Measuring multiqubit systems

We have been learning about multi-qubit states, but we still don’t know what happens if we measure a qubit for the multi-qubit state. In this video, David will tell us about the generalized Born rule.

Prerequisite knowledge

• Born rule
• Multiqubit systems
• Multiqubit gates
• Entanglement

Further thinking

You measure the first qubit of the following state in the X basis and obtain |+>. 

What is the state of the second qubit?

What does the following gate sequence simplify to (U_i means apply gate U to qubit i)?

Solutions

Further reading

For a more detailed explanation on measuring multi-qubit systems see section 4.4 in M. A. Nielsen and I. L. Chuang, Quantum computation and quantum information, 10th anniversary ed. Cambridge ; New York: Cambridge University Press, 2010 and section 1.9 in N. D. Mermin, Quantum Computer Science: An Introduction