Undergrad Calculating qubit purity/entanglement in a quantum computer

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The discussion focuses on calculating the "purity of reduced states" in a quantum computer simulator, a metric previously referred to as "entanglement." The author successfully developed a method to compute these values, which align with those provided by IBM's simulator for various quantum circuits. The document detailing this calculation method is shared for others to utilize. The purity values are consistent across different gate operations, including one-qubit gates and CNOT gates. This information may benefit others working with quantum computing simulations.
Bob Walance
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I had written a simulator for a quantum computer and wanted to be able to calculate the values of the "purity of reduced states".

"Purity of reduced states" is a quantity that IBM provides on their simulator for each qubit. They used to call it "entanglement", and I believe that the two terms are synonymous since all of the qubits in a quantum computer start out in a pure state.

After struggling to find examples for how to calculate these purity/entanglement values for each qubit, I did finally figure it out. Attached is a document describing how to do this. The values calculated with this method jibe with IBM's simulator for various quantum circuits including simple one-qubit gates, CNOT gates, Toffoli gates, and various combinations of those.

Perhaps someone will find this useful.
 

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I am slowly going through the book 'What Is a Quantum Field Theory?' by Michel Talagrand. I came across the following quote: One does not" prove” the basic principles of Quantum Mechanics. The ultimate test for a model is the agreement of its predictions with experiments. Although it may seem trite, it does fit in with my modelling view of QM. The more I think about it, the more I believe it could be saying something quite profound. For example, precisely what is the justification of...

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