Entwicklung von Josephsonschaltungen mit integrierten Flussquanten-Phasenschieberelementen

• Applicant: Professor Dr. Jürgen Niemeyer
• Subject Area: Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
• Term: from 2004 to 2008
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 5436937

• Final Report Year: 2007

Final Report Abstract

This project was focused on the research and development of superconducting digital circuits operating on the principle of Rapid Single Flux Quantum (RSFQ) logic in which the new type of the circuit element, the so-called phase shifter, was successfully implemented. Due to the remarkable property of this element, i.e. the ability to accumulate between its terminals the predefined drop of the superconducting phase cp (-n < cp < jr), one can engineer the reliably operating RSFQ circuits. Moreover, these phase shifting circuits can be naturally integrated with the Josephson circuits operating in quantum regime, i.e. Josephson qubits. The objective of the project was to develop the technological platform for implementation of the superconducting phase shifters and on this basis to realize the fundamental RSFQ circuits with desired characteristics. The passive superconducting phase shifter presents a miniature superconducting loop with the frozen magnetic flux quantized in units of elementary flux quanta Φ0. The optimum number of the frozen flux quanta is one. The operating principle of these phase shifters is based on the non-zero superconducting phase difference between different points of the loop arising because of circulating supercurrent. The diametrically opposite points have in this case the phase difference of Φ0 = n. (Therefore, the circuits exhibiting the behavior similar to that of the so-called pi-junctions can also be easily realized with the help of the phase shifter.) This property of the superconducting loops was earlier used by the group of H. Mooij (Technical University of Delft) for realization of a fixed flux bias in their Al flux qubits. The idea of this project was to incorporate these loops carrying the predefined phase drops into RSFQ logic circuits. As we had demonstrated, the fixed phase differences across the phase shifters indeed made it possible to design the basic RSFQ circuits in the elegant way. The main challenge in integration of the superconducting phase shifters and RSFQ circuits was in modification of the available Nb/AlxOy/Nb trilayer fabrication technology and development of a reliable technique of preparation of the initial phase shifter state with frozen flux. To solve these problem the optimum layout of the circuits was developed for two main concepts of the RSFQ circuitry. The first circuitry concept included Josephson tunnel junctions having standard value of the critical current density, i.e. jc = 1 kA/cm2. Since such Nb trilayer technology was already available, it allowed to start designing and manufacturing of the first circuits with the phase shifting elements already in the beginning of the project. After modificating and adjusting technological parameters this technology allowed fabricating the circuits with phase-shifting elements and checking the principle of their operation. The second circuitry concept included Josephson junctions set to a lower critical current density of y'c = 100 A/cm2. Due to the possibility of achieving sufficienly low absolute values of critical current 7C (about 10 µA) for relatively large size of the junctions (about few micrometers of linear dimensions, that is still feasible applying conventional fabrication methods), this technology is considered as the basis technology for manufacturing superconducting qubits and its RSFQ-logic-circuit environment (RSFQ shell) enabling control and readout. Within the framework of the given project this technology was developed to a high grade of reliability in fabrication, and the circuits including the superconducting phase shifters were successfully realized and characterized. Finding of the optimum design of the RSFQ circuits including Josephson junctions with low values of critical current usually presents a problem. The single-flux-quantum representation of information assumes its storing in the loops of interferometers having superconductor inductance L of the order of Φ0/Ic, which in the case of small critical currents may be extremely large (of the order of 1 nH). The implementation of such inductances require significant area on the chip which, in turn, makes a circuit very sensitive to electromagnetic noise. Moreover, parasitic capacitance of these thin-film inductances may be appreciable. The action of our miniature superconducting phase shifters is equivalent to the action of large inductances carrying a current, therefore, such shifters can do the function of these inductances. It turned out that the idea of using of the phase shifting elements for engineering the singleflux- quantum circuits operating in either classical or quantum regime was surprisingly good. The phase drop arising on the elements of the circuit proved stable behavior whereby its value was predefined by the circuit layout. Using of the phase shifters considerably simplifies the design, makes it very compact and, therefore, less sensitive to fluctuations of the electromagnetic field inducing unwanted interference in the circuit operation. The latter effect is particularly harmful for the quantum regime of operation because it dramatically limits the coherence time of the circuit. The question arises concerning perspective of a wide application of the investigated frozenflux- based phase-shifters in integrated RSFQ and qubit circuits. In contrast to operation of conventional circuits, these circuits first require initialization by setting the loops of all phase shifters in the states with exactly one flux quantum frozen. As we had demonstrated, to a certain extent this problem can be solved by optimization of the design. Accumulation of sufficient experience at low-temperature measurements played also a certain role. However, further improvement of the design of the phase shifters including the current-carrying loops using for their initialization is still needed. In the meantime the alternative technology of fabricating the pi-junctions on the basis of sandwiches of type S-F-S (where S is superconductor and F is ferromagnetic material) has been dramatically improved. (In particular, very promising results were recently obtain in the research group of V. Ryazanov, Institute of Solid State Physics, Chernogolovka). These devices can also ensure the required phase shift and hence can also be incorporated into RSFQ and qubit circuits. In contrast to the frozen-flux-based phase-shifters investigated in this project, the S-F-S junctions do not require initialization and, therefore, easy for operating. Morever, the design of such circuits can be made even more compact. In conclusion, the given DFG project has prepared a solid basis for further investigation of the very perspective superconducting circuits including phase-shinting elements functioning on different physical principles. The development of new elements and technologies like pi-junctions and pi-shifters promote the combination of conventional Josephson junction devices and complementary novel phaseshifting structures. Such an innovation in technology will open the way towards miniaturisation of superconducting digital circuits by considerable reduction in the size of complementary logic cells. In long-term, such developments promote realization of the circuit concepts which combine superconducting structures operating on different physical principles and manufactured applying different technology. The Josephson qubits with integrated RSFQ circuits, including phase-shifting elements, could lead to new logic and memory cell architectures.

