Commits on Sep 5, 2023

1. Track Sabre decay heuristic on physical qubits …

   The custom `decay` heuristic is supposed to penalise increases in depth
   in the output.  The output space of qubits are the physical qubits, so
   the depth is defined in relation to those.  Since its introduction in
   Qiskitgh-4537, this heuristic has instead been tracking the depth on the
   virtual qubits, which due to the swaps is not necessarily related to the
   depth in the output.
   
   Notably, this commit actually makes the depth for routing large
   volumetric circuits slightly _worse_ on average (at least for heavy-hex
   topologies).  This may be because the effect on the heuristic is
   overweighted, or that the depth tracking resets after each gate is
   routed (and occasionally in between) to be flat across all qubits,
   rather than reflecting the actual depth each qubit is subject to.

   

   jakelishman committed Sep 5, 2023
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