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1. Direct triplet sensitization of oligothiophene by quantum dots

   https://doi.org/10.1039/C9SC01648A  

   Xu, Zihao; Jin, Tao; Huang, Yiming; Mulla, Karimulla; Evangelista, Francesco A.; Egap, Eilaf; Lian, Tianquan (January 2019, Chemical Science)

   Effective sensitization of triplet states is essential to many applications, including triplet–triplet annihilation based photon upconversion schemes. This work demonstrates successful triplet sensitization of a CdSe quantum dot (QD)–bound oligothiophene carboxylic acid (T6). Transient absorption spectroscopy provides direct evidence of Dexter-type triplet energy transfer from the QD to the acceptor without populating the singlet excited state or charge transfer intermediates. Analysis of T6 concentration dependent triplet formation kinetics shows that the intrinsic triplet energy transfer rate in 1 : 1 QD–T6 complexes is 0.077 ns −1 and the apparent transfer rate and efficiency can be improved by increasing the acceptor binding strength. This work demonstrates a new class of triplet acceptor molecules for QD-based upconversion systems that are more stable and tunable than the extensively studied polyacenes. 

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2. Bidirectional triplet exciton transfer between silicon nanocrystals and perylene

   https://doi.org/10.1039/d1sc00311a  

   Huang, Tingting; Koh, Timothy T.; Schwan, Joseph; Tran, Tiffany T.-T.; Xia, Pan; Wang, Kefu; Mangolini, Lorenzo; Tang, Ming L.; Roberts, Sean T. (May 2021, Chemical Science)

   null (Ed.)

   Hybrid materials comprised of inorganic quantum dots functionalized with small-molecule organic chromophores have emerged as promising materials for reshaping light's energy content. Quantum dots in these structures can serve as light harvesting antennas that absorb photons and pass their energy to molecules bound to their surface in the form of spin-triplet excitons. Energy passed in this manner can fuel upconversion schemes that use triplet fusion to convert infrared light into visible emission. Likewise, triplet excitons passed in the opposite direction, from molecules to quantum dots, can enable solar cells that use singlet fission to circumvent the Shockley–Queisser limit. Silicon QDs represent a key target for these hybrid materials due to silicon's biocompatibility and preeminence within the solar energy market. However, while triplet transfer from silicon QDs to molecules has been observed, no reports to date have shown evidence of energy moving in the reverse direction. Here, we address this gap by creating silicon QDs functionalized with perylene chromophores that exhibit bidirectional triplet exciton transfer. Using transient absorption, we find triplet transfer from silicon to perylene takes place over 4.2 μs while energy transfer in the reverse direction occurs two orders of magnitude faster, on a 22 ns timescale. To demonstrate this system's utility, we use it to create a photon upconversion system that generates blue emission at 475 nm using photons with wavelengths as long as 730 nm. Our work shows formation of covalent linkages between silicon and organic molecules can provide sufficient electronic coupling to allow efficient bidirectional triplet exchange, enabling new technologies for photon conversion. 

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3. Floquet-enhanced spin swaps

   https://doi.org/10.1038/s41467-021-22415-6  

   Qiao, Haifeng; Kandel, Yadav P.; Dyke, John S.; Fallahi, Saeed; Gardner, Geoffrey C.; Manfra, Michael J.; Barnes, Edwin; Nichol, John M. (December 2021, Nature Communications)

   null (Ed.)

   Abstract The transfer of information between quantum systems is essential for quantum communication and computation. In quantum computers, high connectivity between qubits can improve the efficiency of algorithms, assist in error correction, and enable high-fidelity readout. However, as with all quantum gates, operations to transfer information between qubits can suffer from errors associated with spurious interactions and disorder between qubits, among other things. Here, we harness interactions and disorder between qubits to improve a swap operation for spin eigenstates in semiconductor gate-defined quantum-dot spins. We use a system of four electron spins, which we configure as two exchange-coupled singlet–triplet qubits. Our approach, which relies on the physics underlying discrete time crystals, enhances the quality factor of spin-eigenstate swaps by up to an order of magnitude. Our results show how interactions and disorder in multi-qubit systems can stabilize non-trivial quantum operations and suggest potential uses for non-equilibrium quantum phenomena, like time crystals, in quantum information processing applications. Our results also confirm the long-predicted emergence of effective Ising interactions between exchange-coupled singlet–triplet qubits. 

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4. A high-spin ground-state donor-acceptor conjugated polymer

   https://doi.org/10.1126/sciadv.aav2336  

   London, A. E.; Chen, H.; Sabuj, M. A.; Tropp, J.; Saghayezhian, M.; Eedugurala, N.; Zhang, B. A.; Liu, Y.; Gu, X.; Wong, B. M.; et al (May 2019, Science Advances)

   Interest in high-spin organic materials is driven by opportunities to enable far-reaching fundamental science and develop technologies that integrate light element spin, magnetic, and quantum functionalities. Although extensively studied, the intrinsic instability of these materials complicates synthesis and precludes an understanding of how fundamental properties associated with the nature of the chemical bond and electron pairing in organic materials systems manifest in practical applications. Here, we demonstrate a conjugated polymer semiconductor, based on alternating cyclopentadithiophene and thiadiazoloquinoxaline units, that is a ground-state triplet in its neutral form. Electron paramagnetic resonance and magnetic susceptibility measurements are consistent with a high-to-low spin energy gap of 9.30 × 10 −3 kcal mol −1 . The strongly correlated electronic structure, very narrow bandgap, intramolecular ferromagnetic coupling, high electrical conductivity, solution processability, and robust stability open access to a broad variety of technologically relevant applications once thought of as beyond the current scope of organic semiconductors. 

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5. Adiabatic quantum state transfer in a semiconductor quantum-dot spin chain

   https://doi.org/10.1038/s41467-021-22416-5  

   Kandel, Yadav P.; Qiao, Haifeng; Fallahi, Saeed; Gardner, Geoffrey C.; Manfra, Michael J.; Nichol, John M. (December 2021, Nature Communications)

   null (Ed.)

   Abstract Semiconductor quantum-dot spin qubits are a promising platform for quantum computation, because they are scalable and possess long coherence times. In order to realize this full potential, however, high-fidelity information transfer mechanisms are required for quantum error correction and efficient algorithms. Here, we present evidence of adiabatic quantum-state transfer in a chain of semiconductor quantum-dot electron spins. By adiabatically modifying exchange couplings, we transfer single- and two-spin states between distant electrons in less than 127 ns. We also show that this method can be cascaded for spin-state transfer in long spin chains. Based on simulations, we estimate that the probability to correctly transfer single-spin eigenstates and two-spin singlet states can exceed 0.95 for the experimental parameters studied here. In the future, state and process tomography will be required to verify the transfer of arbitrary single qubit states with a fidelity exceeding the classical bound. Adiabatic quantum-state transfer is robust to noise and pulse-timing errors. This method will be useful for initialization, state distribution, and readout in large spin-qubit arrays for gate-based quantum computing. It also opens up the possibility of universal adiabatic quantum computing in semiconductor quantum-dot spin qubits. 

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