Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session P17: Focus Session: Materials and Device Physics for Quantum Computing II |
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Sponsoring Units: DMP DCMP Chair: John Worlock, University of Utah Room: LACC 404B |
Wednesday, March 23, 2005 11:15AM - 11:51AM |
P17.00001: Controlling Spin and Charge in Quantum Dots and Nanotubes Invited Speaker: This talk summarizes recent experimental progress toward realizing exchange-coupled spin qubits in both few-electron GaAs quantum dots, and gate-defined carbon nanotubes. In the GaAs quantum dots, measurement of the singlet-triplet spin relaxation for separated spins has been realized. These measurements are made at low magnetic fields, and can be studied as a function of magnetic field. We find T2-like relaxation times of order 100 microseconds at 100 mT, with enhanced relaxation at zero field. The experimental method uses pulsed confining gates of a double dot with integrated quantum point contact charge sensors. The relaxation time appears to be limited by hyperfine coupling to nuclear spins. In the carbon nanotube system, we have realized gate-confined double dots and are currently measuring nonlinear double-dot transport, aiming to observe Pauli blockade effects. Up-to-the-minute results will be reported. [Preview Abstract] |
Wednesday, March 23, 2005 11:51AM - 12:03PM |
P17.00002: Transport and charge detection spectroscopy of few electron quantum dot molecular states. Andrew Sachrajda, Michel Pioro-Ladriere, Pawel Hawrylak, Ramin Abolfath, Piotr Zawadzki, Jean Lapointe, Sergei Studenikin . Few electron lateral quantum dot molecules have been fabricated and studied using both transport and charge detection techniques. Such devices are ultimately planned for quantum information applications such as charge and spin based qubits. Measurements have been performed for several specific configurations (n,m) relevant for quantum information where n and m refer to the measured number of electrons in each dot e.g. (5,5) the lowest occupancy configuration with a filling factor 2 state. Measurements were made as a function of magnetic field and the tunnel coupling between the dots. [Preview Abstract] |
Wednesday, March 23, 2005 12:03PM - 12:15PM |
P17.00003: Fermionic Bell state analyzer for spin qubits Hans-Andreas Engel, Daniel Loss In a seminal proposal Knill, Laflamme, and Milburn showed that quantum computation with photons is possible using only linear optics [1]. Partial measurements of a quantum state are sufficient and, most remarkably, coupling qubits with each other via gates is no longer required. It is an important task, therefore, to extend this concept to other qubit systems as well and, in particular, to search for physical realization of such partial measurements for Fermionic qubits. In an important step in this direction, Beenakker et al. recently proposed to combine partial Bell measurements of charge qubits with single qubit operations [2], however, no concrete read-out scheme was discussed. To address and solve this fundamental problem, we propose here to consider spin instead of charge, which is considered a promising candidate for a scalable qubit system [3]. We consider partial Bell state measurements on two spin qubits and argue that it can be performed with available techniques, based on spin-to-charge conversion and charge detection. This opens up the possibility to implement quantum computing without the need of two-qubit gates. \\ \noindent [1] E. Knill, R. Laflamme, and G.J. Milburn, Nature { \bf 409}, 46 (2001). \\ \noindent [2] C.W.J. Beenakker, D.P. DiVincenzo, C. Emary, and M. Kindermann, Phys. Rev. Lett. { \bf 93}, 020501 (2004). \\ \noindent [3] D. Loss and D.P. DiVincenzo, Phys. Rev. A { \bf 57}, 120 (1998). [Preview Abstract] |
Wednesday, March 23, 2005 12:15PM - 12:27PM |
P17.00004: Pulsed-gate measurements of the singlet-triplet relaxation time in a two-electron double quantum dot J. R. Petta, A. C. Johnson, A. Yacoby, C. M. Marcus, M. P. Hanson, A. C. Gossard We use a pulsed-gate technique to measure the singlet-triplet relaxation time in a two-electron double quantum dot when the singlet and triplet states are nearly degenerate. Transitions from the (1,1) to (0,2) charge state involve spin selection rules. Measurements of this transition probability as a function of pulse time and perpendicular magnetic field are used to determine the (1,1) singlet-triplet relaxation time and the (0,2) singlet-triplet splitting. We find a singlet-triplet relaxation time $\geq$70 $\mu$s for our double dot. Experiments aimed at measuring the spin T$_2$ time will be described. [Preview Abstract] |
Wednesday, March 23, 2005 12:27PM - 12:39PM |
