Session V51: Materials for Quantum Information Science - 3 (Novel Materials Systems)

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The future global quantum internet will require high-performance matter-photon interfaces. The highly demanding technological requirements indicate that the matter-photon interfaces currently under study all have potentially unworkable drawbacks, and there is a global race underway to identify the best possible new alternative. For overwhelming commercial and quantum reasons, silicon is the best possible host for such an interface. Silicon is not only the most developed integrated photonics and electronics platform by far, isotopically purified silicon-28 has also set records for quantum lifetimes at both cryogenic and room temperatures [1]. Despite this, the vast majority of research into photon-spin interfaces has notably focused on visible-wavelength colour centres in other materials. In this talk I will introduce a variety of silicon colour centres and discuss their properties in isotopically purified silicon-28. Some of these centres have zero-phonon optical transitions in the telecommunications bands [2], some have long-lived spins in their ground states [3], and some, including the newly rediscovered T centre, have both [4].
[1] K. Saeedi, S. Simmons, J.Z. Salvail, et al. Science 342:830 (2013).
[2] C. Chartrand, L. Bergeron, K.J. Morse, et al. Phys. Rev. B 98:195201 (2018).
[3] K. Morse, R. Abraham, A. DeAbreu, et al. Science Advances 3:e1700930 (2017).
[4] L. Bergeron, C. Chartrand, A.T.K. Kurkjian, et al. PRXQuantum 1 020301 (2020).   

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Realization of practical quantum communication, sensing, and computing critically depends on suitable materials that offer specific combinations of physical properties. Identifying and designing physical systems for use as qubits, the basic units of quantum information, are critical steps in the development of quantum information science. Among the possibilities in the solid state, defect states in wide gap materials stand out for robustness – their quantum states can be manipulated with high fidelity at finite temperature. Solid-state materials such as rare earth ions into wide band gap crystals are one of the most promising candidates for the quantum information processing. We provide an overview of rare earth-doped material properties and summarize some of the most promising oxide host materials studied from ab-initio electronic structure methods that take care of critically important electron correlation, spin orbit coupling, crystal field, quadrupolar and hyperfine interactions, and Zeeman effect to identify and realize quantum states in the gap regime.   

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An aspiration for quantum communication is to combine high quality photonic interfaces with quantum memories. The neutral divacancy (VV⁰) in silicon carbide (SiC) combines both of these attributes in a material compatible with scalable semiconductor technologies. Here, we demonstrate high fidelity initialization (>99%) and gates (99.984%), as well as extended dephasing (40x improvement) and decoherence (>14 ms) times in this system. We then use a single VV⁰ to control nearby nuclear spins with both large and small hyperfine couplings. This creates a multi-register system where electron spins can be entangled with nuclear memories. We discuss how choosing optimal isotopic concentration can maximize the number of available and controllable nuclear spins. Finally, we embed this quantum system into p-i-n diodes. This simple integration allows us to engineer a spin-photon interface which is spectrally narrow (~20 MHz) and widely tunable (~1 THz). These advances open the door to using spins in classical semiconductor devices as building blocks for scalable quantum communication nodes.

References:
A. Bourassa*, C. P. Anderson* et al., Nat. Mat. (2020) [arXiv:2005.07602]
C. P. Anderson*, A. Bourassa* et al., Science 366, 6470, 1225-1230 (2019)   

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Color centers in diamond and other wide-bandgap semiconductors have drawn a great deal of attention recently as candidate material systems for quantum devices. In spite of this, an ideal color-center qubit system has yet to be discovered, and there remain important open questions related to the inner workings of different types of color centers as well as the interactions that occur between centers situated in close proximity. Here we report a series of novel measurements [1] on a high-density sample of a particular color-center type—negatively charged silicon-vacancy (SiV^-) centers in diamond—using collinear optical multidimensional coherent spectroscopy (MDCS). The measurements reveal a hidden population of silicon-vacancy centers, which are not typically observed in photoluminescence, and which exhibit a high degree of spectral inhomogeneity and longer-than-expected single-particle electronic T₂ dephasing times. The phenomenon is likely caused by strain, indicating opportunities for controllably mediating electronic coherence in color-center-based quantum devices.

