• Members 5 posts
    Nov. 8, 2021, 10:30 p.m.

    In this question below, I thought B and E are correct because we need a vector in the second argument. But I was wrong. What is the correct use of the initialize() method?

    Which two of these can be used to initialize a vector of complex amplitudes in a quantum circuit qc?
    A. qc.initialize([0,0,1,1,0])
    B. qc.initialize([0,1,1] , [0,1])
    C. qc.initialize([0,1], 0)
    D. qc.initialize([1,0,0,0])
    E. qc.initialize([1,0,0,0] , [0,1])

  • Members 12 posts
    Nov. 10, 2021, 1:47 p.m.

    I would say the correct answer is C, E. A is wrong becase the dimension is not even for a multi-qubit ($dim=2^n$) system. B is wrong for similar reasons. In general, you need to specify qc.initialize( [some state_vec in dim 2^n], qubits=[0,1,2...]). D is a little tricky here. If you already specify the number of qubits in qc = QuantumCircuit(2). D also works. The rest two have correct arguments according to qiskit API doc: qiskit.org/documentation/stubs/qiskit.circuit.QuantumCircuit.initialize.html

    I also recommend watching this 1MinuteQiskit tutorial on "How Can I Initialize a Multi-Qubit Statevector to Any State?".

    Here is a long example on how to use the initialize( ) method in different scenarios:

    import numpy as np
    from qiskit import QuantumCircuit
    from qiskit.quantum_info import random_statevector
    # Specify number of qubits in the system and a random state vector
    num_qubit = 2
    vec1 = random_statevector(2**1)           # 1-qubit state_vec
    vec2 = random_statevector(2**num_qubit)   # 2-qubit state_vec
    # Initialize the circuit and draw it
    # Case1: Initialize 1-qubit system
    qc1 = QuantumCircuit(1)
    qc1.initialize(vec1, 0)
    qc1.initialize('0', 0)
    # Case2: Initialize 2-qubit system
    qc2 = QuantumCircuit(num_qubit)
    qc2.initialize(vec2, [0,1])
    qc2.initialize(vec2, qc2.qubits)
    # Only initialize a partial system
    qc2.initialize(vec1, 0)