Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session E29: Semiconductor Qubits - Quantum Computing with Donor Spins IFocus Live
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Sponsoring Units: DQI Chair: Matthew Donnelly, Univ of New South Wales |
Tuesday, March 16, 2021 8:00AM - 8:12AM Live |
E29.00001: First-principles calculations of hyperfine interaction, binding energy, and quadrupole coupling for shallow donors Michael Swift, Hartwin Peelaers, Sai Mu, Chris Van de Walle Shallow impurities are central to semiconductor technology. A thorough understanding of the physics of shallow impurities has taken on new urgency in the context of quantum information science, where they form key components of qubits. Because of the large spatial extent of the wave function, first-principles calculations of shallow centers have proved elusive. In addition, the “central cell corrections” that are crucial for accurately describing binding energies and hyperfine parameters are not adequately captured by traditional semi-local functionals in density functional theory, requiring advanced approaches that have proven too computationally demanding. We have developed a methodology that is capable of accurately predicting properties of shallow impurities. It is based on a combination of extrapolating results from supercell calculations carried out using a semi-local functional with performing select calculations using a hybrid functional. We will illustrate the power of this approach with results that provide an explanation for an observed unexpected strain dependence of the hyperfine properties of shallow donors in silicon, a system with applications in atomic clock transitions for silicon-based spin qubits. |
Tuesday, March 16, 2021 8:12AM - 8:24AM Live |
E29.00002: Arrayed Quantum Dot Characterization in Si:P Quantum Devices Fan Fei, Xiqiao Wang, Ranjit Kashid, Jonathan Wyrick, Pradeep Namboodiri, Richard Silver STM based hydrogen lithography is a promising architecture for fabrication of atomic-scale quantum devices. The precision of this technique allows placement of phosphorus atoms into pre-designed lithography patterns with sub-nanometer resolution to form quantum dots. Structures such as double quantum dots or arrays of dopants in Si are a promising platform for analog quantum simulation of the Fermi-Hubbard model. Design, fabrication and characterization of these devices is also useful for future multi-qubit Si:P-based quantum information processing. We present the design and fabrication of double/arrayed quantum dot devices. We compare low temperature transport measurements of the devices with a generalized Hubbard model and find it necessary to include inelastic processes. We extract inter-dot tunnel coupling of a double dot device and demonstrate interesting rectifying behavior. For devices with more than 2 dots, we found disorder is currently inevitable and is crucial to device transport properties. In our Si:P devices, gates/leads are capacitively coupled to the central dot region. Large gate ranges and linear capacitance coupling are essential to producing good finite bias spectroscopy. We will describe fabrication developments to improve device gating performance. |
Tuesday, March 16, 2021 8:24AM - 8:36AM Live |
E29.00003: A chemical model for atomic-precision single-donor incorporation of phosphorus atoms in Si(100)-2x1 Quinn Campbell, Jeffrey Ivie, Justin Koepke, Mitchell Brickson, Peter Schultz, Richard Muller, Ezra Bussmann, Andrew D Baczewski, Andrew M Mounce, Shashank Misra Understanding the statistics of atomic-precision single-phosphorus atom incorporation on Si(100)-2x1 is crucial to the development of analog quantum simulation devices. One method for creating such devices is to use a scanning tunneling microscope to depassivate a few-atom region on H-terminated Si, which is then exposed to a precursor gas that subsequently dissociates such that a donor is incorporated through some chemical pathway. In this talk, we develop a kinetic Monte Carlo model of this process parameterized from first principles calculations to predict the incorporation statistics as a function of the initial depassivation geometry, temperature at dosing and anneal, and pressure of precursor gas. Using our model, we match experimentally measured rates of incorporation and suggest future pathways for the improvement of incorporation rates. |
Tuesday, March 16, 2021 8:36AM - 8:48AM Live |
E29.00004: Transition metal impurities in Silicon: Computational search for semiconductor qubit Cheng-Wei Lee, Adele Tamboli, Meenakshi Singh, Vladan Stevanovic Two classical impurity-semiconductor qubits are 31P in silicon and NV- center in diamond, both of which have their own strengths and weaknesses. The spin subsystem of 31P in Si requires extremely low temperature to initialize while in case of the NV- center, it is initialized by the non-radiative transition, which allows higher operating temperatures. However, diamond manufacturing is not as mature as Si. Herein, we search for candidate Si-based impurity-semiconductor qubits that could benefit from the established silicon technology but with higher operating temperature. Since transition metal impurities are known to form deep states in crystalline Si, we conduct a survey of all 3d and selected heavier transition metals using modern first-principles defect calculations with the purpose of predicting location and spin of defect levels. Specifically, we applied the well-established supercell approach and HSE06 hybrid functional, which accurately reproduces band gap of Si as well as localization of defect states. As a result, we find several candidate impurities that form spin triplet defect states within the electronic bandgap with C3v symmetry. These candidate systems resemble the NV- center in diamond and hold the potential to operate at higher temperature. |
