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Quantum Nanophotonic Materials, Devices, and Systems

Multipair emission effects in quantum dot-based entangled photon sources

Julia Neuwirth, Francesco Basso Basset, Michele B. Rota, Saimon Filipe . Covre da Silva, Klaus D. Jöns, Armando Rastelli, Rinaldo Trotta

Published Oct 4, 2022• 0 citations
Publisher

Abstract

During recent years, quantum dots have become an increasingly established source of highly entangled photons 1. The main motivation for the development of this technology has resided in the expectation that a resonantly driven quantum emitter can offer a path towards on-demand photon pair generation 2. In fact, state-of-the-art sources relying on spontaneous parametric down-conversion intrinsically suffer from multipair emission at high pair generation rates, which causes a tradeoff between brightness and degree of entanglement 3. Despite the key importance of this aspect, the experimental study of how multiphoton emission affects the entanglement properties of quantum dot-based sources has received surprisingly little attention. In this paper we report the investigation of the multipair emission of the source under quasi-deterministic resonant two-photon excitation without filtering the excitation laser using polarization suppression. The focus is on measuring the real multipair emission entering in entanglement-based measurements, minimizing measurement artefacts from the setup and in particular from the excitation source. This is investigated by measuring the second-order correlation function at zero-time delay in several measurement conditions, including spectral filtering. Our work confirms that the multipair emission is provided also for entanglement-based measurement conditions and thus helps the design of efficient photon sources for quantum information and communication technologies.

