Do not understand error in if-else section

Question

I have tried to build a circuit for encryption. One function is:

def selector(qc, a, b, c):
    qc.measure(a, 0)
    if a == 1:
        qc.cswap(a, b, c)
    else:
        qc.id(b)
        qc.id(c)

When I run it in IBM quantum lab, only the else option appears to be working. Why is my a == 1 case never taking place?

Answer 1

Your code is always takes the else branch because you are branching on a == 1 where a is 0 (as you mentioned in your comment selector(fcircuit,0,1,2)).

I have the feeling that you are confusing circuit construction and execution time. In your code measure runs at circuit-execution-time, while your conditional is at circuit-construction-time.

Running a conditional at execution time requires dynamic circuits. In this case:

qc.measure(a, 0)                  # measures a into classical bit 0
with qc.if_test((0, 0)) as else_: # tests if classical bit 0 is 0
    qc.cswap(a, b, c)             # then branch
with else_:
    qc.id(b)                      # else branch
    qc.id(c)                      #

This condition (if_test) is not a Python branching, but a conditional that will run as the circuit is being executed.

Answer 2

When you say only the else is working, I assume you mean the cswap is never taking place. In your code, you compare a to 1, while a is a qubit identifier. What you need to check is the classical register bit to which the measurement has been saved.

qc.measure(a, 0) essentially means "measure qubit a and save the result to classical bit 0".

When you initialized your qc, you have either specified your classical register (in which case you can check its value under index 0 after the measurement) or you can use qc.clbits to see the classical bits of the circuit (see docs.)

Your code then becomes something like if qc.clbits[0] == 1