701 research outputs found
Macroscopic Observables Detecting Genuine Multipartite Entanglement and Partial Inseparability in Many-Body Systems
We show a general approach for detecting genuine multipartite entanglement
(GME) and partial inseparability in many-body-systems by means of macroscopic
observables (such as the energy) only. We show that the obtained criteria, the
"GME gap" and "the k-entanglement gap", detect large areas of genuine
multipartite entanglement and partial entanglement in typical many body states,
which are not detected by other criteria. As genuine multipartite entanglement
is a necessary property for several quantum information theoretic applications
such as e.g. secret sharing or certain kinds of quantum computation, our
methods can be used to select or design appropriate condensed matter systems.Comment: 4 pages, 3 figures, published version, title extende
Magnetic Susceptibility as a Macrosopic Entaglement Witness
We show that magnetic susceptibility can reveal spin entanglement between
individual constituents of a solid, while magnetisation describes their local
properties. We then show that these two thermodynamical quantities satisfy
complementary relation in the quantum-mechanical sense. It describes sharing of
(quantum) information in the solid between spin entanglement and local
properties of its individual constituents. Magnetic susceptibility is shown to
be a macroscopic spin entanglement witness that can be applied without complete
knowledge of the specific model (Hamiltonian) of the solid.Comment: 6 Pages, 2 figures, revtex
Qubit rotation and Berry Phase
A quantized fermion can be represented by a scalar particle encircling a
magnetic flux line. It has the spinor structure which can be constructed from
quantum gates and qubits. We have studied here the role of Berry phase in
removing dynamical phase during one qubit rotation of a quantized fermion. The
entanglement of two qubit inserting spin-echo to one of them results the change
of Berry phase that can be considered as a measure of entanglement. Some effort
is given to study the effect of noise on the Berry phase of spinor and their
entangled states.Comment: 12 page
The elusive source of quantum effectiveness
We discuss two qualities of quantum systems: various correlations existing
between their subsystems and the distingushability of different quantum states.
This is then applied to analysing quantum information processing. While quantum
correlations, or entanglement, are clearly of paramount importance for
efficient pure state manipulations, mixed states present a much richer arena
and reveal a more subtle interplay between correlations and distinguishability.
The current work explores a number of issues related with identifying the
important ingredients needed for quantum information processing. We discuss the
Deutsch-Jozsa algorithm, the Shor algorithm, the Grover algorithm and the power
of a single qubit class of algorithms. One section is dedicated to cluster
states where entanglement is crucial, but its precise role is highly
counter-intuitive. Here we see that distinguishability becomes a more useful
concept.Comment: 8 pages, no figure
Witnesses of non-classicality for simulated hybrid quantum systems
The task of testing whether quantum theory applies to all physical systems
and all scales requires considering situations where a quantum probe interacts
with another system that need not obey quantum theory in full. Important
examples include the cases where a quantum mass probes the gravitational field,
for which a unique quantum theory of gravity does not yet exist, or a quantum
field, such as light, interacts with a macroscopic system, such as a biological
molecule, which may or may not obey unitary quantum theory. In this context a
class of experiments has recently been proposed, where the non-classicality of
a physical system that need not obey quantum theory (the gravitational field)
can be tested indirectly by detecting whether or not the system is capable of
entangling two quantum probes. Here we illustrate some of the subtleties of the
argument, to do with the role of locality of interactions and of
non-classicality, and perform proof-of-principle experiments illustrating the
logic of the proposals, using a Nuclear Magnetic Resonance quantum
computational platform with four qubits.Comment: Revised and extende
Entanglement as a quantum order parameter
We show that the quantum order parameters (QOP) associated with the
transitions between a normal conductor and a superconductor in the BCS and
eta-pairing models and between a Mott-insulator and a superfluid in the
Bose-Hubbard model are directly related to the amount of entanglement existent
in the ground state of each system. This gives a physical meaningful
interpretation to these QOP, which shows the intrinsically quantum nature of
the phase transitions considered.Comment: 5 pages. No figures. Revised version. References adde
Measuring quantumness via anticommutators
We introduce a method to witness the quantumness of a system. The method
relies on the fact that the anticommutator of two classical states is always
positive. We show that there is always a nonpositive anticommutator due to any
two quantum states. We notice that interference depends on the trace of the
anticommutator of two states and it is therefore more natural to detect
quantumness by looking at anticommutators of states rather than their
commutators.Comment: 7 pages, 2 figure
Optimal State Discrimination Using Particle Statistics
We present an application of particle statistics to the problem of optimal
ambiguous discrimination of quantum states. The states to be discriminated are
encoded in the internal degrees of freedom of identical particles, and we use
the bunching and antibunching of the external degrees of freedom to
discriminate between various internal states. We show that we can achieve the
optimal single-shot discrimination probability using only the effects of
particle statistics. We discuss interesting applications of our method to
detecting entanglement and purifying mixed states. Our scheme can easily be
implemented with the current technology
- …