## 11.1 Decoherence simplified

Consider the following qubit-environment interaction: \begin{aligned} |0\rangle|e\rangle &\longmapsto |0\rangle|e_{00}\rangle \\|1\rangle|e\rangle &\longmapsto |1\rangle|e_{11}\rangle \end{aligned} where |e\rangle, |e_{00}\rangle, and |e_{11}\rangle are the states of the environment, which not need to be orthogonal.140 Let |\psi\rangle = \alpha|0\rangle + \beta|1\rangle be the initial state of the qubit. The environment is essentially trying to measure the qubit and, as the result, the two get entangled: \Big( \alpha|0\rangle + \beta|1\rangle \Big) |e\rangle \longmapsto \alpha |0\rangle|e_{00}\rangle + \beta |1\rangle |e_{11}\rangle. This state can also be written as \begin{aligned} \Big( \alpha|0\rangle + \beta|1\rangle \Big) |e\rangle \longmapsto & \Big( \alpha|0\rangle + \beta|1\rangle \Big) \frac{|e_{00}\rangle+|e_{11}\rangle}{2} \\+& \Big( \alpha|0\rangle - \beta|1\rangle \Big) \frac{|e_{00}\rangle-|e_{11}\rangle}{2}. \end{aligned} or as |\psi\rangle|e\rangle \longmapsto \mathbf{1}|\psi\rangle|e_{\mathbf{1}}\rangle + Z|\psi\rangle|e_Z\rangle, where |e_{\mathbf{1}}\rangle = \frac12(|e_{00}\rangle + |e_{11}\rangle) and |e_Z\rangle = \frac12(|e_{00}\rangle - |e_{11}\rangle). We may interpret this expression by saying that two things can happen to the qubit: nothing \mathbf{1} (first term), or phase-flip Z (second term). This, however, should not be taken literally unless the states of the environment, |e_{\mathbf{1}}\rangle and |e_Z\rangle, are orthogonal.141

This process is what we refer to as decoherence.

1. The reason we use two indices in |e_{00}\rangle and |e_{11}\rangle will become clear in a moment, when we consider more general interaction with the environment.↩︎

2. Why not?↩︎