How to obtain qubit's amplitude in qiskit

Question

For example, I have some circuit

circuit = QuantumCircuit(2)

After operations:

circuit.h(0)
circuit.cx(0, 1)

probability amplitudes of the qubits will change.

I know that I can estimate them by measuring qubits a bunch of times and normalizing the counts:

result = execute(circuit, backend, shots=1000).result()
counts  = result.get_counts(circuit)

But is there a way to directly obtain true amplitudes? Yeah, it's impossible in real life, but I hope the information is contained under the hood of qiskit for education purposes.

Answer 1

You can get these "true amplitudes" through the use of the statevector_simulator within Aer. If you execute the circuit on the statevector_simulator backend, it will return a statevector for the circuit. This statevector, when normalized, will return the theoretical probabilities of each state.

Let's take the circuit you posted as an example:

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

# Retrieve the statevector_simulator backend
backend = Aer.get_backend('statevector_simulator')

result = execute(circuit, backend, shots=1000).result()

# Get the statevector from result().
statevector = result.get_statevector(circuit)

# Normalize statevector to receive the true probabilities.

The statevector will be in the form of a list, with each entry representing a possible state. In this example, the statevector would be returned as [0.70710678+0.j 0. +0.j 0. +0.j 0.70710678+0.j]. This can be read as:

• State 00 probability = 0.70710678+0.j (which is $\frac{1}{\sqrt2}$)
• State 01 probability = 0
• State 10 probability = 0
• State 11 probability = 0.70710678+0.j (which is $\frac{1}{\sqrt2}$)

Which, when normalized would give you a 50% probability of the state 00 and a 50% probability of the state 11.