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A while ago I wrote an example of quantum teleportation circuit in the IBM Composer which ran fine as of three days ago. Yesterday, however, when I try to run the circuit, I get the following error:

Failed - QASM2ParseError: '<input>:27,1: needed a start-of-statement token, but instead got /

I am using the IBM Composer and not writing the OpenQASM 2.0 code directly, but of course the composer writes the code automatically. The line that seems to be causing a problem is at the end, and seems to be there due to using 'Freeform alignment' in the composer:

// @columns [0,1,1,1,3,4,5,5,5,6,7,8,8,8,9,10,11,12,13,14]

If I delete that line the code will run, but I lose the freeform alignment and hence visual organization in the composer. Is there a way to keep the freeform alignment line and still get working code?

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According to Open Quantum Assembly Language by Cross et. la.

Comments begin with a pair of forward slashes and end with a new line.

Now, the line

// @columns [0,1,1,1,3,4,5,5,5,6,7,8,8,8,9,10,11,12,13,14]

is the last line in the OpenQasm panel. That is, it doesn't end with a new line. Which means that this error happens due to a bug in IBM Quantum Composer. As a workaround, just add an empty line after it, and your circuit should run without any issue.

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  • $\begingroup$ Thanks, that seems to have been the problem! $\endgroup$ Aug 26, 2023 at 17:31

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