Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session H40: Focus Session: Materials for Quantum Computing II |
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Sponsoring Units: DMP Chair: Britton Plourde, Syracuse University Room: Baltimore Convention Center 343 |
Tuesday, March 14, 2006 11:15AM - 11:27AM |
H40.00001: Microwave absorption by electrons on helium in a tilted magnetic field Solomon Duki, H. Mathur Microwaves can be used to drive transitions between the ground and low-lying excited states of electrons bound to the surface of liquid helium. Such microwave absorption has been observed since the 1970s and is the basis of quantum computing schemes that use electrons on helium. We study theoretically the effect of a tilted magnetic field on the microwave absorption. It is found that the absorption lineshape will change from a symmetric Lorentzian to an asymmetric Fano lineshape as the tilted field is increased. Thus application of the tilted field permits control of the lineshape. [Preview Abstract] |
Tuesday, March 14, 2006 11:27AM - 11:39AM |
H40.00002: Measurement of the charge transfer efficiency of electrons on liquid helium in a CCD-like architecture Guillaume Sabouret, E. A. Shaner, S. A. Lyon Electrons floating on the surface of liquid helium are possible qubits in quantum computing; the necessary two quantum levels can either be the charge states or the spin states of the electrons. Varying electric potentials do not modify spin states, which allows the transport of qubits on the surface of liquid helium using a CCD-like array of underlying gates. A quantum computer will require the controlled movement of single electrons and the reliability of this scheme depends on how readily an electron can be transferred from one gate to another and on the absence of electron traps along the way. We will present a measurement of the charge transfer efficiency (CTE) of electrons clocked back and forth above a short CCD-like structure submerged under liquid helium. This was achieved by using a special clocking sequence that ejects any electrons left above a chosen gate at the end of each clock cycle. The CTE obtained at low clocking frequencies is very high with an electron density of about 0.3 electrons/cm$^{2}$, comparable to the CTE of silicon CCD's with much higher electron densities. We find no evidence for deep electron trapping in our system. [Preview Abstract] |
Tuesday, March 14, 2006 11:39AM - 11:51AM |
H40.00003: A photoelectron source for electrons on liquid helium Shyam Shankar, Guillaume Sabouret, Wei Zhao, Fabrice Amy, Antoine Kahn, S. A. Lyon Manipulation of the spins of electrons on the surface of liquid helium is a promising method to implement a quantum computer. One of the challenges in this method is to generate electrons on the helium surface in a reliable and controlled manner. The traditional approach is to use thermionic emission from a tungsten filament. This approach generates many electrons in an uncontrolled manner and heats the low temperature system. An electron source relying on photoemission from a zinc film has been previously described[1] using a high power light source coupled to a fiber. This work has been reproduced in a more compact manner by using a small modular lamp and proximity focussing. However, zinc films tend to oxidize rather quickly. We are studying the use of alternative low work function materials such as samarium that promise better stability. [1] L. A. Wilen and R. W. Gianetta, Rev. Sci. Instrum. 56(11), November 1985. [Preview Abstract] |
Tuesday, March 14, 2006 11:51AM - 12:27PM |
