I am looking for a way to implement a projection operator like $\mid +\rangle\langle+\mid$ in qiskits opflow module. I tried Plus @ ~Plus but this gives me an error message. There must be a direct implementation of projection operators right? I can't use the operators in qiskit.quantum_info for what I want to do, except if there is a way to convert those operators to the ones in opflow.

Edit: The error message I get from (Plus)@(~Plus) is ValueError: Composition with a Statefunctions in the first operand is not defined.

The reason I think I can't use qiskit.quantum_info's opearotors it that I want to compute the spectrum using qiskit.algorithms.NumPyEigensolver and this seems to me to not work with a quantum_info operator:

>>>plus = Statevector([1/np.sqrt(2),1/np.sqrt(2)]).to_operator()
Operator([[0.5+0.j, 0.5+0.j],
          [0.5+0.j, 0.5+0.j]],
         input_dims=(2,), output_dims=(2,))

>>>solver = NumPyEigensolver()
>>>spectrum = solver.compute_eigenvalues(plus)
AttributeError: 'Operator' object has no attribute 'to_spmatrix'

I am generally confused about the connection and difference between qiskit.quantum_info and qiskit.opflow because they seem to me to have a large overlap. Is one of those an older module and should not be used anymore? Is there a general way to convert objects like operators from one to the other?

  • $\begingroup$ Welcome to QCSE! There is some context information missing from your question that might help folks answer it: what error message you see and why you think that you can't use operators in qiskit.quantum_info. $\endgroup$ Commented Dec 18, 2021 at 0:33

1 Answer 1


You can convert a qiskit.quantum_info operator to a qiskit.opflow operator easily by passing it as a parameter to PrimitiveOp constructor:

from qiskit.opflow import PrimitiveOp

# |+>
sv = Statevector.from_label('+')

# |+><+|
proj = sv.to_operator()

# Convert to opflow operator:
op = PrimitiveOp(proj)

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