Bulletin of the American Physical Society
88th Annual Meeting of the Southeastern Section of the APS
Volume 66, Number 16
Thursday–Saturday, November 18–20, 2021; University Center Club, Florida State University, Tallahassee, Florida
Session E01: Quantum Information and Computing |
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Chair: Steve Hill, FSU Room: East Ballroom |
Thursday, November 18, 2021 2:00PM - 2:30PM |
E01.00001: ADAPT-QAOA: an adaptive quantum algorithm for optimization Invited Speaker: Yanzhu Chen In today’s quantum devices, decoherence sets limits on the circuit depth. Quantum-classical hybrid variational algorithms, which bypass the need for long quantum circuits by alternating between quantum computation and classical optimization, show promise for providing advantage over purely classical computing. In these algorithms, we need a suitably chosen ansatz to ensure the correct solution is reached or well approximated. To that end, an Adaptive Derivative-Assembled Problem-Tailored (ADAPT) protocol has been developed to tailor an ansatz to the problem. First proposed in the context of the variational quantum eigensolver [1], the ADAPT protocol constructs the ansatz iteratively, leading to a shallow quantum circuit and fast convergence [1,2]. One can apply the same approach to the quantum approximate optimization algorithm (QAOA), which was shown to significantly improve convergence of QAOA [3]. In this talk I will review the ADAPT approach, emphasizing its application to QAOA. Specifically, I will discuss the performance of ADAPT-QAOA in solving the Max-Cut problem, and I will examine the intermediate steps of ADAPT-QAOA and explore the role entanglement plays in this algorithm. [1] H. R. Grimsley, S. E. Economou, E. Barnes, N. J. Mayhall. Nat Commun 10, 3007 (2019). [2] H. L. Tang, V.O. Shkolnikov, G. S. Barron, H. R. Grimsley, N. J. Mayhall, E. Barnes, S. E. Economou. PRX QUANTUM 2, 020310 (2021). [3] L. Zhu, H. L. Tang, G. S. Barron, F. A. Calderon-Vargas, N. J. Mayhall, E. Barnes, S. E. Economou. arXiv:2005.10258. [Preview Abstract] |
Thursday, November 18, 2021 2:30PM - 3:00PM |
E01.00002: Rigorous demonstration of electron-nuclear decoupling at a spin clock transition Invited Speaker: Silas Hoffman The ability to design quantum systems that decouple from environmental noise sources is highly desirable for development of quantum technologies with optimal coherence. The chemical tunability of electronic states in magnetic molecules combined with advanced electron spin resonance techniques provides excellent opportunities to address this problem. Indeed, so-called clock transitions (CTs) have been shown to protect molecular spin qubits from magnetic noise, giving rise to significantly enhanced coherence. Here we conduct a spectroscopic and theoretical investigation of this physics, focusing on the role of the nuclear bath. Away from the CT, linear coupling to the nuclear degrees of freedom causes a modulation and decay of electronic coherence, as quantified via spin echo signals generated experimentally and in silico. Meanwhile, the effective electron-nuclear interaction vanishes upon approaching the CT, resulting in perfect decoupling and complete absence of quantum information leakage to the nuclear bath, providing opportunities to characterize other decoherence sources. [Preview Abstract] |
Thursday, November 18, 2021 3:00PM - 3:30PM |
E01.00003: Designing Two-Qubit Gates for Exchange-Only Quantum Computation Invited Speaker: Nick Bonesteel In exchange-only quantum computation, qubits are encoded using three or more spin-1/2 particles and quantum gates can be performed by switching on and off, or “pulsing”, the isotropic exchange interaction between spins. Finding efficient pulse sequences for realizing two-qubit gates in this way is complicated by the large search space of sequences and has typically involved numerical brute force search. Here I will give a simple analytic derivation of the most efficient known exchange-pulse sequence for carrying out a controlled-NOT gate [1], originally found numerically by Fong and Wandzura [2]. I will then show how the ideas behind this derivation can be used to analytically find new pulse sequences for two-qubit gates beyond controlled-NOT [3]. [1] D. Zeuch and N.E. Bonesteel, PRA 93, 010303 (2016). [2] B.H. Fong and S.M. Wandzura, Quantum Inf. Comput. 11, 1003 (2011). [3] D. Zeuch and N.E. Bonesteel, PRB 102, 075311 (2020). [Preview Abstract] |
Thursday, November 18, 2021 3:30PM - 3:42PM |
E01.00004: Massive 9 GHz Hyperfine Clock Transition in a Molecular Spin Qubit Krishnendu Kundu, Jessica White, Samuel Moehring, Jason Yu, Joseph Ziller, Filipp Furche, William Evans, Stephen Hill Spins in molecules have been proposed as potential qubits in quantum computers, enabling chemical tunability of their quantum nature and potential for scaleup via self-assembly. We demonstrate chemical control on the degree of s-orbital mixing into the spin-bearing d-orbital associated with a series of spin-\textonehalf La(II) and Lu(II) molecules. Increased s-orbital character reduces spin-orbit coupling and enhances the electron-nuclear Fermi contact interaction. In one particular Lu(II) complex, we have observed an enormous hyperfine interaction for a molecular system, Aiso $=$ 3467 MHz (more than 1200 G), which, in turn, generates a 9 GHz clock transition. The large magnitude of this hyperfine interaction necessitated high-field W-band EPR in order to fully characterize the electron-nuclear spin Hamiltonian parameters. Meanwhile, pulsed X-band EPR studies reveal an order of magnitude increase in phase memory time, Tm, at the 9 GHz clock transition. These findings suggest new strategies for the development of molecular quantum technologies, akin to trapped ion systems. [Preview Abstract] |
Thursday, November 18, 2021 3:42PM - 3:54PM |
E01.00005: Massive 116 GHz crystal-field clock transition in a tetragonal molecular Ho(III) complex Robert Stewart, Anna Celmina, Emma Regincós, Mark Murrie, Stephen Hill Molecular lanthanide complexes are promising candidates for development of next-generation quantum technologies [1]. High-symmetry structures can give rise to well-isolated crystal-field quasi-doublet ground states, i.e., quantum two-level systems that may serve as a basis for spin qubits. Recent work has shown that the coordination environment around the lanthanide can be tailored to produce an avoided crossing, or clock transition within the ground doublet, leading to significantly enhanced coherence times [2]. Here, we employ single-crystal high-frequency electron paramagnetic resonance (EPR) spectroscopy to interrogate a new molecular Ho(III) complex. An axial coordination environment with four-fold symmetry gives rise to a ground state $m_{J}$~$=$~\textpm 8 crystal-field quasi-doublet with a massive 116~GHz clock transition, where $m_{J}$ denotes the projection of the $J$~$=$~8 spin-orbital moment associated with the Ho(III) ion. These states are further split into eight (2$I$~$+$~1) sub-levels due to the hyperfine interaction with the $I$~$=$~$^{7}$/$_{2}$~nuclear spin. [1] Nat. Chem. 11, 301 -- 309 (2019); https://doi.org/10.1038/s41557-019-0232-y [2] Nature 531, 348 -- 351 (2016); \underline {https://doi.org/10.1038/nature16984} [Preview Abstract] |
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