A quantum computing circuit to swap two bits in Qiskit.

swap.py

def swap(b, a):
    """
    A quantum circuit that swaps the input bits.
    01 => 10
    """
    swap = QuantumCircuit(2, 2)

    if a:
        swap.x(0)
    if b:
        swap.x(1)

    swap.cx(0, 1)
    swap.cx(1, 0)
    swap.cx(0, 1)
    swap.measure_all()

    job = qiskit.execute(swap, qiskit.BasicAer.get_backend('qasm_simulator'))
    return job.result().get_counts()

zclient.py

def swap_test(b, a):
    # Set our input bits to be swapped.
    input = [b, a]
    # Create a string version '10'.
    input_str = '{}{}'.format(input[0], input[1])
    # Swap the bits.
    output = swap(input[0], input[1])
    # Create a string version of the output '01'.
    output_str = list(output.keys())[0].split(' ')[0]
    # Verify the output.
    print('{} => {} ? {}'.format(input_str, output_str, input_str[::-1] == output_str))

swap_test(0, 0)
swap_test(1, 0)
swap_test(0, 1)
swap_test(1, 1)

zoutput.txt

Note, input/output bits are read from right to left.

00 => 00 ? True
10 => 01 ? True
01 => 10 ? True
11 => 11 ? True

zreadme.md

Quantum Circuit to Swap Bits

An example of swapping two bits from input to output in Qiskit.

Demo on IBM Quantum Composer.

Calculating Depth

[0 0]
X1: [0, 1]
CX1: [1, 2]
CX2: [3, 3]
CX3: [4, 4]
M1: [5,4]
M2: [5,5]

print(swap.depth())
# 5