[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Method for Quantum Key Establishment through a Multimode Fiber

摘要： Secure communication became extremely important in the Information Age. Quantum communication protocols have been developed to provide absolutely secure transmission of information. Historically, quantum key distribution (QKD) uses the two-dimensional polarization basis to encode information and the key element of long-distance communication is single-mode optical fiber. However, it intrinsically limits the information content to 1 bit per photon. Nowadays, significant effort is devoted to increase the data carrying capacity of a single quantum secure optical line. Large-core multimode fibers support a multitude of transverse optical modes and can potentially transfer information at much higher density. Here we propose a fundamentally new concept for secure high-dimensional remote key establishment. Our method uses a quantum-secure wavefront shaping procedure to transmit information through a light-scrambling large-core multimode fiber. It naturally works with a high-dimensional alphabet. The main idea is illustrated in Fig. 1. To send a symbol, Alice prepares a wavefront that leads to a focus at a particular point on the fiber output facet. Throughout the fiber, the photon will be present as a disordered superposition of all fiber modes. If an eavesdropper (Eve) intercepts, the photon collapses at a nearly random position on Eve’s detector, which varies even between identical copies of the same symbol. Only at Bob’s end, the photon will be spatially localized and Bob can unambiguously identify the symbol. In contrast to popular QKD protocols, our method allows to detect eavesdropper attacks without using two randomly switched mutually unbiased bases. Moreover, information is sent in a deterministic way, allowing direct decoding at the receiver end without classical postprocessing. Since it is optical fiber based, our method allows to naturally extend secure communication to larger distances. We argue that our method is quantum secure if symbols are encoded into single photons. Moreover, we show that our method does not require the light to be in an entangled (or otherwise special) quantum state; a weak coherent state (even when several photons per symbol are used) still guarantees the security of the protocol. We experimentally characterise the setup for this new type of key exchange method by encoding information into a few-photon light pulse decomposed over guided modes of an easily available step-index multimode fiber.