  Regarded as one of the most fundamental concepts of classical mechanics and
thermodynamics, work has received well-grounded definitions within the quantum
framework since the 1970s, having being successfully applied to many contexts.
Recent developments on the concept have taken place in the emergent field of
quantum thermodynamics, where work is frequently characterized as a stochastic
variable. Notwithstanding this remarkable progress, it is still debatable
whether some sensible notion of work can be posed for a strictly quantum
instance involving a few-particle system prepared in a pure state and abandoned
to its closed autonomous dynamics. By treating work as a quantum mechanical
observable with a well defined classical limit, here we show that this scenario
can be satisfactorily materialized. We prove, by explicit examples, that one
can indeed assign eigensystems to work operators. This paves the way for
frameworks involving quantum superposition and nonlocal steering of work. We
also show that two-point measurement protocols can be inappropriate to describe
work (and other two-time physical quantities), especially in the semiclassical
regime. However subtle it may be, our quantum mechanical notion of work is
experimentally testable and requires an updating of our intuition regarding the
concept of two-time elements of reality. In this context, we derive a
work-energy uncertainty relation, and we illustrate how energy conservation
emerges as an element of physical reality.
