In
quantum mechanics
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistr ...
, negativity is a measure of
quantum entanglement
Quantum entanglement is the phenomenon that occurs when a group of particles are generated, interact, or share spatial proximity in a way such that the quantum state of each particle of the group cannot be described independently of the state of ...
which is easy to compute. It is a measure deriving from the
PPT criterion for
separability.
It has shown to be an
entanglement monotone
In quantum information and quantum computation, an entanglement monotone is a function that quantifies the amount of entanglement present in a quantum state. Any entanglement monotone is a nonnegative function whose value does not increase unde ...
and hence a proper measure of entanglement.
Definition
The negativity of a subsystem
can be defined in terms of a
density matrix
In quantum mechanics, a density matrix (or density operator) is a matrix that describes the quantum state of a physical system. It allows for the calculation of the probabilities of the outcomes of any measurement performed upon this system, using ...
as:
:
where:
*
is the
partial transpose of
with respect to subsystem
*
is the
trace norm or the sum of the singular values of the operator
.
An alternative and equivalent definition is the absolute sum of the negative eigenvalues of
:
:
where
are all of the eigenvalues.
Properties
* Is a
convex function of
:
:
* Is an
entanglement monotone
In quantum information and quantum computation, an entanglement monotone is a function that quantifies the amount of entanglement present in a quantum state. Any entanglement monotone is a nonnegative function whose value does not increase unde ...
:
:
where
is an arbitrary
LOCC operation over
Logarithmic negativity
The logarithmic negativity is an entanglement measure which is easily computable and an upper bound to the
distillable entanglement.
It is defined as
:
where
is the partial transpose operation and
denotes the
trace norm.
It relates to the negativity as follows:
:
Properties
The logarithmic negativity
* can be zero even if the state is entangled (if the state is
PPT entangled).
* does not reduce to the
entropy of entanglement The entropy of entanglement (or entanglement entropy) is a measure of the degree of quantum entanglement between two subsystems constituting a two-part composite quantum system. Given a pure bipartite quantum state of the composite system, it is p ...
on pure states like most other entanglement measures.
* is additive on tensor products:
* is not asymptotically continuous. That means that for a sequence of
bipartite Hilbert spaces
(typically with increasing dimension) we can have a sequence of quantum states
which converges to
(typically with increasing
) in the
trace distance, but the sequence
does not converge to
.
* is an upper bound to the distillable entanglement
References
* This page uses material fro
Quantikilicensed under GNU Free Documentation License 1.2
{{reflist
Quantum information science