Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session A45: Semiconductor Qubits: Si/SiGe Quantum DotsFocus
|
Hide Abstracts |
Sponsoring Units: GQI Chair: Jason Petta, Princeton University Room: 348 |
Monday, March 14, 2016 8:00AM - 8:36AM |
A45.00001: Spin qubits in quantum dots -- beyond nearest-neighbour exchange Invited Speaker: Lieven Vandersypen The spin of a single electron is the canonical two-level quantum system. When isolated in a semiconductor quantum dot, a single electron spin provides a well-controlled and long-lived quantum bit. So far, two-qubit gates in this system have relied on the spin exchange interaction that arises when the wave functions of neighbouring electrons overlap. Furthermore, experimental demonstrations of controlled spin-exchange have been limited to 1D quantum dot arrays only. Here we explore several avenues for scaling beyond 1D arrays with nearest-neighbour coupling. First, we show that second-order tunnel processes allow for coherent spin-exchange between non-nearest neighbour quantum dots. The detuning of the intermediate quantum dot controls the frequency of the exchange-driven oscillations of the spins. Second, we demonstrate shuttling of electrons in quantum dot arrays preserving the spin projection for more than 500 hops. We use this technique to read out multiple spins in a way analogous to the operation of a CCD. Finally, we develop superconducting resonators that are resilient to magnetic field and with a predicted tenfold increase in vacuum electric field amplitudes. This makes coupling spin qubits via superconducting resonators in a circuit-QED approach a realistic possibility. [1] F.R. Braakman et al, Nature Nano 8, 432, 2013 [2] T.A. Baart et al, Nature Nano, accepted, see arXiv:1507.07991 [3] T.A. Baart et al, in preparation [4] N. Samkharadje et al, in preparation [Preview Abstract] |
Monday, March 14, 2016 8:36AM - 8:48AM |
A45.00002: Characterization of accumulation-mode Si/SiGe triple quantum dots T. M. Hazard, D. M. Zajac, X. Mi, S. S. Zhang, J. R. Petta The transition from quantum dots fabricated from doped Si/SiGe quantum wells to undoped accumulation-mode structures has greatly improved the performance of few-electron quantum dots. Our accumulation-mode devices\footnote{D. M. Zajac \textit{et al.}, Appl. Phys. Lett. \textbf{106}, 223507 (2015).} are reconfigurable and allow for operation as single, double, or triple quantum dots. In these devices, we measure typical charging energies $E_{\mathrm{c}}= 5.7$ meV, orbital excited state energies as large as $E_{\mathrm{o}}$ = 2.9 meV, and valley splittings of up to $E_{\mathrm{v}}= 80$ $\mu$eV. With the device configured as a triple quantum dot, we easily reach the (1,1,1) charge configuration. The gate architecture allows the interdot tunnel coupling to be tuned over a wide range, which is important for operation as an exchange-only spin qubit.\footnote{ J. Medford \textit{et al.}, Phys. Rev. Lett. \textbf{111}, 050501 (2013).} [Preview Abstract] |
Monday, March 14, 2016 8:48AM - 9:00AM |
A45.00003: Characterization of a gate-defined double quantum dot in a Si/SiGe nanomembrane T. J. Knapp, R. T. Mohr, Yize Stephanie Li, Brandur Thorgrimsson, Ryan H. Foote, Xian Wu, Daniel R. Ward, D. E. Savage, M. G. Lagally, Mark Friesen, S. N. Coppersmith, M. A. Eriksson We report the characterization of a gate-defined double quantum dot formed in a Si/SiGe nanomembrane. Previously, all heterostructures used to form quantum dots were created using the strain-grading method of strain relaxation, a method that necessarily introduces misfit dislocations into a heterostructure and thereby degrades the reproducibility of quantum devices. Using a SiGe nanomembrane as a virtual substrate eliminates the need for misfit dislocations but requires a wet-transfer process that results in a non-epitaxial interface in close proximity to the quantum dots. We show that this interface does not prevent the formation of quantum dots, and is compatible with a tunable inter-dot tunnel coupling, the identification of spin states, and the measurement of a singlet-to-triplet transition as a function of the applied magnetic field. This work was supported in part by ARO (W911NF-12-0607), NSF (DMR-1206915, PHY-1104660), 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] |
Monday, March 14, 2016 9:00AM - 9:12AM |
