partial_trace

partial_trace(state, trace_systems, dimensions=None, reverse=True)

Partial trace over subsystems of multi-partite matrix.

Note that subsystems are ordered as rho012 = rho0(x)rho1(x)rho2.

Parameters

• state (matrix_like) – a matrix NxN
• trace_systems (list(int)) – a list of subsystems (starting from 0) to trace over.
• dimensions (list(int)) – a list of the dimensions of the subsystems. If this is not set it will assume all subsystems are qubits.
• reverse (bool) – ordering of systems in operator. If True system-0 is the right most system in tensor product. If False system-0 is the left most system in tensor product.

Returns

A density matrix with the appropriate subsystems traced

over.

Return type

matrix_like

Raises

Exception – if input is not a multi-qubit state.