# Fidelity between a pure state and an arbitrary state

1. Dec 31, 2015

### Emil_M

\1. The problem statement, all variables and given/known data
Show that the Fidelity between one pure state $|\Psi\rangle$ and one arbitrary state $\rho$ is given by $F(|\Psi\rangle , \rho)=\sqrt{\langle\Psi|\rho|\Psi\rangle}$ .

2. Relevant equations
The quantum mechanical fidelity is defined as $\begin{equation*} F(\rho,\sigma):= Tr{ \sqrt{\rho^{\frac{1}{2}} \sigma \rho^{\frac{1}{2}}}} \end{equation*}$

3. The attempt at a solution
The operator corresponding to $|\Psi\rangle$ is $\sigma=|\Psi\rangle\langle\Psi|$.
We can make use of the spectral decomposition of $\sigma$ in order to write
$\begin{equation*} \sigma=\sum_{j=1}^N s_j |j\rangle\langle j| \end{equation*}$
It follows, that the Fidelity $F(\sigma, \rho)$ is given by
\begin{align*}
F(\sigma, \rho) & =Tr{\sqrt{\left(|\Psi\rangle\langle\Psi|\right)^{\frac{1}{2}}\rho\left(|\Psi\rangle\langle\Psi|\right)^{\frac{1}{2}}}} \\
&= Tr{\sqrt{|\Psi\rangle\langle\Psi|\rho|\Psi\rangle\langle\Psi|}}\\
&= Tr{\sqrt{\langle\Psi|\rho|\Psi\rangle\;|\Psi\rangle\langle\Psi|}}\\
&= Tr{\sqrt{\langle\Psi|\rho|\Psi\rangle\sum_{j=1}^N s_j |j\rangle\langle j|}}\\
&=\sqrt{\langle\Psi|\rho|\Psi\rangle} \; Tr \sum_{j=1}^N \sqrt{s_j} |j\rangle\langle j|\\
&=\sqrt{\langle\Psi|\rho|\Psi\rangle} \; \sum_{k,j=1}^N \sqrt{s_j} \langle k|j\rangle\langle j|k\rangle\\
&=\sqrt{\langle\Psi|\rho|\Psi\rangle} \; \sum_{j=1}^N \sqrt{s_j}
\end{align*}
This is not the expected result, so I have made a mistake somewhere along the way. I would be really thankful if somebody were to spot it!

Last edited: Dec 31, 2015
2. Dec 31, 2015

### Fightfish

Well, the easiest way to go about doing it is to realise that
$$\sqrt{\langle \psi | \rho | \psi\rangle |\psi\rangle \langle \psi| } = \sqrt{\langle \psi | \rho | \psi\rangle} \sqrt{ |\psi\rangle \langle \psi| } = \sqrt{\langle \psi | \rho | \psi\rangle} |\psi\rangle \langle \psi|$$
and so the trace becomes trivial.

Your method is confusing, but there isn't anything wrong with it. When you perform a spectral decomposition of a projector into its diagonal basis, well its just itself isn't it? So, all the coefficients are zero except for one which is unity. So, the summation in your final expression is indeed 1.

3. Jan 1, 2016

Thanks!

4. Jan 2, 2016

### Emil_M

I still don't get it. Why is it a given that $|\Psi\rangle \langle\Psi|$ is a projector?

Thanks!

5. Jan 2, 2016

### Fightfish

Well, any operator of the form $\hat{P}_{\alpha} = |\alpha \rangle \langle \alpha |$, when acted upon any state ket $|\beta \rangle$, yields the component of $|\beta \rangle$ along the "direction" of $|\alpha \rangle$. So, the operator $\hat{P}_{\alpha}$ is said to project $|\beta \rangle$ into the "direction" of $|\alpha\rangle$, and that's why it is known as a projector.

6. Jan 2, 2016

### Emil_M

Yes, but a projector has to respect the condition $P^2=P$. And the density matrix corresponding to a general qubit, for example, $|\Psi\rangle = a|0\rangle+b|1\rangle$ is thus not a projector, is it?

7. Jan 2, 2016

### Fightfish

$|\Psi\rangle \langle \Psi | \Psi \rangle \langle \Psi | = ?$

8. Jan 2, 2016

### Emil_M

right :)
Thanks for taking the time, don't know why I was so confused.