Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session G37: Focus Session: Semiconductor Qubits - SiGe, Isotopic Purification, and Electrical Control |
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Sponsoring Units: GQI Chair: Andrea Morello, University of New South Wales Room: 212A |
Tuesday, March 3, 2015 11:15AM - 11:51AM |
G37.00001: An Exchange-Only Qubit in Isotopically Enriched $^{28}$Si Invited Speaker: Mark Gyure We demonstrate coherent manipulation and universal control of a qubit composed of a triple quantum dot implemented in an isotopically enhanced Si/SiGe heterostructure, which requires no local AC or DC magnetic fields for operation. Strong control over tunnel rates is enabled by a dopantless, accumulation-only device design, and an integrated measurement dot enables single-shot measurement. Reduction of magnetic noise is achieved via isotopic purification of the silicon quantum well. We demonstrate universal control using composite pulses and employ these pulses for spin-echo-type sequences to measure both magnetic noise and charge noise. The noise measured is sufficiently low to enable the long pulse sequences required for exchange-only quantum information processing. Sponsored by United States Department of Defense. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the United States Department of Defense or the U.S. Government. Approved for public release, distribution unlimited. [Preview Abstract] |
Tuesday, March 3, 2015 11:51AM - 12:03PM |
G37.00002: Accumulation-Only Device Architecture for Si/SiGe Single Quantum Dots T.M. Hazard, D. Zajac, X. Mi, J.R. Petta Accumulation mode devices with overlapping gate architectures have been successfully realized in both Si/SiGe heterostructures [1] and Si MOS devices [2]. The increased control of tunneling rates, inter-dot tunnel couplings and confinement potentials over previous depletion mode designs make the overlapping gate architecture preferable. Material quality and device geometry have important implications for Si/SiGe quantum dots as potential hosts for spin qubits. Here we have fabricated and characterized quantum dot devices made with this accumulation mode architecture. We also perform numerical simulations to optimize device geometry for tight confinement potentials and reduced cross-coupling between accumulation gates. In addition to device improvements, we have also implemented a compact filtering system on the DC gate lines to achieve sub-40 mK electron temperatures.\\[4pt] [1] M. Borselli \textit{et al.}, arXiv:1408.0600v1\\[0pt] [2] M. Veldhorst \textit{et al.}, Nat. Nano. (2014), doi:10.1038/nnano.2014.216 [Preview Abstract] |
Tuesday, March 3, 2015 12:03PM - 12:15PM |
G37.00003: A Reconfigurable Device Architecture for Si/SiGe Quantum Dots D. Zajac, T.M. Hazard, X. Mi, J.R. Petta Depletion mode architectures for gate-defined quantum dots have been successful in the implementation of single, double and triple quantum dots. However, scaling up to more complicated devices presents serious lithographic challenges for depletion mode devices. We present a reconfigurable, accumulation-only mode lateral quantum dot device. We demonstrate full control of the device as both a single quantum dot with a single dot sensor and a double quantum dot with a single dot sensor. We reach the few electron regime in both operating modes. [Preview Abstract] |
Tuesday, March 3, 2015 12:15PM - 12:27PM |
G37.00004: Probing the mobility-limiting mechanisms in undoped Si/SiGe heterostructures Xiao Mi, Thomas M. Hazard, Christopher M. Payette, Ke Wang, David M. Zajac, Jeffrey V. Cady, Jason R. Petta Silicon is an ideal host material for spin-based semiconductor quantum dot qubits due to weak hyperfine coupling and a route to isotopic purification. Si quantum dots formed in undoped Si/SiGe quantum wells have recently allowed measurements that were previously only possible in the GaAs system [1, 2]. We report the growth of Si/SiGe quantum wells with mobilities reaching 260,000 cm$^2$/Vs at a density of $7 \times 10^{11}$ /cm$^2$. We systematically investigate a series of 26 wafers with different growth profiles and impurity levels, and find that the mobility is limited by scattering from both oxygen impurities in the quantum wells and interface charges at the surface of the wafer.\\[4pt] [1] B. M. Maune {\it et al.}, Nature {\bf 481}, 344 (2012).\\[0pt] [2] K. Wang, C. Payette, Y. Dovzhenko, P. W. Deelman, and J. R. Petta, Phys. Rev. Lett. {\bf 111}, 046801 (2013). [Preview Abstract] |
Tuesday, March 3, 2015 12:27PM - 12:39PM |
G37.00005: Understanding the Fong-Wandzura Sequence Daniel Zeuch, N.E. Bonesteel In exchange-only spin quantum computation, logical qubits are encoded into the Hilbert space of three or more spin-1/2 particles (e.g. electron spins in quantum dots), and quantum gates are realized by sequences of Heisenberg exchange operations, or ``exchange-pulses,'' acting on pairs of spins. It is easy to obtain pulse sequences for single-qubit gates, but difficult for entangling 2-qubit gates due to the large Hilbert space of six spins. The shortest known 2-qubit gate sequence, obtained by Fong and Wandzura via a numerical search [1], consists of 12 exchange pulses. Unlike a longer 2-qubit gate sequence constructed analytically in [2], this 12-pulse sequence has, until now, escaped intuitive explanation. Here, we analyze this sequence using techniques introduced in [2]. We find the 12-pulse sequence naturally decomposes into three parts, each consisting of the same partial pulse sequence acting on four spins at a time. This reduced sequence preserves certain total spin quantum numbers in a way that naturally suggests how it can be used to construct a leakage free entangling 2-qubit gate. \\[4pt] [1] B. H. Fong and S. M. Wandzura, Quantum Information \& Computation 11, 1003 (2011).\\[0pt] [2] D. Zeuch, R. Cipri, and N. E. Bonesteel, Phys. Rev. B 90, 045306 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 12:39PM - 12:51PM |
