The Bounded Observer

Daniel John Murray

Jun Muramatsu, Kazuyuki Yoshimura, Peter Davis +2 more

This paper reviews an approach to secret-key distribution based on the bounded observability (BO) model. First, the information-theoretic framework of secret-key agreement from a correlated random source is reviewed. Next, the BO model is introduced. In the context of this model, the BO condition is presented as a necessary and sufficient condition for the possibility of secret-key distribution. This condition describes limits on the information obtained by observation of a random object, and models the practical difficulty of completely observing random physical phenomena. Finally, an implementation of secret-key distribution based on BO in an optical fiber system is described.

Su‐Yong Lee, Yonggi Jo, Taek Jeong +6 more

We propose observable bounds for Gaussian illumination to maximize the signal-to-noise ratio, which minimizes the discrimination error between the presence and absence of a low-reflectivity target using Gaussian states. The observable bounds are achieved with mode-by-mode measurements. In the quantum regime using a two-mode squeezed vacuum state, our observable receiver outperforms the other feasible receivers whereas it cannot approach the quantum Chernoff bound. The corresponding observable cannot be implemented with heterodyne detections due to the additional vacuum noise. In the classical regime using a thermal state, a receiver implemented with a photon number difference measurement approaches its bound regardless of the signal mean photon number, while it asymptotically approaches the classical bound in the limit of a huge idler mean photon number.

Paweł Caban, Beatrix C. Hiesmayr

Bound entanglement, in contrast to free entanglement, cannot be distilled into maximally entangled states by two local observers applying measurements and utilizing classical communication. In this paper we ask whether a relativistic observer classifies states according to being separable, bound or free entangled in the same manner as an unboosted observer. Surprisingly, this turns out not to be the case. And that even if the system in a given inertial frame of reference is separable with respect to the partition momenta versus spins. In detail, we show that if the spin state is initially bound entangled, some boosted observers observe their spin states to be either bound entangled, separable or free entangled. This also explains why a general measure of the entanglement property is difficult to find.