Session S72: Molecular Qubits and Techniques

Molecular Color Centers

Within the realm of quantum information science there are numerous applications that would benefit from tunable atomically precise qubits. Molecules offer a pathway to create systems which can be modified towards specific targets. For example, molecules can be tuned for biocompatibility for quantum sensing applications. Research on developing molecular pathways towards quantum information science will be presented.    

Electronic Structure and Decoherence in Optically Addressable Cr(IV)-based Magnetic Molecules

Great success of point defects in wide-gap semiconductors for quantum information science applications stimulated extensive search for molecules with analogous properties. Recently, molecular spin states of a family of mononuclear Cr(IV)-based molecules have been shown to be initialized and read-out via optical transitions just like a deep nitrogen-vacancy defect in diamond. So far, decoherence mechanisms of the molecular spin states have not been discussed. Here, we investigate the electronic structure of the family of Cr(IV)-based molecules, by employing ab initio multireference quantum chemistry methods including spin-orbit coupling. We compute the excitation energy between the ground-state spin triplet and first-excited spin singlet as well as zero-field splitting of the triplet. Our results are compared to experimental data. As possible sources of spin decoherence, we calculate the magnetic hyperfine and nuclear quadrupole interaction parameters for the ⁵³Cr nucleus within the multireference methods. Furthermore, we study the interactions between the ligand ¹³C or ¹H nucleus and the molecular electronic spin for all ligand C and H sites of the molecules.   

Increase in Spin-Phonon Coupling in Molecular Qubit CuPc from Substrate Effects

Molecular qubits are a promising platform for quantum information systems (QIS). It is understood that to function in devices, regular arrays of qubits supported by a substrate are needed. The substrate imposes mechanical and electronic boundary conditions on the molecule, however the impact of these effects on spin lattice relaxation times (T₁) is not well understood. We perform electronic structure calculations to assess the impact of a graphene substrate (C_gr) on the molecular qubit copper phthalocyanine (CuPc). We separate the impacts arising from the mechanical boundary and electronic boundary conditions of the substrate on the spin-phonon coupling (SPC) of CuPc. Then we use a simple thermal model to predict the impact of the changes in SPC from 0-300K. Our analysis of the vibrational modes with and without C_gr shows that the character and amplitude of the modes that impact T₁ changes, leading to an overall increase in SPC with the surface. We explain these changes by examining how the presence of the substrate alters the symmetry of CuPc. Our work shows a surface can have a large impact on SPC and that ways to reduce this coupling need to be found to fully utilize arrays of molecular qubits.   

Host Matrix Engineering of Optically Addressable Molecular Qubits

Optically addressable molecular spin qubits show promise as tunable, portable, and scalable platforms for quantum sensing [1]. Chemical synthesis methods provide opportunities for bottom-up design of these qubits, a challenge in solid-state spins and quantum dots. In this work, we demonstrate the enhancement of spin coherence and optical contrast properties of chromium (IV)-based molecular qubits through engineering of the host matrix. We detect multiple orientations of the chromium (IV) site through optically detected magnetic resonance. We also measure the optical linewidth of these qubits through all-optical spin sublevel control. We demonstrate improved spin coherence in a non-isostructural host matrix with a significant transverse zero-field splitting which leads to energy levels robust to magnetic-field noise. We model the coherence properties using cluster correlation expansion methods and demonstrate agreement with experimental coherence measurements for four different molecular qubits with varying transverse zero-field splitting values. These host-matrix-engineered molecular systems provide an avenue for coherence-protected sensing in noisy environments.   

Improved spin coherence times of the singlet-triplet system in a self-assembled quantum dot molecule

In a doubly-charged quantum dot molecule, singlet and triplet ground state energies are split by the electron-electron exchange interaction. At the sweet spot, the system is largely decoupled from both magnetic and electric fluctuations, similar to atomic-clock transitions. We directly measure the T₂* coherence time in this system by modulating the laser field with a programmable microwave sequence and perform electron spin rotation via two-photon Raman process. We show that at the sweet spot, the coherence between singlet (S) and triplet (T₀) is dramatically enhanced, reaching tens of nanoseconds, an order of magnitude higher than that of an electron spin in a single quantum dot. Long coherence times open up the feasibility of using self-assembled quantum dot molecules in quantum information application such as photonic cluster state generation.   

Spin Seebeck Effect and Magnon Diffusion Length in Vanadium Tetracyanoethylene (V[TCNE]x, x ~ 2)

V[TCNE]_x is an organic-based ferrimagnetic semiconductor that has rapidly attracted attention for applications in magnon-based quantum information systems due to its low-loss (high-Q) magnetic excitations (α~4×10^-5, Q>7,000) rivaling those of the gold standard material, YIG. Low-damping V[TCNE]_x microstructures are predicted to couple solid state spins over micron distances, establishing the foundation for on-chip interconnected solid state qubits coherently coupled via magnonic excitations. Despite these attractive prospects, a thorough understanding of magnonic transport in V[TCNE]_x is lacking. Here, we present systematic longitudinal spin Seebeck effect (LSSE) studies on V[TCNE]_x, along with a theroetical model based on the bulk magnon spin current produced by a temperature gradient in the devices, allowing us to extract information regarding magnonic transport in V[TCNE]_x. Such information (e.g. magnon DOS, magnon-phonon coupling, magnon thermalization) is critical for understanding the highly coherent magnons in this system. For example, through these measurements we establish an initial lower bound for the magnon diffusion length of 1 micron in V[TCNE]_x at room temperature, competitive with similar studies in single-crystal YIG.   

