A Menagerie of Symmetry Testing Quantum Algorithms

Abstract

This thesis aims to establish notions of symmetry for quantum states and channels as well as describe algorithms to test for these properties on quantum computers. Ideally, the work will serve as a self-contained overview of the subject. We begin by establishing the necessary mathematical background. We show how to generate a notion of symmetry from a discrete, finite group and how this generalizes to a continuous group. We then use these notions to investigate Hamiltonian symmetries. We propose quantum algorithms capable of testing whether a Hamiltonian exhibits symmetry with respect to a group and show that this algorithm is DQC1-Complete. We next discuss tests of symmetry for quantum states. We prove that the acceptance probability of each algorithm is equal to the maximum symmetric fidelity of the state being tested and establish various generalizations of the resource theory of asymmetry. In the next chapter, we show that the analytical form of the acceptance probability of such a test is given by the cycle index polynomial of the symmetric group $S_k$. We derive a family of quantum separability tests, each of which is generated by a finite group; for all such algorithms, we show that the acceptance probability is determined by the cycle index polynomial of the group. Finally, we produce and analyze explicit circuit constructions for these tests, showing that the tests corresponding to the symmetric and cyclic groups can be executed with $O(k^2)$ and $O(k\log(k))$ controlled-SWAP gates, respectively, where $k$ is the number of copies of the state. Finally, we include additional results not previously published; specifically, we give a test for symmetry of a quantum state using density matrix exponentiation, a further result of Hamiltonian symmetry measurements when using Abelian groups, and an alternate Hamiltonian symmetry test construction for a block-encoded Hamiltonian.