[1] C. L. Smallwood, et al., arXiv:2006.02323 (2020)   

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Two-dimensional materials can be crafted with structural precision approaching the atomic scale, enabling quantum defects-by-design. These defects are frequently described as `artificial atoms’ and are emerging optically-addressable spin qubits. However, interactions and coupling of such artificial atoms with each other, in the presence of the lattice, is remarkably underexplored. Here we present the formation of `artificial molecules’ in solids, introducing a new degree of freedom in control of quantum optoelectronic materials. Specifically, in monolayer hexagonal boron nitride as our model system, we observe configuration- and distance-dependent dissociation curves and hybridization of defect orbitals within the bandgap, with bonding/antibonding orbital splitting energies ranging from ~ 10 meV to nearly 1 eV. We calculate the energetics of both cis and trans out-of-plane defect pairs against an in-plane defect pair and find that in-plane pair interacts more strongly. We envision leveraging this chemical degree of freedom of defect complexes to precisely engineer defect properties as quantum memories and quantum emitters for quantum information science.   

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To unambiguously identify defects responsible for the experimentally detected single-photon emissions in hexagonal boron nitride, we theoretically investigate the photoluminescence and electron-hole recombination mechanism (radiative and nonradiative) of defect candidates, especially carbon related defects. The slow supercell convergence of electron-phonon coupling impacts the accuracy of photoluminescence and nonradiative recombination rates. We found the low energy bulk state modes are the main cause for this slow convergence, and provided possible solutions. Furthermore, optical excitation including defect-exciton coupling is computed by solving the Bethe-Salpeter equation. We found large variation of exciton binding energy and radiative lifetime among different defects. At the end, the layer dependence and strain effects on photoluminescence and electron-hole recombination properties will also be discussed to help to unequivocally identify sources of single photon emission.   

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Despite the recognition of two-dimensional (2D) systems as emerging and scalable host materials of single photon emitters or spin qubits, uncontrolled and undetermined chemical nature of these quantum defects has been a roadblock to further development. Leveraging the design of extrinsic defects can circumvent these persistent issues and provide an ultimate solution. Here we established a complete theoretical framework to accurately and systematically design new quantum defects in wide-bandgap 2D systems. In particular, many-body interactions such as defect-exciton couplings are vital for describing excited state properties of defects in ultrathin 2D systems. Meanwhile, nonradiative processes such as phonon-assisted decay and intersystem crossing rates require careful evaluation, which compete together with radiative processes. From a thorough screening of defects based on first-principles calculations, we identified the Ti-vacancy complex as a promising defect in hexagonal boron nitride for spin qubits, with a triplet ground state, large zero-field splitting, and a prominent intersystem crossing rate highly desirable for spin-state initialization and qubit operation.

T. J. Smart, K. Li, J. Xu, and Y. Ping, arXiv:2009.02830 (2020).   

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Defect based single photon emitters in hexagonal boron nitride exhibit promising photophysical properties as a room temperature source of quantum light, however spectral diffusion poses challenges towards utilizing these emitters in quantum applications which require indistinguishable photons. Here we study photoluminescence of hBN single emitters via variable temperature (4K-300K) spectroscopy and compare exfoliated, chemically vapor deposition (CVD) grown and bulk hBN crystals. Furthermore, we propose a method to reduce spectral diffusion by using a conductive substrate ITO. Our method can decrease the inhomogeneous linewidth of hBN single emitters by 45%. Our findings are a step forward towards indistinguishable photon sources at room temperature based on hBN color centers.   

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Point defects in hexagonal boron nitride (hBN) have attracted growing attention as bright single-photon emitters. However, understanding of their atomic structures and radiative properties remains incomplete. Here we employ first-principles calculations to compute the structure, excited states and radiative lifetimes of over 20 native defects and carbon or oxygen impurities in hBN, generating a large data set of light emission energy, polarization and lifetime. The results show exciton energies varying from 0.3 to 4 eV and radiative lifetimes ranging from ns to ms for different candidate structures. Through a Bayesian statistical analysis, we identify highest-likelihood defect emitters with energy and radiative lifetimes in agreement with experiment. Our work advances the microscopic understanding of hBN single-photon emitters and introduces a computational framework to systematically investigate quantum emitters in 2D materials.   