Controlled Color Centers in SP2 Carbon: Defect Characterization Using Resonant Raman Spectroscopy

Fully controllable materials for single photon emission (SPE) are sought, especially in the telecom region. Single-wall carbon nanotubes (SWCNTs) have exact and unique crystal structures. Every SWCNT of the same (n,m), a vector which describes the angle of the graphene lattice and defines the cylinder diameter, has identical optoelectronic properties, e.g., optical bandgap, in the absence of extrinsic effects. A key development is the addition of single-point covalent modifications to the sp2 lattice to introduce atomic-scale color center defects. These defects localize emission, vastly increase quantum yield, and enable fine spectral tuning through a wide range of chemical modifications. Using highly purified, single-chirality sample sets wrapped with DNA, we have developed a modified chemistry to produce distinctly defective lattice structures. Resonance Raman spectroscopy identified unique features beyond the common “D peak” to characterize the defect types.     

Electronic structure of the InSb-CdTe-αSn interface: using CdTe as a barrier

The discovery and design of new inorganic interfaces with desirable properties for semiconductor, spintronic, and quantum devices offers a path to new device functionalities as well as improved performance of electronic circuits. We study the InSb-CdTe-αSn interface via density functional theory (DFT).  InSb is the backbone of Majorana devices for topological quantum computing and Sn is a superconductor that is utilized in Majorana devices. Though superconductivity is observed in the beta phase of Sn, α-Sn is explored here because the materials are lattice matched. CdTe is explored as a passivation layer and tunnel barrier material. The PBE+U method is used, with the Hubbard U parameters found via a machine-learned Bayesian optimization algorithm, which allowed the simulation of large interfaces. We discuss the results of DFT simulations considering the band offsets at the interfaces and the effects of varying the CdTe thickness. We show that once 16 layers of CdTe have been inserted this acts as an effective barrier.   

Broad-band spectroscopy of electronuclear spin qudits based on vanadyl porphyrin molecules

The possibility of encoding several qubits in vanadyl porphyrin molecules hosting a S = 1/2 electronic spin coupled to a I = 7/2 nuclear spin has been explored. A complete study of the spin Hamiltonian and the spin dynamics has been performed via a combination of electron paramagnetic resonance, heat capacity, magnetization and on-chip magnetic spectroscopy experiments performed on single crystals, observing low temperature T₂ of micro-seconds and T₁ longer than a second. For sufficiently strong magnetic fields (B > 0.1 T, corresponding to resonance frequencies of 9–10 GHz) these properties make vanadyl porphyrin molecules suitable qubit realizations. For lower magnetic fields (B < 0.1 T), and lower frequencies (< 2 GHz), spectroscopic signatures of a sizeable electronuclear entanglement arise. This effect generates a larger set of allowed transitions between different electronuclear spin states and removes their degeneracies. Under these conditions, each molecule fulfils the conditions to act as a universal 4-qubit processor or, equivalently, as a d = 16 qudit. These findings widen the catalogue of chemically designed systems able to implement non-trivial quantum functionalities, such as quantum simulations and quantum error correction at the molecular level.   

Lumped element superconducting resonators for electrical control and read-out of molecular spin qubits

Paramagnetic molecular spin systems strongly coupled to superconducting resonators are promising building blocks for hybrid quantum processors. However, the control of specific operations remains a challenge as the spin coherence times are not yet sufficiently long as compared with the coupling rate. Our approach is to electrically control and read-out the spin state of a high-spin (S = 5/2) paramagnetic manganese(II) molecules embedded in its polar piezoelectric diamagnetic Zn(II) host lattice. We have developed lumped-element resonators (LERs) that, through the resonant microwave fields, modify the local ion coordination modulating the magnetic anisotropy that determines the spin qubit Hamiltonian and transition rates. The use of superconducting microwave resonators implies low losses, high-quality factors and multiplexability which are crucial for this application permitting the simultaneous readout of several qubits. M. C. de Ory and S. Roca contributed equally to this work.   

Entanglement of magnon excitations with antiferromagnetic coupling

Quantum entanglement as a major non-classical resource naturally comes about in any quantum technology protocol, including quantum magnonics. We discuss how an antiferromagnetic coupling leads to experimentally detectable bipartite continuous variable entanglement between two ferromagnetic magnon modes [1, 2]. We then present a clear relation between this entanglement and Einstein, Podolsky, and Rosen non-locality. Based on this relation a practical experimental setup for detecting the EPR function of the ground state as well as the magnon-magnon entanglement will be discussed. The setting relies on magnon-photon interaction in a microwave cavity [2].

[1] V. Azimi-Mousolou, et al., Hierarchy of magnon mode entanglement in antiferromagnets, Phys. Rev. B 102, 224418 (2020).

[2] V. Azimi-Mousolou, et al., Magnon-magnon entanglement and its detection in a microwave cavity, (2021). arXiv:2106.06862     

Chemical tuning and on-chip tranmission experiments of clock transitions in molecular spin qubits

We report a sizeable quantum tunnelling splitting for the mononuclear Ni(II) molecular complexes [Ni(Me₆tren)Cl](ClO₄) (1) and [Ni(2-Imdipa)(NCS)](NCS) (2). With their S = 1 ground state and strong anisotropy, these molecules provide a realization of the simplest non-Kramers system (integer spin). The “clock transition” between levels associated with superpositions of m_S = ±1 spin states, with its characteristic non-linear magnetic field dependence, has been directly monitored by heat capacity experiments. The comparison of complex 1 with a Co derivative (S = 3/2), for which tunnelling is forbidden, shows that the clock transition leads to an effective suppression of intermolecular spin–spin interactions. We also show that the splitting admits a chemical tuning via the modification of the ligand shell that determines the magnetic anisotropy. In particular, the weaker magnetic anisotropy of complex 2 makes its qubit frequency compatible with superconducting microwave circuits, and has allowed its direct detection by on-chip broadband transmission experiments.