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For surface codes, the syndrome measurements collapse the errors into either being $X$ or $Z$ errors. All Clifford gates have easy-to-compute commutation relations with $X$ and $Z$ gates. So the idea is not to actually correct the errors, since that would require more quantum operations which are difficult and error-prone, but to simply track the errors and ...


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I expect you get the Noise Model from the calibrated data of the hardware, however I am not sure how often it is updated. I doubt that it is live or even daily. You can check the noise model by running noise_model._local_quantum_errors and noise_model._local_readout_errors For instance: device = provider.get_backend('ibmq_armonk') noise_model = NoiseModel....


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IBM Quantum Systems are calibrated daily, and the system properties update once this calibration sequence is complete. It means that your noise model could change on a daily basis. More information in the documentation: https://quantum-computing.ibm.com/lab/docs/iql/manage/systems/properties https://quantum-computing.ibm.com/admin/docs/admin/calibration-jobs


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As mentioned by @user47787 This is yet another case of floating-point representation error at the machine level. This behaviour is language agnostic and has nothing to do with Python or Numpy or Qiskit. Related Reads: A very old post on stackoverflow discussing this issue - Is Floating Point Math broken ? Python docs describing the same - Floating Point ...


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