The following five sentences, labeled 1 to 5, relate to a single topic. Four of these sentences can be arranged to form a logical paragraph. Identify the sentence that does not fit with the others and enter its number as your answer.

1. In quantum mechanics, the famed observer effect stipulates that the very act of measuring or observing a quantum system invariably alters its state, a radical departure from the passive role of observation in classical physics.
2. This perplexing phenomenon fundamentally undermines the classical ideal of an independent, objective reality, proposing instead that quantum entities do not possess determinate properties until actively engaged by an observational apparatus.
3. The relentless pursuit of quantum supremacy seeks to leverage phenomena such as superposition and entanglement for computational breakthroughs, promising exponential processing power unattainable by current classical architectures.
4. At the heart of this conceptual difficulty lies the "measurement problem," detailing the non-unitary evolution wherein a quantum system transitions from a probabilistic superposition of states to a singular, definite outcome upon interaction with a macroscopic detector.
5. Consequently, the wave function, which mathematically encapsulates the probabilities of a particle's various potential states, is posited to undergo an instantaneous "collapse" into a specific eigenstate precisely at the moment of observation.