12 research outputs found
Introduction to Quantum Information Processing
As a result of the capabilities of quantum information, the science of
quantum information processing is now a prospering, interdisciplinary field
focused on better understanding the possibilities and limitations of the
underlying theory, on developing new applications of quantum information and on
physically realizing controllable quantum devices. The purpose of this primer
is to provide an elementary introduction to quantum information processing, and
then to briefly explain how we hope to exploit the advantages of quantum
information. These two sections can be read independently. For reference, we
have included a glossary of the main terms of quantum information.Comment: 48 pages, to appear in LA Science. Hyperlinked PDF at
http://www.c3.lanl.gov/~knill/qip/prhtml/prpdf.pdf, HTML at
http://www.c3.lanl.gov/~knill/qip/prhtm
Spectral Conditions on the State of a Composite Quantum System Implying its Separability
For any unitarily invariant convex function F on the states of a composite
quantum system which isolates the trace there is a critical constant C such
that F(w)<= C for a state w implies that w is not entangled; and for any
possible D > C there are entangled states v with F(v)=D. Upper- and lower
bounds on C are given. The critical values of some F's for qubit/qubit and
qubit/qutrit bipartite systems are computed. Simple conditions on the spectrum
of a state guaranteeing separability are obtained. It is shown that the thermal
equilbrium states specified by any Hamiltonian of an arbitrary compositum are
separable if the temperature is high enough.Comment: Corrects 1. of Lemma 2, and the (under)statement of Proposition 7 of
the earlier version
Witnessing causal nonseparability
Our common understanding of the physical world deeply relies on the notion
that events are ordered with respect to some time parameter, with past events
serving as causes for future ones. Nonetheless, it was recently found that it
is possible to formulate quantum mechanics without any reference to a global
time or causal structure. The resulting framework includes new kinds of quantum
resources that allow performing tasks - in particular, the violation of causal
inequalities - which are impossible for events ordered according to a global
causal order. However, no physical implementation of such resources is known.
Here we show that a recently demonstrated resource for quantum computation -
the quantum switch - is a genuine example of "indefinite causal order". We do
this by introducing a new tool - the causal witness - which can detect the
causal nonseparability of any quantum resource that is incompatible with a
definite causal order. We show however that the quantum switch does not violate
any causal nequality.Comment: 15 + 12 pages, 5 figures. Published versio