Jahresbericht 2005 - IPHT Jena

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18 MAGNETIK & QUANTENELEKTRONIK / MAGNETICS & QUANTUM ELECTRONICS junctions (JJs) was increased by about one third. The critical current spread was reduced from about 10% to 2–3%. All this together results in a distinct increase of the yield of fully functional quantizer circuits. A new cooperation with the NMi Van Swinden Laboratorium B.V. in Delft, Netherlands, allowed to measure our chips in a bipolar pulse driving mode with pulse repetition frequencies of up to 12 GHz. The pulse pattern generator delivers a repeating pattern of short current pulses in which the desired low frequency output waveform is encoded. By the Josephson quantizer circuit it is transformed into a pattern of quantized voltage pulses. Fig. 1.4 shows an example of the operation of a Josephson quantizer circuit with 2560 JJs. In this case, a 122 kHz ac voltage was delta-sigma modulated in a 64 Mbit long pattern. The used clock frequency was 4 GHz. The left spectrum of Fig. 1.4 is delivered by the generator itself. It can be seen that there are not only the distinct higher harmonics of the fundamental but also other peaks whose origin is not yet clear. The quantizer circuit driven by the same pattern removes all of these unwanted signals as seen in the right hand side spectrum of Fig. 1.4. The amplitude of the fundamental frequency was 9.5 mV with quantum accuracy and the higher harmonics were suppressed by –86 dBc. Currently, the Josephson junction array limits the maximum usable clock frequency to 4.8 GHz and the maximum ac voltage amplitude to 9.5 mV. Therefore, the major task for the next few months is to increase the chip bandwidth in order to increase the maximum ac voltage amplitude. Fig. 1.4: Measured power spectra of a deltasigma modulated 122 kHz sine wave with an amplitude of 9.5 mV. The bipolar pulse pattern generator was clocked at 4 GHz. Left spectrum: Delivered by the generator itself. Right spectrum: Output signal of the JJ array quantizer, pulse-driven by the generator with the same pattern. The suppression of the higher harmonics is –86 dBc. Josephson quantizer and 10 V Josephson voltage standard chips with almost 20,000 JJs are the most highly integrated superconductive circuits provided commercially. Worldwide only HYPRES Inc. (USA) and the IPHT are able to offer such 10 V Josephson voltage standard circuits. In 2005 the IPHT delivered quantizer chips and 10 V chips to several national metrology laboratories. In 2005 a complete microprocessor controlled 10 V Josephson voltage standard system was developed. The system facilitates a variety of DC voltage calibrations and measuring functions: Calibration of secondary DC-reference Zeners and testing of the linearity and accuracy of DCvoltmeters and DC-calibrators in the voltage range of 0 to ±10 V. It was successfully evaluated at the PTB in Braunschweig. A direct comparison of the IPHT Josephson voltage standard with the PTB one showed a voltage difference between both standards of only 0.7 nV, with a measurement uncertainty of 3.4 nV. This corresponds to an accuracy of 7 × 10 –11 . So, the IPHT system compares to the primary standards of the national metrology institutes. 1.2.4 Quantum computing (Evgeni Il’ichev, Miroslav Grajcar, Andrei Izmalkov, Thomas Wagner, Sven Linzen, Uwe Hübner, Simon van der Ploeg, Daniel Wittig) Approximately ten years ago it was demonstrated theoretically that a quantum computer can solve some problems much more effectively than a classical one. This discovery has stimulated an effort to find a physical system which can be used as a qubit, the building block of a quantum computer. Amongst the many systems in physics which can be used as a qubit, one is the so-called persistent current qubit. This qubit consists of a small inductance superconducting loop with three Josephson junctions. Such superconducting qubits have several advantages over qubits based on microscopic systems: there is no principal limitation in their number and they can be accessed and controlled individually. We proposed a specific implementation for adiabatic quantum computing with a set of coupled superconducting flux qubits, which can be fabricated with the present state of the art. We could show that our measurement setup – the impedance measurement technique – can be effectively used to read out the results of the adiabatic evolution algorithm. In order to demonstrate any quantum algorithm, a coupling between qubits must be implemented. We proposed implementing the coupling between two qubits through a shared Josephson junction. We have experimentally demonstrated direct antiferromagnetic Josephson coupling between two persistent current qubits. The coupling strength can be of the order of a Kelvin, and agrees with theoretical predictions to the expected accuracy. Moreover, the resulting coupling not only is strong, but can also be varied independent of other design parameters by choosing the shared junction’s size.
MAGNETIK & QUANTENELEKTRONIK / MAGNETICS & QUANTUM ELECTRONICS We implemented a “twisted” design which exhibits the ferromagnetic coupling. It was done by fabricating and studying a four–qubit circuit (Fig. 1.5) in which the two types of coupling co-exist. This is very promising from the perspectives of realizing nontrivial Ising-spin systems and scalable adiabatic quantum computing. This world-wide first coupling of four superconducting qubits clearly is a highlight of our work in 2005. The acquired data fully agree with a quantum-mechanical description to the experimental accuracy. Fig. 1.5: Electron micrograph of a four-qubit sample. The central junctions A1–A3 couple the Al qubits q1–q4. The surrounding Nb coil is part of the LC tank circuit, used for measurement and global flux biasing. The Nb lines I b1–I b4 allow asymmetric bias tuning. For superconducting qubits the problem of decoherence is one of the most important. In order to get additional information about the qubits dynamics we proposed a new tool – low frequency Rabi spectroscopy for a two-level system. In principal we have analyzed the interaction of a dissipative two-level quantum system with highand low-frequency excitation. The system is continuously and simultaneously irradiated by these two waves. If the frequency of the first signal is close to the level separation, the response of the system exhibits undamped low-frequency oscillations whose amplitude has a clear resonance at the Rabi frequency with the width being dependent on the damping rates of the system. Therefore, by analyzing the experimental data, decoherence as well as relaxation times can be reconstructed. We have also experimentally and theoretically investigated combined superconductor-semiconductor structures. The measurement of the supercurrent-phase relationship of a ballistic Nb/InAs(2DES)/Nb junction in the temperature range from 1.3 K to 9 K using impedance measurement technique showed at low temperatures substantial deviations of the supercurrent-phase relationship from conventional tunnel-junction behaviour, as has to be expected for highly trans- parent interfaces. Reasonable agreement between theory and experiment was found. The whole work of the quantum computing group was honoured already in the preceding year with the Thuringian Research Award. The actual awards ceremony happened in the year 2005, shown in the colored picture. These good results, ever again presented in important scientific journals, also reflect in a steadily growing embedding in international cooperation. To allow for the growing number of measurements coming along with this, a second mK-cryostat had to be set up, what was connected with the preparation of an additional specially prepared room for it. So, in combination of these further improving measurement conditions and the scientific potential of the quantum computing group its position in the emerging field of quantum computing could further be strengthened. 1.2.5 Foundry service (Wolfgang Morgenroth, Ludwig Fritzsch, Torsten May, Jürgen Kunert, Birger Steinbach, Gerd Wende, Hans-Georg Meyer) The Jena Superconductive Electronics Foundry (JeSEF) was founded in 1997. It uses the knowhow and the competitive infrastructure for niobium and YBCO technology of the department of Quantum Electronics. The foundry provides customers with LTS SQUIDs and SQUID systems, RSFQ circuits, Josephson voltage standard circuits, components and systems, and with microfabrication items including the design and fabrication of lithography masks. Further, the foundry offers technological services like single manufacturing steps, using our cleanroom facilities, as well as characterization services of our well equipped laboratories. JeSEF and the department of Quantum Electronics have been ISO 9001:2000 certified since 2002. About 40 orders were processed in 2005. Our main customers for SQUIDs and Microfabrication were Supracon AG, THEVA GmbH, Bruker BioSpin GmbH, and Jenoptik LOS GmbH. Voltage standard circuits and components were delivered to the University of Naples and to the national metrology laboratories of the Netherlands, France, and Korea. 1.2.6 Domain wall motion in small GMR structures (Roland Mattheis, Hardy Köbe, Dominique Schmidt, Uwe Hübner, Wolfgang Morgenroth) The motion of domain walls in narrow stripes represents the key element for new magneto- 19
Page 1 and 2: INSTITUT FÜR PHYSIKALISCHE HOCHTEC
Page 3 and 4: Inhaltsverzeichnis / Content INHALT
Page 5 and 6: A. Vorwort Das IPHT konnte das Jahr
Page 7 and 8: Ein besonderer Höhepunkt war im Ra
Page 9 and 10: ORGANISATION / ORGANIZATION B. Orga
Page 11 and 12: Mitglieder des IPHT e.V. / Members
Page 13 and 14: Finanzen des Instituts / Budget of
Page 15 and 16: MAGNETIK & QUANTENELEKTRONIK / MAGN
Page 19: MAGNETIK & QUANTENELEKTRONIK / MAGN
Page 29 and 30: • Lawrence Berkely National Labor
Page 35 and 36: J. Bierlich, T. Habisreuther, M. Ze
Page 37 and 38: Dr. R. Mattheis Editorial Board IEE
Page 39 and 40: 2. Optik / Optics 2.1 Übersicht Da
Page 41 and 42: Ein wichtiges internationales Forum
Page 43 and 44: Fig. 2.2: Preform absorption spectr
Page 45 and 46: OPTIK / OPTICS Coating material NA
Page 47 and 48: 2.2.8 High-reflectivity draw-tower
Page 49 and 50: a b Fig. 2.17: a. Scheme of cross s
Page 51 and 52: 2.2.13 Monitoring of inhomogeneous
Page 53 and 54: The operation principle is based on
Page 55 and 56: K.-F. Klein, H. S. Eckhardt, C. Vin
Page 57 and 58: J. P. Dakin, W. Ecke, M. Reuter:
Page 59 and 60: W. Ecke, K. Schröder: „Anordnung
Page 61 and 62: 3.1 Überblick 2005 war für den Be
Page 63 and 64: MIKROSYSTEME / MICROSYSTEMS Fig. 3D
Page 65 and 66: 3.2 Scientific Results 3.2.1 Spectr
Page 67 and 68: Spectral-reader-technologies (G. Ni
Page 69 and 70: 3.2.2 Photonic chip systems (W. Fri
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B. DNA-chip Technology Characteriza
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cial flow dynamics inside the micro
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• Mikrotechnik + Sensorik GmbH, J
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P. Rösch, M. Schmitt, K.-D. Peschk
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P. Grigaravicius, K. O. Greulich, S
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• Member of scientific council of
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LASERTECHNIK / LASER TECHNOLOGY Com
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Dank besonderem Engagement aller Mi
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UV optical materials (Alfons Burker
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• PV Silicon GmbH, Erfurt • Sch
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F. Falk „Mikromaterialbearbeitung
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into the Ta 2O 5 layers. Unlike for
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