`Circuit.get_q_register` seems to assume the register is zero indexed

[CalMacCQ]

I've encountered some confusing behaviour with Circuit.get_q_register when the register we're "getting" has qubits which are not indexed starting from 0.

Consider the following example where I create an eight qubit circuit with two registers. I'll then add some CX gates to the circuit in such a way as to leave some the first three qubits without a quantum gate acting upon them.

circ = Circuit()
control_reg = circ.add_q_register("q", 1)
block = circ.add_q_register("b", 7)

for t in (3, 4, 5, 6):
    circ.CX(control_reg[0], block[t])

I can then remove the three blank qubits b[0], b[1], b[2]

circ.remove_blank_wires()

Now what happens if we list the remaining qubits in circ?

Using circ.qubits gives the expected answer

print(circ.qubits)

Output:

[b[3], b[4], b[5], b[6], q[0]] 

However if we use circ_get_q_register to list the qubits in the "b" register we get a stange answer.

print(list(circ.get_q_register("b")))

Output:

[b[0], b[1], b[2], b[3]]

This seems strange. The register has the right number of qubits but I was expecting to see the list

 [b[3], b[4], b[5], b[6]]

It seems that the get_q_register methods is reindexing the qubits to be indexed from b[0]-b[3].

[cqc-alec]

Yes this is a bug (or at the very least a documentation bug) with Circuit::get_q_register() (and also get_c_register()). This method just looks at the total number of qubits with the given name and returns a zero-indexed register of that size.

[cqc-alec]

However it's not clear what the method should do in the case of incomplete registers. Probably return an error.

[CalMacCQ]

Hmm I'd have to think about that. What would be wrong with having it return [b[3], b[4], b[5], b[6]]? Using print(circ.qubits) gives this list if I exclude the q[0].

[cqc-alec]

Hmm I'd have to think about that. What would be wrong with having it return [b[3], b[4], b[5], b[6]]? Using print(circ.qubits) gives this list if I exclude the q[0].

It returns a QubitRegister, not a list of qubits. A QubitRegister is defined only by a name and a size, and refers to a collection of zero-indexed qubits.