Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F37: Focus Session: Semiconductor Qubits - Topological Qubits, Spin-orbit Effects and Micromagnets |
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Sponsoring Units: GQI Chair: Leo Kouwenhoven, Technische Universiteit Delft Room: 212A |
Tuesday, March 3, 2015 8:00AM - 8:36AM |
F37.00001: Superconducting Edge-Mode Transport in InAs/GaSb Double Quantum Wells Invited Speaker: Vlad Pribiag In proximity to a superconductor, topological insulators are predicted to host topological superconductivity, an exotic state of matter that supports Majorana zero-modes. Localized Majorana modes are expected to obey non-Abelian exchange statistics, making them interesting building blocks for topological quantum computing. Here we report supercurrent in the edge modes of Type-II InAs/GaSb quantum wells, a two-dimensional topological insulator (2D TI). By electrostatically-gating the devices we observe superconducting transport in all three regimes of the 2D TI: bulk electrons, edge modes and bulk holes. From superconducting quantum interference measurements, we extract the spatial distribution of the supercurrent in each regime. A clear transition to edge-dominated supercurrent is observed under conditions of high bulk resistivity, which we associate with the 2D topological phase. These experiments establish InAs/GaSb as a promising platform for confinement of Majoranas into localized states, enabling future investigations of non-Abelian statistics. [Preview Abstract] |
Tuesday, March 3, 2015 8:36AM - 8:48AM |
F37.00002: Quasiparticle parity lifetime of bound states in a hybrid superconductor-semiconductor quantum dot Andrew Higginbotham, Sven Albrecht, Gediminas Kirsanskas, Willy Chang, Ferdinand Kuemmeth, Peter Krogstrup, Thomas Jespersen, Jesper Nyg{\aa}rd, Karsten Flensberg, Charles Marcus We measure quasiparticle transport in an InAs nanowire that is half-covered with epitaxial superconducting aluminum, then locally gated to form a quantum dot. We observe negative differential conductance at finite source-drain bias, and temperature dependent even-odd alternations in the Coulomb blockade peak spacings at zero bias. These observations can be understood in terms of a mid-gap semiconductor discrete state and a continuum of BCS quasiparticle states. Comparing with simple models, we bound the discrete state's parity lifetime and the quasiparticle temperature. These results indicate that parity fluctuations are slow, and imply Majorana qubit poisoning times on the order of a millisecond. Additional results indicate that the bound states move to zero energy in a magnetic field, qualitatively consistent with expectations for Majorana fermions in a finite system. [Preview Abstract] |
Tuesday, March 3, 2015 8:48AM - 9:00AM |
F37.00003: Hard proximity induced superconducting gap in semiconductor-superconductor epitaxial hybrids Thomas Jespersen, Peter Krogstrup, Nino Ziino, Sven Albrecht, Willy Chang, Morten Madsen, Erik Johnson, Ferdinand Kuemmeth, Jesper Nyg{\aa}rd, Charles Marcus We present molecular beam epitaxy grown InAs semiconductor nanowires capped with a shell of aluminum (superconductor). The hybrid wires are grown without breaking vacuum, resulting in an epitaxial interface between the two materials as demonstrated by detailed transmission electron microscopy and simulations. The domain matching at the interface is discussed. Incorporating the epitaxial nanowire hybrids in electrical devices we performed detailed tunneling spectroscopy of the proximity induced superconducting gap in the InAs core at 20 mK. We find the sub-gap conductance being at least a factor 200 smaller than the normal state value (gap hardness). This is a significant improvement compared to devices fabricated by conventional lithographic methods and metal evaporation showing no more than a factor of $\sim 5$. The epitaxial hybrids seem to solve the soft gap problem associated with the use of nanowire hybrids for future applications in topological quantum information based on Majorana zero modes. [Preview Abstract] |
Tuesday, March 3, 2015 9:00AM - 9:12AM |
F37.00004: Bound States in ``Majorana Box'' Sven Albrecht, Andrew Higginbotham, Ferdinand Kuemmeth, Peter Krogstrup, Thomas Jespersen, Jesper Nyg{\aa}rd, Charles Marcus We perform bias spectroscopy and observe Coulomb peak motion in InAs quantum dots with an epitaxial superconducting aluminum shell. Varying the length of the aluminum shell and applying a magnetic field, we are able to tune between regimes with 2e and 1e-periodic Coulomb oscillations. The doubling in periodicity reflects a transition from two-electron tunneling to single quasiparticle charging, attributable to a competition between the charging energy and the superconducting energy gap. At high fields below the superconducting-to-normal transition, we observe low-lying features in bias and 1e-periodic Coulomb peaks, both consistent with the presence of a zero-energy discrete state. We discuss these results in the context of proposed experimental signatures of Majorana fermions. [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:24AM |
