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Physical review. B./Physical review. B

Pump power control of photon statistics in a nanowire quantum dot

Dan Dalacu, David B. Northeast, Philip J. Poole, G. C. Aers, Robin L. Williams, Kim A. Owen, Daniel Oblak

Published Sep 1, 2020• 7 citations
Publisher

Abstract

Through fluctuations in the local composition, InAsP quantum dots embedded within site-selected InP nanowires are observed to display biexciton binding energies spanning a range between $\ensuremath{-}0.3\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ and $+2.9\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$. From this range we select dots having energy-degenerate exciton and biexciton emission and observe an excitation rate-mediated transition from sub- to super-Poissonian second-order correlation statistics. Under pulsed excitation, ${g}^{(2)}(\ensuremath{\tau}=0)$ is found to increase from 0.5 at high excitation levels, rising to 28 as the excitation is reduced by two orders of magnitude. The observed second-order correlation statistics are interpreted using both a stochastic model and a rate equation model of the competition between the various excitonic emission processes. Our results demonstrate that nanowire quantum dots represent a promising approach to the efficient generation of twin-photon states.

DOI
10.1103/physrevb.102.115401

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Through fluctuations in the local composition, InAsP quantum dots embedded within site-selected InP nanowires are observed to display biexciton binding energies spanning a range between \ensuremath−0.3meV\ensuremath{-}0.3\phantom{\rule{0.16em}{0ex}}\mathrm{meV}\ensuremath−0.3meV and +2.9meV+2.9\phantom{\rule{0.16em}{0ex}}\mathrm{meV}+2.9meV. From this range we select dots having energy-degenerate exciton and biexciton emission and observe an excitation rate-mediated transition from sub- to super-Poissonian second-order correlation statistics. Under pulsed excitation, g(2)(\ensuremathτ=0){g}^{(2)}(\ensuremath{\tau}=0)g(2)(\ensuremathτ=0) is found to increase from 0.5 at high excitation levels, rising to 28 as the excitation is reduced by two orders of magnitude. The observed second-order correlation statistics are interpreted using both a stochastic model and a rate equation model of the competition between the various excitonic emission processes. Our results demonstrate that nanowire quantum dots represent a promising approach to the efficient generation of twin-photon states.
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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.
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.
In this Letter, we present entanglement generated from a novel structure: a single InAsP quantum dot embedded in an InP nanowire. These structures can grow in a site-controlled way and exhibit high collection efficiency; we detect 0.5 million biexciton counts per second coupled into a single mode fiber with a standard commercial avalanche photo diode. If we correct for the known setup losses and detector efficiency, we get an extraction efficiency of 15(3) %. For the measured polarization entanglement, we observe a fidelity of 0.76(2) to a reference maximally entangled state as well as a concurrence of 0.57(6).
Coherent control of single photon sources is a key requirement for the advancement of photonic quantum technologies. Among them, nanowire-based quantum dot sources are popular due to their potential for on-chip hybrid integration. Here, we demonstrate on-demand single-photon generation [g(2) (0) (X*) = 0.078] from resonantly excited InAsP/InP nanowire quantum dots and observe Rabi oscillations in the dot emission, indicating the successful coherent manipulation of the excitonic states in the nanowire. This work addresses the long-standing challenge of resonantly exciting nanowire-quantum dots. It paves the way for hybrid quantum photonic integration, enabling spin–photon entanglement and matter memories on-chip.