Showing papers by "Stefano Pirandola published in 2014"

Optimality of Gaussian discord.

Abstract: In this Letter we exploit the recently solved conjecture on the bosonic minimum output entropy to show the optimality of Gaussian discord, so that the computation of quantum discord for bipartite Gaussian states can be restricted to local Gaussian measurements. We prove such optimality for a large family of Gaussian states, including all two-mode squeezed thermal states, which are the most typical Gaussian states realized in experiments. Our family also includes other types of Gaussian states and spans their entire set in a suitable limit where they become Choi matrices of Gaussian channels. As a result, we completely characterize the quantum correlations possessed by some of the most important bosonic states in quantum optics and quantum information.

Two-way quantum cryptography at different wavelengths

Abstract: We study the security of two-way quantum cryptography at different wavelengths of the electromagnetic spectrum, from the optical down to the microwave range. In particular, we consider a two-way quantum communication protocol where Gaussian-modulated thermal states are subject to random Gaussian displacements and finally homodyned. We show how its security threshold (in reverse reconciliation) is extremely robust with respect to the preparation noise and able to outperform the security thresholds of one-way protocols at any wavelength. As a result, improved security distances are now accessible for implementing quantum key distribution in the very challenging regime of infrared frequencies.

Continuous-variable-entanglement swapping and its local certification: Entangling distant mechanical modes

Abstract: We introduce a modification of the standard entanglement swapping protocol where the generation of entanglement between two distant modes is realized and verified using only local optical measurements. We show, indeed, that a simple condition on the purity of the initial state involving also an ancillary mode is sufficient to guarantee the success of the protocol by local measurements [M. Abdi, S. Pirandola, P. Tombesi, and D. Vitali, Phys. Rev. Lett. 109, 143601 (2012)]. We apply the proposed protocol to a tripartite optomechanical system where the never interacting mechanical modes become entangled and certified using only local optical measurements.

Post-firewall paradoxes

Abstract: The preeminent view that evaporating black holes should simply be smaller black holes has been challenged by the firewall paradox. In particular, this paradox suggests that something different occurs once a black hole has evaporated to one-half its original surface area. Here we derive variations of the firewall paradox by tracking the thermodynamic entropy within a black hole across its entire lifetime. Our approach sweeps away many unnecessary assumptions, allowing us to demonstrate a paradox exists even after its initial onset (when conventional assumptions render earlier analyses invalid). Our results suggest that not only is the formation of a firewall the most natural resolution, but provides a mechanism for it. Finally, although firewalls cannot have evolved for modest-sized black holes, within the age of the universe, we speculate on the implications if they were ever unambiguously observed.

Optimality of Gaussian Discord

Abstract: Stefano Pirandola, Gaetana Spedalieri, Samuel L. Braunstein, Nicolas J. Cerf, and Seth Lloyd Computer Science, University of York, York YO10 5GH, United Kingdom Ecole Polytechnique de Bruxelles, CP 165, Universite Libre de Bruxelles (ULB), 1050 Brussels, Belgium Research Laboratory of Electronics & Department of Mechanical Engineering, MIT, Cambridge, Massachusetts 02139, USA (Received 9 September 2013; revised manuscript received 5 July 2014; published 3 October 2014)

Microwave Photodetection with Electro-Opto-Mechanical Systems

Abstract: While detection of optical photons is today achieved with very high efficiencies, the detection of microwave fields at the photon level still poses non-trivial experimental challenges. In this Letter we propose a model of microwave photodetector which is based on the use of an electro-opto-mechanical system. Using state of the art technology, we show how single microwave photons can efficiently be converted into optical photons which are then measured by standard optics. The overall quantum efficiency of our microwave detector tends to one for achievable values of the optical and microwave cooperativity parameters. Our scheme could be used for sensing and scanning very faint microwave fields, as they may occur in radio astronomy, satellite and deep space communications.

Two-way quantum cryptography with continuous variables: unconditional security and performances at different wavelengths

Abstract: We analysed the security of two-way QKD against coherent attacks achieving unconditional security in all encoding/decoding configurations. The security at different wavelengths is analysed finding a robust threshold in reverse reconciliation, improving the security distances for QKD in the infrared.