F37.00005: Optimal Control of Majorana Zero Modes Gil Refael, Torsten Karzig, Armin Rahmani, Felix von Oppen Braiding of Majorana zero modes provides a promising platform for quantum information processing, which is topologically protected against errors. Strictly speaking, however, the scheme relies on infinite braiding times as it utilizes the adiabatic limit. Here we study how to minimize nonadiabatic errors for finite braiding times by finding an optimal protocol for the Majorana movement. Interestingly, these protocols are characterized by sharp transitions between Majorana motion at maximal and minimal velocities. We show that these so-called bang-bang protocols can minimize the nonadiabatic transitions of the system by orders of magnitude in comparison with naive protocols. [Preview Abstract] |
Tuesday, March 3, 2015 9:24AM - 9:36AM |
F37.00006: Hyperfine and spin-orbit dynamics in GaAs double quantum dots Shannon Harvey, Michael Shulman, John Nichol, Arijeet Pal, Bertrand Halperin, Vladimir Umansky, Amir Yacoby Semiconductor quantum dots provide a unique platform for single-particle physics and many-body quantum mechanics. In particular, understanding the dynamics of a single electron interacting with a nuclear spin bath is key to improving spin-based quantum information processing, since the hyperfine interaction limits the performance of many spin qubits. We probe the electron-nuclear interaction by measuring the splitting at the anti-crossing between the electron singlet (S) and m$=$1 triplet (T$+)$ states in a GaAs double quantum dot. Using Landau-Zener sweeps, we find that the size of this splitting varies by more than an order of magnitude depending on the magnitude and direction of the external magnetic field. These results are consistent with a competition between the spin orbit interaction and the hyperfine interaction, even though the extracted spin orbit length is much larger than the size of the double quantum dot. We confirm these results by using Landau-Zener sweeps to measure the high-frequency correlations in the S-T$+$ splitting that arise from the Larmor precession of the nuclei. These unexpected results have implications for improving the performance of spin-based quantum information processing, as well as improving our understanding of the central spin problem. [Preview Abstract] |
Tuesday, March 3, 2015 9:36AM - 9:48AM |
F37.00007: Spin-orbit coupling quenches dynamic nuclear polarization in GaAs double quantum dots John Nichol, Shannon Harvey, Michael Shulman, Arijeet Pal, Emmanuel Rashba, Bertrand Halperin, Vladimir Umansky, Amir Yacoby Dynamic nuclear polarization (DNP) occurs in a wide variety of condensed matter systems and enables the transfer of angular momentum from electron to nuclear spins via the hyperfine interaction. In semiconductor devices, DNP is exploited for coherent electron spin manipulation, but the mechanisms limiting the maximum-achievable polarization in gate-defined quantum dots to only a few percent remain incompletely understood. We demonstrate here that spin-orbit coupling quenches DNP in a GaAs double quantum dot, even though the spin-orbit length is much larger than the interdot spacing. The observed suppression of DNP is consistent with theoretical models. These results demonstrate the surprising competition between the hyperfine and spin-orbit interactions in GaAs double quantum dots and highlight the importance of field orientation for efficient DNP. [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:00AM |
F37.00008: Spin relaxation in a nanowire quantum dot due to electrical noises Jo-Tzu Hung, Xuedong Hu Semiconductor nanowire with strong spin-orbit couplings makes fast electrical coherent control feasible for spin qubits. One example is the spin-orbit qubit [1], confined by nanowire based quantum dots made from InAs or InSb. Because of the strong spin-orbit coupling, such a qubit is naturally sensitive to electrical noises. We theoretically investigate the influence of electrical noise on spin-orbit qubit by considering fluctuations from the gates and/or defects. We start from a three-dimensional Hamiltonian, and consider spin-orbit couplings for nanowires with zincblende structure grown along [111] and those with wurtzite structure grown along [001], respectively. We then analyze spin relaxation as we vary the parameters for the system, such as the magnitude and direction of the applied magnetic field, nanowire thickness, and the quantum dot confinement. [1] Nadj-Perge et al., Nature 468, 1084 (2010). [Preview Abstract] |
Tuesday, March 3, 2015 10:00AM - 10:12AM |
F37.00009: Phonon mediated spin relaxation in a moving quantum dot Xinyu Zhao, Peihao Huang, Xuedong Hu We study decoherence of an electron spin qubit that is being transported in a moving quantum dot. Our focus is on spin relaxation due to phonon noise through the spin-orbit interaction. We find that the effective magnetic field caused by the motion of the electron can either enhance or suppress spin relaxation depending on the angle between the moving direction and the external magnetic field. At low external magnetic field ($B \alt 0.5$ T), the suppression effect can be significant, which indicates that a moving quantum dot can maintain spin coherence better than a static dot. We also find that the spin relaxation rate is not a monotonically increasing function of the applied magnetic field when the motion of the electron is taken into account. [Preview Abstract] |