A45.00004: Gate-defined quantum dot devices in undoped Si/SiGe heterostructures for spin qubit applications Christian Volk, Frederico Martins, Charles M. Marcus, Ferdinand Kuemmeth Spin qubits based on few electron quantum dots in semiconductor heterostructures are among the most promising systems for realizing quantum computation. Due to its low concentration of nuclear-spin-carrying isotopes, silicon is of special interest as a host material. We characterize gate-defined double and triple quantum dot devices fabricated from undoped Si/Si$_{0.7}$Ge$_{0.3}$ heterostructures. Our device architecture is based on integrating all accumulation and depletion mode gates in a single gate layer. This allows us to omit the commonly used global accumulation gate in order to achieve a more local control of the potential landscape in the device. We present our recent progress towards implementing spin qubits in these structures. [Preview Abstract] |
Monday, March 14, 2016 9:12AM - 9:24AM |
A45.00005: Observation of multiple exchange oscillation frequencies in Si/SiGe spin qubits Matthew Rakher An all-electrical approach to quantum information processing with spin qubits in Si/SiGe quantum wells relies on the ability to quickly turn on and off the exchange interaction between electrons in neighboring quantum dots [1]. The quality of gates enabled by this technique depends critically on reliably achieving a specific value of exchange coupling for a given control voltage. In recent experiments [2], we have observed multiple exchange oscillation frequencies at the same control bias for several different devices. In particular, Fourier transforms of exchange oscillations measured as a function of evolution time reveal the presence of multiple frequencies over a wide range of pulse amplitudes. The data are suggestive of unwanted population of an excited singlet-triplet manifold that behaves similarly with bias as the qubit ground state pair. The occupation of excited singlet-triplet states can degrade gate performance in exchange-based quantum devices and we outline methods to observe and investigate these states. [1] K. Eng et al, Science Advances 1 (2015) [2] M.D. Reed et al, arxiv:1508.01223 (2015) [Preview Abstract] |
Monday, March 14, 2016 9:24AM - 9:36AM |
A45.00006: Predicting the valley physics of silicon quantum dots directly from a device layout John King Gamble, Patrick Harvey-Collard, N. Tobias Jacobson, Andrew D. Bacewski, Erik Nielsen, In\`{e}s Monta\~{n}o, Martin Rudolph, Malcolm S. Carroll, Richard P. Muller Qubits made from electrostatically-defined quantum dots in Si-based systems are excellent candidates for quantum information processing applications. However, the multi-valley structure of silicon's band structure provides additional challenges for the few-electron physics critical to qubit manipulation. Here, we present a theory for valley physics that is predictive, in that we take as input the real physical device geometry and experimental voltage operation schedule, and with minimal approximation compute the resulting valley physics. We present both effective mass theory and atomistic tight-binding calculations for two distinct metal-oxide-semiconductor (MOS) quantum dot systems, directly comparing them to experimental measurements of the valley splitting. We conclude by assessing these detailed simulations’ utility for engineering desired valley physics in future devices. 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] |
Monday, March 14, 2016 9:36AM - 9:48AM |
A45.00007: Gate fidelity and coherence time of an electron spin in a Si/SiGe quantum dot Erika Kawakami, Thibaut Jullien, Pasquale Scarlino, D. R. Ward, D. E. Savage, M. G. Lagally, V. V. Dobrovitski, Mark Friesen, S. N. Coppersmith, M. A. Eriksson, L. M. K. Vandersypen Electron spins in Si/SiGe quantum dots are one of the most promising candidates for a quantum bit for their potential scalability and long dephasing time. We realized coherent control of an individual electron spin in a single quantum dot (QD), lithographically defined in a Si/SiGe 2D electron gas. Spin rotations are achieved by applying microwave excitation to one of the gates, which oscillates the electron wave function back and forth in the gradient field produced by cobalt micromagnets fabricated near the dot. Thanks to the long intrinsic dephasing time $T_2^*$ = 900 ns and Rabi frequency of 1.4 MHz, we were able to obtain an average single qubit gate fidelity of an electron spin in a Si/SiGe quantum dot of 99 \%, measured via randomized benchmarking. The dephasing time is extended to 70 $\mu$s using Hahn echo, and up to 400 $\mu$s with multipulse dynamical decoupling (128 $\pi$ pulses). We extract the noise spectrum in the range of 5 kHz -1 MHz using dynamical decoupling and show that the gate fidelity is well explained by this noise characteristic. We discuss the mechanism that induces this noise and is responsible for decoherence. [Preview Abstract] |
