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Emerging Imaging and Sensing Technologies for Security and Defence

Towards high-dimensional quantum key distribution over long-distance free-space links (Conference Presentation)

Sebastian Ecker, Fabian Steinlechner, Matthias Fink, Bo Liu, Jessica Bavaresco, Marcus Huber, Thomas Scheidl, Rupert Ursin

Published Oct 11, 2018• 1 citation
Publisher

Abstract

The distribution of quantum entanglement between distant parties is a key challenge in the pursuit of a worldwide quantum network. Quantum repeaters and optical satellite links have both been proposed to overcome the distance limitations of fiber-based quantum networks. In order to test the viability of a world spanning quantum satellite network, several proof-of-concept studies have already demonstrated high-fidelity transmission of photonic entanglement via terrestrial long-distance free-space links. With the recent launch of the Micius quantum satellite by the Chinese Academy of Sciences, the technological challenge of implementing this scheme in situ was overcome. All of these free-space experiments utilized entanglement in a two-dimensional state space, which in most cases was encoded in the polarization of photons. However, high-dimensional entanglement can yield significant benefits in quantum key distribution by increasing the secure key rate and enhancing the resilience to eavesdropping attacks. Energy-time entanglement is an established photonic degree of freedom (DOF); it is routinely used for the distribution of entanglement in fiber-based quantum cryptography networks but has only recently been considered as a viable option for atmospheric free-space quantum communications. The dimensionality can be further increased by exploiting simultaneous entanglement in several DOF. This so-called hyperentanglement has previously been used in various quantum protocols, but not yet been demonstrated outside a protected laboratory environment. For our proof-of-concept experiment (doi:10.1038/ncomms15971), we use energy-time and polarization hyperentanglement to distribute, for the first time, 4-dimensional entanglement via a 1.2-km-long intra-city free-space link. A source of hyperentangled photons and a detection module (Alice) were located in our lab and a receiver station (Bob) was located at a different university building. The source produced polarization entangled photon pairs via type-0 down-conversion in a Sagnac loop configuration. The emission time of a photon pair is uncertain within the significantly longer coherence time of the pump laser, thus resulting in an energy-time and polarization hyperentangled state. One photon was guided to a local measurement module and the partner photon was guided to a transmitter telescope on the roof of the institute. After transmission over the 1.2-km-long free-space link, the photons were received by a telescope and detected in Bob’s measurement module. The measurement modules for Alice and Bob each featured a polarization and an energy-time analyzer. We observe high-visibility two-photon interference in both polarization and energy-time subspaces. The measured visibilities certify entanglement in both subspaces individually. Additionally, they establish a lower bound on the Bell-state fidelity of the hyperentangled state of 0.94, thus certifying genuine 4-dimensional entanglement and 1.47 ebits of entanglement of formation. We have thus successfully distributed hyperentangled photon pairs via an intra-city free-space link under conditions of strong atmospheric turbulence. The transmission of quantum information embedded in a genuine high-dimensional state space under real-world link conditions is a first important step towards real-world implementations of advanced quantum information processing protocols in the future. In particular, it enables the implementation of high-dimensional QKD protocols over long-distance free-space links, and, ultimately, over satellite links with only minor changes to existing mission proposals.

