Where is definitive documentation for OpenQASM 2.0 and 3.0

Question

I have gotten conflicting information about OpenQASM; some sources point to version 3 while others say 3 is not ready. I want to refer to a document that will tell me what instructions actually work with the official library.

Answer 1

OpenQASM 2 is defined by https://arxiv.org/abs/1707.03429 It may focus is to describe a gate-based quantum circuit. There are several examples here on what it can be done with it. There are several parsers for OpenQASM 2 (for example: in Qiskit and NuQasm2.

• Pro: stable and well-supported by many tools.
• Con: Not very expressive. Some use-cases cannot be written in 2.0

OpenQASM 3 is live document in https://qiskit.github.io/openqasm/ . The broad document that govern many of the design decisions is https://arxiv.org/abs/2104.14722 . The detailed specification is currently work-in-progress that is open for participation here. A reference parser is being written as part of the same effort.

• Pro: More complete language with classical support.
• Con: Some aspects of it are in active discussion and might change quickly.

TL;DR Depending on your goal and focus, OpenQASM 2 might be enough. If you need classical computing support, inputs, or other feature not covered by OpenQASM 2, you will need OpenQASM 3. In the last case, keeping an eye on the last changes might be important.