Monday, March 14, 2016 9:48AM - 10:00AM |
A45.00008: Epitaxial deposition of highly enriched $^{28}$Si films with \textless 1 nm roughness K. J. Dwyer, Hyun-Soo Kim, A. N. Ramanayaka, D. S. Simons, Vladimir Oleshko, J. M. Pomeroy Low temperature epitaxial deposition of thin films with less than 1 nm rms roughness is achieved using a $^{28}$Si ion beam deposition source. These films are enriched \textit{in situ} to \textless 140 ppb $^{29}$Si isotope fraction for quantum computing devices. Removal of the 4.7 {\%} $^{29}$Si nuclear spins in natural silicon allows for exceedingly long coherence (T$_{2})$ times of qubits, making incorporation of highly enriched $^{28}$Si into devices critical for solid state quantum information. Low roughness epitaxial $^{28}$Si thin films are achieved by depositing in an island growth mode at temperatures of 300 \textdegree C to 400 \textdegree C, and the morphology is verified using scanning tunneling microscopy. Further, the crystalline quality of the films is shown using cross-sectional transmission electron microscopy. Finally, the chemical purity and broader electrical properties of the $^{28}$Si films are assessed by secondary ion mass spectroscopy as well as capacitance--voltage profiling, schottky diode measurements, and hall measurements. [Preview Abstract] |
Monday, March 14, 2016 10:00AM - 10:12AM |
A45.00009: First measurements of charge carrier density and mobility of in-situ enriched $^{\mathrm{28}}$Si. A. N. Ramanayaka, K. J. Dwyer, Hyun-Soo Kim, M. D. Stewart, Jr., J. M. Pomeroy Magnetotransport in top gated Hall bar devices is investigated to characterize the electrical properties of in-situ enriched $^{\mathrm{28}}$Si. Isotopically enriched $^{\mathrm{28}}$Si is an ideal candidate for quantum information processing devices as the elimination of unpaired nuclear spins improves the fidelity of the quantum information. Using mass filtered ion beam deposition we, in-situ, enrich and deposit epitaxial $^{\mathrm{28}}$Si, achieving several orders of magnitude better enrichment compared to other techniques. In order to explore the electrical properties and optimize the growth conditions of in-situ enriched $^{\mathrm{28}}$Si we perform magnetotransport measurements on top gated Hall bar devices at temperatures ranging from 300 K to cryogenic temperatures and at moderate magnetic fields. Here, we report on the charge carrier density and mobility extracted from such experiments, and will be compared among different growth conditions of in-situ enriched $^{\mathrm{28}}$Si. [Preview Abstract] |
Monday, March 14, 2016 10:12AM - 10:24AM |
A45.00010: Thermal oxidation of Si/SiGe heterostructures for use in quantum dot qubits Samuel F. Neyens, Ryan H. Foote, T. J. Knapp, Thomas McJunkin, D. E. Savage, M. G. Lagally, S. N. Coppersmith, M. A. Eriksson Here we demonstrate dry thermal oxidation of a Si/SiGe heterostructure at 700$^{\circ}$C and use a Hall bar device to measure the mobility after oxidation to be 43,000 cm$^{2}$V$^{-1}$s$^{-1}$ at a carrier density of 4.1$\times$10$^{11}$ cm$^{-2}$. Surprisingly, we find no significant reduction in mobility compared with an Al$_{2}$O$_{3}$ device made with atomic layer deposition on the same heterostructure, indicating thermal oxidation can be used to process Si/SiGe quantum dot devices. This result provides a path for investigating improvements to the gate oxide in Si/SiGe qubit devices, whose performance is believed to be limited by charge noise in the oxide layer. This work was supported in part by ARO (W911NF-12-0607) and NSF (DMR-1206915 and PHY-1104660). Development and maintenance of the growth facilities used for fabricating samples is supported by DOE (DE-FG02-03ER46028). This research utilized NSF-supported shared facilities at the University of Wisconsin-Madison. [Preview Abstract] |
