Large-scale quantum photonics requires the integration of several elements on the same chip, including quantum emitters and memories, active photonics, and single-photon detectors. In this talk, I will report on i) our recent work integrating superconducting nanowire single-photon detectors (SNSPD) with mechanically reconfigurable integrated photonics, and ii) our recently developed method for integration of SNSPDs onto arbitrary photonic substrates.
Quantum networks are fundamental for communication protocols and their implementation and validation become hard as the network's complexity increases. In this talk, we show the experimental implementation of a hybrid network exploiting different technologies a quantum dot and a nonlinear crystal.
The network is used to share a photonic entangled state among three nodes connected through a 270 m free-space channel and fiber links.
We also demonstrate the non-local behavior of the implemented network through the violation of a Bell-like Inequality. Our results pave the way toward the realization of more complex networks and the implementation of quantum communication protocols in urban environments, leveraging the capabilities of hybrid-photonic technologies.
Semiconductor quantum dots (QDs) are able to confine single charges on the nanoscale in all three dimensions of space, making them excellent systems for exploring quantum phenomena. In particular, QDs have demonstrated outstanding performance as sources of entangled and indistinguishable photon pairs, properties highly desired in the fields of quantum communication and -information processing. Here I report on the advances of QDs as potential resources for photonic quantum networks, which allow to overcome the fundamental range limitations of single photon-based applications. After an introduction to the underlying mechanisms of entangled photon pair generation, I demonstrate several building blocks of quantum networks, with quantum key distribution as a prime application.
The prospect of quantum networks is pushing technical advances in entangled photon generation, and different solutions have now the potential to coexist.
After reviewing the development that led to implement entanglement-based quantum key distribution using a quantum dot—a technology motivated by the goal of on-demand operation—in an urban free-space optical link, we present its extension to a three-node quantum network including a source based on spontaneous parametric down conversion. Using separable measurements, we combine intrinsically independent sources to demonstrate a significant violation of a Bell-like inequality associated to nonlocal correlations in a tripartite hybrid network.
Tunable entangled photon emitters based on cavity-enhanced GaAs quantum dots on micromachined piezoelectric substrates was recorded at SPIE Optics + Photonics held in San Diego, California, United States 2022.
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.
Quantum networks play a crucial role for distributed quantum information processing, enabling the establishment of entanglement and quantum communication among distant nodes. Firstly, we use a coherently driven quantum dot to experimentally demonstrate a modified Ekert quantum key distribution protocol with two quantum channel approaches: both a 250-m-long single-mode fiber and in free-space, connecting two buildings within the campus of Sapienza University in Rome. Second, we included an independent SPDC source to construct a hybrid network (quantum dot and SPDC) to violating a suitable non-linear Bell inequality, thus demonstrating the nonlocal behavior of the correlations among the nodes of the network.
Entanglement-based quantum key distribution (QKD) promises enhanced robustness against eavesdropping and compatibility with future quantum networks. Among other sources, semiconductor quantum dots (QDs) can generate polarization-entangled photon pairs with near-unity entanglement fidelity and a multiphoton emission probability close to zero even at maximum brightness. These properties have been demonstrated under resonant two-photon excitation (TPE) and at operation temperatures below 10 K. However, source blinking is often reported under TPE conditions, limiting the maximum achievable photon rate. In addition, operation temperatures reachable with compact cryocoolers could facilitate the widespread deployment of QDs, e.g., in satellite-based quantum communication. We demonstrate blinking-free emission of highly entangled photon pairs from GaAs QDs embedded in a p-i-n diode. High fidelity entanglement persists at temperatures of at least 20 K, which we use to implement fiber-based QKD between two buildings with an average raw key rate of 55 bits / s and a qubit error rate of 8.4%. We are confident that by combining electrical control with already demonstrated photonic and strain engineering, QDs will keep approaching the ideal source of entangled photons for real world applications.
We investigate the indistinguishability of single photons generated from strain-free GaAs/AlGaAs quantum dots using pulsed resonance fluorescence techniques. In pulsed two-photon interference measurements we observe a single photon indistinguishability with a raw visibility of 95%. This can be traced back to the short intrinsic lifetime of excitons and trions confined in the GaAs quantum dots and demonstrates that for this material system the generation of single photons is possible with near-unity indistinguishability even without Purcell enhancement.
