10 research outputs found
Small sets of complementary observables
Two observables are called complementary if preparing a physical object in an
eigenstate of one of them yields a completely random result in a measurement of
the other. We investigate small sets of complementary observables that cannot
be extended by yet another complementary observable. We construct explicit
examples of the unextendible sets up to dimension and conjecture certain
small sets to be unextendible in higher dimensions. Our constructions provide
three complementary measurements, only one observable away from the ultimate
minimum of two observables in the set. Almost all of our examples in finite
dimension allow to discriminate pure states from some mixed states, and shed
light on the complex topology of the Bloch space of higher-dimensional quantum
systems
Information-Disturbance Tradeoff in Quantum State Discrimination
When discriminating between two pure quantum states, there exists a
quantitative tradeoff between the information retrieved by the measurement and
the disturbance caused on the unknown state. We derive the optimal tradeoff and
provide the corresponding quantum measurement. Such an optimal measurement
smoothly interpolates between the two limiting cases of maximal information
extraction and no measurement at all.Comment: 5 pages, 2 (low-quality) figures. Eq. (20) corrected. Final published
versio
Direct sampling of exponential phase moments of smoothed Wigner functions
We investigate exponential phase moments of the s-parametrized
quasidistributions (smoothed Wigner functions). We show that the knowledge of
these moments as functions of s provides, together with photon-number
statistics, a complete description of the quantum state. We demonstrate that
the exponential phase moments can be directly sampled from the data recorded in
balanced homodyne detection and we present simple expressions for the sampling
kernels. The phase moments are Fourier coefficients of phase distributions
obtained from the quasidistributions via integration over the radial variable
in polar coordinates. We performed Monte Carlo simulations of the homodyne
detection and we demonstrate the feasibility of direct sampling of the moments
and subsequent reconstruction of the phase distribution.Comment: RevTeX, 8 pages, 6 figures, accepted Phys. Rev.
Entanglement distribution and quantum discord
Establishing entanglement between distant parties is one of the most
important problems of quantum technology, since long-distance entanglement is
an essential part of such fundamental tasks as quantum cryptography or quantum
teleportation. In this lecture we review basic properties of entanglement and
quantum discord, and discuss recent results on entanglement distribution and
the role of quantum discord therein. We also review entanglement distribution
with separable states, and discuss important problems which still remain open.
One such open problem is a possible advantage of indirect entanglement
distribution, when compared to direct distribution protocols.Comment: 7 pages, 2 figures, contribution to "Lectures on general quantum
correlations and their applications", edited by Felipe Fanchini, Diogo
Soares-Pinto, and Gerardo Adess
Quantum Discord and Entanglement Distribution as the Flow of Correlations Through a Dissipative Quantum System
In this paper, we study the propagation of quantum correlations in open quantum systems using quantum discord as their measure. The role of system-environment correlations in discord dynamics and some operational interpretations of discord are discussed, in particular, activation of correlations into entanglement. The quantum nature of correlations is studied in systems of optical modes, that is, Gaussian quantum states. A counter-intuitive scheme of entanglement distribution by an auxiliary mode, which remains separable at all times, is analyzed to unveil the synergy of coherence and dissipation in quantum protocols with mixed states.</p