NOVEL QUBIT QUANTUM CRYPTOGRAPHY PROTOCOLS Quantum Secret Sharing

This report summarises the ongoing work in QuComm on the demonstration of novel quantum communication protocols, in particular multi-party protocols or protocols which use more than 2-states (or more than one qubit). Such new protocols will extend the capabilities of quantum communication by increasing the channel capacity, by making the systems less sensitive to channel noise. Examples of such protocols include multi-party quantum cryptography (using qutrits and systems with higher Hilbert-space dimension), encoding more than one qubit per photon, “entangled-photon laser”. Work concerning error correction and entanglement purification, although relevant for D20, are presented in D15. Report Version: 1 Report Preparation Date: 2001-06-29 Classification: Public Project funded by the European Community 1 under the “Information Society Technologies” Programme (1998-2002) QuComm IST-1999-10033 Long Distance Photonic Quantum Communication QuComm Deliverable D20 Dissemination level: Public 2 INTRODUCTION In experimental Quantum communication, most work has so far been devoted to either pointto-point quantum key distribution, two-party entanglement protocols such as teleportation, and only recently to three and four party entanglement studies. This report summarises the ongoing experimental and theoretical work in QuComm on the demonstration of novel quantum communication protocols, in particular multiparty protocols and protocols which use more than 2-states (more than one qubit). These protocols will extend the capabilities of quantum communication by increasing the channel capacity, by making the systems less sensitive to channel noise. Examples of such protocols include multi-party quantum cryptography (secret sharing), mutlilevel encoding (six-state quantum cryptography, or quantum cryptography using qutrits and systems with higher Hilbert-space dimension), encoding more than one qubit per photon, quantum error correction, and entanglement purification. NOVEL QUBIT QUANTUM CRYPTOGRAPHY PROTOCOLS Quantum Secret Sharing Going beyond two-party quantum cryptography, in QuComm we have realised the first multiparty quantum cryptography schemequantum secret sharing. Classical secret sharing schemes was introduced in [1,2] ,and the basic idea in the simplest case is that a secret is shared between two persons, say from Alice to Bob and Charlie, in such a way that it can only be reconstructed if both Bob and Charlie collaborate, (see Fig. 1). In a more general setting, notably for secure key management, a m-out-of-n protocol (or (m,n)-threshold scheme) with 1 < m <= n spreads a secret to n participants in a way that any m participants can reconstruct it. Quantum secret sharing was first introduced by Zukowski, Hillery, Buzek and Berthiaume [3] using GHZ-correlations. The interesting aspect of exploring quantum mechanics for secret sharing, is that it allows for the unconditionally secure distribution of the information to the participants.