We investigate the indistinguishability of single photons generated from strain-free GaAs/AlGaAs quantum dots using pulsed resonance fluorescence techniques. In pulsed two-photon interference measurements we observe a single photon indistinguishability with a raw visibility of 95%. This can be traced back to the short intrinsic lifetime of excitons and trions confined in the GaAs quantum dots and demonstrates that for this material system the generation of single photons is possible with near-unity indistinguishability even without Purcell enhancement.
Epitaxially grown self-assembled quantum dots (QDs) are promising candidates for an efficient single-photon generation. In order to maximize the number of photons extracted from the device, QDs are frequently embedded into photonic structures such as micro-cavities. Once the QD is positioned inside an optical cavity, the Purcell effect ensures that light is emitted predominantly into the cavity mode.
In this contribution, we demonstrate results of emission tuning of QDs inserted in micro-cavities. A sample containing an InAs/GaAs QDs embedded in a planar cavity based on Bragg reflectors has been integrated onto the PMN-PT piezo crystal. Subsequently, micro-cavities have been fabricated by electron-beam lithography and reactive ion etching.
The application of external stress produces linear shifts of QDs emission which could be tuned into the resonance with fundamental cavity mode and allow enhancement of QD emission due to the Purcell effect.
Semiconductor quantum dots (QDs) combine high optical activity and the possibility of integration in a myriad of devices. Here, we demonstrate the integration of (Al)GaAs QDs in nanophononic strings and show that the excitonic two-level system of the QD couples to the confined phononic modes in string. For the chosen design the coupling is mediated via the valence band deformation potential and shear strain. Finite element modelling (FEM) shows that the optomechanical coupling parameter γom>0.15 meV/nm, exceeding that of vibrating nanowire architectures by one order of magnitude1.
Among different solid-state emitters of quantum light, epitaxial GaAs quantum dots (QDs) have recently emerged as nearly-ideal sources of triggered polarization entangled photon pairs [1]. This property, combined with the strongly suppressed probability of multiphoton emission under resonant excitation [2] has allowed the implementation of quantum teleportation using photons subsequently emitted by a single QD [3].
In this talk I will discuss the peculiar properties of GaAs QDs [4,5], their performance as emitters of both single and entangled photons, as well as possible methods to engineer their light emission characteristics [1,4] to meet the demanding requirements imposed by photonic quantum technologies.
[1] D. Huber et al. Phys. Rev. Lett. 121, 033902 (2018).
[2] L. Schweickert et al. Appl. Phys. Lett. 112, 093106 (2018).
[3] M. Reindl et al. Science Adv. (2018).
[4] X. Yuan et al. Nat. Comm. 9, 3058 (2018).
[5] M. Reindl et al. https://arxiv.org/abs/1901.11251.
We demonstrate direct control over the level of lateral quantum coupling between two self-assembled InGaAs/GaAs
quantum dots. This coupled system, which we also refer to as a lateral quantum dot molecule, was produced using a
unique technique which combines molecular beam epitaxy and in-situ atomic layer etching. Atomic force microscopy
measurements show that each molecule consists of two structurally distinct dots, which are aligned along the [1-10]
direction. Each molecule exhibits a characteristic photoluminescence spectrum primarily consisting of two neutral
excitonic and two biexcitonic transitions. The various transitions have been investigated using micro-photoluminescence
measurements as a function of excitation power density, time, and applied electric field. Photon statistics experiments
between the excitonic emission lines display strong antibunching in the second-order cross-correlation function which
confirms that the two dots are quantum coupled. Cascaded emission between corresponding biexcitonic and excitonic
emission has also been observed. Using a parallel electric field we can control the quantum coupling between the dots.
This control manifests itself as an ability to reversibly switch the relative intensities of the two neutral excitonic
transitions. Furthermore, detailed studies of the emission energies of the two neutral excitonic transitions as a function of
parallel lateral electric field show a clear anomalous Stark shift which further demonstrates the presence of quantum
coupling between the dots. In addition, this shift allows for a reasonable estimate of the coupling energy. Finally, a
simple one-dimensional model, which assumes that the coupling is due to electron tunneling, is used to qualitatively
describe the observed effects.
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