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1. Deconvolving Contributions to Decoherence in Molecular Electron Spin Qubits: A Dynamic Ligand Field Approach

   https://doi.org/10.1002/chem.202100845  

   Mirzoyan, Ruben ; Kazmierczak, Nathanael P. ; Hadt, Ryan G. ( May 2021 , Chemistry – A European Journal)

   In the past decade, transition metal complexes have gained momentum as electron spin‐based quantum bit (qubit) candidates due to their synthetic tunability and long achievable coherence times. The decoherence of magnetic quantum states imposes a limit on the use of these qubits for quantum information technologies, such as quantum computing, sensing, and communication. With rapid recent development in the field of molecular quantum information science, a variety of chemical design principles for prolonging coherence in molecular transition metal qubits have been proposed. Here the spin‐spin, motional, and spin‐phonon regimes of decoherence are delineated, outlining design principles for each. It is shown how dynamic ligand field models can provide insights into the intramolecular vibrational contributions in the spin‐phonon decoherence regime. This minireview aims to inform the development of molecular quantum technologies tailored for different environments and conditions.

    

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2. Quantum communication with itinerant surface acoustic wave phonons

   https://doi.org/10.1038/s41534-021-00511-1  

   Dumur, É. ; Satzinger, K. J. ; Peairs, G. A. ; Chou, M. -H. ; Bienfait, A. ; Chang, H. -S. ; Conner, C. R. ; Grebel, J. ; Povey, R. G. ; Zhong, Y. P. ; et al ( December 2021 , npj Quantum Information)

   Surface acoustic waves are commonly used in classical electronics applications, and their use in quantum systems is beginning to be explored, as evidenced by recent experiments using acoustic Fabry–Pérot resonators. Here we explore their use for quantum communication, where we demonstrate a single-phonon surface acoustic wave transmission line, which links two physically separated qubit nodes. Each node comprises a microwave phonon transducer, an externally controlled superconducting variable coupler, and a superconducting qubit. Using this system, precisely shaped individual itinerant phonons are used to coherently transfer quantum information between the two physically distinct quantum nodes, enabling the high-fidelity node-to-node transfer of quantum states as well as the generation of a two-node Bell state. We further explore the dispersive interactions between an itinerant phonon emitted from one node and interacting with the superconducting qubit in the remote node. The observed interactions between the phonon and the remote qubit promise future quantum-optics-style experiments with itinerant phonons.

    

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3. Super-resolved time–frequency measurements of coupled phonon dynamics in a 2D quantum material

   https://doi.org/10.1038/s41598-022-22055-w  

   Gentry, Christian ; Liao, Chen-Ting ; You, Wenjing ; Ryan, Sinéad A. ; Varner, Baldwin Akin ; Shi, Xun ; Guan, Meng-Xue ; Gray, Thomas ; Temple, Doyle ; Meng, Sheng ; et al ( November 2022 , Scientific Reports)

   Methods to probe and understand the dynamic response of materials following impulsive excitation are important for many fields, from materials and energy sciences to chemical and neuroscience. To design more efficient nano, energy, and quantum devices, new methods are needed to uncover the dominant excitations and reaction pathways. In this work, we implement a newly-developed superlet transform—a super-resolution time-frequency analytical method—to analyze and extract phonon dynamics in a laser-excited two-dimensional (2D) quantum material. This quasi-2D system, 1T-TaSe₂, supports both equilibrium and metastable light-induced charge density wave (CDW) phases mediated by strongly coupled phonons. We compare the effectiveness of the superlet transform to standard time-frequency techniques. We find that the superlet transform is superior in both time and frequency resolution, and use it to observe and validate novel physics. In particular, we show fluence-dependent changes in the coupled dynamics of three phonon modes that are similar in frequency, including the CDW amplitude mode, that clearly demonstrate a change in the dominant charge-phonon couplings. More interestingly, the frequencies of the three phonon modes, including the strongly-coupled CDW amplitude mode, remain time- and fluence-independent, which is unusual compared to previously investigated materials. Our study opens a new avenue for capturing the coherent evolution and couplings of strongly-coupled materials and quantum systems.

    

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4. Emergent interface vibrational structure of oxide superlattices

   https://doi.org/10.1038/s41586-021-04238-z  

   Hoglund, Eric R. ; Bao, De-Liang ; O’Hara, Andrew ; Makarem, Sara ; Piontkowski, Zachary T. ; Matson, Joseph R. ; Yadav, Ajay K. ; Haislmaier, Ryan C. ; Engel-Herbert, Roman ; Ihlefeld, Jon F. ; et al ( January 2022 , Nature)

   Abstract As the length scales of materials decrease, the heterogeneities associated with interfaces become almost as important as the surrounding materials. This has led to extensive studies of emergent electronic and magnetic interface properties in superlattices 1–9 . However, the interfacial vibrations that affect the phonon-mediated properties, such as thermal conductivity 10,11 , are measured using macroscopic techniques that lack spatial resolution. Although it is accepted that intrinsic phonons change near boundaries 12,13 , the physical mechanisms and length scales through which interfacial effects influence materials remain unclear. Here we demonstrate the localized vibrational response of interfaces in strontium titanate–calcium titanate superlattices by combining advanced scanning transmission electron microscopy imaging and spectroscopy, density functional theory calculations and ultrafast optical spectroscopy. Structurally diffuse interfaces that bridge the bounding materials are observed and this local structure creates phonon modes that determine the global response of the superlattice once the spacing of the interfaces approaches the phonon spatial extent. Our results provide direct visualization of the progression of the local atomic structure and interface vibrations as they come to determine the vibrational response of an entire superlattice. Direct observation of such local atomic and vibrational phenomena demonstrates that their spatial extent needs to be quantified to understand macroscopic behaviour. Tailoring interfaces, and knowing their local vibrational response, provides a means of pursuing designer solids with emergent infrared and thermal responses. 

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5. Enhanced hot electron lifetimes in quantum wells with inhibited phonon coupling

   https://doi.org/10.1038/s41598-018-30894-9  

   Esmaielpour, Hamidreza ; Whiteside, Vincent R. ; Piyathilaka, Herath P. ; Vijeyaragunathan, Sangeetha ; Wang, Bin ; Adcock-Smith, Echo ; Roberts, Kenneth P. ; Mishima, Tetsuya D. ; Santos, Michael B. ; Bristow, Alan D. ; et al ( August 2018 , Scientific Reports)

   Hot electrons established by the absorption of high-energy photons typically thermalize on a picosecond time scale in a semiconductor, dissipating energy via various phonon-mediated relaxation pathways. Here it is shown that a strong hot carrier distribution can be produced using a type-II quantum well structure. In such systems it is shown that the dominant hot carrier thermalization process is limited by the radiative recombination lifetime of electrons with reduced wavefunction overlap with holes. It is proposed that the subsequent reabsorption of acoustic and optical phonons is facilitated by a mismatch in phonon dispersions at the InAs-AlAsSb interface and serves to further stabilize hot electrons in this system. This lengthens the time scale for thermalization to nanoseconds and results in a hot electron distribution with a temperature of 490 K for a quantum well structure under steady-state illumination at room temperature.

    

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