P17.00005: Enhancement Single Electron Transistor for Quantum Computing B. Hu, G. M. Jones, A. S. Mampahzy, C. H. Yang, M. J. Yang, Y. B. Lyanda-Geller We propose a novel scheme to build single spin quantum dots as the building block for quantum computing. In contrast to the depletion mode single electron transistors (SETs) commonly used for creating quantum dot qubits, our approach is based on an enhancement mode SET using InAs/GaSb composite quantum wells through bandgap engineering. The enhancement mode SETs host no electrons at zero applied voltage, compared to thousands of electrons in depletion dots to start with. When a voltage is applied to a single top metal gate, two symmetric tunneling barriers are created between GaSb and InAs quantum wells. These tunneling barriers define an InAs quantum dot and a single electron can tunnel there. This novel approach has a number of advantages for scalable quantum computing. In this talk, we will discuss the structure design, quantum dot simulation, and device fabrication. We will also present experimental results that provide proof-of-principle demonstrations. [Preview Abstract] |
Wednesday, March 23, 2005 12:39PM - 12:51PM |
P17.00006: Anisotropic exchange interactions between electron spins in coupled semiconductor quantum dots \c{S}.C. B\u{a}descu, T.L. Reinecke, Y. Lyanda-Geller Electron spins in semiconductor quantum dots (QDs) are of considerable interest for qu-bits in quantum computing. Quantum gates (QGs) can be obtained by pulsing the exchange interaction between two spins by external fields. The anisotropic parts of the exchange determine the pulse shapes and also provide a dephasing mechanism affecting the gate fidelity. We have derived an anisotropic term caused by the electron-electron interaction, by treating the Coulomb interaction and the \textbf{k}$\cdot $\textbf{p} band mixing on an equal footing. This term arises from the coupling of spins in addition to single spin effects such as spin-orbit coupling, asymmetry of the confining potential, and inversion asymmetry of the bulk material. The anisotropic contribution obtained here can represent $\sim $ 10 $^{-2 }$ of the isotropic exchange, which has a significant effect on gate shapes and fidelity. We use a general model of elliptically shaped dots in arbitrarily oriented magnetic fields. We give results for vertically coupled InAs/GaAs quantum dots and laterally coupled GaAs electrostatic dots, and we describe the operation of the XOR gates. [Preview Abstract] |
Wednesday, March 23, 2005 12:51PM - 1:03PM |
P17.00007: CCD-like Architecture for Quantum Computing on Liquid Helium Guillaume Sabouret, Stephen Lyon Electrons floating above liquid helium form a 2-dimentional gas with high mobilities at low densities due to the absence of a lattice and associated defects and impurities. There are almost no spin-orbit interactions or other magnetic coupling with the surroundings which in turn leads to a long electron spin coherence time. This system therefore lends itself to a quantum computing architecture similar to those proposed for electron spins in semiconductors but with mobile qubits. Scalability requires the ability to shift individual electrons around the surface to either store them in remote locations that can be thought of as memory or to bring pairs close to one another to interact and realize the two-qubit operations. We present a CCD-like architecture for transporting spins above the surface of liquid helium and the ongoing work pertaining to it. [Preview Abstract] |
Wednesday, March 23, 2005 1:03PM - 1:15PM |
P17.00008: Imaging Coherent Electron Flow in a Two-Dimensional Electron Gas with a Perpendicular Magnetic Field K.E. Aidala, A.C. Bleszynski, R.M. Westervelt, M.P. Hanson, A.C. Gossard Scanning probe microscopy (SPM) at liquid He temperatures can be used to image electron flow from a quantum point contact (QPC) in a GaAs/AlGaAs two-dimensional electron gas (2DEG) in a perpendicular magnetic field. A charged SPM tip creates a perturbation directly beneath it, which scatters electrons incident from the QPC (1). Recording the changes in the QPC conductance as we vary tip position creates an image of the flow, including interference fringes that provide information about the accumulated phase of the electrons. Our new SPM allows us to reach He-3 temperatures and to apply a magnetic field. At zero magnetic field with the QPC biased to the first conductance plateau, the image shows a well-defined branch of electron flow. Introducing a magnetic field of about 50 mT breaks the time-reversal symmetry, by enclosing magnetic flux inside the football-shaped roundtrip path, and the amplitude of the conductance decays in the image at longer distances until the branch is no longer visible. (1) Topinka, M.A., Westervelt, R.M., Heller, E.J. ``Imaging Electron Flow.'' Physics Today 56 (12) 47-52 DEC 2003 [Preview Abstract] |
Wednesday, March 23, 2005 1:15PM - 1:27PM |
P17.00009: Imaging Electrons in Few-Electron Quantum Dots P. Fallahi, A.C. Bleszynski, R.M. Westervelt, E.J. Heller, M. Hanson, A.C. Gossard Single-electron quantum dots are important candidates for quantum information processing. We have developed a new technique to image electrons inside a single-electron quantum dot in the Coulomb blockade regime, using a scanning probe microscope (SPM) at liquid He temperatures (1). A single-electron quantum dot was formed in a two-dimensional electron gas (2DEG) inside a GaAs/AlGaAs heterostructure by surface gates. Spatial images of an electron inside the dot were obtained by fixing the tip voltage and recording the dot conductance while scanning the SPM tip above the quantum dot. The images show a ring of increased conductance about the center of the dot, where the dot conductance is on the Coulomb blockade conductance peak between 0 and 1 electrons. Simulations show that this technique can be used to extract the wavefunction of electrons inside the dot if the tip perturbation is narrower than the wave function (2). A charged SPM tip promised to be a useful tool for manipulating electrons in quantum dot circuits. 1) P. Fallahi, A.C. Bleszynski, \textit{et al} submitted to Nanoletters. 2) P. Fallahi,\textit{ et al }Proc. 27 Int. Conf. on Physics and Semiconductors (ICPS27), Flagstaff, July 26-30, 2004, in press. \newline *This work was supported at Harvard University by DARPA DAAD19-01-1-0659 and by the NSEC, NSF PHY-01-17795, and at UCSB by QUEST NSF Science and Technology Center. [Preview Abstract] |
Wednesday, March 23, 2005 1:27PM - 1:39PM |
P17.00010: Local Gating of Nanostructures with a Low-Temperature Scanned Probe Microscope M. Jura, M.A. Topinka, L.S. Moore, D. Goldhaber-Gordon, L. Urban, A. Yazdani, L.N. Pfeiffer, K.W. West We have recently constructed a low-temperature scanned probe microscope designed to operate down to dilution refrigerator temperatures. There has been increasing interest in gaining spatial information about transport through nanostructures by using scanned probe microscopy$^{1-3}$. Our microscope incorporates a commercial stick-slip positioner for coarse approach and alignment to nanostructures. Here we present our solutions to some of the challenges faced in designing such an instrument, as well as a first set of low-temperature scanned probe images obtained from several nanostructures fabricated on different high-mobility GaAs/AlGaAs 2DEG heterostructures. \newline\newline $^{1}$ M.A. Topinka et al., Science \textbf{289}, 2323 (2000). \newline $^{2}$ R. Crook et al., Phys. Rev. Lett.,\textbf{ 91}, 246803 (2003). \newline $^{3}$ A. Pioda et al., Phys. Rev. Lett., \textbf{93}, 216801 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 1:39PM - 1:51PM |
P17.00011: Spin Qubit Quantum Computing with RKKY interaction Ying Zhang, Sankar Das Sarma Motivated by recent theoretical work that demonstrated the importance of the nonlocal coupling between two localized spins via RKKY interaction, we investigate the feasibility of GaAs quantum dot spin-qubit quantum computing scheme utilizing RKKY interactions as a means of spin exchange operation. We estimate the strength and decoherence of RKKY spin exchange with comparison to direct nearest neighbor spin exchange. We also estimate errors associated with RKKY spin interaction and develop schemes of the corresponding error corrections. This work is supported by LPS, ARO, and ARDA. [Preview Abstract] |
Wednesday, March 23, 2005 1:51PM - 2:03PM |
P17.00012: Spin Exchange Interaction between Localized and Itinerant Carriers Guy Ramon, Thomas Reinecke, Lu Sham The exchange interaction between localized and itinerant carriersis a key requirement in indirect spin coupling mechanisms. These interactions have been investigated intensively due to their potential applications in spintronics and in implementations of gates for quantum computation. A quantitative evaluation of spin coupling effects requires an accurate description of this exchange interaction, which has not been available to date. Here we present a microscopic formulation of the spin exchange interaction between localized and itinerant carriers. The effects of correlation and hybridization of continuum and localized states are included by performing a set of nested canonical transformations on an Anderson-type Hamiltonian, bringing it to an s-d spin exchange form. These results are extended to address the problem of spin exchange interaction between a localized electron and an itinerant exciton. This formulation also facilitates the understanding of magnetic properties of Kondo systems. In particular we address the interplay between RKKY and Kondo interactions. [Preview Abstract] |
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