G37.00006: Entangling multi-dot spin qubits V. Srinivasa, J.M. Taylor, C. Tahan Single quantum bits encoded in the spins of two or three electrons confined within multiple semiconductor quantum dots provide practical advantages over individual electron spin qubits, in terms of both faster control via applied electric fields and protection from collective decoherence mechanisms. However, implementing rapid and robust entangling gates between these multi-electron, multi-dot qubits remains a current challenge. While the exchange interaction gives rise to rapid gates, it is limited in range and requires accompanying methods for suppressing leakage. Alternatively, the long-range Coulomb interaction can be used to couple both adjacent and spatially separated qubits, and rapid gates are possible through microwave manipulation of the extended charge distribution associated with a multi-dot system. We theoretically investigate different approaches for entangling qubits in double [1] and triple dots. By analyzing the coupling in the presence of charge noise and relaxation, we identify optimal regimes of operation for two-qubit gates and compare their performance for GaAs and Si quantum dots. \\[4pt] [1] V. Srinivasa and J. M. Taylor, arXiv:1408.4740 (2014). [Preview Abstract] |
Tuesday, March 3, 2015 12:51PM - 1:03PM |
G37.00007: Full-scope modeling of semiconductor devices for quantum information processing John King Gamble, Andrew Baczewski, Adam Frees, N. Tobias Jacobson, Ines Montano, Richard P. Muller, Erik Nielsen Recent outstanding experimental advances in semiconductor-based quantum information processing have placed the fundamental building blocks of a quantum computer within reach. Typical computational simulation of these devices either focuses on the large-scale, semiclassical device physics or more detailed quantum mechanics within an idealized physical system. Here, we present results for full-scope simulation, where detailed multi-valley effective mass theory is coupled to large-scale device physics. This enables the simulation of the quantum properties and operation of a device directly from a physical design. This work opens the door for physics-targeted device optimization and an unprecedented level of predictive power. The authors gratefully acknowledge support from the Sandia National Laboratories Truman Fellowship Program, which is funded by the Laboratory Directed Research and Development (LDRD) Program. Sandia is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 3, 2015 1:03PM - 1:15PM |
G37.00008: Electron Spin Relaxation in Si/SiGe Quantum Dot Ensembles Ryan M. Jock, J.-H. He, A.M. Tyryshkin, S.A. Lyon, C.-H. Lee, S.-H. Huang, C.W. Liu Single electron spin states in Si/SiGe quantum dots have shown promise as qubits for quantum information processing. Our previous ensemble microwave measurements of electron spins in gated Si/SiGe quantum dots have displayed relaxation (T$_{\mathrm{1}})$ and coherence (T$_{\mathrm{2}})$ times of 250 $\mu $s at 350 mK. These experiments used conventional X-band (10 GHz) pulsed Electron Spin Resonance (pESR), on a large area, double-gated, undoped Si/SiGe heterostructure, in which holes with a 300 nm diameter and 700 nm period were lithographically defined in the lower gate. Quantum dots were electrostatically induced in a natural Si quantum well, with their confinement potential controlled by the gates. Electron spin coherence in these first generation quantum dot devices was observed to be T$_{\mathrm{1}}$-limited at stronger confining potentials. By tailoring the quantum dot size and spacing we can modify the electron confinement barrier and electron wave function size, helping us probe the mechanisms limiting coherence in silicon quantum dots. We will report results on dots with lithographic diameters of 150 to 300 nm and a 375 to 700 nm period. The device with smaller dots and larger spacing displays an extended electron relaxation times (T$_{\mathrm{1\thinspace }}=$ 1-2 ms) at 350 mK. Furthermore, we observe a T$_{\mathrm{2}}$ of 310 $\mu $s, that is neither T$_{\mathrm{1}}$-limited nor temperature dependent. This narrows the field of possible coherence limiting mechanisms, which will be discussed. [Preview Abstract] |
Tuesday, March 3, 2015 1:15PM - 1:27PM |
