entangled photon field rather than simply scaling parameters for a monotonic improvement. Extending to a realistic CdSe/CdS core-shell quantum dot containing many excitonic states coupled to the quantum field, we demonstrate that increasing the biphoton arrival-time entanglement (closer arrival time) enhances constructive pathway interference and expands accessible excitation channels while preserving better energy-conservation in excitation than classical light when generating biexcitons. Furthermore, tuning the time-correlation properties enables selective excitation of closely spaced biexciton states. These results establish entangled photons as tools for selective excitation and control of higher-order excited states in quantum-confined systems.
The biexciton-exciton cascade in quantum dots (QDs) yields entangled photon pairs, and recent developments in engineering photonic structures around the dot for efficient light extraction and proper control of the fine-structure splitting, has led to significant breakthroughs in achieving an ideal entangled photon source -- exhibiting properties such as high extraction efficiency, low multi-photon emission and high entanglement fidelity. This thesis presents our development in enhancing the performance of entangled photon generation of QDs towards near-unity efficiency and fidelity. 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Moreover, our calculations show that proper analysis of the time-energy uncertainty relation provides the necessary means to measure a high entanglement fidelity even in the presence of fine-structure splitting; since, a detection system with high timing resolution can overlook the which-path information, as the uncertainty in measuring the energy of the photons will be larger than the fine-structure splitting. Therefore, realizing near-unity entanglement fidelity using QDs is merely a technical challenge, i.e., resonant excitation of the QD, in order to suppress multi-photon emission, and use of a detection system with low timing jitter and dark-count rates. \n \nAs a next step to achieve this goal, we have performed the resonant two-photon excitation scheme for the first time on a QD in a photonic nanowire, which resulted in an enhancement of the performance of our source. We managed to improve the single-photon purity of the source, g^(2)(0), by two orders of magnitude; moreover, analysis of the emission spectrum reveals that this scheme increases the pair-production efficiency to values above 93% for the biexciton-exciton cascade. Taking the efficiency of our experimental setup into consideration, the results indicate an approximately eight-fold enhancement of the pair-extraction efficiency as compared to the quasi-resonant excitation scheme (12.55% vs. 1.6%). Based on these results, two-photon state tomography on our source, under resonant two-photon excitation and performed by a detection system with high timing resolution and low dark-count rates, yields near-unity entanglement fidelity. By considering the enhancement in pair-extraction efficiency that we obtained, QDs in photonic nanowires can surpass the performance of spontaneous parametric down conversion sources in terms of entanglement fidelity and pair-extraction efficiency. \n \nEven though we have shown a finite fine-structure splitting does not degrade the entanglement fidelity in QDs, for various application in quantum information removing the fine-structure splitting is beneficial. Therefore, we have proposed a universal all-optical approach for removing the fine-structure splitting. In this scheme, the energy of the photons are shifted by using a pair of quarter-wave plates and fast-rotating half-wave plates after they have been emitted; and as a result, the energy difference between the two decay paths in the biexciton-exciton cascade is erased. This method is applicable to any QD source and can be easily implemented using the currently available technology; moreover, no further sample processing will be required in order to achieve zero fine-structure splitting.
Photons which are generated in a two-photon cascade process have an underlying time correlation since the spontaneous emission of the upper level populates the intermediate state. This correlation leads to a reduction of the purity of the photon emitted from the intermediate state. Here we characterize this time correlation for the biexciton-exciton cascade of an InAs/GaAs quantum dot. We show that the correlation can be reduced by tuning the biexciton transition in resonance to a planar distributed Bragg reflector cavity. The enhanced and inhibited emission into the cavity accelerates the biexciton emission and slows down the exciton emission thus reduces the correlation and increases the purity of the exciton photon. This is essential for schemes like creating time-bin entangled photon pairs from quantum dot systems.
A well controlled, triggered source of entangled photons is desirable for many applications in quantum information processing. The two-photon cascade from a biexciton state, two electrons and holes, confined in a quantum dot can be such a source of polarisation-entangled photons provided the two decay paths from the biexciton carry no “which-path” information. In recent years many techniques have been employed to make the two optical decay paths from the biexciton state indistinguishable. A number of these techniques will be discussed and entangled-photon emission from a single quantum dot with a high fidelity in the expected Bell-state from the cascade will be demonstrated. This source of entangled photons allows optical interferometry beyond the limits imposed by the photon wavelength. Interference fringes of the entangled biphoton state reveals a periodicity half of that obtained with the single photon, and much less than that of the pump laser. High fringe visibility indicates that biphoton interference is less sensitive to decoherence than interference of two sequential single photons. The effect of the exciton fine-structure splitting on our entangled photon source will be shown. Surprisingly the entanglement is found to persist despite relatively large separations between the intermediate energy levels of up to 4µeV. Measurements demonstrate that entanglement of the photon pair is robust to the dephasing of the intermediate exciton state responsible for the first order coherence time of either single photon. We distinguish between the first-order coherence time, and a parameter defined as the cross-coherence time, this is illustrated in the figure.
We report a study on coherence of excitation of single quantum dots. We address the coherent excitation of biexcitons, the process that is indispensable for deterministic photon pair generation in quantum dots. Based on theoretical modeling we optimized the duration of the excitation pulse in our experiment to minimize the laser-induced dephasing and increase the biexciton-to-background single-exciton occupation probability. An additional effect of this approach is a high degree of time-bin entanglement with a concurrence of up to 0.78(6) and a 0.88(3) overlap with a maximally entangled state.
We investigate the effect of Cooper pair injection in shifting the biexciton energy level of low-symmetry (C2v{C}_{2v}C2v​) quantum dots (QDs) exhibiting nontrivial fine structure splitting. Coupling QDs to the superconducting coherent state forms extra fine structures by intermixing the ground and biexcitonic states where spectroscopic separation of neutral exciton and biexciton can be diminished, yielding a system to be utilized in the time reordering scheme. The separability of exciton and biexciton energy levels is ascribed to the corresponding direct, exchange, and correlation energies calculated here through the configuration interaction method. We demonstrate the possibility of enhancing photon entanglement concurrence via providing an energy coincidence for biexciton-exciton (XXXXXX \ensuremath{\rightarrow} XXX) and exciton-ground (XXX \ensuremath{\rightarrow} 0) emissions within the weak coupling regime.
The strong confinement of semiconductor excitons in a quantum dot gives rise to atomlike behavior. The full benefit of such a structure is best observed in resonant excitation where the excited state can be deterministically populated and coherently manipulated. Because of the large refractive index and device geometry it remains challenging to observe resonantly excited emission that is free from laser scattering in III/V self-assembled quantum dots. Here we exploit the biexciton binding energy to create an extremely clean single photon source via two-photon resonant excitation of an InAs/GaAs quantum dot. We observe complete suppression of the excitation laser and multiphoton emissions. Additionally, we perform full coherent control of the ground-biexciton state qubit and observe an extended coherence time using an all-optical echo technique. The deterministic coherent photon pair creation makes this system suitable for the generation of time-bin entanglement and experiments on the interaction of photons from dissimilar sources.