Session Z68: Molecular Spins for Next Generation Quantum Technologies

Exploring chirality for spin control in quantum information

The interplay between structural chirality and magnetism is at the origin of many fascinating phenomena, and molecular materials are very promising candidates. For instance, giant magneto-chiral dichroism has been observed on a molecular helix^. As far as control is concerned, chiral materials can exhibit a linear spin-electric field with the possibility to control magnetic anisotropy and, more exotically, magnetic exchange² by means of an electric field. Electric fields have been also employed to promote electron transport through a chiral structure grafted on a  metallic surface, thus providing evidence of a spin-filtering mechanism without the need for a magnetic field. Such phenomenon, known as chirality-induced spin selectivity, CISS,³ works also at high temperatures, a significant advantage for spin-based quantum information and other applications.  To fully harness the potential of CISS we have started to investigate the CISS phenomenon at the intramolecular level by using magnetic resonance techniques to directly quantify the spin polarization. We have recently proposed experiments⁴ to be performed on a donor-acceptor dyad linked by a chiral bridge, where the electron transfer (ET) is promoted by light excitation of the donor or acceptor. The effect of spin-filtering in the ET can be detected by time-resolved EPR spectroscopy.⁵ In addition, microwave pulses can be used to transfer the chirality-induced spin polarization to a molecular spin qubit,  thus enabling an innovative control mechanism.

 

1          Sessoli, R., et al. , A. Strong magneto-chiral dichroism in a paramagnetic molecular helix observed by hard X-rays. Nat. Phys. 11, 69-74 (2015).

2          Fittipaldi, M., et al.  Electric field modulation of magnetic exchange in molecular helices. Nat. Mater. 18, 329-334 (2019).

3          Naaman, R., Paltiel, Y. & Waldeck, D. H. Chiral molecules and the electron spin. Nat. Rev. Chem. 3, 250-260 (2019).

4          Chiesa, A., et al.  Assessing the Nature of Chiral-Induced Spin Selectivity by Magnetic Resonance. J. Phys. Chem. Lett. 12, 6341-6347 (2021).

5          Privitera, A., et al. . Direct detection of spin polarization in photoinduced charge transfer through a chiral bridge. Chem. Sci. 13, 12208 - 12218 (2022).   

Using EPR to Guide the Engineering of Organometalic Molecular Qubits

Spin coherence enables magnetic molecules to function as qubits [1]. The lifetime of the qubit must be suficiently long to enable coherent spin manipulations at convenient operational temperatures [1,2]. One impediment is the occurrence of quantum decoherence, which limits the qubit operational lifetime. Inorporation of ligands that provide nuclear spin-free environments into molecules as well as operating at atomic clock transitions are two of the most improtant strategies used nowadays to overcome this problem.  In this talk we show that sufficiently long phase memory times allowing quantum spin manipulations even at ambient temperature are achievable in low-valent molecular systems with either C₃ or C₄ symmetry, even when they are rich in nuclear spins.  Examples include low-valent lanthanide and transition metal complexes bearing cyclopentadinyl derivatives as ligands, aryloxides or imides. The exceptionally long coherence times of thes esystems enabled mapping the spin densities that infiorm on decoherence path, with the aid of pulsed EPR methods: HYSCORE, ESEEM, ENDOR.  For LnCp’₃K (Ln = Sc, Y, La or Lu), coherent Rabi oscillations were measured, including at 300 K in a single crystal [2]. Ac magnetic susceptibility data confirmed slow magnetic relaxation at low temperatures, associated with very long spin lattice relaxation times, which could be nicely characterised by pulsed EPR methods. Despite many ¹H and ¹³C nuclei present in these molecules, all electronuclear transitions of the compounds enabled to be coherently manipulated, indicative that the qubit memory time is protected in these systems due to a reduced orbital angular momentum of the ground spin state.

[1] S. McAdams, A. M. Ariciu, A. Kostopulos, J. Walsh, F. Tuna, Coord. Chem. Rev. 2017, 346, 216-339.

[2] A. M. Ariciu, D. H. Woen, D. N. Huh, A. K. Kostopoulos, C. A. P. Goodwin, N. F. Chilton, E. J. L. McInnes, R. E. P. Winpenny, W. J. Evans, F. Tuna, Nature Commun. 2019, 10, 3330.

