QN Seminar 2013
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QN Seminar 2013 Abstract 18 December 2013 Coherent Oscillations in a Si/SiGe Quantum Dot Hybrid Qubit Mark Eriksson, Universit of Wisconsin-Madison I will discuss measurement and manipulation of the recently proposed quantum dot hybrid qubit, in these experiments formed in a Si/SiGe double quantum dot. X-rotations on the Bloch sphere are performed by pulsing a gate voltage so that the detuning of a double quantum dot makes the (1,2) and (2,1) occupation ground states degenerate [2]. The resulting rotations occur at approximately 5 GHz and reveal an experimentally measured visibility greater than 80%. Z-rotations on the Bloch sphere are performed by pulsing a gate voltage away from the (1,2)-(2,1) degeneracy point, resulting in oscillations at a rate of approximately 10 * GHz and a measured visibility greater than 85%. The T2 time at this detuning is greater than 15 ns, many times longer than the 100 ps gate operation time. Methods for future improvements of the oscillation visibility will be discussed. This work was supported in part by ARO (W911NF-12-0607), NSF (DMR-1206915), and the United States Department of Defense. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the US Government. [1] Z. Shi, et al., Phys. Rev. Lett. 108, 140503 (2012). [2] Teck Seng Koh, et al., Phys. Rev. Lett. 109, 250503 (2012). Abstract 11 December 2013 Microscopic Origin of the 0.7-Anomaly in Quantum Point Contacts Stefan Ludwig, Ludwig-Maximilians-Universität, München Quantum point contacts are the fundamental building blocks of semiconducting quantum circuits. The conductance of such a one-dimensional constriction can be controlled by gate voltages and is quantized in units of GQ = 2e2=h. However, the conductance also exhibits an unexpected shoulder at ' 0:7GQ, known as the ’0.7- anomaly’ [1], whose origin is still subject to controversial discussions. The most prominent proposed theoretical explanations exclude each other: one has evoked spontaneous spin polarization [2] and the other one the formation of a quasi-bound state leading to the Kondo effect [3,4]. In this talk I present an alternative approach based on our experiments and numerical calculations, performed in Jan von Delft’s group. We evoke no assumptions beyond experimental evidences. Our model offers a detailed microscopic explanation for both the 0.7-anomaly and the zero-bias peak accompanying it: their common origin is a pronounced maximum in the local density of states at the bottom of the lowest onedimensional subband of the point contact, which causes an anomalous enhancement of the back-scattering rate and the magnetic spin susceptibility. We find that the 0.7-anomaly is a Fermi-liquid feature while neither spontaneous spin polarization nor a bound state are needed for its explanation [5]. Characterization of Qubit Dephasing by Landau-Zener Interferometry If time permits, I will then present some fairly new data on a two-electron charge qubit defined in a double quantum dot. We have performed Landau-Zener interferometry and established it as a straightforward tool to fully characterize a qubit and its dephasing environment [6]. It is, e. g., possible to determine the characteristic time scales T2 (coherence time) and T? 2 (ensemble average dephasing) individually and without the need of pulsed gate measurements. References [1] Thomas, K. J. et al, Phys. Rev. Lett. 77, 135 (1996). [2] Reilly, D. J. et al., Phys. Rev. Lett. 89, 246801 (2002). [3] Cronenwett, S. M. et al., Phys. Rev. Lett. 88, 226805 (2002). [4] Meir, Y., et al., Phys. Rev. Lett. 89, 196802 (2002) [5] Bauer, F., et al., Nature 501, 73 (2013). [6] Forster, F. et al., arXiv:1309.5907 (2013). Abstract 4 December 2013 Modern 3D optical and electron microscopy for mapping neuronal circuits Kevin Briggman, NINDS/NIH The neuronal circuitry of the retina is the first stage in the processing of visual information by vertebrate brains. The retina transduces photons into chemical and electrical signals and is capable of encoding visual information across an extreme dynamic range from single photon responses in the dark to near saturating conditions on a bright sunny day. In addition, the retina encodes spatial and temporal patterns of light including the presence of edges, color opponency, and the direction of motion. However, how retinal neurons actually perform these sophisticated computations is not well understood. The major problem is that we simply do not comprehensively know the anatomical synaptic connectivity of neurons in the retina. This is in large part due to the disparate length scales involved; the neurites (including dendrites and axons) of neurons can be as thin as 50 nm and yet extend over many millimeters. The tortuous trajectories of these processes in the nervous system necessitate the ability to acquire large volumes at high resolution in all 3 spatial dimensions. High lateral (x-y) resolutions of 5-10 nm are easily achieved in scanning electron microscopes, but the major challenge has been to