Tuesday, March 16, 2021 8:48AM - 9:00AM Live |
E29.00005: Understanding charge-stability diagrams of dopant arrays in Si Garnett Bryant, Emily Townsend, Maicol Ochoa, Fan Fei, Xiqiao Wang, Richard Silver Atomically precise fabrication of dopant arrays in Si provides exciting opportunities to perform quantum simulations, study the dynamics of engineered quantum systems, and develop atomic-scale quantum materials. We describe theoretical simulations done for two-dimensional arrays of dopants in Si implemented with an extended range Fermi-Hubbard model and supported by atomistic modelling of the array states. Simulations are done with and without dopant disorder, as a function of the electron-electron interaction to test the limits of weak and strong interaction. Hund’s rule defines the nature of the charged array ground states for large on-site electron-electron repulsion. Ground states for charged arrays can be highly (quasi) degenerate, providing multiple transport channels. Disorder splits these degeneracies, helping define the charge boundaries in charge-stability diagrams. We consider n x m arrays of different sizes to identify the array states that are probed in transport. Results are used to understand charge-stability diagrams recently obtained for two-dimensional arrays of dopants in Si. Implications for using dopant arrays as a quantum lab on a chip are discussed. |
Tuesday, March 16, 2021 9:00AM - 9:12AM Live |
E29.00006: Tunneling Statistics and Spin Readout of Few-donor Quantum Dots in Silicon Xiqiao Wang, Ranjit Kashid, Jonathan Wyrick, Fan Fei, Pradeep Namboodiri, Albert Rigosi, Richard Silver The long electron spin coherence and relaxation times achievable in donor-based quantum dots in silicon provide a promising route towards scalable solid-state quantum computation. High-fidelity single-shot spin readout and initialization are essential for implementing fault-tolerant quantum algorithms. However, unwanted dynamic interactions between the quantum dot electron and its environment and charge sensor can significantly limit the spin readout fidelity. Here we present our recent spin readout and initialization measurements in few-donor cluster quantum dots using STM-patterned single-electron transistor charge sensors in the strong response regime. Using spectroscopic measurements of single-electron loading/unloading rates and analysis of spin-selective tunneling statistics, we investigate the charge and spin dynamics between the few-donor quantum dot and charge sensor where spin-to-charge conversion and random telegraph switching occur. We discuss the impact of charge noise and the charge sensor's density of states fluctuations on the spin-state readout and initialization, as well as perspectives on device design, fabrication, and measurement conditions to optimize spin-readout fidelity for robust single electron spin qubit manipulation. |
Tuesday, March 16, 2021 9:12AM - 9:24AM Live |
E29.00007: Properties of shallow donor ensembles in ZnO for quantum memory applications Vasileios Niaouris, Christian Zimmermann, Xiayu Linpeng, Maria L. K. Viitaniemi, Yusuke Kozuka, Masashi Kawasaki, Kai-Mei Fu Neutral shallow donors (D0) in ZnO, such as AlZn, GaZn, InZn, are promising solid-state spin qubits1. D0 optically couples to the neutral donor bound exciton (D0X) with high radiative efficiency, potentially enabling photon-mediated quantum entanglement schemes both between donors, and donors and trapped-ions2. Here we report on the optical ensemble properties of ZnO donors, toward quantum memory applications (QMA). The optical D0X transition exhibits an inhomogeneous linewidth of 20 GHz measured via photoluminescence excitation, and optical absorption. The 0-field optical depth (OD) for the Al and Ga transitions is 10 and 6, respectively, and dramatically decreases at field due to optical pumping. We will further present spectral hole burning measurements to determine the upper bound homogeneous linewidth and discuss the diffrent line broadening contributions. The large OD, homogeneous optical properties and potential to extend spin-coherence times indicate ZnO donor ensembles are promising for QMA. |
Tuesday, March 16, 2021 9:24AM - 9:36AM Live |
E29.00008: The electronic structure and transport properties of phosphorus arrays and phosphorus clusters in silicon nanodevices. Maicol Ochoa, Keyi Liu, Emily Townsend, Michal Gawelczyk, Michal Zielinski, Garnett Bryant Donor-based quantum devices in silicon are attractive for universal quantum computing and analog quantum simulations, providing great control over the quantum states of these devices. We present theoretical atomistic calculations and a detailed analysis of the electronic and transport properties of phosphorus dopant arrays and clusters in Si quantum devices. Our method consists of self-consistent calculations of the electron energy levels in P-doped Si devices using representative tight-binding Hamiltonians with solutions to the Poisson equation to account for external potentials. We identify the electronic states and charge distribution in linear, triangular, and square dopant arrays of different sizes and under the influence of a range of gate and source/drain potentials. We rationalize our findings in terms of dopant wavefunction overlaps, symmetries of electronic states, and group-theory methods. For large dopant spacing, arrays act as weakly-coupled distinct sites. For small dopant spacing, arrays act as giant clusters. We identify this transition and discuss how it affects transport. Our simulations allow us to understand the stability diagrams in these devices, demonstrating that our approach accurately describes transport through multi-dopant quantum devices. |
