Walkthrough ‐ Quantum III.E: Cascade

Learning Objectives

• The player is challenged to distinguish between +Z and +X eigenstates (while given an ensemble of four copies of each state).
• This level serves as a more advanced introduction to single-qubit state tomography, requiring a good understanding of measurement probabilities.

Solution

There are many viable strategies for solving this problem; the one presented here is an extension of the strategy used to solve QII.B.

1. Each sequence of identical qubits is distributed over four separate lines.
2. The qubit on the first line is measured with axis set at -45°.
3. The qubit on the second line is measured (on the +Z axis is the previous result was a 0, else measured on the -X axis).
4. The qubit on the third line is measured (on the +Z axis is the previous result was a 0, else measured on the -X axis).
5. The qubit on the fourth line is measured (on the +Z axis is the previous result was a 0, else measured on the -X axis), and the result is sent to the output.

Level File (copy then paste into Qubit Factory)

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