Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A35: Semiconducting Quantum Computing with DonorsFocus
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Sponsoring Units: DQI Chair: Ryan Jock, Sandia National Laboratories Room: BCEC 205B |
Monday, March 4, 2019 8:00AM - 8:12AM |
A35.00001: Modeling dopants in silicon: application to atomic-scale Si qubit systems. Keyi Liu, Garnett Bryant, Michal Zielinski Dopants in silicon are strong candidates for qubits in scalable, atom-based, solid-state quantum systems due to their long decoherence times and Si nanofabrication infrastructure. In these devices, an impurity atom binds a donor electron at low temperatures and information is stored either in the electron or the dopant nuclear spin. Typically, tight-binding (TB) theory is expected to provide a good computational model with reasonable precision. However, calculations based on simple central cell potential cutoff failed to predict the well-established energy degeneracies for a variety of bulk Si tight-binding models, hinting missing corrections in the model. We present TB calculations using several new corrections including the induced nearest neighbor hopping, the varying screened potential, and the orthogonalization of the on-site wavefunctions. We also discuss the consequences of applying these atomic scale corrections on the dopant models, including effects on the hyperfine interactions and STM imaging of dopants. Finally, we discuss how these models can be potential guides for experiments in many-body physics using atom-based devices. |
Monday, March 4, 2019 8:12AM - 8:24AM |
A35.00002: Using multivalley effective mass theory to probe the phosphorous donor effective potential in silicon Luke Pendo, Xuedong Hu Multivalley effective mass (MEM) theories combine physical intuition with an efficient use of computational resources. However, the most appropriate form of effective potential to use with MEM theory remains an open question. Here we develop an MEM theory for an electron confined to a phosphorus donor in Silicon, and explore the most useful form of effective potential that would allow us to accurately predict both spectrum and wavefunction of the electron. We employ a variational method with a freely extensible set of symmetrized Gauss- or Slater-type atomic basis states, with representatives from all five irreducible representations of the Td point-symmetry group. We employ stochastic optimization to complete both variational minimization and model parameter fitting, which allows for parameter spaces of large dimensionality. We consider an effective potential with tetrahedrally symmetric central cell corrections and a dynamic dielectric, as well as exchange-correlation effects, and we also explore effects of external perturbations such as an applied electric field. Our investigation here lays a solid foundation for studies of electronic states and interactions of multiple donors. |
Monday, March 4, 2019 8:24AM - 9:00AM |
A35.00003: Quantum computation and simulation with dopants in silicon Invited Speaker: Sven Rogge Bottom-up dopant engineering in silicon reached a level of control where devices can be reproducibly fabricated at the atomic scale with high yield. This talk focuses on the progress of single dopant atom placement in the context of quantum computation and simulation. Silicon offers a particularly interesting platform for quantum bits (qubits) because when isotopically purified it acts as a “semiconductor vacuum” for spins. This leads to extraordinary coherence that is used to realise donor atom based qubits. One and two qubit gates have been achieved with phosphorus qubits in silicon. High-bandwidth dispersive readout has been implemented and single-shot capability has been demonstrated with this technique. Spatially resolved tunnelling experiments that reveal the spectrum and quantum state image of single atoms and tunnel coupled arrangements of atoms will be discussed. This technique enabled the design and verification of a robust scheme to achieve exchange coupling of an two dimensional array of dopants that is immune to placement errors of the atoms. In addition, the fabrication of strongly coupled donor arrays that represent a hardware implementation of a Hubbard simulator will be presented. Quasi-particle tunnelling maps of spin-resolved states with atomic resolution reveal interference processes from which the entanglement entropy and Hubbard interactions are quantified. This represents a first stepping stone towards artificial quantum matter with up to 30 spins to implement complex highly correlated systems. |
Monday, March 4, 2019 9:00AM - 9:12AM |
A35.00004: Decoherence of donors in silicon at millikelvin temperatures Patrice Bertet, Vishal Ranjan, Bartolo Albanese, Sebastian Probst, Gengli Zhang, Emmanuel Flurin, Denis Vion, Daniel Esteve, Ren-Bao Liu, John Morton Donors in silicon are model spin systems [1] and potential candidates for implementing solid-state quantum bits. In that perspective, a detailed understanding of their coherence properties is needed. I will present measurements of Bismuth donors spin relaxation and coherence times at millikelvin temperatures obtained with a home-made spectrometer working at the quantum limit of sensitivity based on superconducting micro-resonators and Josephson parametric amplifiers [2], and I will discuss the various mechanisms at play. |
Monday, March 4, 2019 9:12AM - 9:24AM |
