No, it is not always necessary to measure all qubits in a quantum computer. In fact, one of the key advantages of quantum computing is the ability to perform computations on qubits without measuring them until the final result is obtained. This is known as quantum parallelism.
Quantum parallelism allows a quantum computer to process a vast number of possible solutions simultaneously by operating on all the superposition states of the qubits involved. Through quantum algorithms and quantum gates, computations can be performed on the qubits without explicitly measuring their individual states.
However, there are cases where measurements are necessary. In some quantum algorithms, measurement is used at specific points to extract information about the quantum state or to determine the final result of a computation. Measurements collapse the superposition of the qubits into definite classical states, providing an output that can be observed and interpreted.
Additionally, measurements are also important for error correction and error detection in quantum computing. Quantum error correction requires measuring specific properties of the qubits to detect and correct errors that may occur due to noise and other environmental factors.
So while not all qubits need to be measured during the intermediate steps of a quantum computation, measurements are often employed strategically at certain stages to extract results or perform error correction. The specific measurement strategy depends on the quantum algorithm and the goals of the computation.