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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Entanglement generation in semiconductor nanostructures

Laia Ginés, Junior R. Gonzales-Ureta, Magdalena Moczała-Dusanowska, Jonathan Jurkat, Sven Höfling, Christian Schneider, Ana Predojević

Published Jun 21, 2021• 0 citations
Publisher

Abstract

Single self-assembled quantum dots are established emitters of single photons and entangled photon pairs. The entanglement of photons generated by quantum dots has been shown in polarization [1] , in time bin [2] , and also as hyperentanglement [3] . However, the achievable degree of entanglement and readiness of the source for use in quantum communication protocols, depend on several additional functionalities such as high collection efficiency, coherence of the emitted photon pairs, and fabrication scalability.

DOI
10.1109/cleo/europe-eqec52157.2021.9542051

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Single self-assembled quantum dots are established emitters of single photons and entangled photon pairs. The entanglement of photons generated by quantum dots has been shown in polarization [1] , in time bin [2] , and also as hyperentanglement [3] . However, the achievable degree of entanglement and readiness of the source for use in quantum communication protocols, depend on several additional functionalities such as high collection efficiency, coherence of the emitted photon pairs, and fabrication scalability.
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The quick development of quantum technologies and quantum communication demands a continuous search for more efficient and with higher performance quantum sources and devices. In particular, quantum dots are firmly established sources of single photon emitters and entangled photon pairs, with entanglement of photons emitted by a single quantum dot probed in polarization[1], time-bin[2] and hyperentanglement[3]. Although spontaneous parametric down conversion sources (SPDC) are widely used as entangled photon sources with the highest values of entanglement[4], are being reached by quantum dot devices[5]. With similar quality as SPDC sources, QDs however stand out as sources with sub-Poissonian statistics, essential for safe quantum communication. Still, most of the quantum dots devices exhibit low extraction efficiencies. Attempts to maximize the extraction efficiency involve sophisticated engineered photonic systems[6,7]. Furthermore, the accurate alignment of the quantum dot-cavity system is crucial for an effective quantum dotcavity coupling and hence an emission enhancement into free space[8]. All of this implies the need of intricated nanofabrication processes. Here we present a simple, scalable photonic device that grants efficient creation of entangled photon pairs from InAs quantum dots. The device, spontaneously formed during the growth process, is based on pairs of bottom/top distributed bragg reflectors (DBRs). A propagating deformation in the DBRs, leads to the formation of self-aligned dimples on top of the QDs, resulting in a self-aligned quantum-dot cavity system[9]. Therefore, costly and complex deterministic nanofabrication techniques are not required. In this work, the device performance was characterized, showing experimental collection efficiencies of 0.17, with theoretically pair extraction efficiencies above 0.45. Furthermore, a Purcell enhancement up to 1.7 was obtained. Finally, the device potential to generate time-bin entangled photons was investigated performing state tomography measurements. The subsequent reconstruction of the density matrix yields a concurrence of 0.70(10) and a fidelity of 0.84(5).
Summary form only given. Entanglement, a fundamental resource of quantum technologies, is a unique quantum mechanical attribute. In various systems it can be achieved in various individual degrees of freedom, however some of those systems are able to create entanglement in multiple degrees of freedom simultaneously: hyper-entanglement [1]. Here, we report on demonstration of polarization [2] and time-bin [3] hyper-entangled photons emitted from a single quantum dot. By applying two-photon resonant and coherent excitation on a quantum dot system with marginal fine structure splitting we yield fidelities to the maximally entangled state of 0.80(3) and 0.87(4) in polarization and time-bin, respectively. Quantum enhanced communication schemes rely on Bell-state measurements, which find their simplest realization in the interference of two photons at a beam splitter. However, this method is efficiency limited [4] and a complete Bell state analysis of a photon pair entangled in one degree of freedom using only linear optics is impossible. This limitation can be overcome if the photon pair is entangled in more than one degree of freedom, a hyper-entangled state. Using such states in such schemes does not only reduce the resource overhead [5] it also offers the possibility to increase the success rate significantly [6].
The realization of a functional quantum repeater is one of the major research goals in long-distance quantum communication. Among the different approaches that are being followed, the one relying on quantum memories interfaced with deterministic quantum emitters is considered as among one of the most promising solutions. In this work, we focus on memory-based quantum-repeater schemes that rely on semiconductor quantum dots for the generation of polarization entangled photons. Going through the most relevant figures of merit related to efficiency of the photon source, we select significant developments in fabrication, processing and tuning techniques aimed at combining high degree of entanglement with on-demand pair generation, with a special focus on the progress achieved in the representative case of the GaAs system. We proceed to offer a perspective on integration with quantum memories, both highlighting preliminary works on natural-artificial atomic interfaces and commenting a wide choice of currently available and potentially viable memory solutions in terms of wavelength, bandwidth and noise-requirements. To complete the overview, we also present recent implementations of entanglement-based quantum communication protocols with quantum dots and highlight the next challenges ahead for the implementation of practical quantum networks.