Introducing quantum measurements
In quantum computing, whenever we start talking about making measurements, we are referring to the fact that the quantum system is interacting with the environment. Whenever qubits interact with an external environment, the phenomenon of decoherence takes place and they no longer maintain their quantum properties, which severely alters their quantum state. This configuration is what we describe as an open system, where the qubits interact with the environment and the time evolution is not inflexible.
The process of measurement can only take place in an open system because the measurement device has to interact with the quantum system and states to extract information from it. After the measurement process, the whole quantum system is severely altered and disturbed.
During the measurement process, a phenomenon known as the collapse of the wave function occurs. Prior to measurement of the quantum system, the quantum system lives in the superposition...