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Revamp ChebyshevPolynomial #1213

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Merged
merged 2 commits into from
Jul 30, 2024
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charlesyuan314
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  • Migrate ChebyshevPolynomial to the modern BlockEncoding interface by changing the base class, attribute type, and signature.
  • Clarify in the docstring that ChebyshevPolynomial only works correctly with block encodings $\mathcal{B}[A]$ of Hermitian matrices $A$.
  • Make ChebyshevPolynomial correct for use with non-Hermitian block encodings $\mathcal{B}[A] \neq \mathcal{B}[A]^\dagger$, following the existing reference.

The last change involves invoking $\mathcal{B}[A]^\dagger$. Most of our block encodings are not Hermitian, making this necessary for correctness. The total number of invocations of $\mathcal{B}[A]$ and its adjoint remains equal to the specified order of the Chebyshev polynomial.

There are no existing users of ChebyshevPolynomial to update.

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@fdmalone fdmalone left a comment

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Nice!

@charlesyuan314 charlesyuan314 enabled auto-merge (squash) July 30, 2024 18:55
@charlesyuan314 charlesyuan314 merged commit 22e5538 into quantumlib:main Jul 30, 2024
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@charlesyuan314 charlesyuan314 deleted the chebyshev branch July 30, 2024 20:01
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