Add global-phase support to control-flow builders #10800
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This formalises the concept of global phase within control-flow scopes, and teaches the control-flow builders to treat modifications to the global phase as being conditional on the control-flow block being entered.
jakelishman
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Changelog: Bugfix
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mtreinish
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Oct 11, 2023
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| if self._control_flow_scopes: | ||
| return self._control_flow_scopes[-1].global_phase |
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This is a necessary part of the interface being added in this PR, but is there really a use case for:
with qc.while_loop((qc.clbits[1], False)):
qc.h(0)
print(qc.global_phase)It seems pretty esoteric. But it's good we handle it
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Typically you wouldn't need to actually get the global_phase for your own use, but things like __iadd__, which is a fairly natural way of writing things, use both a get and a set by default.
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Summary
This formalises the concept of global phase within control-flow scopes, and teaches the control-flow builders to treat modifications to the global phase as being conditional on the control-flow block being entered.
Details and comments
I turned this up during trying to write #10786, and felt like we needed a formalisation of what we treat "global phase" as during control-flow scopes.
This PR defines the "global phase" of a block as "the amount by which the phase of all program qubits will be advanced if this block is taken", which I think is consistent with how we have been de facto treating this via basis-translator recursion into control-flow blocks. For the most part, this shouldn't be observable in an actual execution of a quantum program, but you could say take a single shot of a statevector simulation, and this interpretation would cause the resultant statevector to have a well-defined global phase consistent with our existent conventions.