Séminaire du professeur invité LIP6 Norbert Lütkenhaus
Intervenant(s) : Norbert Lütkenhaus
Title: Optical Quantum Communication Protocols
Quantum communication can offer qualitative and quantitative advantages over classical communication. Best known are Quantum Key Distribution protocols which aim at qualitative advantages: no information theoretic secure key can be established by classical communication alone without extra assumption about channel behaviour. Since a while, it is also known that quantum communication can reduce the amount of classical communication needed to solve some communication task. The quantitative quantum advantage can be exponential (from O(sqrt(n)) to O(log n)) for enabling a third party to compare two separate data files of length n (in a specialized setting), or it can be quadratic (from O(n) to O(sqrt(n)) in the case of a two parties who want to find a joint free time slot in their calendars with n time slots. However, up to recently, these quantitative advantages have been of high conceptional interest, but of no practical relevance due to the apparent difficulty of implementing these schemes. This changed with the development of protocols that use standard tool in modern (classical) optical communication. The first implementations realized and demonstrated the quantum advantage, including here at UPMC! The optical approaches to quantum communication protocols also allow another quantitative advantage connected to secure multi-party computations: for example, for the comparison of data files of length n, we can show that the referee learns at most O(log n) bits about the input of the two input sets of data, also an exponential improvement over what is possible with classical communication. In this seminar I will introduce the basic concepts of quantum communication and quantum information complexity, as compared to their classical counterpart, show the principles that allow a quantum advantage, and show how the optical protocols realize that advantage.