repeatedly cut tens of thousands of thin sections (each at least <50 nm) from a block of tissue. Historically, tissue sectioning is performed manually using an ultramicrotome which is tedious, error-prone and is limited to, at best, 50 nm sections. We have therefore automated both image acquisition and sectioning by developing a serial block-face scanning electron microscopy (SBEM) technique. This method allows us to image large 3-dimensional EM datasets, typically hundreds of microns on a side, at nearly isotropic voxel resolutions of 12 x 12 x 25 nm3. I will discuss the technical aspects of SBEM in relation to other current volume electron microscopy techniques and present how we have used this technique to reconstruct the connectivity of the mouse retina. In particular, I will stress the power of combining large-scale functional recordings of neurons with subsequent anatomical reconstruction of neuronal circuits. I will also discuss our efforts to automate the analysis of large (multi- terabyte) 3D EM datasets. Abstract 27 November 2013 Quantum Annealing and the D-Wave Devices Matthias Troyer, ETH Zürich, and Microsoft Research Quantum annealing - a finite temperature version of the quantum adiabatic algorithm - combines the classical technology of slow thermal cooling with quantum mechanical tunneling, to try bring a physical system towards its ground state. The Canadian company D-Wave systems has recently built and sold programmable devices that are designed to use this effect to find solutions to optimization problems. I will present results of experiments designed to shed light on crucial questions about these controversial devices: are these devices quantum or classical? Are they faster than classical devices? What is their potential? Abstract 13 November 2013 Josephson junction-based coherent caloritronic nanocircuits Francesco Giazotto, NEST, Instituto Nanoscienze-CN & Scuola Normale Superiore, Pisa, Italy The Josephson effect [1] represents perhaps the prototype of macroscopic phase coherence and is at the basis of the most widespread interferometer, i.e., the superconducting quantum interference device (SQUID) [2]. Yet, in analogy to electric interference, Maki and Griffin [3] predicted in 1965 that thermal current flowing through a temperature-biased Josephson tunnel junction is a stationary periodic function of the quantum phase difference between the superconductors. The interplay between quasiparticles and Cooper pairs condensate is at the origin of such phase-dependent heat current, and is unique to Josephson junctions. In this scenario, a temperature-biased SQUID would allow heat currents to interfere [4, 5] thus implementing the thermal version of the electric Josephson interferometer. In this presentation we shall initially report the first experimental realization of a heat interferometer [6,7]. We investigate heat exchange between two normal metal electrodes kept at different temperatures and tunnel- coupled to each other through a thermal `modulator' [5] in the form of a DC-SQUID. Heat transport in the system is found to be phase dependent, in agreement with the original prediction. Besides offering remarkable insight into thermal transport in Josephson junctions, our results represent a significant step toward phase- coherent mastering of heat in solid-state nanocircuits, and pave the way to the design of novel-concept coherent caloritronic devices, for instance, heat transistors, thermal splitters and diodes [8] which exploit phase-dependent heat transfer peculiar to the Josephson effect. In this latter context, we shall also present the concept for a further development of a Josephson heat interferometer based on a double superconducting loop [9] which allows, in principle, enhanced control over heat transport. We shall finally conclude presenting experimental results on the first prototypical quantum diffractor for thermal flux [10]. Specifically, thermal diffraction manifests itself with a peculiar modulation of the electron temperature in a small metallic electrode nearby-contacted to a Josephson junction when sweeping the magnetic flux Φ [11]. Remarkably, the observed temperature dependence exhibits Φ-symmetry and a clear reminiscence with a Fraunhofer-like modulation pattern, as expected fingerprints for a quantum diffraction phenomenon. Our results confirm a pristine prediction of quantum heat transport and, joined with double- junction heat interferometry demonstrated in [6], exemplify the complementary and conclusive proof of the existence of phase-dependent thermal currents in Josephson-coupled superconductors. This approach combined with well-known methods for phase-biasing superconducting circuits provides with a novel tool for mastering heat fluxes at the nanoscale. References [1] B. D. Josephson, Phys. Lett. 1, 251 (1962) [2] J. Clarke and A. I. Braginski, The SQUID Handbook (Wiley-VCH, 2004) [3] K. Maki and A. Griffin, Phys. Rev. Lett. 15, 921 (1965) [4] G. D. Guttman, E. Ben-Jacob, and J. Bergman, Phys. Rev. B 57, 2717 (1998) [5] F. Giazotto and M. J.