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Control experimental d'entrellaçament en múltiples dimensions per QKD basat en fibra òptica multi-core

Sánchez Blanca, Adrián

Published Jul 11, 2023• 0 citations
OpenAlex

Abstract

Quantum Key Distribution (QKD) is a crucial step in quantum cryptography protocols, as a secure distribution of the secret key is determinant in the performance of the system. Some QKD protocols like E91 require the distribution of entangled photon pairs between the emitter and the receiver in order to guarantee a safe transfer of information. However, the distribution of entangled quantum states is a remarkably challenging task, due to the fragility of this physical phenomenon. In this project, a new method for the distribution of high-dimensional entangled photon pairs through an optical fiber is presented. This method is based on the spatial modulation of the output beam of an Entangled Photon Source (EPS) in order to increase the coupling efficiency into a multi-core optical fiber (MCF), which otherwise is highly inefficient. In order to perform the spatial modulation, a LCoS phase-only Spatial Light Modulator (SLM) has been used. The presented method is designed for the modulation of a two-dimensional entangled photon beam, both entangled spatially and in polarization. So, the aim is not only to improve the overall single photon efficiency, but also the maximize as much as possible the coupling of entangled photon pairs, always maintaining the spatial correlations and the polarization entanglement. The system has been tested for a 810$nm$ (NIR) central wavelength EPS, coupled into a 1m long MCF. The obtained results for the single photon efficiency are not as good as initially expected, due to limitations found in the setup along the course of the experiment. Nevertheless, the spatial correlation maintenance and the efficiency in the coupling of entangled photon pairs compared to the obtained single photon efficiency have resulted to be remarkable. The overall conclusion of this project is that the presented method has the potential to achieve much higher coupling efficiencies with some changes in the setup, always maintaining the entanglement. The versatility of the computed phase masks for the spatial modulation allows to adapt the shape of the beam to the used fiber, and also opens a door for other possible future applications. This applications can range from the implementation of the Hong-Ou-Mandel effect in optical fibers to the modulation of high-dimensional entangled photon pairs in other degrees of freedom, like frequency, OAM or time.

