The last two decades have witnessed a revolution in our understanding of the role of information in quantum mechanics and have led to the notion of the quantum computer. Similarly we now better understand the flow of information that occurs when measurements are made and we understand in a precise sense that ideal quantum measurements are ones that do not waste any information. Finally we now understand that it is not just the experimentalist who makes intentional measurements but any ‘observer’ including the environment surrounding a quantum system. The quantum coherence that is essential for the operation of any quantum computer is maintained only as long as no information is lost to the environment.

This talk will present a simple introductory tutorial on how real measurements work by entangling the state of the quantum system with the state of the measurement apparatus. This will allow us to understand and how the choice of what is measured affects the state of the quantum system being observed.