  How may we quantify the value of physical resources, such as entangled
quantum states, heat baths or lasers? Existing resource theories give us
partial answers; however, these rely on idealizations, like perfectly
independent copies of states or exact knowledge of a quantum state. Here we
introduce the general tool of currencies to quantify realistic descriptions of
resources, applicable in experimental settings when we do not have perfect
control over a physical system, when only the neighbourhood of a state or some
of its properties are known, or when there is no obvious way to decompose a
global space into subsystems. Currencies are a special set of resources chosen
to quantify all others - like Bell pairs in LOCC or a lifted weight in
thermodynamics. We show that from very weak assumptions on the theory we can
already find useful currencies that give us necessary and sufficient conditions
for resource conversion, and we build up more results as we impose further
structure. This work is an application of Resource theories of knowledge
[arXiv:1511.08818], generalizing axiomatic approaches to thermodynamic entropy,
work and currencies made of local copies.
