Quantum Cryptography with Imperfect Apparatus

Quantum key distribution, first proposed by Bennett and Brassard, provides a possible key distribution scheme whose security depends only on the quantum laws of physics. So far the protocol has been proved secure even under channel noise and detector faults of the receiver, but is vulnerable if the photon source used is imperfect. In this paper we propose and give a concrete design for a new concept, {\it self-checking source}, which requires the manufacturer of the photon source to provide certain tests; these tests are designed such that, if passed, the source is guaranteed to be adequate for the security of the quantum key distribution protocol, even though the testing devices may not be built to the original specification. The main mathematical result is a structural theorem which states that, for any state in a Hilbert space, if certain EPR-type equations are satisfied, the state must be essentially the orthogonal sum of EPR pairs.Comment: 7 pages (To appear in FOCS 98

Fair Deals for Watershed Services: Lessons from a Multi-country Action-learning Project

Payments for ecosystem services make good sense. In the case of watershed ecosystems, downstream beneficiaries of wise upstream land and water stewardship should compensate these upstream stewards. These 'payments for watershed services' (PWS) should contribute to the costs of watershed management and, if upstream communities are also characterised by poverty, these payments should contribute to local development and poverty reduction as well. Debates about both conservation and development have seen a wave of excitement about payments for watershed services in recent years. But on the ground an equivalent surge of action is harder to see. IIED and its partners have been building on earlier international case study work to set up new PWS schemes - to 'learn by doing' and to improve our understanding of the opportunities and the challenges.This report is about the complex business of trying to put a simple conservation and development idea into practice. The idea is that watershed degradation in developing countries might be better tackled than it currently is if downstream beneficiaries of wise land use in watershed areas paid for these benefits. There are some examples around the world of this idea being put into practice - this report reviews these and describes what happened when teams in six developing countries set about exploring how the idea works on the ground

Why Quantum Bit Commitment And Ideal Quantum Coin Tossing Are Impossible

There had been well known claims of unconditionally secure quantum protocols for bit commitment. However, we, and independently Mayers, showed that all proposed quantum bit commitment schemes are, in principle, insecure because the sender, Alice, can almost always cheat successfully by using an Einstein-Podolsky-Rosen (EPR) type of attack and delaying her measurements. One might wonder if secure quantum bit commitment protocols exist at all. We answer this question by showing that the same type of attack by Alice will, in principle, break any bit commitment scheme. The cheating strategy generally requires a quantum computer. We emphasize the generality of this ``no-go theorem'': Unconditionally secure bit commitment schemes based on quantum mechanics---fully quantum, classical or quantum but with measurements---are all ruled out by this result. Since bit commitment is a useful primitive for building up more sophisticated protocols such as zero-knowledge proofs, our results cast very serious doubt on the security of quantum cryptography in the so-called ``post-cold-war'' applications. We also show that ideal quantum coin tossing is impossible because of the EPR attack. This no-go theorem for ideal quantum coin tossing may help to shed some lights on the possibility of non-ideal protocols.Comment: We emphasize the generality of this "no-go theorem". All bit commitment schemes---fully quantum, classical and quantum but with measurements---are shown to be necessarily insecure. Accepted for publication in a special issue of Physica D. About 18 pages in elsart.sty. This is an extended version of an earlier manuscript (quant-ph/9605026) which has appeared in the proceedings of PHYSCOMP'9