Week 7, summary 
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1. Quantum noise. 
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Obstacles for quantum error correction. 
Discretization of noise. Bit flips and phase flips,   
Pauli matrices. 


2. Quantum error correcting codes. 
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Shor's nine qubit code. 


3. good quantum error correcting codes. 
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Calderbank Shor Steane (CSS) codes.