Qiskit / qiskit / 17041136902

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Commit Message

Fix composition of `final_layout` (#14919) (#14920)

The previous implementation had it backwards.  There are two ways to
think about the `final_layout`, which is a `Layout` object mapping
"virtual" objects (qubits at the start of the circuit) to "physical"
indices (qubits at the end of the circuit):

- It is a "goes to" permutation that you would _prepend_ to the
  _unrouted_ circuit to have the same effect as routing.

- It is a "comes from" permutation that you would _append_ to the
  _routed_ circuit to undo the effects of routing.

If you have permutation `A` that is already applied, and you're newly
applying permutation `B`, then in the first interpretation, you want to
transform
    A and then (B and then circuit)
into
    (A and then B) and then circuit.

In the second interpretation, you instead want to turn
    (circuit then "undo B") then "undo A"
into
    circuit then ("undo B" then "undo A")

The composition "undo B then undo A" is the same as "undo (A then B)"
by standard inverse rules.

In both cases, the logic is implemented as `previous.compose(new)` in
terms of `Layout.compose`.

Co-authored-by: Jake Lishman <jake.lishman@ibm.com>

Run Details

3 of 5 new or added lines in 4 files covered. (60.0%)

21 existing lines in 4 files now uncovered.

80006 of 91199 relevant lines covered (87.73%)

512755.97 hits per line