Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J17: Focus Session: Materials and Device Physics for Quantum Computing |
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Sponsoring Units: DMP DCMP Chair: Carlo Piermaroochi, Michigan State University Room: LACC 404B |
Tuesday, March 22, 2005 11:15AM - 11:27AM |
J17.00001: Single electron transistor (SET) devices for probing single donors in Si and for microscopic CV characterization L. Sun, K. R. Brown, B. E. Kane We will describe SET devices fabricated in our group for measurement of single donors. The Al/Al$_{2}$O$_{3}$/Al SET is fabricated with standard electron-beam lithography and double-angle thermal evaporation. A SiO$_{2}$ barrier layer about 20 nm thick isolates the SET from the lightly n-doped silicon, and the substrate is heavily boron doped using high energy ion implantation, and hence conducting at low temperature, beginning a few hundred nm below the SiO$_{2}$/Si interface. We will describe our fabrication process and characterization of our devices made with capacitance voltage (CV) measurement. CV measurement is a traditional tool for probing doping, carrier densities and interface trap densities. It can be extended to microscopic regime using SET electrometers. In the SET measurements, after the voltage on the substrate was swept, the recorded coulomb blockade peaks were counted and the capacitance could be extracted. We will compare the macroscopic and microscopic CV measurements in our devices. Finally, we will present the results of electrostatic modeling of our device design and discuss improvements to the design that will enhance the sensitivity to motion of single electrons at donors.. [Preview Abstract] |
Tuesday, March 22, 2005 11:27AM - 11:39AM |
J17.00002: Field ionization of individual donors in Si measured with a single electron transistor K. R. Brown, L. Sun, B. E. Kane Many proposals for spin qubits in semiconductors rely on spin-charge conversion combined with charge measurement for determination of the final state. In pursuit of such a measurement we have engineered devices consisting of a single electron transistor (SET) on a lightly n-doped, oxidized (thickness 25 nm) Si wafer. A p++-doped region 250 nm below the oxide serves as a back gate for donor ionization, creating an electric field towards the SET island and perpendicular to the surface. It also pins the Fermi level in the substrate, so that we can empty or fill the donors above by applying an appropriate bias. To empty the donors we apply a strong positive voltage to the back gate, pulling any donor electrons into the substrate. To fill the donors we apply a negative voltage and momentarily shine an LED at the sample, flooding it with electron-hole pairs. We will present recent results that give strong evidence for the filling and emptying of donors and for their ionization and recapture in an electric field as measured by an SET. We will conclude with a discussion of our efforts to extend these techniques to a measurement of spin. [Preview Abstract] |
Tuesday, March 22, 2005 11:39AM - 11:51AM |
J17.00003: Detection of Quantum State of Electrons Bound to Shallow Donors via Resonance Fluorescence: Ensemble Measurements Dan Allen, Mark Sherwin Electrons bound to shallow donors in GaAs have a hydrogenic spectrum with principal transitions in the THz range and below the optical phonon energy. The 1S and 2P levels serve as a model solid-state qubit. Bound excitons can be resonantly excited from donors in their 1S state. Since the probability of decay to the 1S state is high, donors in the 1S state scatter light; donors in excited states do not. This is similar to the cycling transition used for readout in ion trap quantum computers. When the bound exciton recombines, some energy may be transferred to the donor, leaving it in an excited state. This Auger process is the limiting factor in making nondemolition measurement of the qubit. We present measurements of the change in resonance fluorescence of ensembles of donor-bound electrons due to THz excitation, corresponding to a decrease in the fraction of electrons in the 1S state. Resonance fluorescence is less destructive to the qubit than photoconductivity-style measurements, and may be used to measure lifetimes of states of bound electrons. Research supported by CNID, DARPA-QUIST, and Sun Microsystems. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J17.00004: Quantum computing on long-lived donor states of Li in Si V.N. Smelyanskiy, A.G. Petukhov, V.V. Osipov We predict a gigantically long lifetime of several excited states in the ground-state ({\em 1s}) manifold of an interstitial lithium donor in silicon. The nature of this effect roots in the anomalous level structure of the {\em 1s} Li manifold under external stress. In particular, the coupling via the deformation potential between the lowest two states of the opposite parity is very weak and occurs via intervalley phonon transitions only. We propose to use these states under the controlled ac and dc mechanical stress to process quantum information. We find an unusual form of the elastic-dipole interaction between different donors. This interaction scales with the inter-donor distance $R$ as $R^{-3}$ or $R^{-5}$ for the transitions between the states of the same or opposite parity, respectively. The long-range $R^{-3}$ interaction provides an extremely high fidelity mechanism for 2-qubit operations. [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J17.00005: Gate voltage control of exchange interaction for phosphorous donors in silicon. Angbo Fang, Yia-Chung Chang, John R. Tucker We perform realistic calculations for coupled phospherous donors in silicon delta-doping sheet, which is relevant for silicon-based quantum computation. With the help of generalized unrestricted Hartree-Fock method, we study the influence of valley-orbit interaction on the exchange coupling. We also solve the tunable gate potential by Poisson's equation and study the gate votage dependence of the exchange splitting. The implications are examined for silicon-based quantum computer architecture, where phospherous donor electron spin encodes logic qubit and exchange interaction is employed to generate entanglement among qubits. [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J17.00006: Pulsed Electrically Detected Magnetic Resonance of 2D Electrons in a Si/SiGe Quantum Well Alexei Tyryshkin, Stephen Lyon, Wolfgang Jantsch, Friedrich Schaffler We have developed a new method of pulsed EDMR (Electrically Detected Magnetic Resonance) and applied it to measure spin relaxation times of 2D electrons in a Si/SiGe quantum well. The method is based on spin-dependent transport in the 2D channel: Conduction electrons scatter off each other, and their scattering cross section depends on the relative orientation of their spins [1]. The initial, thermal polarization of 2D electron spins (at H=350 mT and T=4 K) is altered by applying the resonant 10 GHz microwave pulses. A change in the spin polarization results in a variation of the device conductivity ($\sim $10$^{-4})$, and its recovery back to the thermal equilibrium is measured after the microwave pulse. The recovery time measures the spin relaxation, and we find T$_{1}$ = 1.4 $\mu $s for 2D electrons in a modulation-doped Si quantum well, the same time as we measure with conventional pulsed spin resonance. This new pulsed EDMR method will allow the measurement of T$_{1}$ and T$_{2}$ on small semiconductor structures with sensitivity down to a few spins, possibly a single spin. [1] Ghosh and Silsbee, Phys. Rev. B 42, 12508(1992). [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J17.00007: Microwave Spectroscopy of the Valley Splitting in a Silicon/Silicon-Germanium Two Dimensional Electron Gas Srijit Goswami, J.L. Truitt, Charles Tahan, L.J. Klein, K.A. Slinker, D.W. van der Weide, S.N. Coppersmith, Robert Joynt, R.H. Blick, Mark A. Eriksson, J.O. Chu, P.M. Mooney The strain in silicon/silicon-germanium quantum wells reduces the usual six-fold degeneracy of the silicon conduction band, leaving a pair of degenerate bands in the growth direction. Quantum confinement in the silicon well further splits this degeneracy, leading to a small, but extremely important energy gap (the valley splitting) between these lowest two levels. We perform microwave spectroscopy, electron valley resonance (EVR), between these two states. Transport measurements at 0.25 K in a silicon/silicon-germanium two dimensional electron gas (2DEG) are used to detect microwave absorption at the valley splitting energy. The lineshapes are similar to those observed in electrically detected electron spin resonance signals. The valley splitting is found to increase linearly with an applied perpendicular magnetic field. The valley splitting peak shows a dramatic (seven-fold) increase in width as the temperature is increased from 0.23~K to 0.35 K. These results indicate that in moderate magnetic fields the silicon valley degeneracy can be completely removed in low temperature quantum devices. [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J17.00008: Valley splitting in low-density quantum-confined heterostructures: Superposition, not Spin! Timothy Boykin, Gerhard Klimeck, S. Coppersmith, Mark Friesen, Paul von Allmen, Seungwon Lee, Fabiano Oyafuso Although valley splitting in Si quantum-confined heterostructures has