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.
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.
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.
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.
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.
High-dimensional entanglement not only offers a high security level for quantum communication but also promises improved information capacity and noise resistance of the system. However, due to various constraints on different high-dimensional degrees of freedom, whether these advantages can bring improvement to the actual implementation is still not well proven. Here we present a scheme to fully utilize these advantages over long-distance noisy fiber channels. We exploit polarization and time-bin hyperentanglement to achieve high-dimensional coding, and observe significant enhancements in secure key rates and noise tolerance that surpass the capabilities of qubit systems. Moreover, the demonstration achieves a distribution up to 50 km, which is the longest distance for high-dimensional entanglement-based quantum key distribution up to date, to our knowledge. Our demonstration validates the potential of high-dimensional entanglement for quantum communications over long-distance noisy channels, paving the way for a resilient and resource-efficient quantum network.
This thesis is focused on the development of advanced techniques for quantum communications in free-space channels. At present there are several demonstrations of quantum communication protocols at long distances, many of these exploit optical fiber as transmission channel. The optical fiber is very advantageous in that it is scarcely influenced by external conditions and allows to connect remote localities that are not in direct line of sight. On the other hand, it presents strong limitations in transmission distance because of dispersion and attenuation. It is therefore necessary to explore new transmission channels in order to allow a global spread of quantum communication. The free-space channels are good candidates, both the vertical ones (between earth and space) and the horizontal ones. The study of these is of fundamental importance for the extension of quantum communication over a global scale. Since the eighties to present day have been defined several quantum communication protocols such as Bennet-Brassard 84 (BB84), Bennet 92 (B92), Ekert 91, Decoy State, Dense Coding, etc.. These protocols present different characteristic such as reliability, security and key rate. These three parameters are very important for the characterization of a communication protocol and are directly related to each other. The development of new quantum communication protocols that maximize these parameters is very important for future progress of quantum communications. This thesis is divided into three parts. In the first part we consider the characteristics and the behaviour of a free-space horizontal quantum channel. We study the impact of atmospheric turbulence in the case of single beam propagation and twin-beam propagation. We then measure the losses of the channel and analyse the effect of turbulence on photon statistics. Finally, we propose a method that exploits the turbulence to improve the signal to noise ratio (SNR) of the channel. In the second part we take into consideration the security and efficiency of Quantum Key Distribution (QKD) protocols . We propose the experimental demonstration of the B92 protocol with non-maximally entangled states . Using this kind of states allow us to improve the security and the key rate of this protocol. Finally, in the last part, we presented a possible way to improve the quantum channel capacity exploiting hyperentangled photon pairs. We designed and experimentally tested a system that can be used to transmit hyperentangled states at long distances.
Quantum communication uses the laws of quantum physics to encode and decode information and hence provides much more efficient ways of communication than are classically possible. Additionally employing quantum entanglement offers access to a new domain of communication protocols, such as Quantum Dense Coding, Quantum Teleportation or Quantum Cryptography. Photonic realizations allow implementing quantum communication over long distances. To achieve quantum networking on a global scale, photonic free-space links via satellites are a promising future trend. Entanglement-based quantum communication schemes rely on an efficient source of high-quality quantum-entangled photons. Over the last decade extensive effort has been put into the development and enhancement of highly efficient sources. Using spontaneous parametric down conversion (SPDC) in nonlinear crystals turned out to be the most successful method of creating polarization-entangled photon pairs. To date, periodically poled crystals allow collinear setups of very high brightness. We present a highly efficient source of polarization-entangled photons whose performance in pair-production rate, phase stability and compactness by far exceeds that of previous approaches. The superior brightness and quality of entanglement of our source enabled us to cope with high link attenuation, thus allowing to bridge long free-space distances. We performed a proof-of-concept experiment between the Canary Islands of La Palma and Tenerife making use of the European Space Agency's (ESA) optical ground station (OGS). Therefore we used space hardware within a quantum communication free-space experiment. We successfully emulated conditions that are expected for a LEO-satellite down link and we faithfully transmitted a maximally quantum-entangled state over a long-distance free-space link of 144km. These results pave the way for first quantum communication experiments in space.