Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session W26: Focus Session: Semiconductor Qubits - Progress in Si |
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Sponsoring Units: GQI Chair: Charles Tahan, Laboratory for Physical Sciences, University of Maryland Room: 328 |
Thursday, March 21, 2013 2:30PM - 3:06PM |
W26.00001: Robust few-electron quantum dot devices in nuclear spin engineered Si/SiGe Invited Speaker: Dominique Bougeard Spins in gate-defined quantum dots are currently discussed as one of the most promising scalable qubit architecture. Since the identification of the hyperfine interaction as a dominant spin qubit decoherence mechanism, Si/SiGe heterostructures have been receiving steadily increasing attention for realizing devices almost free of nuclear spin carrying isotopes. Building Si/SiGe heterostructures from material enriched in nuclear spin-free isotopes brings new perspectives of reaching a regime of further improved decoherence times compared to Si/SiGe of natural isotope composition. In such isotopically engineered heterostructures, the decoherence is predicted to no longer be governed by the hyperfine interaction with the nuclear spin bath, but solely by dipolar interactions. In the first part of my presentation I will review the development of two-dimensional electron systems in 28Si for spin qubit applications in my group and discuss few electron double quantum dot devices based on these heterostructures. Being able to avoid hyperfine-induced decoherence then brings a second major limitation for the realization of robust spin qubits into focus. Indeed, the manipulation of such qubits relies on Coulomb interactions, enabling electronic noise to cause decoherence. Charge traps in the heterostructure may contribute to decoherence through a fluctuation of charges or through dipolar interactions of the spin degree of freedom of the trap and the qubit. In the second part of my talk I will present our recent study of charge noise in modulation-doped Si/SiGe heterostructures and discuss device and heterostructure designs which efficiently suppress charge noise. [Preview Abstract] |
Thursday, March 21, 2013 3:06PM - 3:18PM |
W26.00002: \textit{In situ} isotopic enrichment and growth of $^{28}$Si for quantum information Kevin Dwyer, Joshua Pomeroy Starting from natural abundance silane gas, we deposit $^{28}$Si films enriched \textit{in situ} to 99.9{\%} in support of solid state quantum information systems. Isotopically enriched materials such as $^{28}$Si are known to act as a ``solid state vacuum'' allowing for qubits with coherence (T$_{2})$ times of minutes. Quantum coherent devices rely on long T$_{2}$ times, but nuclear spin impurities are a major cause of decoherence. Isotopically enriching materials to eliminate stray nuclear spins (such as the 4.7{\%} $^{29}$Si in natural silicon) greatly improves coherence. Our objective is to produce silicon that is not only isotopically enriched, but chemically pure and defect free. We crack and ionize a natural abundance source gas, magnetically mass filter the ions in a beam line, and deposit the enriched material hyperthermal energies. In addition to our first $^{28}$Si samples assessed by SIMS to be enriched to \textgreater\ 99.9{\%}, we previously implanted $^{22}$Ne enriched at 99.4{\%} (9.2{\%} natural abundance) as proof of principle and have also grown $^{12}$C films enriched at \textgreater\ 99.996{\%} (98.9{\%} natural abundance). To our knowledge, no other effort is actively producing enriched solid silicon directly from natural abundance silane. Ongoing improvements are leading us towards our goal of $^{28}$Si enriched to \textgreater\ 99.99{\%} and epitaxial deposition. [Preview Abstract] |
Thursday, March 21, 2013 3:18PM - 3:30PM |
W26.00003: Coherence time of the nuclear spin of ionized phosphorus donors in $^{28}$Si at liquid He and room temperature Michael L.W. Thewalt, Kamyar Saeedi, Stephanie Simmons, John J.L. Morton Remarkable coherence times have recently been reported for the nuclear spin of dilute neutral $^{31}$P in highly enriched $^{28}$Si [1]. For ionized $^{\mathrm{31}}$P, the removal of the hyperfine-coupled electron should result in a nuclear spin even more decoupled from the environment, and hence even longer coherence times at cryogenic temperatures. The coherence time of ionized $^{31}$P was recently observed in natural Si, and while the nuclear coherence time was indeed much longer than the electron coherence time measured in the same device, it was limited to 18 ms due to both the presence of $^{29}$Si as well as the readout mechanism being employed [2]. Here we report on coherence time measurements for ionized $^{31}$P in the same $^{28}$Si samples used for the previous [1] neutral donor study. In addition to the promise of longer cryogenic coherence times, the removal of the hyperfine-coupled electron should result in a profound change in the temperature dependence of T$_{2}$. For the neutral donor, the electron T$_{1}$ decreases very rapidly with increasing temperature, and even at 4.2 K the nuclear T$_{2}$ is limited by the electron T$_{1}$ [1]. This mechanism is absent for the ionized donor, and we will report on nuclear coherence time measurements for ionized $^{31}$P at room temperature.\\[4pt] [1] M. Steger et al., Science 336, 1280 (2012).\\[0pt] [2] L. Dreher et al., Phys. Rev. Lett. 108, 027602 (2012). [Preview Abstract] |
Thursday, March 21, 2013 3:30PM - 3:42PM |
W26.00004: Decoherence of Neutral $^{31}$P Donor Nuclear Spins by $^{29}$Si E.S. Petersen, A.M. Tyryshkin, S.A. Lyon, J.J.L. Morton, K.M. Itoh, M.L.W. Thewalt NMR data from degenerately doped Si:P has suggested that the coherence of $^{31}$P nuclear spins can be limited to a few ms in natural Si by spectral diffusion from $^{29}$Si [1]. Here we report measurements of the nuclear spin coherence of neutral isolated $^{31}$P donors in lightly-doped ($\sim $10$^{15}$ /cm$^{3})$ Si with $^{\mathrm{29}}$Si concentrations from 1\% to 50\%. Pulsed ENDOR at X-band microwave frequency and a magnetic field of 0.35 T was used to measure the nuclear spins. The light doping and measurement temperature of 1.7K ensured that neither electron spin flips nor flip-flops limited the nuclear T$_{2}$. We find that the resulting echo intensity decays are nonexponential, and the time to reach 1/e is inversely proportional to the $^{29}$Si density. The nuclear decoherence time for natural silicon is found to be approximately 1 second, about 2000 times longer than donor electron spins in natural Si.\\[4pt] [1] G.P. Carver et al., Phys. Rev. B 3, 4285 (1971). [Preview Abstract] |
Thursday, March 21, 2013 3:42PM - 3:54PM |
W26.00005: Spin measurement in an undoped Si/SiGe double quantum dot incorporating a micromagnet Xian Wu, Jonathan Prance, Daniel Ward, John Gamble, Donald Savage, Max Lagally, Mark Friesen, Susan Coppersmith, Mark Eriksson We present recent measurements on a double dot formed in an accumulation mode undoped Si/SiGe heterostructure. The double dot incorporates a proximal micromagnet to generate a stable magnetic field difference between the quantum dots. By measuring the ground state and excited state spectrum of this double dot as a function of in-plane magnetic field we identify the (1,1) and (2,0) charge degeneracy point. Using single-shot readout we measure transitions between the (2,0) singlet and the (1,1) triplet states. This method enables the identification of the crossing as a function of detuning between the (1,1) triplet states (both the first and second excited states) and the (2,0) singlet state. We also present data showing that this undoped device has good charge stability and can be measured with high frequency (up to 500MHz) voltage pulses. [Preview Abstract] |
Thursday, March 21, 2013 3:54PM - 4:06PM |
W26.00006: Valley-orbit hybrid states in Si quantum dots John King Gamble, Mark Friesen, S.N. Coppersmith The conduction band for electrons in layered Si nanostructures oriented along (001) has two low-lying valleys. Most theoretical treatments assume that these valleys are decoupled from the long-wavelength physics of electron confinement. In this work, we show that even a minimal amount of disorder (a single atomic step at the quantum well interface) is sufficient to mix valley states and electron orbitals, causing a significant distortion of the long-wavelength electron envelope. For physically realistic electric fields and dot sizes, this valley-orbit coupling impacts all electronic states in Si quantum dots, implying that one must always consider valley-orbit hybrid states, rather than distinct valley and orbital degrees of freedom. We discuss the ramifications of our results on silicon quantum dot qubits. [Preview Abstract] |
Thursday, March 21, 2013 4:06PM - 4:18PM |
W26.00007: A new mechanism for spin and valley relaxation in silicon quantum dots Rusko Ruskov, Charles Tahan We consider spin and valley relaxation in imperfect silicon quantum dots with 1 to 3 electrons. Phonons, spin-orbit coupling, and the electrostatic confining potential of the dot all play roles in both the functional dependence on key parameters (say magnetic field) and the quantitative magnitude of the relaxation rate. Level mixing in the dot allows for spin relaxation via phonons and also explains anti-crossing behavior of dot levels as a function of magnetic field. We show that valley state relaxation can be fast in realistic dots and that spin relaxation can be a few orders of magnitude longer. Our results compare favorably to recent experimental data including the power dependence on magnetic field, location of relaxation hot spots, and the magnitude of the relaxation rates themselves. Some of this work is in collaboration with A. Dzurak group at the University of New South Wales, Australia. [Preview Abstract] |
Thursday, March 21, 2013 4:18PM - 4:30PM |
W26.00008: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 4:30PM - 4:42PM |