G37.00009: Electrical assessments of $^{28}$Si enriched and deposited \textit{in situ} to \textless 1 ppm $^{29}$Si Joshua Pomeroy, Kevin Dwyer, Hyun-Soo Kim We are enriching $^{28}$Si to better than 99.9999{\%}, depositing epitaxial films, and measuring materials properties to improve our deposition process so that our enriched films can be used to fabricate quantum devices. Recent reports of spin-echo measurements in donor ensembles and single spins have both demonstrated spectacular coherence time and line width improvements due to enriched $^{28}$Si. In order to realize the benefits of our enrichment, the electrical properties of our films need to be similar quality to commercial wafers. Therefore, we are using C-V, g-V, Hall and other techniques commonly used for quantifying defect densities, mobility, carrier density, etc. to benchmark the viability of using our enriched films for quantum device fabrication. [Preview Abstract] |
Tuesday, March 3, 2015 1:27PM - 1:39PM |
G37.00010: Dependence of$^{29}$Si concentration on deposition temperature in $^{28}$Si epilayers Kevin Dwyer, Joshua Pomeroy, Hyun Soo Kim, David Simons In an effort to gain predictive power for the enrichment of $^{28}$Si epilayers deposited at elevated temperatures, we correlate the $^{29}$Si concentration due to natural abundance silane adsorption with measured SIMS values. We have previously shown very high enrichments up to 99.99996 {\%} (0.3 ppm $^{29}$Si) using mass filtered ion beam deposition. However, the incorporation at higher deposition temperatures of naturally abundant silane gas from our ion source has the potential to reduce the final film enrichment. Knowledge of the expected reduction in $^{29}$Si concentration is important because removal of the 4.7{\%} $^{29}$Si nuclear spins in natural silicon allows for exceedingly long coherence (T$_{2})$ times of qubits, approaching an hour at room temperature for $^{31}$P nuclear spins. This makes incorporation of highly enriched $^{28}$Si into devices critical for solid state quantum information. [Preview Abstract] |
Tuesday, March 3, 2015 1:39PM - 1:51PM |
G37.00011: Feasibility study of simultaneous capacitance detection during STM of silicon Hyun Kim, Kevin Dwyer, Joshua Pomeroy We are examining the feasibility of capacitance detection during STM to image buried metal nanostructures in silicon. As the hydrogen STM lithography for quantum information enables us to fabricate the atomically precise devices such as single atom qubits, the accurate alignment of metal contacts such as electrodes to the buried nanostructures on the surface becomes very challenging. Using SCM with STM gives benefits to locate the buried nanostructures and image the surface morphology simultaneously. [Preview Abstract] |
Tuesday, March 3, 2015 1:51PM - 2:03PM |
G37.00012: Two-Dimensional Electron Gases in Nanomembrane-based Epitaxial Si/SiGe Heterostructures Yize Li, Pornsatit Sookchoo, Xiaorui Cui, Robert Mohr, Donald Savage, Ryan Foote, R.B. Jacobson, Jose Sanchez-Perez, Xian Wu, Dan Ward, Susan Coppersmith, Mark Eriksson, Max Lagally To assess possible improvements in the electronic performance of two-dimensional electron gases (2DEGs) in silicon, SiGe/Si/SiGe heterostructures are grown on fully elastically relaxed single-crystal SiGe nanomembranes fabricated through a strain engineering approach. This procedure eliminates the formation of dislocations in the heterostructure. Top-gated Hall bar devices are fabricated to enable magnetoresistance and Hall effect measurements. Both Shubnikov de Haas oscillations and the quantum Hall effect are observed at low temperatures, demonstrating the formation of high-quality 2DEGs. Values of charge carrier mobility as a function of carrier density extracted from these measurements are at least as high or higher than those obtained from companion measurements made on heterostructures grown on conventional strain graded substrates. In all samples impurity scattering appears to limit the mobility. [Preview Abstract] |
Tuesday, March 3, 2015 2:03PM - 2:15PM |
G37.00013: Fabrication and characterization of gate-defined quantum dots in Si/SiGe nanomembranes T.J. Knapp, Robert T. Mohr, Yize Stephanie Li, Ryan H. Foote, Xian Wu, Dan R. Ward, Donald E. Savage, M.G. Lagally, Susan N. Coppersmith, M.A. Eriksson We have fabricated gate-defined quantum dots in Si/SiGe heterostructures grown on single crystal nanomembranes, implementing strain relaxation by release into liquid solution. Such heterostructures are much more uniform than those grown using conventional strain relaxation, and they therefore offer a promising path to the fabrication of large numbers of nearly identical quantum dots. Conventional strain-grading methods result in lateral strain inhomogeneities and mosaic tilt (small rotations of the crystal axes from location to location), phenomena that make the heterostructure nonuniform across a wafer. We show that nanomembranes, which enable the formation of Si/SiGe heterostructures that are free of such problems, are robust enough for successful fabrication of quantum dots defined by two layers of electrostatic gates separated by an Al$_{2}$O$_{3}$ dielectric layer. We report electrical characterization of these quantum dots at cryogenic temperatures. This work was supported in part by ARO (W911NF-12-0607), NSF (DMR-1206915), and the United States Department of Defense. The views and conclusions contained in this document are those of the author and should not be interpreted as representing the official policies, either expressly or implied, of the US Government. [Preview Abstract] |
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