Tuesday, March 3, 2015 10:12AM - 10:24AM |
F37.00010: Orbital hyperfine interaction and qubit dephasing in carbon nanotube quantum dots Andras Palyi, Gabor Csiszar Hyperfine interaction (HF) is of key importance for the functionality of solid-state quantum information processing, as it affects qubit coherence and enables nuclear-spin quantum memories. In this work, we complete the theory of the basic hyperfine interaction mechanisms (Fermi contact, dipolar, orbital) in carbon nanotube quantum dots by providing a theoretical description of the orbital HF. We find that orbital HF induces an interaction between the nuclear spins of the nanotube lattice and the valley degree of freedom of the electrons confined in the quantum dot. We show that the resulting nuclear-spin--electron-valley interaction (i) is approximately of Ising type, (ii) is essentially local, in the sense that an effective atomic interaction strength can be defined, and (iii) has a strength that is comparable to the combined strength of Fermi contact and dipolar interactions. We argue that orbital HF provides a new decoherence mechanism for single-electron valley qubits and spin-valley qubits in a range of multi-valley materials. We explicitly evaluate the corresponding inhomogeneous dephasing time $T_2^*$ for a nanotube-based valley qubit. [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 10:36AM |
F37.00011: Hall magnetometry of micromagnets for single-electron spin qubits Dany Lachance-Quirion, Julien Camirand Lemyre, Laurent Bergeron, Michel Pioro-Ladri\`{e}re The coherence time of a single-electron spin can reach tens of milliseconds when placed in the right environment [1]. The electric-dipole interaction between such a single spin and an electric field can be engineered by the inhomogeneous magnetic field of a micromagnet [2]. This effective spin-orbit interaction can be used to manipulate the spin through electric-dipole spin resonance [2], but also to couple a single spin to the electric field of a microwave cavity in the circuit QED architecture [3]. We selected the material and improved the shape of the micromagnet in order to maximize magnetic field gradients and remanence. We perform Hall magnetometry of those improved micromagnets using Hall bars electrostatically defined in an AlGaAs/GaAs two-dimensional electron gas. The gate-voltage dependent width of the Hall bar enables us to map the averaged magnetic field of the micromagnet, which validates simulations of the inhomogeneous magnetic field profile created by the magnet. We can therefore deduce that our micromagnets can produce magnetic field differences over 200 nm of more than 200 mT.\\[4pt] [1] M. Veldhorst et al., Nat. Nano. 1 (2014).\\[0pt] [2] M. Pioro-Ladri\`{e}re et al., Nat. Phys. 4, 2 (2008).\\[0pt] [3] X. Hu, Y. Liu, and F. Nori, Phys. Rev. B 86, 1 (2012). [Preview Abstract] |
Tuesday, March 3, 2015 10:36AM - 10:48AM |
F37.00012: Integration of nanomagnet for spin rotations in quantum dots Julien Camirand Lemyre, Dany Lachance-Quirion, Michel Pioro-Ladri\`{e}re Integrating micrometric size ferromagnets to quantum dots have proven a powerful tool to rotate single spins. In previous approaches, the distance between the magnet and the quantum dots were limiting the magnetic field gradients, thus impeding spin rotation speeds in both GaAs [1] and Si [2] quantum dots. In this work, we first reduce the size of the magnet to improve the field gradients in the dots. Furthermore, we avoid the thick insulating layer between the magnet and the gates by patterning oxidized aluminum gates with a 2 nm thick native oxide. This allows us to bring the nanomagnet closer to the quantum dots, hence increasing the magnetic field gradients by a factor of ten compared to previous structures. \\[4pt] [1] M. Pioro-Ladri\`{e}re \textit{et al}., Nat. Phys., 4(10):776-779, 10 2008.\\[2pt] [2] E. Kawakami \textit{et al}. Nat Nano, 9(9):666-670, 09 2014. [Preview Abstract] |
Tuesday, March 3, 2015 10:48AM - 11:00AM |
F37.00013: High Fidelity Singlet-Triplet $\mathbf{S - T_-}$ Qubits in Inhomogeneous Magnetic Fields Clement Wong, Mark Eriksson, Sue Coppersmith, Mark Friesen We propose an optimal set of quantum gates for a singlet-triplet qubit in a double quantum dot with two electrons utilizing the $S$-$T_-$ subspace. Qubit rotations are driven by the applied magnetic field and an orthogonal field gradient provided by a micromagnet. We optimize the fidelity of this qubit as a function of magnetic fields, taking advantage of ``sweet spots" where the rotation frequencies are independent of the energy level detuning, providing protection against charge noise. We simulate gate operations and qubit rotations in the presence of quasistatic noise from charge and nuclear spins as well as leakage to nonqubit states, and predict that in silicon quantum dots gate fidelities greater than $99\%$ can be achieved for two nearly-orthogonal rotation axes. Preprint: arXiv:1410.2310 [Preview Abstract] |
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