W26.00009: Interactions and valley-orbit coupling in Si quantum dots Luyao Jiang, C. H. Yang, Zhaodi Pan, Andrea Morello, Andrew Dzurak, Dimitrie Culcer The valley-orbit coupling in a few-electron Si quantum dot is a function of its occupation number N, and for N \textgreater 1 is in principle renormalized by the electron-electron Coulomb interaction, which is known to be strong. We study the interaction renormalization of the valley-orbit coupling for 2 $\le $ N $\le $ 4, showing that, counterintuitively, interaction effects on the valley-orbit coupling are weak. For N $=$ 2 the renormalization is suppressed by valley interference, while for N $=$ 3 all renormalization terms are zero due to spinor overlaps, and for N $=$ 4 interaction renormalization terms cancel between different pairs of electrons. Experimental observations reveal no evidence of interaction effects on the valley-orbit coupling, consistent with these findings. [Preview Abstract] |
Thursday, March 21, 2013 4:42PM - 4:54PM |
W26.00010: Genetic Design of Enhanced Valley Splitting towards a Spin Qubit in Silicon Lijun Zhang, Jun-Wei Luo, Andre Saraiva, Belita Koiller, Alex Zunger The quantum state of an electron in the Si conduction band holds exceptional promise for quantum computing, owing to its attractive spin coherence properties and adaptability to standard electronics. A paramount challenge is the orbital degeneracy of the lowest conduction band of Si, which is potentially a serious source of decoherence for spin qubits. Hence, isolating a single electron valley state by creating a sufficiently large valley splitting (VS) is a prerequisite for the realization of Si-based spin qubits. Previous explorations of Si quantum wells confined by Si-Ge alloy barriers led thus far to a limited VS of the order of 1 meV or smaller. Here we demonstrate, via an atomically resolved pseudopotential theory, that the monolayer ordering of Si-Ge barriers within reach of modern superlattice growth techniques can be harnessed to enhance the VS by up to one order of magnitude compared to disordered random alloy barriers. A biologically inspired genetic-algorithm search allowed us to identify magic atomic layer sequences of the superlattice barriers that isolate single electron valley state in Si with VS as large as $\sim$9 meV. These results may provide a roadmap for reliable spin-only quantum computing in Si. [Preview Abstract] |
Thursday, March 21, 2013 4:54PM - 5:06PM |
W26.00011: Impact of the valley degree of freedom on the control of donor electrons near a Si/SiO$_2$ interface Andre Saraiva, Alejandra Baena, Maria Calder\'on, Belita Koiller We analyze the valley composition of one electron bound to a shallow donor close to a Si/barrier interface as a function of an applied electric field within a multivalley effective mass model. Switching from low to high fields, the electron ground state is drawn from the donor site into the interface, leaving the donor partially ionized. Valley splitting at the interface occurs due to the valley-orbit coupling, $V_{vo}^I = |V_{vo}^I| e^{i \theta}$. At intermediate electric fields, close to a characteristic shuttling field, the electron states may constitute hybridized states with valley compositions different from the donor and the interface ground states. The full spectrum shows crossings and anticrossings as the field varies. The degree of level repulsion depends on the relative valley compositions, which vary with $|V_{vo}^I|$, $\theta$ and the interface-donor distance. We focus on the valley configurations of the states involved in the donor-interface tunneling process, given by the anticrossing of the three lowest levels. A sequence of two anticrossings takes place and the complex phase theta affects the symmetries of the eigenstates and level anticrossing gaps. Implications of our results on the practical manipulation of donor electrons in Si nanostructures are discussed. [Preview Abstract] |
Thursday, March 21, 2013 5:06PM - 5:18PM |
W26.00012: Localization of Si/SiO$_2$ Interface States: Properties and Physical Implications Belita Koiller, Amintor Dusko, Andre Saraiva Interface states (IS) form spontaneously at some semiconductor-barrier interfaces and they may improve or hinder electronic control and coherence for semiconductor-based qubits. Intrinsic Si/SiO$_2$ IS and its hybridization to the Si bulk states were recently investigated within tight binding (TB) models [1]. From the simplest model (1D), new insights emerge regarding the IS's energy and hybridization with the band states. In this work the 1D TB Hamiltonian is further explored, here within a Green's function formalism. The problem is solved exactly via a decimation technique based on renormalization group ideas [2]. The IS thus obtained are strictly related to the junction of two semi-infinite chains modeling the Si material and the SiO$_2$ barrier, excluding possible contributions from parameters (e.g. chain length) previously invoked [1]. We obtain the energy of IS as well as the exponential longer (shorter) localization lengths into the Si (barrier) material. The IS may be probed experimentally by an external electric field, which modulates the capacitance of the system, or by the spacing between the two lowest levels, related to the valley splitting [1].\\[4pt] [1] Saraiva et al, Phys. Rev. B 82, 245314 (2010).\\[0pt] [2] da Siva and Koiller, Solid State Commun. 40, 215 (1981) [Preview Abstract] |
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