Simultaneous transmission of hyper-entanglement in three degrees of freedom through a multicore fiber
Lukas Achatz, Lukas Bulla, Sebastian Ecker, Evelyn A. Ortega, Michael Bartokos, Juan Carlos Alvarado-Zacarias, Rodrigo Amezcua‐Correa, Martin Bohmann, Rupert Ursin, Marcus Huber
Abstract Entanglement distribution is at the heart of most quantum communication protocols. Inevitable loss of photons along quantum channels is a major obstacle for distributing entangled photons over long distances, as the no-cloning theorem forbids the information to simply be amplified along the way as is done in classical communication. It is therefore desirable for every successfully transmitted photon pair to carry as much entanglement as possible. Spontaneous parametric down-conversion (SPDC) creates photons entangled in multiple high-dimensional degrees of freedom simultaneously, often referred to as hyper-entanglement. In this work, we use a multicore fiber (MCF) to show that energy-time and polarization degrees of freedom can simultaneously be transmitted in multiple fiber cores, even maintaining path entanglement across the cores. We verify a fidelity to the ideal Bell state of at least 95% in all degrees of freedom. Furthermore, because the entangled photons are created with a center wavelength of 1560 nm, our approach can readily be integrated into modern telecommunication infrastructure, thus paving the way for high-rate quantum key distribution and many other entanglement-based quantum communication protocols.
Abstract Entanglement distribution is at the heart of most quantum communication protocols. Inevitable loss of photons along quantum channels is a major obstacle for distributing entangled photons over long distances, as the no-cloning theorem forbids the information to simply be amplified along the way as is done in classical communication. It is therefore desirable for every successfully transmitted photon pair to carry as much entanglement as possible. Spontaneous parametric down-conversion (SPDC) creates photons entangled in multiple high-dimensional degrees of freedom simultaneously, often referred to as hyper-entanglement. In this work, we use a multicore fiber (MCF) to show that energy-time and polarization degrees of freedom can simultaneously be transmitted in multiple fiber cores, even maintaining path entanglement across the cores. We verify a fidelity to the ideal Bell state of at least 95% in all degrees of freedom. Furthermore, because the entangled photons are created with a center wavelength of 1560 nm, our approach can readily be integrated into modern telecommunication infrastructure, thus paving the way for high-rate quantum key distribution and many other entanglement-based quantum communication protocols.
Quantum key distribution has shown promising advances in information transmission security. In particular, quantum communication schemes based on entanglement have enormous potential to enhance the security and efficiency of information transmission. Therefore, today’s efforts address overcoming issues related to high data transmission and long-distance implementations to enable real-world quantum communication applications. Here, we experimentally demonstrate a multiplexing technique to enhance the secret key rate by exploiting the momentum correlation of photon pairs generated by spontaneous parametric downconversion. We distributed polarization-entangled photon pairs into opposite cores within a 19-core multicore fiber. We estimated the secret key rates in a configuration with 6 and 12 cores from the entanglement visibility after transmission through a 411-m-long multicore fiber. Our results can lead to the transmission of more information per photon and the establishment of novel quantum networks with one common communication channel, which opens the possibility of deploying new quantum channels over longer distances.
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.
The development and wide application of quantum technologies highly depend on the capacity of the communication channels distributing entanglement.Space-division multiplexing (SDM) enhances data channel transmission capacity in classical telecommunication and bears the potential to transfer the idea to quantum communication using current infrastructure.Here, we demonstrate a SDM of polarizationentangled photons over a 411 m long 19-core multicore fiber distributing polarization-entangled photon pairs through up to 12 channels simultaneously.The quality of the multiplexed transfer is evidenced by high polarization visibility and Clauser-Horne-Shimony-Holt (CHSH) Bell inequality violation for each pair of opposite cores.Our distribution scheme shows high stability over 24 h without any active polarization stabilization and can be effortlessly adapted to a higher number of channels.This technique increases the quantum-channel capacity and allows the reliable implementation of quantum networks of multiple users based on a single entangled photon pair source.
In a quantum network involving multiple communicating parties, an important goal is to establish high-quality pairwise entanglement among the users without introducing multiple entangled-photon sources which would necessarily complicate the overall network setup. Moreover, it is preferable that the pairwise entanglement of photons is in the time-bin degree of freedom as the photonic time-bin qubit is ideally suited for fiber-optic distribution. Here, we report an experimental demonstration of a field-deployable quantum communication network involving multiple users, all of whom share pairwise entanglement in the time-bin degree of freedom of photons. In particular, by utilizing a single spontaneous-parametric down-conversion source which produces a broadband pair of photons and the wavelength-division demultiplexing/multiplexing technology, all the communicating parties within the network are always simultaneously ready for quantum communication. To further demonstrate the practical feasibility of a quantum network with time-bin entanglement over a wavelength-multiplexed fiber network, we demonstrate entangled-photon quantum key distribution with three users, each separated by 60 km of optical fibers.
The development and wide application of quantum technologies highly depend on the capacity of the communication channels distributing entanglement. Space-division multiplexing (SDM) enhanced channel capacities in classical telecommunication and bears the potential to transfer the idea to quantum communication using current infrastructure. Here, we demonstrate an SDM of polarization-entangled photons over a 411m long 19-core multicore fiber distributing polarization-entangled photon pairs through up to 12 channels simultaneously. The quality of the multiplexed transfer is evidenced by high polarization visibility and CHSH Bell inequality violation for each pair of opposite cores. Our distribution scheme shows high stability over 24 hours without any active polarization stabilization and can be effortlessly adapted to a higher number of channels. This technique increases the quantum-channel capacity and allows the reliable implementation of quantum networks of multiple users based on a single entangled-photon pair source.
This thesis explores a method to enhance the practical utility of hyper-entangled photons generatedin periodically poled silica fiber for quantum communication applications. Hyper-entanglement, which involves quantum correlations in multiple degrees of freedom simultaneously, offers increased information capacity per particle compared to single-degree entanglement. Periodically poled sil- ica fiber has shown promise in generating high-quality, broadband polarization-frequency hyper- entangled photon pairs collinearly through type-II spontaneous parametric down-conversion. How- ever, the collinear nature of these photon pairs poses a challenge for applications requiring hyper- entanglement, as they occupy the same spatial mode. This research develops a technique for sep- arating these photon pairs while preserving their information capacity, measures the resulting en- tanglement quality degradation, and discusses potential improvements. This work contributes to the development of efficient fiber-based hyper-entanglement sources, advancing the field of practical quantum technologies and long-distance quantum communication.