Calculating qubit purity/entanglement in a quantum computer

In summary, calculating qubit purity and entanglement in a quantum computer involves assessing the degree of coherence and correlations between qubits. Purity measures how much a quantum state deviates from a pure state, typically calculated using the density matrix. Entanglement quantifies the non-classical correlations between qubits, often analyzed through metrics like the von Neumann entropy or concurrence. These calculations are crucial for understanding the performance and reliability of quantum computations, as they provide insights into the system's state fidelity and the effectiveness of quantum algorithms.
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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.
 

Attachments

  • Calculating qubit purity-entanglement.pdf
    81.8 KB · Views: 20

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