DOI
10.1117/12.2632421

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During recent years, quantum dots have become an increasingly established source of highly entangled photons 1. The main motivation for the development of this technology has resided in the expectation that a resonantly driven quantum emitter can offer a path towards on-demand photon pair generation 2. In fact, state-of-the-art sources relying on spontaneous parametric down-conversion intrinsically suffer from multipair emission at high pair generation rates, which causes a tradeoff between brightness and degree of entanglement 3. Despite the key importance of this aspect, the experimental study of how multiphoton emission affects the entanglement properties of quantum dot-based sources has received surprisingly little attention. In this paper we report the investigation of the multipair emission of the source under quasi-deterministic resonant two-photon excitation without filtering the excitation laser using polarization suppression. The focus is on measuring the real multipair emission entering in entanglement-based measurements, minimizing measurement artefacts from the setup and in particular from the excitation source. This is investigated by measuring the second-order correlation function at zero-time delay in several measurement conditions, including spectral filtering. Our work confirms that the multipair emission is provided also for entanglement-based measurement conditions and thus helps the design of efficient photon sources for quantum information and communication technologies.
We investigate the effect of multiphoton emission on polarization-entangled photon pairs from a coherently driven quantum dot by comparing quantum state tomography and second-order autocorrelation measurements as a function of the excitation power. We observe that the relative (absolute) multiphoton emission probability is as low as ${p}_{m}\phantom{\rule{4pt}{0ex}}=(5.6\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}[{p}_{2}=(1.5\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}]$ at the maximum source brightness, with a negligible effect on the degree of entanglement. In contrast with probabilistic sources of entangled photons, the multiphoton emission probability and the degree of entanglement remain practically unchanged against the excitation power over multiple Rabi cycles, while observing oscillations in the second-order autocorrelation function by more than one order of magnitude. Our results, explained by a model which links the second-order autocorrelation function to the actual multiphoton contribution in the two-photon density matrix, highlight that quantum dots can be regarded as a multipair-free source of entangled photons in the solid state.
Abstract Single-photon sources based on semiconductor quantum dots find several applications in quantum information processing due to their high single-photon indistinguishability, on-demand generation, and low multiphoton emission. In this context, the generation of entangled photons represents a challenging task with a possible solution relying on the interference in probabilistic gates of identical photons emitted at different pulses from the same source. In this work, we implement this approach via a simple and compact design that generates entangled photon pairs in the polarization degree of freedom. We operate the proposed platform with single photons produced through two different pumping schemes, the resonant excited one and the longitudinal-acoustic phonon-assisted configuration. We then characterize the produced entangled two-photon states by developing a complete model taking into account relevant experimental parameters, such as the second-order correlation function, Hong–Ou–Mandel visibility, multiphoton emission and pump laser filtering. Our source shows long-term stability and high quality of the generated entangled states, thus constituting a reliable building block for optical quantum technologies.
The generation of on-demand, optimally entangled photon pairs remains one of the most formidable challenges in the quantum optics and quantum information community. Despite the fact that recent developments in this area have opened new doors leading toward the realization of sources exhibiting either high brightness or near-unity entanglement fidelity, the challenges to achieve both together persist. Here, we will provide a historical review on the development of quantum dots (QDs) for entangled photon generation, with a focus on nanowire QDs, and address the latest research performed on nanowire QDs, including measuring entanglement fidelity, light-extraction efficiency, dephasing mechanisms, and the detrimental effects of detection systems on the measured values of entanglement fidelity. Additionally, we will discuss results recently observed pertaining to resonant excitation of a nanowire QD, revealing the potential of such sources to outperform spontaneous parametric down-conversion (SPDC) sources, providing a viable solution to the current challenges in quantum optics and quantum information.
Entangled photon pairs are a ubiquitous resource in quantum technologies, used in quantum key distribution and quantum networking as well as fundamental tests of non-locality. For scalable quantum networks, pairs that are indistinguishable in all unentangled degrees of freedom are essential, as they enable high-fidelity entanglement swapping across network nodes. To date the most-studied sources of "swappable" entangled photon pairs have been based on spontaneous parametric down-conversion (SPDC) in non-linear crystals. However, the probabilistic nature and unavoidable trade-off between brightness and unwanted multi-photon emission limits their performance in lossy channels. Here, we demonstrate a high-fidelity source of "swappable" entangled photon pairs using a semiconductor quantum dot (QD) coupled to a tunable microcavity. By actively modulating the QD emission between orthogonal polarisation states, delaying one path in a low-loss Herriott cell, and recombining the two on a balanced beam splitter, we generate entangled photon pairs with a fidelity of 96.1±0.596.1\pm0.596.1±0.5 %. We identify and mitigate fidelity-limiting factors, achieving a maximum fidelity of 98.1±0.598.1\pm0.598.1±0.5 % through time-resolved post-selection. The scheme suppresses residual multi-photon events concentrated near the excitation pulse and has only a modest impact on the rate. Furthermore, the photons are mutually indistinguishable, enabling efficient entanglement swapping. Our results establish semiconductor QDs as a viable platform for quantum network-compatible swappable entangled photon pair generation, with feasible entanglement generation rates exceeding 0.5 Gpairs/s.
Abstract Spontaneous two-photon emission is a process in which the transition between two quantum states happens via simultaneous emissions of two identical photons. Such second-order quantum radiation is of paramount importance in astrophysics, atomic physics, and quantum technology. In particular, on-demand spontaneous two-photon emission from single quantum emitters has long been envisioned to revolutionize photonic quantum science and technology. To date, experimental realizations of two-photon emission at the single quanta level are still highly challenging yet elusive due to their low probabilities. In this work, we explore spontaneous two-photon emission from a single semiconductor quantum dot deterministically coupled to a high-quality microcavity in which the cavity resonances greatly enhance both excitation and emission processes. When the cavity is resonant to the half energy of the biexciton, the strong vacuum field in the cavity mode drives the biexciton to the ground state and simultaneously emits two photons into the cavity mode, resulting in two-photon emissions with a record brightness comparable to the competing single-photon process and sub-nanowatt power consumption. For the first time, the quantum nature associated with spontaneous two-photon emissions in the cavity quantum electrodynamics regime has been investigated via photon statistics measurements. Furthermore, spontaneous two-photon emission is exploited to build unconventional entangled quantum light sources with the unique advantage of combining the best of two worlds: near-unity entanglement fidelity for spontaneous parametric down-conversion source and on-demand photon emissions for atomic quantum emitters. Our work provides unprecedented insights into the two-photon process in the quantum regime and opens an unexplored avenue for pursuing optically reconfigurable quantum light sources with nonlinear quantum radiations.