Can I get a result with more detail from Result.get_statevector?

Question

I'd like to get a statevector result through Result.get_statevector in Qiskit. However, since my circuit has too many qubits(13qubits) to represent the whole statevector results at a time, I'm searching how I can see the skipped results.

For example,

circuit.save_statevector()

job= execute(circuit, simulator)
result = job.result()

job_result = result.get_statevector(circuit)

after such code I could see the result like,

[ 0.0221-0.j 0.0221-0.j 0.0221-0.j ... -0. -0.j -0. -0.j 0. +0.j]

Is there any way to see these omitted results or to handle 'qiskit_aer.backends.compatibility.Statevector' class?

Answer 1

If you really want to look at the numerical complex value of each single amplitude in the statevector, you can use something like the following:

from qiskit.quantum_info import Statevector
from qiskit.visualization import array_to_latex

num_qubits = 13
qc = circuit.remove_final_measurements(inplace=False)
array_to_latex(Statevector(qc), max_size=2**num_qubits)

Note that you have to remove all the final measurements (if present) to create the Statevector object without errors.

However, this is likely to crash because $2^{13} = 8192$ may be already too many complex numbers to be visualized properly. What you could do instead, is to fill an histogram by using the probabilities vector $p = \left( \left | \alpha_1 \right |^2, \left | \alpha_2 \right |^2,\dots, \left | \alpha_{2^n} \right |^2 \right)$, where $\alpha_i$ is the $i$-th amplitude in your $n$-qubits statevector. This histogram can be easily created in matplotlib by the following code:

import pylab as plt
    
x = range(2**num_qubits)
p = Statevector(qc).probabilities()
plt.bar(x=x, height=p)

Answer 2

You can simply use the Statevector's draw() method:

job_result.draw('latex', max_size = 32)

where, job_result is the variable in your code.

This way, you get a nice-looking output, and you can control the number of displayed components via max_size parameter.