H40.00004: Oxygen Instabilities and the Electronic Properties of Oxide Tunnel Barrier Layers. Invited Speaker: Very thin AlO$_{x}$ layers, usually formed by the controlled dose, RT oxidation of the surface of Al thin films, have long been the most successful and most reproducible approach for the fabrication of high-quality superconducting Josephson junctions, as well as for the formation of the gate-insulator for myriad non-conventional electronics studies. AlO$_{x}$ has also been widely employed as the barrier in magnetic tunnel junctions (MTJs). In recent years, however, it has become clear that for demanding applications, most notably Josephson junction qubit devices, the performance of the AlO$_{x}$ barrier layer is far below what is needed, particularly with respect to 1/f noise behavior; a determination that has given new urgency to the task of better understanding and significantly improving the properties of this barrier layer material, or of developing a better alternative. I will describe the results of a series of scanning tunneling spectroscopy, XPS and electronic transport studies on ultra-thin AlOx layers, in both half-formed and fully-formed junction configurations. These results demonstrate the inherently unstable nature of a significant portion of the oxygen component of the oxide. As the result of this instability, the process that forms a completed AlOx tunnel junction is much more complex than the reproducibility of the process might otherwise suggest. I will compare this character of amorphous AlOx barrier layers with the oxygen stability in crystalline MgO layers such as are currently being employed in MTJs, and on the basis of these findings discuss some approaches that might lead to more stable, lower noise Josephson junctions. Finally I will discuss some preliminary results that suggest that a low T tunnel-junction 1/f noise level that is much more than 10x less than the best that has been reported to date may indeed be achievable. [Preview Abstract] |
Tuesday, March 14, 2006 12:27PM - 12:39PM |
H40.00005: Eliminating Junction Fluctuators using Epitaxial Barrier in Josephson Qubit Seongshik Oh, K. Cicak, M. S. Allman, Jeffrey S. Kline, K. D. Osborn, G. Prokopenko, M. A. Sillanpaa, A. J. Sirois, J. A. Strong, J. D. Whittaker, R. W. Simmonds, D. P. Pappas, R. McDermott, John M. Martinis Before a multi-qubit solid-state quantum computer is realized, many obstacles need to be overcome. The most significant problem with any solid- state qubit implementation is strong coupling to sources of decoherence in the environment. Accordingly, identifying and removing these sources is an important prerequisite for construction of a solid-state quantum computer. Among other things, all the present-day superconducting qubits use amorphous AlO$_{x}$ as a tunnel barrier. However, there is growing evidence that the amorphous AlO$_{x}$ tunnel barriers have undesirable two-level fluctuators that adversely affect the qubit. Along this line, we have fabricated the first epitaxial Josephson phase qubit with single-crystal Al$_{2}$O$_{3}$ barrier and observed substantially reduced density of two-level fluctuators in this new type of qubit. This is the first clear evidence showing the relationship between crystallinity of the tunnel barrier and the two-level fluctuators. With this new epitaxial Josephson qubit technology, we are one-step closer to realization of a practical multi-qubit solid-state quantum computer. This talk will be focused more on the fabrication side of the epitaxial qubit and the following talk will discuss the details of its measurement. [Preview Abstract] |
Tuesday, March 14, 2006 12:39PM - 12:51PM |
H40.00006: Improved Phase Qubits with Re-Al$_{2}$O$_{3}$-Al Junctions with Epitaxial Barriers. K. Cicak , M.S. Allman , Jeffrey S. Kline , Seongshik Oh , K.D. Osborn, G. Prokopenko , M.A. Sillanpaa , A.J. Sirois , J.A. Strong , J.D. Whittaker, John M. Martinis, D.P. Pappas , R.W. Simmonds In our efforts to identify and eliminate sources of decoherence in our qubits we have established that dielectric loss, both from substrate and from insulating layers in the device, can compromise qubit coherence. This has lead to significant improvements of our phase qubits. Now we show that the structural quality of the Josephson junction barrier itself can significantly improve qubit performance in a specific way. As compared to qubits with amorphous barriers, our latest measurements using qubits fabricated with epitaxial Josephson tunnel barriers show a significant decrease in density and coupling strength of individual spurious resonator defects. [Preview Abstract] |
Tuesday, March 14, 2006 12:51PM - 1:03PM |
