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.
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.
Quantum Entanglement is the essence of quantum physics and inspires fundamental questions about the principles of nature. Moreover it is also the basis for emerging technologies of quantum information processing such as quantum cryptography, quantum teleportation and quantum computation. Bell's discovery, that correlations measured on entangled quantum systems are at variance with a local realistic picture led to a flurry of experiments confirming the quantum predictions. However, it is still experimentally undecided whether quantum entanglement can survive global distances, as predicted by quantum theory. Here we report the violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality measured by two observers separated by 144 km between the Canary Islands of La Palma and Tenerife via an optical free-space link using the Optical Ground Station (OGS) of the European Space Agency (ESA). Furthermore we used the entangled pairs to generate a quantum cryptographic key under experimental conditions and constraints characteristic for a Space-to-ground experiment. The distance in our experiment exceeds all previous free-space experiments by more than one order of magnitude and exploits the limit for ground-based free-space communication; significantly longer distances can only be reached using air- or space-based platforms. The range achieved thereby demonstrates the feasibility of quantum communication in space, involving satellites or the International Space Station (ISS).
Entanglement-based quantum cryptography has the appealing advantage of intuitively more evident security. While originally, weak laser pulse schemes were implemented earlier as technologically simpler, it is now possible to build entanglement-based quantum key distribution systems on a technically equally advanced level. The existing polarization-based systems as developed in Vienna now cover distances of the order of 100 km in fiber and of 144 km in free space. In a recent fiber experiment, an asymmetric source is used such that one photon at the 1.550 nm telecom wavelength is transmitted to Bob, while the other photon at 810 nm is locally measured by Alice. It turns out that polarization entanglement is rather robust, certainly over distances of 100 km in fibers. In a recent long-distance free-space experiment, one photon was sent over 144 km from the Canary Island of La Palma to the island of Tenerife, while again the other photon was measured locally. The receiving station uses the OGS telescope operated by the European Space Agency ESA. This experiment opens up the possibility for future quantum key distribution using satellites.
Experimental realization of entanglement-based quantum key distribution (QKD) in a daylight-uplink (ground-to-satellite) communication channel is highly challenging due to the very low signal-to-noise ratio (SNR) of this configuration, and it has yet to be successfully implemented to date. While current research efforts focus on weak coherent pulse (WCP) sources for daylight QKD, we present a proof-of-principle laboratory demonstration emulating daylight-uplink QKD in a free-space-fiber hybrid configuration using polarization-entangled photon pairs generated via a spontaneous parametric down-conversion (SPDC) process. The emulated uplink channel attenuation reaches up to 50 dB, equivalent to low-earth-orbit (LEO) distances. Furthermore, the detected noise level (up to a few MHz) in the receiver telescope is representative of daylight conditions. The enhancement of SNR is achieved by implementing rigorous filtering on spatial, spectral, and temporal modes. Our ultra-bright source of entangled photons, characterized by a narrow spectral bandwidth of 0.54 nm FWHM, ensures minimal signal attenuation. Using a theoretical model, we estimate the conditional feasibility of entanglement-based QKD for daylight-uplink LEO satellite links up to approximately 400 km, subject to the experimentally motivated system assumptions adopted in this work, including efficient single-mode fiber (SMF) coupling performance.
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.
In the field of quantum communication the teleportation1 of single quanta plays a fundamental role in numerous quantum information-processing protocols. Quantum teleportation allows to faithfully transfer unknown quantum states over arbitrary distances and constitutes a method to circumvent the no-cloning theorem2. Even formally completely independent particles can become entangled via the process of entanglement swapping3. In a future quantum communication network4 this will be of utmost importance, enabling quantum computers to become globally interconnected. In order to prove the feasibility of quantum teleportation under optical link attenuations that will arise in a future space-application scenario, we extended the communication distance to 143 km, employing an optical free-space link between the two Canary Islands of La Palma and Tenerife. This work proofs the feasibility of ground-based freespace quantum teleportation. With our setup we were able to achieve coincidence production rates and fidelities to cope with the optical link attenuation, resulting from various experimental and technical challenges, which will arise in a quantum transmission between a ground-based transmitter and a low-earth-orbiting satellite receiver5. In our experiment we gained an average state fidelity for the teleported quantum states of more than 6 standard deviations beyond the classical limit of 2/3 and a process fidelity of 0.710(42). We expect that many of the features implemented in this experiment will be key blocks for future investigations.
We report the demonstration of a bulk, intrinsically phase-stable source of polarization- and time-energy-entangled photon pairs at 810nm and 1550nm, directly coupled into single-mode optical fibers. This highly non-degenerate wavelength combination is well suited for hybrid quantum communication networks, enabling low-loss transmission in optical fibers at 1550nm while maintaining efficient free-space propagation and detection at 810nm. The source is based on spontaneous parametric down-conversion in a periodically poled lithium niobate crystal embedded in a polarization Sagnac interferometer, providing inherent stability and dual-degree-of-freedom entanglement.
We measure a spectral brightness of B = 4800 pair/s/mW/GHz, with fiber coupling efficiencies exceeding 0.48 at both wavelengths. The entanglement quality is characterized by high-visibility two-photon interference, yielding net visibilities of 0.995 in the polarization basis and 0.991 in the energy-time basis. These performances demonstrate a compact and robust entanglement source compatible with hybrid fiber/free-space quantum key distribution architectures, and suitable for future ground-to-satellite quantum communication links.