Partial trace operation throws up error once it is broken

[zohimchandani]

Informations

• Qiskit Aer version: 0.10.2
• Python version: 3.6.9
• Operating system: Mac OS

What is the current behavior?

See minimal code example below

Steps to reproduce the problem

# Import Qiskit
from qiskit import QuantumCircuit
import qiskit.quantum_info as qi
from qiskit import Aer, execute

simulator = Aer.get_backend('statevector_simulator', seed = 100)


qc_AB = QuantumCircuit(2)
qc_AB.h(0)
qc_AB.cx(0,1)

sv = execute(qc_AB, simulator).result().get_statevector()

rho_B = qi.partial_trace(sv,[0]) #trace out qubit 0

Now If I provide an empty list to trace out then it throws up an error which is expected

rho_B = qi.partial_trace(sv, [])  

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-26-a145dac87eb0> in <module>
----> 1 rho_B = qi.partial_trace(sv,[])

~/PQKs/pqks/lib/python3.6/site-packages/qiskit/quantum_info/states/utils.py in partial_trace(state, qargs)
     56     if isinstance(state, Statevector):
     57         trace_systems = len(state._op_shape.dims_l()) - 1 - np.array(qargs)
---> 58         arr = state._data.reshape(state._op_shape.tensor_shape)
     59         rho = np.tensordot(arr, arr.conj(), axes=(trace_systems, trace_systems))
     60         rho = np.reshape(rho, traced_shape.shape)

ValueError: cannot reshape array of size 4 into shape (2,2,2,2)

After the error, if I now recall a previous instruction that successfully ran then the error reappears.

rho_B = qi.partial_trace(sv,[0]) #trace out qubit 0

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-27-8cbae0b90ecc> in <module>
----> 1 rho_B = qi.partial_trace(sv,[0])

~/PQKs/pqks/lib/python3.6/site-packages/qiskit/quantum_info/states/utils.py in partial_trace(state, qargs)
     56     if isinstance(state, Statevector):
     57         trace_systems = len(state._op_shape.dims_l()) - 1 - np.array(qargs)
---> 58         arr = state._data.reshape(state._op_shape.tensor_shape)
     59         rho = np.tensordot(arr, arr.conj(), axes=(trace_systems, trace_systems))
     60         rho = np.reshape(rho, traced_shape.shape)

ValueError: cannot reshape array of size 4 into shape (2,2,2,2)

It seems as if the qi.partial_trace operation is in some state that once it is broken, it cant function again?

If I use .get_statevector().data the above issue disappears.

What is the expected behavior?

Operation should not be state-bound and work regardless?

Suggested solutions

[jakelishman]

I've transferred this to Terra, because the issue isn't related to Aer, it's just about qiskit.quantum_info, which is in Terra:

In [1]: from qiskit.quantum_info import Statevector, partial_trace
   ...: sv = Statevector([0, np.sqrt(0.5), np.sqrt(0.5), 0])
   ...: try:
   ...:     partial_trace(sv, [])
   ...: except ValueError:
   ...:     pass
   ...: partial_trace(sv, [0])

gives

ValueError                                Traceback (most recent call last)
Input In [1], in <module>
      5 except ValueError:
      6     pass
----> 7 partial_trace(sv, [0])

File ~/code/qiskit/terra/qiskit/quantum_info/states/utils.py:57, in partial_trace(state, qargs)
     55 if isinstance(state, Statevector):
     56     trace_systems = len(state._op_shape.dims_l()) - 1 - np.array(qargs)
---> 57     arr = state._data.reshape(state._op_shape.tensor_shape)
     58     rho = np.tensordot(arr, arr.conj(), axes=(trace_systems, trace_systems))
     59     rho = np.reshape(rho, traced_shape.shape)

ValueError: cannot reshape array of size 4 into shape (2,2,2,2)

I'm guessing that somewhere along the line, Statevector._op_shape gets mutated in place, and never fixed because of the exception.

[jakelishman]

Also, I'm not super convinced that passing an empty list should be an error - I'd have thought we should just return the density matrix of the state.