qiskit.quantum_info.diamond_norm

diamond_norm(choi, **kwargs)

Return the diamond norm of the input quantum channel object.

This function computes the completely-bounded trace-norm (often referred to as the diamond-norm) of the input quantum channel object using the semidefinite-program from reference [1].

Parameters

• choi (Choi or QuantumChannel) – a quantum channel object or Choi-matrix array.
• kwargs – optional arguments to pass to CVXPY solver.

Returns

The completely-bounded trace norm

$\|\mathcal{E}\|_{\diamond}$.

Return type

float

Raises

QiskitError – if CVXPY package cannot be found.

Additional Information:

The input to this function is typically not a CPTP quantum channel, but rather the difference between two quantum channels $\|\Delta\mathcal{E}\|_\diamond$ where $\Delta\mathcal{E} = \mathcal{E}_1 - \mathcal{E}_2$.

Reference:

J. Watrous. “Simpler semidefinite programs for completely bounded norms”, arXiv:1207.5726 [quant-ph] (2012).