Showing papers by "Margaret D. Reid published in 2000"

Quantum cryptography with a predetermined key, using continuous variable Einstein-Podolsky-Rosen correlations

Abstract: Correlations of the type discussed by EPR in their original 1935 paradox for continuous variables exist for the quadrature phase amplitudes of two spatially separated fields. These correlations were first experimentally reported in 1992. We propose to use such EPR beams in quantum cryptography, to transmit with high efficiency messages in such a way that the receiver and sender may later determine whether eavesdropping has occurred. The merit of the new proposal is in the possibility of transmitting a reasonably secure yet predetermined key. This would allow relay of a cryptographic key over long distances in the presence of lossy channels.

Incompatibility of macroscopic local realism with quantum mechanics in measurements with macroscopic uncertainties

Abstract: We show that quantum mechanics predicts a contradiction with local hidden variable theories for photon number measurements which have limited resolving power, to the point of imposing an uncertainty in the photon number result which is macroscopic in absolute terms. We show how this can be interpreted as a failure of a new premise, macroscopic local realism.

Violations of Bell inequalities for measurements with macroscopic uncertainties: What it means to violate macroscopic local realism

Abstract: We suggest a method to test the premise of “macroscopic local realism” that is sufficient to derive Bell inequalities when measurements of photon numbers are only accurate to an uncertainty of order n photons, where n is macroscopic. Macroscopic local realism is only sufficient to imply, in the context of the original Einstein-Podolsky-Rosen argument, fuzzy “elements of reality” that have a macroscopic indeterminacy. We show therefore how the violation of local realism in the presence of macroscopic uncertainties implies the failure of macroscopic local realism. Quantum states violating this macroscopic local realism are presented.

Quantum cryptography using continuous variable Einstein-Podolsky-Rosen correlations and quadrature phase amplitude measurements

Abstract: Summary form only given. We show how such Einstein-Podolsky-Rosen (EPR) correlated beams may be used for quantum cryptography. The high detection efficiencies give a very much reduced overall loss factor, which may make it possible to transmit directly and efficiently a predetermined message, later checking providing a means to check security. In contrast photon-based proposals are limited by poor photon count detection efficiencies, and rely in practice on deciphering a sequence (key) a posteriori from infrequent detected photons.

Macroscopic local realism incompatible with quantum mechanics: failure of local realism where measurements give macroscopic uncertainties

Abstract: Summary form only given. We define the premise of "macroscopic local realism" so that its failure is more surprising than failure of the local realism addressed previously. Macroscopic local realism is a premise stating the following. If one can predict the result of a measurement at A by performing a simultaneous measurement on a spatially separated system B, then the result of the measurement at A is predetermined but described by an Einstein-Podolsky-Rosen "element of reality," which has an indeterminacy in each of its possible values, so that only values macroscopically different to those predicted are excluded.