4 Analyze quantum states
This chapter covers
- Using quantum measurements to extract information about quantum states
- Implementing quantum measurements using Q# and Qiskit
- Analyzing quantum measurement outcomes
- Writing tests to validate information extracted using measurements
As we discussed in chapter 2, a typical quantum algorithm follows a standard sequence of steps (see figure 4.1): prepare the initial state of the quantum system, modify it as prescribed by the algorithm, and then extract the answer using measurements. In chapter 2, we learned to implement the first step, preparing the quantum system in the given state, and in chapter 3, we covered the second step, implementing the unitary transformations to modify the quantum state as required by the algorithm.
Figure 4.1 Any quantum algorithm can be broken down into several steps: prepare the initial state, evolve it following the algorithm, and measure the end state to get the result. In this chapter, we’ll learn to work with the final step, measuring the state of the system to extract information about it.
In this chapter, we’ll focus on the final step of a quantum algorithm: extracting the information about the state of the system. We will learn to use quantum measurements — the only way to get information out of a quantum system.
One of the most prominent ways in which quantum computing differs from classical is the difference in how you read out information about the system. In the classical world: