Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P27: Focus Session: Semiconductor Qubits - Spin Readout, Backaction, and Valley Physics in Silicon |
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Sponsoring Units: GQI Chair: Irfan Siddiqi, University of California, Berkeley Room: C155 |
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P27.00001: Development of a Si/SiO$_2$–based double quantum dot charge qubit with dispersive microwave readout M.G. House, E. Henry, A. Schmidt, O. Naaman, I. Siddiqi, H. Pan, M. Xiao, H.W. Jiang Coupling of a high-Q microwave resonator to superconducting qubits has been successfully used to prepare, manipulate, and read out the state of a single qubit, and to mediate interactions between qubits. Our work is geared toward implementing this architecture in a semiconductor qubit. We present the design and development of a lateral quantum dot in which a superconducting microwave resonator is capacitively coupled to a double dot charge qubit. The device is a silicon MOSFET structure with a global gate which is used to accumulate electrons at a Si/SiO$_2$ interface. A set of smaller gates are used to deplete these electrons to define a double quantum dot and adjacent conduction channels. Two of these depletion gates connect directly to the conductors of a 6 GHz co-planar stripline resonator. We present measurements of transport and conventional charge sensing used to characterize the double quantum dot, and demonstrate that it is possible to reach the few-electron regime in this system. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P27.00002: Dispersive microwave readout of a double quantum dot charge qubit in silicon Edward Henry, Andrew Schmidt, Mathew House, Ofer Naaman, H. Pan, Ming Xiao, Hong-wen Jiang, Irfan Siddiqi Microwave resonators coupled to quantum systems have been used for fast dispersive measurement in many different architectures in solid state and atomic physics. The electronic states of a semiconductor quantum dot represent a promising candidate for quantum information processing. Our work is geared toward developing a fast, non-demolition readout of semiconductor qubit by coupling to a superconducting resonant circuit. We report on microwave measurements of a lateral quantum dot, realized using a silicon MOSFET structure, where the charge degree of freedom is capacitively coupled to a shorted quarter wave 6 GHz resonator. We characterize the sensitivity of this charge detection scheme and its implications for qubit readout fidelity. [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P27.00003: Radio Frequency Single Electron Transistors on Si/SiGe Mingyun Yuan, Zhen Yang, A.J. Rimberg, M.A. Eriksson, D.E. Savage Superconducting single electron transistors (S-SETs) are ideal for charge state readout due to their high sensitivity and low back-action. Upon successful formation of quantum dots(QDs) on Si/SiGe, aluminum S-SETs are added in the vicinity of the QDs. Coupling of the S-SET to the QD is confirmed by using the S-SET to perform sensing of the QD charge state at 0.3 K. We have formed a matching network for an SET with an off-chip inductor. The reflection coefficient of the radio frequency(RF) signal is shown to be modulated by the SET resistance. Efforts to develop an on-chip matching network and perform charge sensing with the RF-SETs are in progress. Recent experimental results will be discussed. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P27.00004: Two-detector two-qubit correlated continuous measurements and their implications for quantum computing Rusko Ruskov, Charles Tahan We calculate the full counting statistics for a system of two interacting qubits which are simultaneously measured by weakly coupled linear detectors. Two approaches are considered based on rate equations for the full system-detectors density matrix and on quantum filtering equations. Implications for the assessment of quantumness in physical devices based on charge qubits are considered. In addition we consider applications of such systems to practical quantum computing in silicon and/or GaAs quantum dots. [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P27.00005: Backaction due to Resonant Phonon Absorption in Quantum Dots Measured by a Quantum Point Contact Carolyn Young, Aashish Clerk Recent experiments have observed unexplained periodic resonances in the charging diagrams of both double [1] and triple [2] quantum dots (DQDs and TQDs). These resonances correspond to the generation of inelastic transitions, driven by energy transfer from a biased quantum point contact (QPC) charge detector used for measurement. In this talk, we present theoretical results describing