The concept of "collapse" in quantum mechanics refers to the phenomenon where the quantum state of a system changes instantaneously upon measurement. When a qubit is observed or measured, its state transitions from a superposition of possible states to a definite value corresponding to the measurement outcome.
The reason for this collapse can be understood through the mathematical formalism of quantum mechanics. The state of a qubit is typically described by a superposition of basis states, such as |0⟩ and |1⟩. These basis states represent the two orthogonal states that the qubit can be in. However, until a measurement is made, the qubit can exist in a linear combination of these basis states, with certain probability amplitudes associated with each state.
When a measurement is performed on the qubit, it "collapses" into one of the basis states with a probability determined by the squared magnitude of the corresponding probability amplitude. The measurement outcome provides a definite result, and the qubit's state is no longer described by a superposition but rather by the specific outcome of the measurement.
The exact mechanism underlying the collapse is still a topic of debate and interpretation in quantum mechanics. One interpretation is the Copenhagen interpretation, which views the collapse as a fundamental and intrinsic property of quantum systems. Other interpretations, such as the many-worlds interpretation, propose that the apparent collapse is a result of the branching of the universe into multiple parallel realities, each corresponding to a different measurement outcome.
It's important to note that the act of measurement itself is a complex process that involves interaction between the quantum system and the measuring apparatus. The measurement process entangles the system with the measuring device, leading to the apparent collapse of the system's state.
In summary, the collapse of a qubit's state upon measurement is a fundamental aspect of quantum mechanics. It is a consequence of the probabilistic nature of quantum systems and the interaction between the system and the measuring apparatus during the measurement process.