Support manual variables QuantumCircuit copy methods
#10963
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Changelog: New Feature
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This adds the representation of `expr.Var` nodes that own their own storage locations, and consequently are not backed by existing Qiskit objects (`Clbit` or `ClassicalRegister`). This is the base of the ability for Qiskit to represent manual classical-value storage in `QuantumCircuit`, and the base for how manual storage will be implemented.
This does not yet add the implementation of `QuantumCircuit.store`, which will come later as part of expanding the full API of `QuantumCircuit` to be able to support these runtime variables. The `is_lvalue` helper is added generally to the `classical.expr` module because it's generally useful, while `types.cast_kind` is moved from being a private method in `expr` to a public-API function so `Store` can use it. These now come with associated unit tests.
This adds all the new `QuantumCircuit` methods discussed in the variable-declaration RFC[^1], and threads the support for them through the methods that are called in turn, such as `QuantumCircuit.append`. It does yet not add support to methods such as `copy` or `compose`, which will be done in a follow-up. The APIs discussed in the RFC necessitated making `Var` nodes hashable. This is done in this commit, as it is logically connected. These nodes now have enforced immutability, which is technically a minor breaking change, but in practice required for the properties of such expressions to be tracked correctly through circuits. A helper attribute `Var.standalone` is added to unify the handling of whether a variable is an old-style existing-memory wrapper, or a new "proper" variable with its own memory. [^1]: Qiskit/RFCs#50
This commit adds support to the `QuantumCircuit` methods `copy` and `copy_empty_like` for manual variables. This involves the non-trivial extension to the original RFC[^1] that variables can now be uninitialised; this is somewhat required for the logic of how the `Store` instruction works and the existence of `QuantumCircuit.copy_empty_like`; a variable could be initialised with the result of a `measure` that no longer exists, therefore it must be possible for variables to be uninitialised. This was not originally intended to be possible in the design document, but is somewhat required for logical consistency. A method `add_uninitialized_var` is added, so that the behaviour of `copy_empty_like` is not an awkward special case only possible through that method, but instead a complete part of the data model that must be reasoned about. The method however is deliberately a bit less ergononmic to type and to use, because really users _should_ use `add_var` in almost all circumstances. [^1]: Qiskit/RFCs#50
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Summary
This commit adds support to the
QuantumCircuitmethodscopyandcopy_empty_likefor manual variables. This involves the non-trivial extension to the original RFC1 that variables can now be uninitialised; this is somewhat required for the logic of how theStoreinstruction works and the existence ofQuantumCircuit.copy_empty_like; a variable could be initialised with the result of ameasurethat no longer exists, therefore it must be possible for variables to be uninitialised.This was not originally intended to be possible in the design document, but is somewhat required for logical consistency. A method
add_uninitialized_varis added, so that the behaviour ofcopy_empty_likeis not an awkward special case only possible through that method, but instead a complete part of the data model that must be reasoned about. The method however is deliberately a bit less ergononmic to type and to use, because really users should useadd_varin almost all circumstances.Details and comments
Depends on #10962.
Close #10959.
Footnotes
https://github.com/Qiskit/RFCs/pull/50 ↩