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Silicon carbide is an industrially mature host material of promising spin defects, including the nitrogen vacancy (NV) center, for which coherent control at room temperature in the 4H polytype [1] has been recently demonstrated. However, the electronic properties of this center are still controversial, as conflicting experimental and computational results [2-3] have been reported in the literature. We report density functional theory calculations and spectrally resolved optically detected magnetic resonance measurements of NV in 4H-SiC, which in agreement with each other and with one recent report [3]. We find that the calculations agree with experiment only when we consider large supercells (>2000 atoms) and eliminate any spurious source of strain in the calculations. In addition, we compute zero-field splitting, hyperfine and quadrupole tensors to build a spin Hamiltonian and we compute coherence times. We find significant enhancement in T₂ and T₂* for basal defects compared to the axial ones, and we discuss similarities and differences with the corresponding quantities obtained for the divacancy in SiC.

[1] Wang, J.-F. et al. Phys. Rev. Lett 124, 223601 (2020).
[2] Csóré, A. et al. Phys. Rev. B 96, 085204 (2017).
[3] Zargaleh, S. A. et al. Phys. Rev. B 98, 214113 (2018).   

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Single photon sources are elementary building blocks for photonic quantum technologies. The recent discovery of quantum light emission from color centers defects in hexagonal boron nitride (hBN), has introduced a promising candidate for a room temperature single photon sourcl. However, a number of properties that determine the emission characteristics of these centers are not well understood.
In this work, we investigate the spectral and intensity fluctuations of emission from hBN color centers triggered by laser pulses. By tagging every photon count, we measure the Mandel parameter (Q) over several orders of magnitude of observation time scale. We report transformation of sub-Poissonian statistics of intensity fluctuations observed over the shorter (tens of nanosecond) times to super-Poissonian over the longer (tens of microseconds) times. We quantify the probability of two-photons events for hBN color centers and compare it to a reference Poissonian and quasi-thermal source of same average power. We compare the experimental results with those from light-emitter interaction model calculations. Our results may shine light on some of the open questions about hBN color centers, from their origins to their photophysical properties.   

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Spin defects in semiconductors, for example nitrogen-vacancy center in diamond, are promising spin qubits to build scalable quantum technologies, including quantum sensing and communication technologies. Despite ongoing efforts in the literature, predicting the singlet states of spin defects is still a challenging task, due to their strongly correlated nature. Using a recently developed quantum embedding theory [1], we present first-principles calculations of strongly correlated states of spin defects in diamond [2]. Within this theory, effective Hamiltonians are constructed, which can be solved by classical and quantum computers; the latter promise a much more favorable scaling as a function of system size than the former. In particular, we report a study of the neutral group-IV vacancy complexes in diamond, and we discuss their strongly correlated excited states. Our results provide valuable predictions for experiments aimed at optical manipulation of these defects.

[1] He Ma, Marco Govoni and Giulia Galli. npj Comput Mater 6, 85 (2020).
[2] He Ma, Nan Sheng, Marco Govoni, and Giulia Galli. Phys. Chem. Chem. Phys. (2020).   

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Point defects in two-dimensional semiconductors have attracted much attention for their potential as single-photon sources for next-generation quantum information science. Here, we present studies of a novel point defect, a negatively-charged carbon impurity substituting for a chalcogen atom, in transition-metal dichalcogenides (TMDs) monolayers that has been recently realized experimentally [arXiv:2008.12196] and can be generated deliberately with atomic precision. Using density functional theory calculations and a model spin-boson Hamiltonian, we show that the defect introduces a spin-1/2 two-level quantum system deep in the TMD band gap with strong phonon sidebands. We identify a few specific phonon modes that significantly couple to the spin-split defect states. Interestingly our calculations predict that this coupling is sensitive to the defect spin state, which in turn is responsible in generating phonon sidebands with different fine structure depending on the spin channel. Finally, we will discuss the possibility of the defect state being used as a next-generation single-photon emitter.