DOI
10.1117/12.2326600

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The distribution of quantum entanglement between distant parties is a key challenge in the pursuit of a worldwide quantum network. Quantum repeaters and optical satellite links have both been proposed to overcome the distance limitations of fiber-based quantum networks. In order to test the viability of a world spanning quantum satellite network, several proof-of-concept studies have already demonstrated high-fidelity transmission of photonic entanglement via terrestrial long-distance free-space links. With the recent launch of the Micius quantum satellite by the Chinese Academy of Sciences, the technological challenge of implementing this scheme in situ was overcome. All of these free-space experiments utilized entanglement in a two-dimensional state space, which in most cases was encoded in the polarization of photons. However, high-dimensional entanglement can yield significant benefits in quantum key distribution by increasing the secure key rate and enhancing the resilience to eavesdropping attacks. Energy-time entanglement is an established photonic degree of freedom (DOF); it is routinely used for the distribution of entanglement in fiber-based quantum cryptography networks but has only recently been considered as a viable option for atmospheric free-space quantum communications. The dimensionality can be further increased by exploiting simultaneous entanglement in several DOF. This so-called hyperentanglement has previously been used in various quantum protocols, but not yet been demonstrated outside a protected laboratory environment. For our proof-of-concept experiment (doi:10.1038/ncomms15971), we use energy-time and polarization hyperentanglement to distribute, for the first time, 4-dimensional entanglement via a 1.2-km-long intra-city free-space link. A source of hyperentangled photons and a detection module (Alice) were located in our lab and a receiver station (Bob) was located at a different university building. The source produced polarization entangled photon pairs via type-0 down-conversion in a Sagnac loop configuration. The emission time of a photon pair is uncertain within the significantly longer coherence time of the pump laser, thus resulting in an energy-time and polarization hyperentangled state. One photon was guided to a local measurement module and the partner photon was guided to a transmitter telescope on the roof of the institute. After transmission over the 1.2-km-long free-space link, the photons were received by a telescope and detected in Bob’s measurement module. The measurement modules for Alice and Bob each featured a polarization and an energy-time analyzer. We observe high-visibility two-photon interference in both polarization and energy-time subspaces. The measured visibilities certify entanglement in both subspaces individually. Additionally, they establish a lower bound on the Bell-state fidelity of the hyperentangled state of 0.94, thus certifying genuine 4-dimensional entanglement and 1.47 ebits of entanglement of formation. We have thus successfully distributed hyperentangled photon pairs via an intra-city free-space link under conditions of strong atmospheric turbulence. The transmission of quantum information embedded in a genuine high-dimensional state space under real-world link conditions is a first important step towards real-world implementations of advanced quantum information processing protocols in the future. In particular, it enables the implementation of high-dimensional QKD protocols over long-distance free-space links, and, ultimately, over satellite links with only minor changes to existing mission proposals.
Photonic entanglement is a fundamental resource in quantum information processing and its distribution between distant parties is a key challenge in quantum communications. Optical satellite links allow transmitting entangled photons over longer distances than currently possible on ground and could provide a path towards global quantum communication networks. The majority of current photonic free-space quantum communication systems, including two recent quantum satellite experiments, use two-dimensional polarization encoding, where each photon can carry at most a single bit of quantum information. Encoding additional qubits in the high-dimensional spatio-temporal degrees of freedom could not only increase channel capacities but also improve robustness with respect to noise and eavesdropping in quantum cryptography. Here, we give an overview of our efforts towards exploiting high-dimensional entanglement in long-distance free-space quantum communications. We report on the results of a first feasibility study, in which we used polarization / energy-time hyperentanglement to increase the dimensionality of the state space and transmit genuine 4-dimensional entanglement via an intra-city free-space link. We discuss how we intend to extend this approach to applications in noise-tolerant large-alphabet quantum key distribution via free-space links, and, ultimately, experiments with satellites.
Quantum entanglement is a fundamental resource in quantum information processing and its distribution between distant parties is a key challenge in quantum communications. Increasing the dimensionality of entanglement has been shown to improve robustness and channel capacities in secure quantum communications. Here we report on the distribution of genuine high-dimensional entanglement via a 1.2-km-long free-space link across Vienna. We exploit hyperentanglement, that is, simultaneous entanglement in polarization and energy-time bases, to encode quantum information, and observe high-visibility interference for successive correlation measurements in each degree of freedom. These visibilities impose lower bounds on entanglement in each subspace individually and certify four-dimensional entanglement for the hyperentangled system. The high-fidelity transmission of high-dimensional entanglement under real-world atmospheric link conditions represents an important step towards long-distance quantum communications with more complex quantum systems and the implementation of advanced quantum experiments with satellite links.
