When a qubit in a superposition of states (0 and 1) is measured, it collapses into one of the two possible classical states with a probability determined by the coefficients of its superposition. The act of measurement causes the quantum system to "choose" a definite state, destroying the superposition.
The probability of obtaining the classical state 0 or 1 upon measurement is given by the squared magnitudes of the coefficients associated with those states in the superposition. For example, if a qubit is in a superposition state represented as α|0⟩ + β|1⟩, where α and β are complex probability amplitudes, the probability of measuring 0 would be |α|^2, and the probability of measuring 1 would be |β|^2. It's important to note that the sum of the probabilities for all possible outcomes must equal 1.
Once the measurement is made and the qubit collapses into a definite state, it becomes a classical bit and can no longer exhibit quantum properties like superposition or entanglement. The measurement outcome is deterministic, meaning it will yield a specific result, but the outcome of a single measurement on an individual qubit cannot be predicted precisely due to the probabilistic nature of quantum mechanics.
It's worth mentioning that the act of measurement can affect other entangled qubits as well. When multiple qubits are entangled, measuring one qubit can instantaneously affect the state of the other entangled qubits, regardless of their physical separation. This phenomenon is known as quantum entanglement and is a key aspect of quantum computing and quantum information processing.