  In this Letter we investigate the concept of quantum work and its
measurability from the viewpoint of quantum measurement theory. Very often,
quantum work and fluctuation theorems are discussed in the framework of
projective two-point measurement (TPM) schemes. According to a well known no-go
theorem, there is no work observable which satisfies both (i) an average work
condition and (ii) the TPM statistics for diagonal input states. Such
projective measurements represent a restrictive class among all possible
measurements. It is desirable, both from a theoretical and experimental point
of view, to extend the scheme to the general case including suitably designed
unsharp measurements. This shifts the focus to the question what information
about work and its fluctuations one is able to extract from such generalized
measurements. We show that the no-go theorem no longer holds if the observables
in a TPM scheme are jointly measurable for any intermediate unitary evolution.
We explicitly construct a model with unsharp energy measurements and derive
bounds for the visibility that ensure joint measurability. In such an unsharp
scenario a single work measurement apparatus can be constructed that allows us
to determine the correct average work and to obtain free energy differences
with the help of a Jarzynski equality.
