In quantum mechanics, measurement is indeed a process that interrupts the unitary evolution of a quantum system. When a measurement is made on a quantum system, its state "collapses" to one of the eigenstates of the observable being measured, and subsequent evolution follows the rules of classical probability.
However, it's important to note that interaction with a quantum system does not necessarily imply measurement. There are situations where a quantum system can interact with its environment or other particles without undergoing a measurement. In such cases, the unitary evolution of the system continues, governed by the Schrödinger equation or the appropriate evolution equation of the quantum system.
To clarify, an interaction without measurement can cause entanglement or superposition to evolve and change the state of the system, but it doesn't collapse the state into a definite outcome. The system can still evolve unitarily while entangled with the interacting entity.
For example, let's consider the case of a particle with a spin interacting with its surrounding environment. The interaction may cause the entanglement of the particle's spin state with the environment's degrees of freedom. In this scenario, the combined system of the particle and environment evolves unitarily according to the laws of quantum mechanics. The particle's spin state can become entangled with the environment, resulting in a more complex quantum state.
To extract information about the particle's spin, a measurement or interaction specifically designed to extract that information is needed. This measurement process disrupts the unitary evolution and causes the collapse of the particle's state into a definite spin value.
In summary, measurement is not the only thing that halts the unitary evolution of a quantum system. While measurement collapses the state into a definite outcome, interactions without measurement can lead to entanglement and continued unitary evolution of the quantum system.