Three-intensity decoy state method for device independent quantum key distribution with basis dependent errors

We study the measurement device independent quantum key distribution (MDIQKD) in practice with limited resource, when there are only 3 different states in implementing the decoy-state method and when there are basis dependent coding errors. We present general formulas for the decoy-state method for two-pulse sources with 3 different states, which can be applied to the recently proposed MDIQKD with imperfect single-photon source such as the coherent states or the heralded states from the parametric down conversion. We point out that the existing result for secure QKD with source coding errors does not always hold. We find that very accurate source coding is not necessary. In particular, we loosen the precision of existing result by several magnitude orders for secure QKD.Comment: Published version with Eq.(17) corrected. We emphasize that our major result (Eq.16) for the decoy-state part can be applied to generate a key rate very close to the ideal case of using infinite different coherent states, as was numerically demonstrated in Ref.[21]. Published in PRA, 2013, Ja

Unconditional security of coherent-state quantum key distribution with strong phase-reference pulse

We prove the unconditional security of a quantum key distribution protocol in which bit values are encoded in the phase of a weak coherent-state pulse relative to a strong reference pulse. In contrast to implementations in which a weak pulse is used as a substitute for a single-photon source, the achievable key rate is found to decrease only linearly with the transmission of the channel.Comment: 4 pages, 3 figure

Unconditionally secure key distillation from multi-photons

In this paper, we prove that the unconditionally secure key can be surprisingly extracted from {\it multi}-photon emission part in the photon polarization-based QKD. One example is shown by explicitly proving that one can indeed generate an unconditionally secure key from Alice's two-photon emission part in ``Quantum cryptography protocols robust against photon number splitting attacks for weak laser pulses implementations'' proposed by V. Scarani {\it et al.,} in Phys. Rev. Lett. {\bf 92}, 057901 (2004), which is called SARG04. This protocol uses the same four states as in BB84 and differs only in the classical post-processing protocol. It is, thus, interesting to see how the classical post-processing of quantum key distribution might qualitatively change its security. We also show that one can generate an unconditionally secure key from the single to the four-photon part in a generalized SARG04 that uses six states. Finally, we also compare the bit error rate threshold of these protocols with the one in BB84 and the original six-state protocol assuming a depolarizing channel.Comment: The title has changed again. We considerably improved our presentation, and furthermore we proposed & analyzed a security of a modified SARG04 protocol, which uses six state

A violation of the uncertainty principle implies a violation of the second law of thermodynamics

Uncertainty relations state that there exist certain incompatible measurements, to which the outcomes cannot be simultaneously predicted. While the exact incompatibility of quantum measurements dictated by such uncertainty relations can be inferred from the mathematical formalism of quantum theory, the question remains whether there is any more fundamental reason for the uncertainty relations to have this exact form. What, if any, would be the operational consequences if we were able to go beyond any of these uncertainty relations? We give a strong argument that justifies uncertainty relations in quantum theory by showing that violating them implies that it is also possible to violate the second law of thermodynamics. More precisely, we show that violating the uncertainty relations in quantum mechanics leads to a thermodynamic cycle with positive net work gain, which is very unlikely to exist in nature.Comment: 8 pages, revte

Testing CPT Symmetry with Supernova Neutrinos

Diagnosing core of supernova requires favor-dependent reconstruction of three species of neutrino spectra, \nu_e, \bar{\nu}_{e} and \nu_x (a collective notation for \nu_{\mu}, \bar{\nu}_{\mu}, \nu_{\tau}, and \bar{\nu}_{\tau}). We point out that, assuming the information available, CPT symmetry can be tested with supernova neutrinos. We classify all possible level crossing patterns of neutrinos and antineutrinos into six cases and show that half of them contains only the CPT violating mass and mixing patterns. We discuss how additional informations from terrestrial experiments help identifying CPT violation by narrowing down the possible flux patterns. Although the method may not be good at precision test, it is particularly suited to uncover gross violation of CPT such as different mass patterns of neutrinos and antineutrinos. The power of the method is due to the nature of level crossing in supernova which results in the sensitivity to neutrino mass hierarchy and to the unique characteristics of in situ preparation of both \nu and \bar{\nu} beams. Implications of our discussion to the conventional analyses with CPT conservation are also briefly mentioned.Comment: 24 pages, 4 figures, discussion added on narrowing down flux patterns by terrestrial measuremen