# qiskit.quantum_info.diamond_norm¶

diamond_norm(choi, **kwargs)[source]

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.

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 $$\Delta\mathcal{E} = \mathcal{E}_1 - \mathcal{E}_2$$.
This function requires the optional CVXPY package to be installed. Any additional kwargs will be passed to the cvxpy.solve function. See the CVXPY documentation for information on available SDP solvers.