H40.00007: Epitaxial growth of Mo/Al$_{2}$O$_{3}$/Mo trilayers for Josephson junction qubits Jeffrey S. Kline, Kristine M. Lang, Seongshik Oh, Kevin D. Osborn, Raymond W. Simmonds, Robert McDermott, John M. Martinis, David P. Pappas The growth of ultrathin epitaxial Al$_{2}$O$_{3}$ tunnel barriers on Re has been proven to reduce the number of spurious resonators in Josephson phase qubits when compared to qubits fabricated with amorphous tunnel barriers. Other refractory metals might also be used as base layers for epitaxial Al$_{2}$O$_{3}$ growth. In this work, Mo films were deposited onto Al$_{2}$O$_{3}\left( {11\bar {2}0} \right)$ substrates by UHV magnetron sputtering. To achieve epitaxy, the substrate was held at elevated temperature during the deposition. Post-deposition high temperature anneals increase the terrace size and occurrence of step-bunching as observed by STM. Ultrathin films of Al$_{2}$O$_{3}$ were then deposited at room temperature onto the Mo base layer by reactive evaporation of Al in a controlled oxygen background. A high temperature anneal in an oxygen background crystallizes the amorphous Al$_{2}$O$_{3}$ layer. Finally, the structure is capped with polycrystalline Mo by sputter deposition at room temperature. Dielectric loss measurements on LC oscillators fabricated from these structures will be reported. [Preview Abstract] |
Tuesday, March 14, 2006 1:03PM - 1:15PM |
H40.00008: Josephson Tunnel-Junctions Fabricated From Epitaxial Niobium-Based Multi-Layers Paul B. Welander, Tim J. McArdle, James N. Eckstein We report on the investigation of Josephson tunnel-junctions fabricated from epitaxial multi-layers. The foundation of these devices is the growth of a single-crystal niobium base electrode on sapphire. Niobium films grown near 800 $^{\circ} $C typically have critical temperatures around 9.4 K and residual resistance ratios above 100. Diffraction measurements show excellent crystallinity, and microscopy reveals surfaces with mono-layer step-edges and rms roughness less than 0.2 nm. Tunnel barriers are formed using a range of methods. On one hand, epitaxial alumina is grown on niobium above 700 $^{\circ} $C by evaporating aluminum metal in an oxygen gas background of about 1 microtorr. On the other hand, an aluminum single- crystal film about 20 nm thick is deposited on niobium at room temperature and then oxidized in 10-100 torr oxygen gas for about one hour. The counter-electrode in both cases is amorphous niobium deposited at ambient temperature. The latter method of barrier formation produces tunnel-junctions with critical current densities around 100 A/cm$^2$. [Preview Abstract] |
Tuesday, March 14, 2006 1:15PM - 1:27PM |
H40.00009: Enhanced Macroscopic Quantum Tunneling in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ Intrinsic Josephson Junction Stacks X.Y. Jin, J. Lisenfeld, Y. Koval, A. Lukashenko, A.V. Ustinov, P. M\"uller We have investigated macroscopic quantum tunneling in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ intrinsic Josephson junctions and performed microwave absorption experiments. Classical-to- quantum crossover temperatures of up to 700mK were found. Plasma frequencies of up to 1.13 THz have been observed. In uniformly switching intrinsic Josephson stacks, the escape rate from zero- voltage state is enhanced by several orders of magnitude in comparison to single intrinsic junctions having comparable junction parameters. This enhancement is due to the unique stacking structure of intrinsic Josephson junctions. [Preview Abstract] |
Tuesday, March 14, 2006 1:27PM - 1:39PM |
H40.00010: Coupled Quantum Dots in the Kondo regime: interference and filtering effects. Luis Dias da Silva, Nancy Sandler, Kevin Ingersent, Sergio Ulloa Double quantum-dot systems (DQDs) provide a vast array of possibilities for both theoretical and experimental investigations of the Kondo regime. In this work, we propose DQDs as a possible experimental realization of a Kondo impurity coupled to an effective structured (non-constant) density of states (DoS). We consider a DQD in parallel configuration coupled to metallic leads. By changing the lead-dot and dot-dot couplings, the effective hybridization function for an individual dot displays sharp resonances and/or pseudogaps, allowing for an experimental probe into the transition between both regimes. Using numerical renormalization