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Quantum Key Distribution (QKD) is a crucial step in quantum cryptography protocols, as a secure distribution of the secret key is determinant in the performance of the system. Some QKD protocols like E91 require the distribution of entangled photon pairs between the emitter and the receiver in order to guarantee a safe transfer of information. However, the distribution of entangled quantum states is a remarkably challenging task, due to the fragility of this physical phenomenon. In this project, a new method for the distribution of high-dimensional entangled photon pairs through an optical fiber is presented. This method is based on the spatial modulation of the output beam of an Entangled Photon Source (EPS) in order to increase the coupling efficiency into a multi-core optical fiber (MCF), which otherwise is highly inefficient. In order to perform the spatial modulation, a LCoS phase-only Spatial Light Modulator (SLM) has been used. The presented method is designed for the modulation of a two-dimensional entangled photon beam, both entangled spatially and in polarization. So, the aim is not only to improve the overall single photon efficiency, but also the maximize as much as possible the coupling of entangled photon pairs, always maintaining the spatial correlations and the polarization entanglement. The system has been tested for a 810nmnmnm (NIR) central wavelength EPS, coupled into a 1m long MCF. The obtained results for the single photon efficiency are not as good as initially expected, due to limitations found in the setup along the course of the experiment. Nevertheless, the spatial correlation maintenance and the efficiency in the coupling of entangled photon pairs compared to the obtained single photon efficiency have resulted to be remarkable. The overall conclusion of this project is that the presented method has the potential to achieve much higher coupling efficiencies with some changes in the setup, always maintaining the entanglement. The versatility of the computed phase masks for the spatial modulation allows to adapt the shape of the beam to the used fiber, and also opens a door for other possible future applications. This applications can range from the implementation of the Hong-Ou-Mandel effect in optical fibers to the modulation of high-dimensional entangled photon pairs in other degrees of freedom, like frequency, OAM or time.
The second quantum revolution has driven rapid advancements in quantum technologies, particularly in quantum communication and computing, where the manipulation and distribution of quantum entanglement are crucial. Prior studies have explored entanglement distribution via multi-core fibres (MCF), which allow space-division multiplexing (SDM) and spatial information encoding. Particularly, in [1], polarization-entangled photon pairs from a spontaneous parametric down-conversion (SPDC)-based entangled photon source (EPS) were transmitted through opposite cores of a MCF. However, the ring-shaped intensity of the source output led to non-optimal core overlap, achieving coupling efficiencies below 1%. In this work, we focus on enhancing the coupling efficiency of 810 nm polarization-entangled photons into the six internal cores of a 19-core MCF designed for this wavelength. We achieve this by shaping the SPDC ring into Gaussian modes with a phase-only liquid crystal on Silicon (LCoS) spatial light modulator (SLM). The phase mask in Fig. 1(a) is used in the configuration shown in Fig. 1(b).
Rapid recent advances in quantum technology are giving rise to potential new applications in a wide range of fields. In particular, efficient generation and distribution of quantum entanglement is crucial for many use cases in quantum communication. In this field, multi-core optical fibers (MCF) are especially interesting, as they enable space-division multiplexing, increasing data transmission capacity of the channel. In this work, we present a flexible system for efficient and scalable quantum communication through MCF, using phase-only spatial light modulation (SLM). Firstly, we utilise SLM to enhance the coupling efficiency of near-infrared polarization-entangled photon pairs into a 19-core MCF. Secondly, we present a novel SLM-based system which corrects random polarization transformations incurred over propagation through fiber, for all MCF cores in parallel. The quality of the distributed entanglement is certified by high polarization visibility and violation of the Clauser–Horne–Shimony–Holt (CHSH) inequality for every core pair. Our results show potential for implementation in real-world quantum communication networks, as well as enabling further advanced quantum experiments.
Spatial division multiplexing is a key technology for increasing the capacity of the classical optical fiber networks. It also provides additional degrees of freedom for photon entanglement and transmission in quantum communication networks. This paper describes steps toward generating a four-dimensional, spatially-entangled state and its distribution over multi-km-long four-core fibers. We generate four mutually coherent beams using a spatial light modulator and investigate their simultaneous coupling into a four-core fiber. We also verify the stability of both the relative phase and polarization between the cores over a 10 km of a four-core fiber.
Spatial-division multiplexing is a key technology for increasing the capacity of classical optical fiber networks. It also provides additional degrees of freedom for photon entanglement and transmission in quantum communication networks. This paper describes steps toward generating a four-dimensional, spatially-entangled state and its distribution over multi-km-long four-core fibers. We generate four mutually coherent beams using a spatial light modulator and investigate their simultaneous coupling into a four-core fiber. We verify the stability of both the relative phase and polarization between the cores over10 km\mathbf{1 0 ~ k m}10 kmof a four-core fiber. We also characterize the polarization mode dispersion of the fiber and find its principal states of polarization to mitigate the effects of random polarization-dependent delay across the fiber cores.
High-dimensional entangled photons pairs are interesting for quantum information and cryptography: Compared to the well-known 2D polarization case, the stronger non-local quantum correlations could improve noise resistance or security, and the larger amount of information per photon increases the available bandwidth. One implementation is to use entanglement in the spatial degree of freedom of twin photons created by spontaneous parametric down-conversion, which is equivalent to orbital angular momentum entanglement, this has been proven to be an excellent model system. The use of optical fiber technology for distribution of such photons has only very recently been practically demonstrated and is of fundamental and applied interest. It poses a big challenge compared to the established time and frequency domain methods: For spatially entangled photons, fiber transport requires the use of multimode fibers, and mode coupling and intermodal dispersion therein must be minimized not to destroy the spatial quantum correlations. We demonstrate that these shortcomings of conventional multimode fibers can be overcome by using a hollow-core photonic crystal fiber, which follows the paradigm to mimic free-space transport as good as possible, and are able to confirm entanglement of the fiber-transported photons. Fiber transport of spatially entangled photons is largely unexplored yet, therefore we discuss the main complications, the interplay of intermodal dispersion and mode mixing, the influence of external stress and core deformations, and consider the pros and cons of various fiber types.
In contemporary society, communication security has become increasingly important. The security of the current cryptosystems, based on mathematical assumptions, will not be guaranteed when quantum computers become available [1]. This has spurred investigations into new security technologies based on quantum physics. In order to exchange secure information between users, quantum key distribution (QKD), a branch of Quantum Communications (QCs), provides good prospects for ultimate security based on the laws of quantum mechanics [2–7]. Most of QKD systems are implemented in a point-to-point link using bulky, discrete and expensive devices. Consequently, a large scale deployment of this technology has not been achieved. In a future scenario, where QCs will become standard technology, and where infrastructures like banks and government buildings, will be connected through a quantum network, different requirements in terms of key generation are needed. A solution may be represented by new technologies applied to quantum world. In particular multicore fiber (MCF) open a new scenario for quantum communications, from high-dimensional (HD) spatial entanglement generation, to HD QKD and multi-user key generations, to HD-entanglement distribution. Furthermore, MCFs are expected as a good candidate for overcoming the capacity limit of a current optical communication system, as example the record capacity of 661 Tbits/s was obtained last year with a 30-cores fiber [8]. Proof of concept experiment has already proved the coexistence of classical and quantum communications transmitted into different cores of MCF [9]. On the other hand, photonic integration has played a critical role in recent quantum information revolution by integrating functionalities of traditional discrete bulky components into ultra-compact chips [10, 11]. In fact, integrated photonic circuits provides excellent performances (compacts, good optical phase stability, access to new degrees of freedom), and are particularly suitable for the manipulation of quantum states. Some recent experiments have already demonstrated conventional binary QKD systems, using polarization and phase reference degrees of freedom [12, 13]. Moreover, by using integrated solution new high-dimensional quantum states can be generated and propagated. Based on compact silicon photonic integrated circuits, we here show how a MCF can be used for quantum communications protocols by proving decoy-state HD-QKD and multi-users quantum key generations.