Quantum entanglement based communication schemes have many advantages. High-dimensional encoding has been shown to improve robustness and channel capacities in secure quantum communications. The transmission of genuine high-dimensional entanglement under real-world atmospheric link conditions, however, is an ongoing experimental challenge. We report on a proof-of-concept experiment in which we, for the first time, use energy-time and polarization hyperentanglement to transmit 4-dimensional entanglement via a 1.2-km-long intra-city free-space link. We discuss how this approach can be adapted for high-dimensional free-space QKD and report on the progress of ongoing experiments.
Free-space quantum key distribution has recently achieved several milestones, such as \nthe launch and results of the first quantum satellite, Micius. The emergence of quantum satellites has certainly made progress towards the realization of a global quantum cryptographic network. In this thesis, two challenges in the development of an optical quantum ground station \nfor a free-space quantum satellite link are studied. The first is the development of a high brightness, fiber pigtailed waveguide that is to be used as a polarization entangled photon source. The high pair production rate is required in order to meet the requirements for a satellite up-link configuration. The portability, robustness and ease of alignment were motivations for choosing a fiber pigtailed source. Certain challenges that are fundamental to the source design were characterized and several solutions to these challenges were investigated. The other main investigation in this thesis, is the development of a passive polarization \ncompensation using polarization maintaining fibers. The birefringence in standard single mode optical fibers causes random polarization rotations to the light passing through the fiber. Polarization maintaining fibers, though very high in birefringence, are used with entangled photons and techniques from reference frame independent quantum key distribution protocols are shown to compensate for random polarization rotations while preserving the entanglement. In addition, the feasibility of the protocol using the polarization maintaining fibers is investigated. \nThrough various studies, experiments, and component design, the feasibility of a pigtailed waveguide entangled photon source has been shown to need further investigation, while the feasibility of implementing polarization maintaining fibers to the ground station has been shown to be effective. It is particularly effective as a passive polarization compensation system that uses entanglement, however a similar concept is effective for non-entangled single photons. This work contributes to a long line of achievements leading towards satellite implementations of quantum key distribution for an eventual global quantum cryptographic network.
Abstract Quantum key distribution (QKD) is a pioneering quantum technology on the brink of widespread deployment. Nevertheless, the distribution of secret keys beyond a few 100 km at practical rates remains a major challenge. One approach to circumvent lossy terrestrial transmission of entangled photon pairs is the deployment of optical satellite links. Optimizing these non-static quantum links to yield the highest possible key rate is essential for their successful operation. We therefore developed a high-brightness polarization-entangled photon pair source and a receiver module with a fast steering mirror capable of satellite tracking. We employed this state-of-the-art hardware to distribute photons over a terrestrial free-space link with a distance of 143 km, and extracted secure key rates up to 300 bits per second. Contrary to fiber-based links, the channel loss in satellite downlinks is time-varying and the link time is limited to a few minutes. We therefore propose a model-based optimization of link parameters based on current channel and receiver conditions. This model and our field test will prove helpful in the design and operation of future satellite missions and advance the distribution of secret keys at high rates on a global scale.
Quantum communication rates in terrestrial quantum networks are fundamentally limited by fiber loss, even in the presence of quantum repeaters. Satellites offer a solution for long-distance communication, with the most commonly explored scenario involving prepare-and-measure protocols connecting from orbit to a trusted-node ground station via free-space down-links. In contrast, up-link scenarios allow for entanglement to be distributed between a satellite and remote end-users in terrestrial networks, eliminating any trust requirement on the ground station. Here, we demonstrate an ultra-bright source of far-non-degenerate entangled photons and perform quantum key distribution in emulated high-loss satellite scenarios. With a loss profile corresponding to that of one of the pioneering Micius up-link experiments, and a terrestrial end-user separated by 10 km of telecom fiber, we achieve secure key bit accumulation of 5.2 kbit in a single emulated overpass in the asymptotic limit. Our results confirm the viability of upcoming low-Earth orbit receiver satellite missions.