Litinski transform (based on Rustiq)

[alexanderivrii]

Summary

Implements the Litinski transform as a Qiskit transformation pass, as requested in #14657.

The input to the pass is a circuit with Clifford and single-qubit RZ-rotation gates, and the output is a circuit with multi-qubit Pauli rotations (implemented as PauliEvolutionGate gates) followed by Clifford gates. (Note that by returning the Clifford gates rather than the Clifford operator we preserve the global phase of the circuit).

Currently the set of supported Clifford gates is ["cx", "cz", "h", "s", "sdg", "sx", "sxdg", "x", "y", "z"] and the set of supported RZ-rotations is ["t", "tdg", "rz"] (we automatically convert T and Tdg gates to RZ-rotations, note that this changes the global phase of the circuit).

Details and comments

This is a joint work with @Cryoris. The implementation calls Simon Martiel's Rustiq code, a package that is already used in Qiskit for synthesizing Pauli Evolution gates. In parallel, @ShellyGarion is working on a native Qiskit rust implementation of Pauli and Clifford classes (see #14677), that will allow to replace the call to Rustiq by a Qiskit-native implementation. The reason for this is that longer-term we need to be able to extend/adjust the implementation and it would be nice to have the flexibility to do this directly in Qiskit.

ToDo:

• support parametric RZ-rotations
• release notes
• support for barriers?
• convert special-angle RZ-gates to Clifford gates?

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[coveralls]

Pull Request Test Coverage Report for Build 16354607606

Details

• 107 of 117 (91.45%) changed or added relevant lines in 6 files are covered.
• 7 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.01%) to 87.779%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/transpiler/passes/optimization/litinski_transformation.py	7	8	87.5%
crates/transpiler/src/passes/litinski_transformation.rs	96	105	91.43%

Files with Coverage Reduction	New Missed Lines	%
crates/circuit/src/symbol_expr.rs	1	73.98%
crates/qasm2/src/expr.rs	1	93.63%
crates/qasm2/src/lex.rs	5	92.27%

Totals
Change from base Build 16327707788:	0.01%
Covered Lines:	81590
Relevant Lines:	92949

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💛 - Coveralls