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qutip / qutip-notebooks

A collection of IPython notebooks using QuTiP: examples, tutorials, development test, etc.
BSD 3-Clause "New" or "Revised" License
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negative expectation values in tutorial #50

Closed marekyggdrasil closed 5 years ago

marekyggdrasil commented 6 years ago

Obviously I'm new to QuTip, while browsing I noticed y-axis in those graphs is marked as expectation value however expectation value is representing a measurement it cannot be negative.

https://github.com/qutip/qutip-notebooks/blob/0adaa9a51b98aa6c513a978c05e810a20fb7530b/examples/qubit-dynamics.ipynb

nathanshammah commented 5 years ago

Probabilities cannot be negative. Measurements of observables can be negative. For example a spin projected on a given direction can point up (+1) or down (-1) and oscillate, as shown in these plots.