5 research outputs found
Efficiency in Quantum Key Distribution Protocols with Entangled Gaussian States
Quantum key distribution (QKD) refers to specific quantum strategies which
permit the secure distribution of a secret key between two parties that wish to
communicate secretly. Quantum cryptography has proven unconditionally secure in
ideal scenarios and has been successfully implemented using quantum states with
finite (discrete) as well as infinite (continuous) degrees of freedom. Here, we
analyze the efficiency of QKD protocols that use as a resource entangled
gaussian states and gaussian operations only. In this framework, it has already
been shown that QKD is possible (M. Navascu\'es et al. Phys. Rev. Lett. 94,
010502 (2005)) but the issue of its efficiency has not been considered. We
propose a figure of merit (the efficiency ) to quantify the number of
classical correlated bits that can be used to distill a key from a sample of
entangled states. We relate the efficiency of the protocol to the
entanglement and purity of the states shared between the parties.Comment: 13 pages, 2 figures, OSID style, published versio
High-bandwidth squeezed light at 1550 nm from a compact monolithic PPKTP cavity
We report the generation of squeezed vacuum states of light at 1550 nm with a
broadband quantum noise reduction of up to 4.8 dB ranging from 5 MHz to 1.2 GHz
sideband frequency. We used a custom-designed 2.6 mm long biconvex
periodically-poled potassium titanyl phosphate (PPKTP) crystal. It featured
reflectively coated end surfaces, 2.26 GHz of linewidth and generated the
squeezing via optical parametric amplification. Two homodyne detectors with
different quantum efficiencies and bandwidths were used to characterize the
non-classical noise suppression. We measured squeezing values of up to 4.8 dB
from 5 to 100 MHz and up to 3 dB from 100 MHz to 1.2 GHz. The squeezed vacuum
measurements were limited by detection loss. We propose an improved detection
scheme to measure up to 10 dB squeezing over 1 GHz. Our results of GHz
bandwidth squeezed light generation provide new prospects for high-speed
quantum key distribution.Comment: 8 pages, 4 figure
Quantum Information with Continuous Variable systems
This thesis deals with the study of quantum communication protocols with
Continuous Variable (CV) systems. Continuous Variable systems are those
described by canonical conjugated coordinates x and p endowed with infinite
dimensional Hilbert spaces, thus involving a complex mathematical structure. A
special class of CV states, are the so-called Gaussian states. With them, it
has been possible to implement certain quantum tasks as quantum teleportation,
quantum cryptography and quantum computation with fantastic experimental
success. The importance of Gaussian states is two-fold; firstly, its structural
mathematical description makes them much more amenable than any other CV
system. Secondly, its production, manipulation and detection with current
optical technology can be done with a very high degree of accuracy and control.
Nevertheless, it is known that in spite of their exceptional role within the
space of all Continuous Variable states, in fact, Gaussian states are not
always the best candidates to perform quantum information tasks. Thus
non-Gaussian states emerge as potentially good candidates for communication and
computation purposes.Comment: PhD Thesis in Universitat Autonoma de Barcelona. Published by the
Lambert Academic Publishing (LAP) on March 18, 2011. ISBN-13:
978-3-8443-1948-