toqito.state_metrics.trace_norm

toqito.state_metrics.trace_norm(rho)[source]

Compute the trace norm of the state [WikTn].

The trace norm \(||\rho||_1\) of a density matrix \(\rho\) is the sum of the singular values of \(\rho\). The singular values are the roots of the eigenvalues of \(\rho \rho^*\).

Examples

Consider the following Bell state

\[u = \frac{1}{\sqrt{2}} \left( |00 \rangle + |11 \rangle \right) \in \mathcal{X}.\]

The corresponding density matrix of \(u\) may be calculated by:

\[\begin{split}\rho = u u^* = \begin{pmatrix} 1 & 0 & 0 & 1 \\ 0 & 0 & 0 & 0 \\ 0 & 0 & 0 & 0 \\ 1 & 0 & 0 & 1 \end{pmatrix} \in \text{D}(\mathcal{X}).\end{split}\]

It can be observed using toqito that \(||\rho||_1 = 1\) as follows.

>>> from toqito.states import bell
>>> from toqito.state_metrics import trace_norm
>>> rho = bell(0) * bell(0).conj().T
>>> trace_norm(rho)
0.9999999999999999

References

Parameters:

rho – Density operator.

Returns:

The trace norm of rho.