  A collapse-free version of quantum theory is introduced to study the role of
the projection postulate. We assume "passive" measurements that do not update
quantum states while measurement outcomes still occur probabilistically, in
accordance with Born's rule. All other defining features of quantum theory,
such as the Hilbert space setting, are retained. The resulting quantum-like
theory has only one type of dynamics, namely unitary evolution. Passive quantum
theory shares many features with standard quantum theory. These include
preparational uncertainty relations, the impossibility to dynamically clone
unknown quantum states and the absence of signalling. However, striking
differences emerge when protocols involve post-measurement states. For example,
in the collapse-free setting, no ensemble is needed to reconstruct the state of
a system by passively measuring a tomographically complete set of observables -
a single system will do. Effectively, the state becomes an observable quantity,
with implications for both the ontology of the theory and its computational
power. At the same time, the theory is not locally tomographic and passive
measurements do not create Bell-type correlations in composite systems.

Quantum Physics