Abstract
Entanglement is an important resource that allows quantum technologies to go
beyond the classically possible. There are many ways quantum systems can be
entangled, ranging from the archetypal two-qubit case to more exotic scenarios
of entanglement in high dimensions or between many parties. Consequently, a
plethora of entanglement quantifiers and classifiers exist, corresponding to
different operational paradigms and mathematical techniques. However, for most
quantum systems, exactly quantifying the amount of entanglement is extremely
demanding, if at all possible. This is further exacerbated by the difficulty of
experimentally controlling and measuring complex quantum states. Consequently,
there are various approaches for experimentally detecting and certifying
entanglement when exact quantification is not an option, with a particular
focus on practically implementable methods and resource efficiency. The
applicability and performance of these methods strongly depends on the
assumptions one is willing to make regarding the involved quantum states and
measurements, in short, on the available prior information about the quantum
system. In this review we discuss the most commonly used paradigmatic
quantifiers of entanglement. For these, we survey state-of-the-art detection
and certification methods, including their respective underlying assumptions,
from both a theoretical and experimental point of view.
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