Tuesday, March 16, 2021 9:36AM - 10:12AM Live |
E29.00009: Atom-Based Silicon Devices for Quantum Computing and Analog Quantum Simulation Invited Speaker: Richard Silver NIST is using atomically precise fabrication to develop devices for use in quantum information |
Tuesday, March 16, 2021 10:12AM - 10:24AM Live |
E29.00010: The improved point-charge model for dopants in Si and applications to atomic-scale system simulations Keyi Liu, Piotr T. Rozanski, Michal Zielinski, Garnett Bryant Dopants in silicon are strong candidates for qubits in scalable solid-state quantum systems. Tight-binding (TB) theory has been used to provide a good atomic-scale model when a central cell correction is fit to experimental binding energies for one choice of the bulk Si TB parameters. However, this model fails to predict the correct dopant level energy degeneracies for other well-established TB models. We argue that the point-charge dopant model with a simple central cell correction is missing vital contributions from the dopant potential. We have developed a first principles-based dopant model with several new corrections that are obtained explicitly through self-consistent field calculations to evaluate the appropriate dopant matrix elements rather than by fitting to experiment. We find that these new corrections greatly improve our predictability of the underlying dopant physics, as all bulk Si TB parameters produce the correct ordering of dopant levels in our new model, and give us flexible tunability of numerical values of the dopant level to arbitrary precision. Results are discussed to show the effect on inter-dopant exchange coupling and the level structure of dopant clusters. Finally, we discuss how these models can effectively simulate many-body physics in atom arrays. |
Tuesday, March 16, 2021 10:24AM - 10:36AM Live |
E29.00011: The impact of donor incorporation statistics on analog quantum simulations of Hubbard physics in near-atomic precision donor arrays Mitchell Brickson, Quinn Campbell, Jeffrey Ivie, Justin Koepke, Peter Schultz, Richard Muller, Ezra Bussmann, Andrew D Baczewski, Shashank Misra Atomic precision advanced manufacturing (APAM) is a promising approach for analog quantum simulation of strongly correlated systems. APAM relies on scanning tunneling microscope lithography to place single P donors precisely in Si. Assessing the impact of experimentally demonstrated stochastic donor incorporation on device performance is vital to understanding the limits of analog quantum simulation. Using a nonequilibrium Green’s function formalism, we simulate transport characteristics of Hubbard models based-on P donors in Si, and the effects of probabilistic donor incorporation on these transport characteristics. Using our model, we find limits on the Hamiltonians one can target without losing prominent physical features of the model to missing donors. |
Tuesday, March 16, 2021 10:36AM - 10:48AM Live |
E29.00012: Digital Hydrogen Depassivation Lithography for Improving Precison of Donor Placement in Si John Randall, James H.G. Owen, Ehud Fuchs, Robin Santini Hydrogen depassivation lithography (HDL) has been used to place dopants on the Si surface for Si donor qubits, single electron transistors1, and shows promise for analog quantum simulation devices2. We use image analysis to identify lithography pixels which are defined as two dimers along a dimer row3. During lithography, the tip moves over the pixels to be exposed, following vectors defined from the device pattern. There is a tip position tolerance of +/- 1.3Å. However, thermal drift, tip changes, creep, and hysteresis all make keeping this tolerance during lithography difficult. |
Tuesday, March 16, 2021 10:48AM - 11:00AM Live |
E29.00013: Dopant Precursor Adsorption into a Single-Dimer Window on Si(100): Computational Screening of Precursor-Resist Combinations Matthew Radue, Yifei Mo, Robert E Butera Envisioned spin-based quantum computers present the tremendous fabrication challenge of creating an atomically precise dopant array in Si. To meet this challenge, lithographic strategies have been successfully developed to incorporate a single phosphorus atom within a few-dimer window on Si(100). A growing number of dopant precursors, including acceptors for hole-based qubits and superconducting Si devices, and resists, such as halogens in place of hydrogen, are being explored to further expand the application space for atomic-precision fabrication. While recent ideas posit the use of single-dimer windows to control dopant placement, whether or not the precursor will “fit” into such a tight window has yet to be explored for new lithographic material systems. This is especially uncertain for relatively large precursors combined with relatively large resist atoms. In this study, density functional theory is used to calculate the initial adsorption configurations and adsorption pathways of common acceptor and donor precursors into a single-dimer window. Various resist atoms are used to identify which precursor-resist combinations are precluded on the basis of steric hindrances. |
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