    

Emulation of Majorana zero modes in molecular magnets

Majorana zero modes (MZMs) have garnered interest owing to their non-Abelian exchange statistics and potential application for quantum computing. While theoretical proposals have suggested that MZMs could be realized in heterostructures of superconductors and semiconductors, definitive detection of MZMs remains inconclusive. Rather than realizing MZMs in electronic systems, several proposals have suggested emulation of MZMs in quantum spins.

In this talk, I will discuss a suite of molecular magnets, modeled as coupled quantum spins, which emulate MZMs. Taking advantage of various transformations from spins to fermions, I show that MZMs can be effectively mimicked in a Co trimer, Blume-Capel models, and (partial) Kitaev lattices. While topological protection of MZMs is typically lost, some of the spectacular properties for which MZMs are famous persist in these spin systems. In particular, I demonstrate the analogue of braiding MZMs in molecular magnets and discuss how it can be leveraged to perform some quantum computing operations.    

Molecular Clock Qubits

This talk will focus on recent efforts aimed at protecting molecular spin qubits from decoherence caused by surrounding electron and nuclear spins (the spin bath), with emphasis on so-called clock transitions – avoided level crossings associated with the Zeeman splitting of spin states [1]. Spin clock transitions provide an optimal operating point at which the transition frequency, f, becomes insensitive to the local magnetic field, B₀. In this way, a clock qubit is immune to magnetic noise [2]. There are several strategies for generating spin clock transitions. All that is needed is an interaction term in the spin Hamiltonian that does not commute with the Zeeman interaction. For molecules with integer spin states, zero-field splitting interactions do the job. Alternatively, clock transitions may be generated in molecules possessing half-integer spin states via the on-site hyperfine interaction, a strategy that is employed widely in trapped-ion quantum devices. Crucially, in the molecular case, the hyperfine interaction can be synthetically controlled to maximize unpaired electron spin density at the relevant nucleus. A recent example involving a Lu^II ([Xe]4f¹⁴5d¹) organometallic compound has demonstrated that this is possible by varying the degree of s-orbital mixing into the spin-bearing d-orbital [3]. This approach has the added advantage of increasing the s-orbital character, thus reducing spin-orbit coupling that, in turn, suppresses spin-lattice relaxation. The talk will provide an overview of various synthetic strategies that have been employed for developing molecular clock qubits, together with the spectroscopic studies that demonstrate their enhanced coherence. The findings are supported by exact quantum dynamics simulations that demonstrate decoupling of a spin qubit from the nuclear bath at a clock transition [4].

[1] Gaita-Ariño et al., Nat. Chem. 11, (2019).

[2] Shiddiq et al., Nature 531, 348 (2016).

[3] Kundu et al., Nat. Chem. 14, 392 (2022).

[4] Kundu et al., arXiv:2106.05185 [quant-ph].   

Coherent spin-electric control in a molecular nanomagnet at clock transitions

Magnetic fields are challenging to localise to short length scales because their sources are electrical currents. Conversely, the control of molecular spins using electric fields is particularly valuable for molecular spintronics because strong electric fields can easily be generated and shielded in within a small volume, allowing addressing of individual spin-carrying molecules in a device [1]. Recently, this has been demonstrated in several molecular magnets [2-4]. However, the spin-electric field couplings (SEC) for those molecules are relatively weak, raising the quest of exploring the pathways for enhancing SEC through molecular engineering.

Here we show that one strategy is to identify a molecule with a significant molecular electrical polarisability and a spin spectrum that is highly sensitive to a structural degree of freedom. We study a molecular nanomagnet, [Ho(W₅O₁₈)₂]^9- [5], in which a small structural distortion leads to a series of ESR clock transitions and a molecular electric dipole. This electric dipole allows us to control electrically the clock transition frequency to an unprecedented level. We demonstrate coherent electrical control of the molecular spin states and independently manipulation for the two magnetically indistinguishable inversion-related molecules in the same unit cell of the crystal [6]. These results pave the way for the use of molecular components in quantum or classical spintronic technologies in which local electrical control can surpass the performance of conventional magnetic spin control.

References:

[1] M. Trif, et al., Phys. Rev. Lett., 101, 217201 (2008)

[2] A. Boudalis, et al., Chemistry - A European Journal, 24, 14896 (2018)

[3] J. Liu, et al., Phys. Rev. Lett., 122, 037202 (2019)

[4] M. Fittipaldi, et al., Nat. Materials, 4, 329 (2019)

[5] M. Shiddiq, et al., Nature, 531, 348 (2016)

[6] J. Liu, et al., Nat. Physics, 17, 1205 (2021)