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Pan Stanford Publishing eBooks

Entangled Photon Pair Emission and Interference from Single Quantum Dots

R. M. Stevenson, Robert James Young, A. J. Shields

Published Mar 9, 2010• 0 citations
Publisher

Abstract

We review in this chapter our recent realization and development of entangled light emission from individual InAs quantum dots. We describe how energetic which-path information is circumvented by attention to the confinement energy of the dot, resulting in generation of entangled photon pairs with up to 76% fidelity. Improvements in device design are detailed that reveal the importance of controlling the charge of the dot in order to maximize both emission intensity and fidelity. Finally we present interferometric measurements of the entangled biphoton state that shows a two-fold enhancement of the de Broglie wavelength compared to single photons. © 2008 by Pan Stanford Publishing Pte. Ltd. All rights reserved.

DOI
10.1142/9789814241199_0015

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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.

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Entangled photon pairs from a quantum dot source

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Abstract We present the results of recent experiments designed to manipulate the splitting in quantum dots, to eradicate the which‐path information that prevents entangled photon emission. We demonstrate that dots with splitting within the homogeneous linewidth can be grown directly by carefully controlling the thickness of the dot layer. Polarization dependent correlation measurements on these dots show characteristic features of entanglement, only present for dots with small splitting. Our results indicate that for the first time, we have observed triggered entangled photon pair emission from a quantum dot. The ability to trigger the emission of just one photon pair has advantages over other schemes to generate entangled photons, including scalability of the applications. Finally, we note that the device, once optimized, could be implemented by a simple LED‐like structure. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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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. 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Improved fidelity of triggered entangled photons from single quantum dots

Robert James Young, R. M. Stevenson, P. Atkinson +3 more

We demonstrate the triggered emission of polarization-entangled photon pairs from the biexciton cascade of a single InAs quantum dot embedded in a GaAs/AlAs planar microcavity. Improvements in the sample design blue shifts the wetting layer to reduce the contribution of background light in the measurements. Results presented show that >70% of the detected photon pairs are entangled. The high fidelity of the state that we determine is sufficient to satisfy numerous tests for entanglement. The improved quality of entanglement represents a significant step towards the realization of a practical quantum dot source compatible with applications in quantum information.

arXiv:quant-ph/0601187•New Journal of Physics•Feb 23, 2006•285 citations
We review in this chapter our recent realization and development of entangled light emission from individual InAs quantum dots. We describe how energetic which-path information is circumvented by attention to the confinement energy of the dot, resulting in generation of entangled photon pairs with up to 76% fidelity. Improvements in device design are detailed that reveal the importance of controlling the charge of the dot in order to maximize both emission intensity and fidelity. Finally we present interferometric measurements of the entangled biphoton state that shows a two-fold enhancement of the de Broglie wavelength compared to single photons. © 2008 by Pan Stanford Publishing Pte. Ltd. All rights reserved.
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.
Abstract We present the results of recent experiments designed to manipulate the splitting in quantum dots, to eradicate the which‐path information that prevents entangled photon emission. We demonstrate that dots with splitting within the homogeneous linewidth can be grown directly by carefully controlling the thickness of the dot layer. Polarization dependent correlation measurements on these dots show characteristic features of entanglement, only present for dots with small splitting. Our results indicate that for the first time, we have observed triggered entangled photon pair emission from a quantum dot. The ability to trigger the emission of just one photon pair has advantages over other schemes to generate entangled photons, including scalability of the applications. Finally, we note that the device, once optimized, could be implemented by a simple LED‐like structure. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Semiconductor quantum dots (QDs) have emerged as a premier solid-state platform for the deterministic generation of nonclassical light, offering a compelling pathway toward scalable quantum photonic systems. While single-photon emission from QDs has reached a high level of maturity, the realization of high-fidelity entangled photon-pair sources remains an active and rapidly evolving frontier. In this review, we survey the recent progress in QD-based entangled photon sources, highlighting the conceptual evolution from the established biexciton-exciton cascade to the emerging paradigm of spontaneous two-photon emission. We further examine how advances in nanophotonic architectures and coherent control strategies are redefining fundamental performance limits, enabling concurrent improvements in source brightness, coherence, and entanglement fidelity. Finally, we discuss the key physical and technological challenges that must be addressed to bridge the gap between laboratory demonstrations and large-scale deployment. We conclude by outlining the future opportunities for integrating QD-based entangled photon sources into practical quantum communication, computation, and sensing platforms.
Semiconductor quantum dots (QDs) have emerged as a premier solid-state platform for the deterministic generation of nonclassical light, offering a compelling pathway toward scalable quantum photonic systems. While single-photon emission from QDs has reached a high level of maturity, the realization of high-fidelity entangled photon-pair sources remains an active and rapidly evolving frontier. In this review, we survey the recent progress in QD-based entangled photon sources, highlighting the conceptual evolution from the established biexciton–exciton cascade to the emerging paradigm of spontaneous two-photon emission. We further examine how advances in nanophotonic architectures and coherent control strategies are redefining fundamental performance limits, enabling concurrent improvements in source brightness, coherence, and entanglement fidelity. Finally, we discuss the key physical and technological challenges that must be addressed to bridge the gap between laboratory demonstrations and large-scale deployment. We conclude by outlining the future opportunities for integrating QD-based entangled photon sources into practical quantum communication, computation, and sensing platforms.
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. We study InAsP quantum dots in photonic nanowires for efficient light extraction and to reveal the effect of a quantum dot with high nuclear spin on the entanglement fidelity. \n \nRevealing the ultimate limit of entanglement in QDs is still a challenge, and it is generally believed that a sample made of atoms possessing high nuclear spins exhibits limited entanglement due to the dephasing caused by spin-spin interactions. Moreover, it is assumed that the fine-structure splitting of QDs degrades the entanglement fidelity, as it introduces a which-path information in the biexciton-exciton cascade. We have performed two-photon state tomography on InAsP QDs in InP nanowires, comprising of nuclear spin 9/2, and have realized that such a source is capable of generating entangled photon pairs with negligible dephasing during the emission lifetime, with a peak concurrence C = 77% and a count-weighted average concurrence 62%. This result was obtained by comparing the outcome of our measurements with a theoretical model that assumes no dephasing, however, takes into account the details of the detection system, i.e., dark-count rates and timing resolution, as well as, specific features of the QD, e.g., emission lifetime, fine-structure splitting, multi-photon emission probability, etc. Proper engineering of tapered nanowires allows for extraction of the emitted photons with high efficiency, which makes it possible to perform a complete two-photon state tomography and monitoring the evolution of the exciton state, as well as, the entanglement fidelity of the photon-pair, during the emission lifetime. This enabled us to reveal the effect of an imperfect detection system, as well as, multi-photon emission in recording low values for entanglement fidelity. 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.
We demonstrate the triggered emission of polarization-entangled photon pairs from the biexciton cascade of a single InAs quantum dot embedded in a GaAs/AlAs planar microcavity. Improvements in the sample design blue shifts the wetting layer to reduce the contribution of background light in the measurements. Results presented show that >70% of the detected photon pairs are entangled. The high fidelity of the state that we determine is sufficient to satisfy numerous tests for entanglement. The improved quality of entanglement represents a significant step towards the realization of a practical quantum dot source compatible with applications in quantum information.