Monday, March 14, 2016 10:24AM - 10:36AM |
A45.00011: Electrode-induced In-plane Strain Variation in Si Quantum Well Joonkyu Park, Youngjun Ahn, Donald Savage, Jonathan Prance, Christine Simmons, Max Lagally, Susan Coppersmith, Martin Holt, Mark Eriksson, Paul Evans Silicon quantum devices are often formed in electrostatically defined quantum dots within Si/SiGe heterostructures incorporating a strained silicon quantum well. Structural variations within the quantum well arise from several sources, including the plastic relaxation of the SiGe substrate and stresses arising from electrodes. The residual stress in the electrode causes an elastic bending distortion of the quantum well that modifies the energy by which the two split-off conduction minima in the silicon quantum well are shifted by biaxial strain. We report a synchrotron hard x-ray nanobeam diffraction study of the quantum well distortion (i) near isolated Pd electrodes and (ii) within a complex quantum dot pattern. The strain difference between the two interfaces of the 10-nm-thick silicon quantum well has a magnitude of up to 10$^{\mathrm{-5}}$ in (i) while it is as large as 10$^{\mathrm{-4}}$ in (ii) which is far larger than the strain difference arising from the plastic relaxation of the SiGe substrate. Mechanical analysis using the edge-force model, shows that the residual stress in the Pd electrode was 350 MPa. We expect that similar effects will arise in all quantum electronic systems with metal-electrode-defined devices. [Preview Abstract] |
Monday, March 14, 2016 10:36AM - 10:48AM |
A45.00012: Are quantum dots in unexpected locations due to strain? Neil Zimmerman, Ted Thorbeck It is a fairly common occurrence that, in top-gated Si quantum dots, the dots appear in reproducible but unexpected positions.~ For instance, sometimes a group will make gates in order to electrostatically generate tunnel barriers, but discover that the quantum dot is formed underneath the gate rather than between two barrier gates.~ We will discuss the possibility that such quantum dots arise from the mechanical strain induced by the gate.~ The model is simple:~ i) We simulate metal or polysilicon gates on top of a Si/SiO{\$}\textunderscore 2{\$} wafer, and calculate the stress and strain from differential thermal contraction of the materials; ii) Using the fact that the energy of the Si conduction band depends on strain through the deformation potential, we then convert the strain modulation to a potential energy modulation.~ As an example, we find that, for a single Al gate, there is a potential well directly underneath the gate with the size of a few meV, in agreement with recent experimental results.~ We also show that polysilicon gates will not produce such strain-induced quantum dots. [Preview Abstract] |
Monday, March 14, 2016 10:48AM - 11:00AM |
A45.00013: Atomic scale quantum circuits in Si A. Dusko, M. Korkusinski, A. Saraiva, A. Delgado, B. Koiller, P. Hawrylak The atomic scale circuits in Si are now realized by manipulation of dangling bonds on Si surface or incorporating dopant atoms in Si by STM techniques. We describe the electronic properties of these atomic scale quantum dot circuits (QDC) by the extended Hubbard-Kanamori Hamiltonian (HK), including on site Coulomb repulsion ($U)$ and interdot hopping ($t)$, direct interaction ($V)$ and exchange ($J)$ terms. The interdot terms strongly depend on dopant position ($R_{D} )$ in Si lattice---small changes in $R_{D} $ strongly impact $t$, $V$and $J$. We study how disorder in $R_{D} $ impacts QDC electronic properties, in particular the interplay of disorder and interactions. With no disorder in $R_{D} $ the energy spectrum (ES) of quantum dot chain at half-filling as a function of $U/t(V,J=0)$shows a transition from ES dominated by kinetic energy ($U/t<<1)$ to ES dominated by Coulomb interactions for $U/t>>1$. The excited states group by single particle energy spacing (Hubbard bands) for weak (strong) interactions. In the weak interaction regime, disorder leads to localization, which strongly affects the electronic properties. We explore the effect of interactions and disorder on HK atomic scale circuits and potential many-body localized phases using Lanczos and Density Matrix Renormalization Group approaches. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700