Ammar Eltigani

This PR is the second in a series to implement qubit routing capabilities in Cirq. It (1) adds a base class AbstractInitialMapper (2) an implementation of this base class LineInitialMapper...

Created routing utilities subdirectory in cirq-core/transformers and added MappingManager module

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This PR is the first in a series to implement qubit routing capabilities in Cirq. It (1) creates a new subdirectory in cirq-core/transformers named 'routing' where all qubit routing utilities...

[Routing] Add initial mapping strategy for circuit with small # of qubits.

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**Summarize the task** Currently `LineInitialMapper` is the only initial mapping (placement) strategy Cirq offers for routing. Adding more strategies like graph monomorphism could improve routing performance on deep circuits with...

**Summarize the task** The current implementation of `RouteCQC` unrolls all CircuitOperations before routing. We want to enhance the transformer by allowing recursive routing of CircuitOperations. **Acceptance criteria - when is...

[Routing] Adding use of ancillas.

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**Summarize the task** Enable use of ancillas to shorten the shortest path along qubits that need to be adjacent during routing. **Acceptance criteria - when is the task considered done?**...

[Routing] Add support for directed device graphs.

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**Summarize the task** Currently, if a user wants to route their circuit on a device that has asymmetric coupling (i.e. you can execute a CNOT(q1, q2) but not CNOT(q2, q1))...