been studied for many years, it is far less well understood than one might expect. Because valley degeneracy is problematic in spin quantum computing as a potential source of decoherence and other difficulties it is essential that its origins be thoroughly explained. We explain the valley splitting in Si quantum wells using a simple tight-binding model which eliminates the artificial valley coupling constants found in multiband/multi-valley effective mass treatments. The results of the simple tight-binding model agree well with multiband tight-binding results, and explain the changing parity of the ground state, and the behavior of the splitting as a function of well width. The results show that the valley splitting has nothing to do with spin, but is instead purely due to the superposition of states in the quantum well. [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J17.00009: Long-lived spin and valley states in silicon for quantum information processing Charles Tahan, Mark Friesen, Robert Joynt Quantum electronics in silicon offers both challenges and opportunities for information technology. Here we take a challenge, the doubly degenerate conduction band minima of silicon quantum wells, and make it into an opportunity, stable valley states for quantum storage and control. We calculate the coupling between the two valley states using both tight-binding and approximate-analytic techniques for a lateral quantum dot. This determines the valley relaxation and optical excitation rates. Not only are the relaxation times uncharacteristically long for excited orbital states, but we find that for finite quantum wells there are 'magic' electric fields where the coupling goes to zero, suppressing valley relaxation and excitation. In the process we derive new expressions for single-valley orbital relaxation (which is very fast) and qubit spin-flip times (due to spin-orbit coupling) and compare them to valley state relaxation. We discuss important implications for 'valley qubits' in silicon quantum information processing and technology. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J17.00010: Fabrication of Encapsulated H-Passivated Si(111) Surfaces for 2D Electron Systems R. N. McFarland, K. Eng, B. E. Kane H-passivated silicon surfaces may provide an excellent high-mobility substrate for 2-D electron systems (2-DES) and, potentially, atomic-scale quantum devices. We have prepared H-Si (111) surfaces \textit{ex situ} using NH$_{4}$F and incorporated these atomically flat surfaces into novel field effect devices. Using Si-SiO$_{2}$ contact bonding, we encapsulate the H-Si (111) surface in a vacuum cavity, which both isolates the surface from the environment and provides a dielectric through which we can gate electrons. However, successful bonding requires both surfaces being bonded to be atomically flat (rms roughness $<$ 0.5 nm). We have observed that making ohmic contact to the 2-DES via P implantation into the Si (111) affects the surface flatness where significant height variations are created at the contact boundaries due to differential oxidation and etch rates between doped and undoped Si regions. Such topographical irregularities can inhibit contact bonding. Using AFM, we have studied these topographic features on our device surfaces, and report methods for obtaining an overall rms surface roughness $<$ 0.2 nm and for reducing the doping-induced height difference to $\le $ 0.5nm by controlling implantation, annealing, and etching parameters. Finally, we discuss ongoing work and the possible implications for quantum computing architectures. [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J17.00011: Electron Transport of a 2-D Electron System Gated on a Hydrogen-Passivated Si (111) Surface via a Vacuum-Silicon Interface K. Eng, R. N. McFarland, B. E. Kane Creating a 2-D electron system to couple with atoms on a surface or to nuclear spins buried in semiconductors are non-trivial due to the inherent presence of disorder at the semiconductor-dielectric interface. However, it has been shown that the H-passivated Si surface in vacuum is an ideal candidate to build such devices because it can be atomically flat and entirely free of dangling bonds. We will discuss the characterization of a new field effect transistor which creates a 2D electron system on a H-passivated Si (111) surface gated through a vacuum-silicon interface. The H-Si (111) surface is preserved and encapsulated in a vacuum cavity via contact bonding of two silicon substrates (*). We report for the first time low temperature electron transport on H-Si (111) surfaces. Hall mobilities at 4.2K were measured to be $\sim $ 4000 cm$^{2}$/Vs which are higher than previous results in Si (111) MOSFETs. We also investigated the possible six-fold degeneracy of the H-Si (111) surface through low temperature (T$<$4K) magnetoconductance measurements up to B=12T. Results of the longevity of these new devices along with potential applications in quantum computing will also be discussed. * Discussed in talk given by R. N. McFarland, ``Fabrication of Encapsulated H-Passivated Si (111) Surfaces for 2-D Electron Systems''. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J17.00012: Top-gated Quantum Dots in Silicon / Silicon-Germanium Two-Dimensional Electron Gases Keith A. Slinker, K.L.M. Lewis, C.C. Haselby, Srijit Goswami, L.J. Klein, J.L. Truitt, D.E. Savage, J.O. Chu, D.W. van der Weide, S.N. Coppersmith, P.M. Mooney, Mark Eriksson Electrons in silicon/silicon-germanium two-dimensional electron gas quantum dots are a promising architecture for spin based quantum computation. Top gated quantum dots allow precise tuning of electron shape and interdot coupling. We report the observation of Coulomb blockade in Si/SiGe quantum dots defined by a combination of etching and metal top gating. A narrow channel or mesa is fabricated by electron beam lithography and subsequent reactive ion etching. Metal gates are deposited across the channel to define the leads of the dot. The sides of dot are defined by surface depletion from the etched sidewalls. Low temperature measurements (250mK) show a single electron charging energy of about 0.8meV. We use an etch- defined side gate to vary the potential in the dot, observing several conductance oscillations as the blockade is lifted, with a period of 280mV. [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 1:51PM |
J17.00013: Quantum Dot Spin Qubits: Decoherence from Nearby Impurities Mark Friesen We study the undesired exchange coupling between quantum dot spin qubits and other nearby electronic spins trapped on dopant impurities. Such coupling is a source of decoherence. The problem is treated in the context of a 2DEG heterostructure, with strong, local electric fields. For silicon-based systems, we develop the theory of the Stark effect for P:Si in a degenerate conduction band. We investigate the resulting Stark energy spectrum and the field dependence of the valley composition parameters. [Preview Abstract] |
Tuesday, March 22, 2005 1:51PM - 2:03PM |
J17.00014: Single spin detection in endohedral fullerenes Paul Delaney, Andreas Larsson, Jim Greer Reading out single spins is challenging. We study the endohedral system of a nitrogen atom trapped inside C$_{60}$. The qubit here is the total electronic spin of the three valence nitrogen 2p electrons (a spin quartet). We propose a method of measuring this spin by placing the endohedral fullerene in a circuit and passing a current through it. If an electron hops onto the fullerene it becomes an anion, and we use the energy splitting between the triplet and quintuplet state of the N@C$_{60}$ anion to make the spin-polarisation of the current passed by the fullerene depend on the state of the qubit inside it. We estimate the size of this energy splitting and of the hopping matrix element between the fullerene and a nearby source or drain electrode. From these data we estimate temperature ranges and experimental geometries necessary for our read-out scheme. [Preview Abstract] |
Tuesday, March 22, 2005 2:03PM - 2:15PM |
J17.00015: Electron Spin Resonance on a Single Carbon Nanotube Christopher Rutherglen, Peter Burke Little is known about the spin properties of carbon nanotubes (CNT) such as their spin-coherence time. We are in process of directly determining the electron spin coherence time of a single walled carbon nanotube by measuring the microwave reflection (S11) off a single CNT in a magnetic field at 0.3K. We expect to observe resonant microwave absorption at the Zeeman frequency, which is 27GHz/Tesla. The linewidth of these absorption peaks will provide a direct measurement of the spin-coherence time of the CNT electrons which is currently lacking in the research literature. Absorption peaks associated with the Coulomb energy, the quantum energy level separation, the energy mismatch between bands are also expected to be measured. A homodyne reflectometer has been constructed in our lab that can resolve S11 changes of 1 part in 10\^{}5. We expect that our technique of measuring the microwave reflection off of a single nanostructure will be a power spectroscopic tool to investigate a wide variety of quantum excitations in nanostructures, an important prerequisite for powerful quantum information processing based on integrated nanosystems. [Preview Abstract] |
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