How to get amplitudes only for chosen qubits?

Question

For computation, I use N working qubits and M ancilla qubits.

qubits = QuantumRegister(N, name='q')
ancilla = QuantumRegister(M, name='anc')
circuit = QuantumCircuit(qubits, ancilla)

Then at the end of a program to get state vector I do:

result = execute(circuit, Aer.backend, shots=shots).result()
return result.get_statevector(circuit)

Because of ancilla qubits are usually used for computation/uncomputation steps it means that the end states of them are $|0..0 \rangle$ (at least in some cases) and I am not interested in them due to unnessasary information about M ancilla qubits contained in get_statevector(circuit).

Is it possible to get state vector so it will show amplitudes only for N working qubit?

I have an idea to solve this equation to find $S_N$ (state vector of N working qubits): $$ S_N \otimes I_M = S_{M+N}$$ but probably qiskit can do it internally.

Answer 1

I think that the statevector_simulator will always return the statevector for all the qubits in the circuit. You can however add snapshots over a subset of qubits, so you could add this right at the end of your circuit and use the information you get from that instead.

from qiskit.extensions.simulator import snapshot
qc.snapshot("one_qubit", qubits=[0])
qc.snapshot("many_qubits", qubits=[0,2])

backend = Aer.get_backend('statevector_simulator')
result = execute(qc, backend).result()

snapshots = result.data()['snapshots']['statevector'].items()

Answer 2

If I can be sure that all ancilla qubits are in $ | 0...0 \rangle$ then that their state vector is: \begin{bmatrix} 1 \\ 0 \\ ... \\ 0 \end{bmatrix}

with $2^M\times 1$ dimension.

So I can take the first $2^N $ elements out of result.get_statevector(circuit) to find the state vector of working qubits.

But it's just a partial solution.