Abstract
Quantum communication aims to provide absolutely secure transmission of secret information. State-of-the-art methods encode symbols into single photons or coherent light with much less than one photon on average. For long-distance communication, typically a single-mode fiber is used and significant effort has been devoted already to increase the data carrying capacity of a single optical line. Here we propose and demonstrate a fundamentally new concept for remote key establishment. Our method allows high-dimensional alphabets using spatial degrees of freedom by transmitting information through a light-scrambling multimode fiber and exploiting the no-cloning theorem. Eavesdropper attacks can be detected without using randomly switched mutually unbiased bases. We prove the security against a common class of intercept-resend and beam-splitting attacks with single-photon Fock states and with weak coherent light. Since it is optical fiber based, our method allows to naturally extend secure communication to larger distances. We experimentally demonstrate this new type of key exchange method by encoding information into a few-photon light pulse decomposed over guided modes of an easily available multimode fiber.
Highlights
The importance of secure communication is rapidly growing [1]
In the original and best-known quantum key distribution (QKD) method – BB84 proposed by Bennett and Brassard [5] – the security is based on the fact that the polarization of a single photon can be prepared and measured along well-defined directions
We give a quantitative argument why the method is still secure with weak coherent light and imperfect wavefront shaping without any adaptation to the method or the setup
Summary
The importance of secure communication is rapidly growing [1]. We use cryptography in everyday life often without noticing, for example, when we conduct financial transactions via the internet. The security of conventional cryptography is based on shared secret keys or on computational assumptions, such as the presumed hardness of factoring [2]. This means that it is vulnerable to unanticipated advances in hardware or algorithms. Quantum cryptography in theory provides unconditional secure communication, assuming only that an eavesdropper (Eve) is restricted by the laws of physics: the quantum no-cloning theorem forbids to replicate an unknown quantum state [3]. In the original and best-known quantum key distribution (QKD) method – BB84 proposed by Bennett and Brassard [5] – the security is based on the fact that the polarization of a single photon can be prepared and measured along well-defined directions. Key-distribution methods do not by themselves communicate useful information, but such a communication can follow with the proven-secure one-time-pad method after the secure key is built up
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