A35.00005: Desorption and lithographic patterning of halogen-terminated Si(100)-(2x1) using STM K.J. Dwyer, Jennifer E. DeMell, Michael Dreyer, Robert E Butera Scanning tunneling microscopy (STM)-based hydrogen depassivation lithography is a well-established technique used for fabricating atomic-scale devices in Si. Incorporation of donor atoms from PH3 into lithographic patterns in a Si surface allows for the formation of metallic wires, electrostatically defined quantum dots, and precise placement of donor atom qubits for quantum information (QI) research. However, interest in acceptor dopants and hole-based devices in QI and other fields necessitates the development of alternate precursor and/or resist chemistries for STM device fabrication. Here, we present results on the passivation and selective depassivation characteristics of halogen resists (Cl and Br) used for STM lithography on Si(100)-(2x1) at low and elevated temperatures (77 K, 300 K, 400 K). We explore STM tip-induced desorption and lithography as a function of tip bias, tunnel current, and electron dose. Through these studies, we demonstrate halogen lithography in an atomically precise mode where single Si dimer-wide features are depassivated, as well as a field emission mode for patterning larger areas of the halogen-terminated Si surface. This work moves us closer to realizing a resist and acceptor combination for fabricating atomic-scale, hole-based devices for QI. |
Monday, March 4, 2019 9:24AM - 9:36AM |
A35.00006: Suppressing spectral diffusion in phosphorus-doped silicon via optical excitation in high magnetic fields Lihuang Zhu, Johan Van Tol, Chandrasekhar Ramanathan The phosphorus donor impurity in silicon is a promising candidate for spin-based quantum devices. Recent experiments have shown that above-band gap optical excitation can result in strong hyperpolarization of the donor nuclear spins [1,2]. Here we show that low-power above-band-gap excitation can also extend the phase memory time of the donor electron spins in a low-concentration (∼3.3 - 3.5 × 1015 cm-3) phosphorus-doped natural abundance silicon sample. A two-pulse Hahn echo experiment at 8.5T and 4K was used to measure the decay of the echo amplitude with time. The non-exponential decays (∼exp(-(t/TSD)n) suggest that the phase memory time is dominated by spectral diffusion due to the 29Si spins [3]. TSD was measured to be 110 µs in the dark and with sub-bandgap excitation, rising to over 180 µs with 1050 nm laser excitation. With 980 nm excitation, TSD was observed to increase with applied laser power, saturating at 200 µs. |
Monday, March 4, 2019 9:36AM - 9:48AM |
A35.00007: Machine Learning approach to the inverse problem in STM imaging of dopant-based quantum devices Piotr T. R�?a?ski, Martyna Patera, Garnett Bryant, Michal Zielinski Atomic-scale solid-state qubits could be implemented using scanned-probe lithography to place two or more phosphorus dopants in silicon close to each other. Scanning tunnelling microscopy (STM) has been used to image individual dopants and to find dopant positions in the host silicon lattice based on that image. Determining the geometry of two-dopant qubits will be an essential step in device fabrication, however, double dopant-based devices will lead to a more challenging problem due to the complicated inter-valley wave-function interference patterns. Here we propose a theoretical solution to that problem. We utilize a multi-million atom tight-binding method, accounting for d-orbitals, surface passivation and surface reconstruction. Further, we use a machine learning approach to determine the positions of both dopants based on STM images generated with tight-binding simulations. From that we derive a set of rules for imaging two dopants and discuss possible generalizations for structures with a larger number of dopants. |
Monday, March 4, 2019 9:48AM - 10:00AM |
A35.00008: Fabrication of Single Donor and Single Electron Transistors for Quantum Technologies Rick Silver, Ranjit Kashid, Xiqiao Wang, Jonathan Wyrick, Pradeep Namboodiri, Scott W Schmucker, Andrew J Murphy, Michael David Stewart, Neil Zimmerman NIST is developing atomically precise, atom-based electronic devices for use in quantum information processing (QIP) and quantum materials research. We are using hydrogen-based scanning probe lithography to enable deterministic placement of individual dopant atoms with atomically aligned contacts and gates to fabricate single electron transistors for use in spin-to-charge conversion and single atom devices for use as qubits. |
Monday, March 4, 2019 10:00AM - 10:12AM |
A35.00009: Investigating the impact of laser illumination upon coherence times of electron spin qubits bound to donors in silicon David Wise, Naitik Panjwani, Siddharth Dhomkar, John Morton Donor spin qubits in silicon have long represented an attractive proposition for a qubit architecture. They have long coherence times, fast gate times and development can leverage techniques of the semiconductor industry. Another potential spin qubit, the optically active crystal defect - of which the NV centre in diamond is the most prominent example - has demonstrated exceptional single qubit properties particularly with regards to high fidelity single-shot read out. |
Monday, March 4, 2019 10:12AM - 10:24AM |
A35.00010: Electronic transport along atomically placed P ribbons in Si Belita Koiller, Amintor Dusko, Caio Lewenkopf Atomically precise placement of dopants in Si permits creating P nanowires by design. High-resolution images show that these wires are few atoms wide with some positioning disorder with respect to the Si structure sites, which is expected to lead to electronic localization. Experiments, however, report good transport properties in quasi-1D P nanoribbons. We investigate their electronic properties using an effective single-particle approach based on a linear combination of donor orbitals (LCDO), keeping the ground state donor orbitals' oscillatory behavior due to interference among the states at the Si conduction band minima. Our model for the P positioning errors accounts for the presently achievable placement precision. |
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