Dmitry Grinko

Symmetry plays a crucial role in the design and analysis of quantum protocols. This result shows a canonical circuit decomposition of a quantum comb with $G\times H$ symmetry for compact groups $G$ and $H$ using the corresponding Clebsch--Gordan transforms. By using this circuit decomposition, we propose a parametrized quantum comb with group symmetry, and derive the optimal quantum comb which transforms an unknown unitary operation $U\in \SU(d)$ to its inverse $U^\dagger$ or transpose $U^\mathsf{T}$. From numerics, we find a deterministic and exact unitary transposition protocol for $d=3$ with $7$ queries to $U$, which is improved over the protocol shown in [Y.-A. Chen et al., arXiv:2403.04704], which requires $13$ queries to $U$. We also provide the simulation of random unitaries for any compact group $G$ using the compressed oracle, which can be implemented efficiently for the unitary group. The precision of our simulation for the unitary group is improved over the path-recording oracle introduced in [F. Ma and H.-Y. Huang, arXiv:2410.10116].

Port-based teleportation (PBT) is a variant of quantum teleportation that, unlike the canonical protocol by Bennett et al., does not require a correction operation on the teleported state. Since its introduction by Ishizaka and Hiroshima in 2008, no efficient implementation of PBT was known. We close this long-standing gap by building on our recent results on representations of partially transposed permutation matrix algebras and mixed quantum Schur transform. We construct efficient quantum algorithms for probabilistic and deterministic PBT protocols on n ports of arbitrary local dimension, both for EPR and optimized resource states. We describe two constructions based on different encodings of the Gelfand-Tsetlin basis for n qudits: a standard encoding that achieves O(n) time and O(nlog(n)) space complexity, and a Yamanouchi encoding that achieves O(n^2) time and O(log(n)) space complexity, both for constant local dimension and target error. We also describe efficient circuits for preparing the optimal resource states.

The quantum analogue of the equality function, known as the quantum state identity problem, is the task of deciding whether n unknown quantum states are equal or unequal, given the promise that all states are either pairwise orthogonal or identical. Under the one-sided error requirement, it is known that the permutation test is optimal for this task, and for two input states this coincides with the well-known Swap test. Until now, the optimal measurement in the general two-sided error regime was unknown. Under more specific promises, the problem can be solved approximately or even optimally with simpler tests, such as the circle test. This work attempts to capture the underlying structure of (fine-grained formulations of) the quantum state identity problem. Using tools from semi-definite programming and representation theory, we (i) give an optimal test for any input distribution without the one-sided error requirement by writing the problem as an SDP, giving the exact solutions to the primal and dual programs and showing that the two values coincide; (ii) propose a general G-test which uses an arbitrary subgroup G of S_n, giving an analytic expression of the performance of the specific test, and (iii) give an approximation of the permutation test using only a classical permutation and n−1 Swap tests.