group methods, we calculate the dot's spectral function in different regimes. For a dot weakly coupled to the leads and strongly coupled to the second dot, the effective DoS has a sharp resonance with width $\Delta$ and the spectral density shows a splitting in the Kondo resonance for $T_K>\Delta$, although the Kondo singlet is \textit{preserved}. Furthermore, for small inter-dot coupling, second order dot-dot interactions through the conduction electrons lead to the formation of a pseudo-gap. The spectral density goes to zero as a power-law $|\epsilon-\epsilon_F|^2$ and the Kondo screening is suppressed. Supported by NFS-NIRT. [Preview Abstract] |
Tuesday, March 14, 2006 1:39PM - 1:51PM |
H40.00011: Spin polarized current in a double quantum dot with time-dependent interdot tunneling Ernesto Cota, Francisco Mireles, Fernando Rojas, Sergio E. Ulloa A bipolar spin filter has been proposed using a few-electron double quantum dot (DQD) in a `hanging-dot' configuration [1], and tunable by controlling the molecular hybridization (interdot tunneling) between dots. The bipolar nature of the electron spin current arises from a singlet-triplet transition in the ground state of the two-electron DQD. The transition occurs due to the competition among Zeeman energy, exchange interaction and interdot coupling, favoring a lower (higher) energy for the triplet (singlet) configuration in the weak (large) interdot tunneling regime. In this work we use a density matrix equation to study the dynamical behavior of the DQD, by considering the adiabatic variation of the interdot tunneling in time. We report on the spin-polarized current through the DQD as a function of the relevant physical parameters. A bipolar spin filtering effect in the time domain is observed to arise on time scales smaller than the relevant coherent and spin relaxation times in typical QDs. Spin-orbit coupling is included in our approach but found to not significantly affect our results. [1] F. Mireles, E. Cota, F. Rojas and S.E. Ulloa, submitted. [Preview Abstract] |
Tuesday, March 14, 2006 1:51PM - 2:03PM |
H40.00012: Charge sensing in Si/SiGe quantum dots using single electron transistors Feng Pan, Tim Gilheart, Alexander Rimberg, Keith Slinker, Lisa McGuire, Mark Eriksson, J. O. Chu Qubit schemes based on silicon technology have obvious economic appeal as well as compelling physical motivations, such as a long spin-spin dephasing time. Proposed silicon solid-state qubit schemes include quantum dots coupled to fast readout devices, such as a quantum point contact or single electron transistor (SET). Recently, Si/SiGe quantum dots defined by Schottky gates deposited on a Si/SiGe heterostructure containing a high mobility two-dimensional electron gas have been fabricated.[1] Here we report the integration of a SET with such a Si/SiGe quantum dot. Several such devices have been produced, and recent measurements, including transport and sensing of the dot charge with the SET, will be discussed. [1] K A Slinker et al 2005 New J. Phys. {\bf 7} 246 [Preview Abstract] |
Tuesday, March 14, 2006 2:03PM - 2:15PM |
H40.00013: Few Electron Si/SiGe Quantum Dots Levente Klein, Srijit Goswami, Don Savage, Keith Slinker, Lisa McGuire, Max Lagally, Mark Eriksson Fully top-gated silicon/silicon-germanium quantum dots have been fabricated and characterized at low temperatures. For the quantum dots with few electron occupation, by varying the gate voltages two regimes may be accessed: at large negative voltages sharp Coulomb diamonds and classical Coulomb blockade is observed with a charging energy of 3.8 meV. At more positive gate voltage, co-tunneling features appear and a zero-bias conductance anomaly is observed with a behavior closely resembling the quantum dot Kondo effect. The increased zero-bias conductance vanishes with increasing temperature, and this peak splits into two peaks at finite drain-source voltage with increasing perpendicular magnetic field. The peak splitting increases linearly with increasing magnetic field. We discuss the various conductance regimes and the application of these types of quantum dots for quantum information processing. [Preview Abstract] |
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