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Analyzing Quantum Boolean Circuits: Complexity of Generalized-CNOT Manipulations

This work explores the intricacies of manipulating Quantum Boolean Circuits (QBCs) formed by Generalized-CNOT gates, which feature one target and multiple controls. We delve into the architectural components of quantum algorithms, including fixed quantum elements and uniform superposition techniques, like Hadamards. The study highlights local transformation rules proposed by Iwama et al., discussing their universality and implications for circuit optimization. Additionally, we examine the computational complexity of determining circuit identities, revealing that certain transformations could require exponentially many steps.

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Analyzing Quantum Boolean Circuits: Complexity of Generalized-CNOT Manipulations

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  1. On the Complexity of Manipulating Quantum Boolean Circuits Vincent Liew

  2. Generalized-CNOT Circuits • Generalized-CNOT gates have one target, multiple controls • We call a circuit composed of these gates is a Quantum Boolean Circuit (QBC)

  3. Why Quantum Boolean Circuits? • Quantum algorithms usually have two components: • Quantum (fixed for each instance) • Uniform superposition (Hadamards) • Quantum Fourier Transform • Boolean Oracle (specific to instance of problem) • Indicator function in Grover

  4. Local Transformation Rules (1) • Iwama et al. gave a set of local transformation rules for QBCs • Finitely many rules • Each rule is an identity on a bounded number of gates • Useful for heuristic optimization

  5. Local Transformation Rules (2) • Easy commutes:

  6. Local Transformation Rules (3) • Harder commuting:

  7. Universality via Canonical Form • Iwama et al. proved that these (with a few more) rules are universal. • Every circuit with one “work-qubit” may be transformed into canonical form.

  8. TOFFLIDENTITY • Given a circuit with one-control-CNOT and Toffoli, is it the identity? • Recently proven CoNP-complete • Should require exponentially many transformations to get identity. Can we prove this?

  9. An Explicit Circuit • We have a circuit which requires exponentially many steps for Iwama’s transformation procedure:

  10. Forced to Commute • After commuting one CNOT over, the middle looks like (after rearranging): Original Middle “Copied” Middle

  11. A Rough Calculation • Each CNOT we move over makes a partial “copy” of the middle section. • Assume that nothing created will cancel in the middle.

  12. What Have We Learned? • Conjecture: A similar circuit family will exhibit exponentiality in general. • How do we show no polynomial series of transforms can do it? • The difficulty of deciding CoNPvsNP allowed us to predict that Iwama’s procedure might take exponentially many steps in general.

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