how quantum backaction due to hot phonons, generated by the out-of-equilibrium QPC, can lead to excited state occupation under certain ``blocking'' conditions that result in slow ground state filling. We propose that recent experiments can be understood in terms of resonant phonon absorption in DQDs and TQDs; a process complementary to resonant phonon emission [3]. Our results shed light on an important contribution to the backaction of the QPC readout scheme widely used for QD-based quantum computation. \\[4pt] [1] D. Harbusch et al., Phys. Rev. Lett., 104, 196801 (2010). \\[0pt] [2] L. Gaudreau et al., App. Phys. Lett., 99, 193101 (2009). \\[0pt] [3] U. Gasser et al., Phys. Rev. B, 79, 035303 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P27.00006: Measurement fidelity in the presence of coherent dynamics or dissipation Jian-Qiang You, S. Ashhab, Franco Nori We analyze the problem of a charge qubit probed by a quantum point contact when the measurement is concurrent with Hamiltonian-induced coherent dynamics or dissipation. This additional dynamics changes the state of the qubit before the measurement is completed. As a result, the measurement fidelity is reduced. We calculate the reduction in measurement fidelity in these cases. References: S. Ashhab, J. Q. You, and F. Nori, New J. Phys. 11, 083017 (2009); Phys. Scr. T137, 014005 (2009). [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P27.00007: Probing coherent tunneling in semiconductor quantum dots using electromechanical backaction Jamie Gardner, Aashish Clerk Self-assembled quantum dots have been studied intensely because of their possible applications to quantum information processing. While such dots are difficult to characterize using direct electrical transport measurements, it has recently been shown both theoretically [1] and experimentally [2] that a capacitively coupled AFM cantilever can serve as a sensitive probe of dot charge dynamics and electronic level structure. This sensitivity is based on the fact that the dot, which is tunnel-coupled to electrons in a reservoir, acts as a dissipative bath for the cantilever. Here, we extend previous theoretical work to describe an AFM cantilever coupled to a double quantum dot. Unlike a single-dot, the double-dot system exhibits both incoherent tunneling to the leads and coherent tunneling between the dots. We find that the cantilever's motion is affected by both kinds of tunneling and can yield significant information even in regimes where the total double-dot charge does not fluctuate. Cantilever dynamics can also be used to learn about the strength of dephasing processes in the double-dot. After presenting the theoretical approach to this problem, we will discuss the results in the context of current experimental efforts using InAs dots. These effects should also be accessible in a variety of other quantum dot setups. [1] S. D. Bennett, et al., Phys. Rev. Lett. \textbf{104}, 017203 (2010). [2] L. Cockins, et al., Proc. Nat. Acad. Sci. \textbf{107}, 9496 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P27.00008: Few-electron states in SiGe double quantum dot structures with non-planar interfaces A.A. Kiselev, R.S. Ross, M.F. Gyure Valley-orbit effects of planar, non-planar, and imperfect heterointerfaces (both on the intra- and inter-dot scale) are directly captured in numerical simulations and analyzed theoretically for electrostatically defined accumulation mode (001) SiGe multi-dot structures. Our modeling is facilitated by explicitly allowing for an arbitrary and spatially inhomogeneous stacking of heterolayers in the active area of the device. Here we focus on results obtained for a double quantum dot (DQD) system, establishing the detailed structure of few-electron states, and, for two electrons, their spin- and valley-selective dynamics when the system is driven by pulse-modulating dot gate potentials. We identify valley-related avoided crossings and evaluate their strength affecting adiabaticity of applied bias sweeps. We consider a number of experimentally relevant scenarios stemming from (i) macroscopic interface imperfections, e.g., interface steps, and (ii) randomness of the substitutional solid solution in the SiGe barrier layers. Our findings are critically compared with results available for single valley III-V DQDs. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P27.00009: Extended interface states enhance valley splitting in Si/SiO2 Andre Saraiva, Belita Koiller, Mark Friesen Interface disorder and its effect on valley degeneracy in the conduction band present an important theoretical challenge for operating spin qubit in silicon. Here, we demonstrate and investigate a counterintuitive effect occurring at Si/SiO2 interfaces. By applying tight binding methods, we show that intrinsic interface states can hybridize with conventional valley states to produce an anomalously large ground state energy gap. Such hybridization effects have not previously been explored in detail for valley splitting. We find that the splitting can be enhanced by disorder in the chemical bonds at the interface, in agreement with recent experiments. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P27.00010: Interface-mediated intervalley coupling in Si Belita Koiller, A.L. Saraiva, M.J. Calderon, Xuedong Hu, S. Das Sarma The conduction band degeneracy in Si is detrimental to spin qubits, for which a nondegenerate ground orbital state is desirable. The Si valley degeneracy is reduced to 2 near an interface with an insulator, and it may be lifted by the spatially abrupt change in the crystal potential. Basic physical mechanisms for Si/barrier mediated valley coupling in different situations are addressed here. Theoretical studies of the interface-induced valley splitting in Si are presented. Abrupt and smooth interface profiles are considered, and the full plane wave expansions of the Bloch functions at the conduction band minima are included. Simple criteria are suggested for optimal fabrication parameters affecting the valley splitting, emphasizing the relevance of different interface-related properties. Refs: A.L.Saraiva et al, PRB 80, 081305 R (2009); arXiv:1006.3338 [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P27.00011: Atomistic simulations of multi-valley silicon double quantum dots in the presence of disorder in the few electron regime Rajib Rahman, Erik Nielsen, Richard Muller, Malcolm Carroll The singlet-triplet based silicon double quantum dot (DQD) is a promising system for implementing a long-lived and controllable quantum bit. The multiple valleys present in silicon, however, may complicate the operation of such a qubit if the valley splitting is small. The valley splitting is affected by a large number of factors including interface roughness, lattice miscuts, electric and magnetic fields, barrier material, and alloy disorder. We employ an atomistic tight-binding (TB) method and a full configuration interaction (CI) to investigate few electron states of a multi-valley Si DQD. This unprecedented approach involving few million atoms allows us to investigate the role of atomic scale disorder (i.e., random alloy effects or interface roughness) on the energy levels and spin configurations of many electron DQDs. [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P27.00012: Engineering anisotropic exchange interactions between quantum dot spin qubits Yun-Pil Shim, Mark Friesen We present a method for engineering anisotropic exchange interactions between quantum dot spin qubits using a Heisenberg antiferromagnetic spin chain as a spin bus. An external magnetic field is applied to create XXZ interactions between spin qubits that are weakly connected to a spin bus whose ground state is non-degenerate. We analyze the dependence of the anisotropy of the effective interaction on the external field and on the length of the spin bus. We show that the tunable XXZ interaction mediated by the spin bus can be used to generate multi-qubit entanglement and to efficiently implement universal gates based on encoded qubits. We also show that the operation of the spin bus is qualitatively different when the spin bus is near one of its magnetic field-induced quantum phase transitions. In this case, the qubits interact with a bus pseudo-spin and the resulting entanglement between pairs of qubits is enhanced. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P27.00013: Asymmetric Quantum Pd Films for Enhanced Hydrogen Separation Guangfen Wu, Wenguang Zhu, Jinlan Wang, Zhenyu Zhang Based on density functional theory calculations and numerical simulations, we have investigated the permeation of H2 through ultra-thin Pd quantum films. The H2 flux can be highly increased by the elevation of the chemisorption-well on the permeate side without significantly blocking the subsurface-surface penetration. We find that Cu-coated asymmetric Pd quantum films (with the Cu monolayer on the permeate side) will enhance the capability for H2 separation: the recombination barrier for H is reduced from 1.36 to 0.79 eV, while the subsurface-surface penetration barrier is only slightly increased from 0.04 to 0.10 eV. Numerical simulations show enhanced H2 flux by 5 orders of magnitude as an upper-limit for asymmetric Pd films over symmetric ones under similar conditions. [Preview Abstract] |
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