How to obtain wavefunction from quibit registers

[yangjy0826]

Description

Hi, I found in this link that there is a way to obtain wavefunction from quibit registers, and an example is given:

import qiskit
from qiskit import QuantumProgram
import qiskit.extensions.qiskit_simulator

qp = QuantumProgram()
qr = qp.create_quantum_register('qr', 2);
cr = qp.create_classical_register('cr', 2);
circ = qp.create_circuit('bell', [qr], [cr])
circ.h(qr[0])
circ.cx(qr[0], qr[1])
circ.barrier(qr)
circ.save(1, qr)
circ.barrier(qr)
circ.measure(qr, cr)

backend = 'local_qiskit_simulator'
config = {'data': ['saved_quantum_state',
                   'saved_density',
                   'saved_quantum_state_ket',
                   'saved_probabilities']}
result = qp.execute('bell', shots=10, backend=backend, config=config)
result.get_data('bell')

its output is:

{'counts': {'00': 5, '11': 5},
 'saved_probabilities': {'1': [0.5, 0.0, 0.0, 0.5]},
 'saved_quantum_states': [{1: array([0.70710678+0.j, 0.        +0.j, 0.        +0.j, 0.70710678+0.j])},
  {1: array([0.70710678+0.j, 0.        +0.j, 0.        +0.j, 0.70710678+0.j])},
  {1: array([0.70710678+0.j, 0.        +0.j, 0.        +0.j, 0.70710678+0.j])},
  {1: array([0.70710678+0.j, 0.        +0.j, 0.        +0.j, 0.70710678+0.j])},
  {1: array([0.70710678+0.j, 0.        +0.j, 0.        +0.j, 0.70710678+0.j])},
  {1: array([0.70710678+0.j, 0.        +0.j, 0.        +0.j, 0.70710678+0.j])},
  {1: array([0.70710678+0.j, 0.        +0.j, 0.        +0.j, 0.70710678+0.j])},
  {1: array([0.70710678+0.j, 0.        +0.j, 0.        +0.j, 0.70710678+0.j])},
  {1: array([0.70710678+0.j, 0.        +0.j, 0.        +0.j, 0.70710678+0.j])},
  {1: array([0.70710678+0.j, 0.        +0.j, 0.        +0.j, 0.70710678+0.j])}],
 'saved_quantum_states_ket': [{'1': {'00': [0.707106781186548, 0.0],
    '11': [0.707106781186547, 0.0]}},
  {'1': {'00': [0.707106781186548, 0.0], '11': [0.707106781186547, 0.0]}},
  {'1': {'00': [0.707106781186548, 0.0], '11': [0.707106781186547, 0.0]}},
  {'1': {'00': [0.707106781186548, 0.0], '11': [0.707106781186547, 0.0]}},
  {'1': {'00': [0.707106781186548, 0.0], '11': [0.707106781186547, 0.0]}},
  {'1': {'00': [0.707106781186548, 0.0], '11': [0.707106781186547, 0.0]}},
  {'1': {'00': [0.707106781186548, 0.0], '11': [0.707106781186547, 0.0]}},
  {'1': {'00': [0.707106781186548, 0.0], '11': [0.707106781186547, 0.0]}},
  {'1': {'00': [0.707106781186548, 0.0], '11': [0.707106781186547, 0.0]}},
  {'1': {'00': [0.707106781186548, 0.0], '11': [0.707106781186547, 0.0]}}],
 'time_taken': 0.000511}

But when I was trying the code, I find that QuantumProgram and qiskit.extensions.qiskit_simulator can't be imported successfully. I am not sure whether it's because of the updating of Qiskit version. Then how can I obtain the wavefunction now?

Thank you!

Your Environment

• QISKit version: 0.6.0
• Python version: 3.6

[yangjy0826]

I searched for the Changelog, and found that in the Deprecated part of version 0.5.0, there is a sentence saying ''Simulators no longer return wavefunction by setting shots=1. Instead, use the local_statevector_simulator, or explicitly ask for snapshot.'' What does it mean?

[chriseclectic]

That standard way to the get the final state vector is to use the statevector_simulator backend to execute a single shot of a circuit with no reset or measure instructions. For example:

import qiskit
from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit
from qiskit import Aer

qr = qiskit.QuantumRegister(2)
circ = qiskit.QuantumCircuit(qr)
circ.h(qr[0])
circ.cx(qr[0], qr[1])

backend = Aer.get_backend('statevector_simulator')
result = qiskit.execute(circ, backend=backend).result()
result.get_data(circ)

This will return the output

{'statevector': array([0.70710678+0.j, 0.        +0.j, 0.        +0.j, 0.70710678+0.j])}

You may also use result.get_statevector(circ) to directly get the np array from the output of the statevector simulator.

[yangjy0826]

Thank you so much! This is exactly what I'm looking for.

[chriseclectic]

No problem!