Multi-Qubit Systems, Entanglement, and Quantum Weirdness

Abstract

Quantum state entanglement is a crucial ingredient in many quantum algorithms. This chapter introduces the description of multi-qubit quantum systems and then explains what it means to say the state of that system is entangled. If the quantum state of the system is an entangled state, none of the constituents of the system are described by their own individual states. For a two-qubit entangled state, measurements of one of the qubits apparently affect the measurement results of the other even though the two qubits may be far apart. The change in basis states, introduced in Chapter 8, is applied to a two-qubit system, which will be used to illustrate Bell’s theorem. That theorem tells us that the quantum description of the world profoundly violates almost every concept we have about material properties and the measurement of those properties.