Publications

• B. Dimov, D.V. Balashov, M.I. Khabipov, Th. Ortlepp, F.-Im. Buchholz, A.B. Zorin, J. Niemeyer, F.H. Uhlmann, "Superconductive passive phase shifters for integrated RSFQ digital circuits", Supercond. Sei. Techno!., vol. 20 (2007), pp. S332-S335 (Siehe online unter: doi:10.1088/0953-2048/20/lI/S07.)
  
  
• A.B. Zorin, E. Tolkacheva, M.I. Khabipov, F.-Im, Buchholz, J. Niemeyer, "Dynamics of Josephson junctions and single-flux-quantum networks with superconductorinsulator- normal metal junction shunts" Phys. Rev. B, vol., 74 (2006), pp. 014508-1- 014508-7, dx.doi.org/10.1103/PhvsRevB.74.Q14508.
  
  
• B. Dimov, D.V. Balashov, M.I, Khabipov, Th. Ortlepp, F,-Im. Buchholz, A.B. Zorin, J. Niemeyer, F.H. Uhlmann, "Superconductive passive phase phifters for integrated RSFQ digital circuits", Conference Program and Extended Abstracts (2007), P-B07-1- P-B07-2, 11th International Superconductive Electronics Conference (ISEC 2007), Washington D.C., USA.
  
  
• B. Dimov, D.V. Balashov, M.I. Khabipov, A.B. Zorin, D. Hagedorn, F.-Im. Buchholz, F.H. Uhltnann, J. Niemeyer, "RSFQ digital devices including a Pi-Shifter", Tagung Kryoelektronische Bauelemente 2005, Kryo'05, Bad Herrenalb, Germanay, 09.-11.10.2005,.
  
  
• B. Dimov, D.V. Balashov, M.I. Khabipov, Th. Ortlepp, F.-Im. Buchholz, A.B. Zorin, J. Niemeyer, F.H. Uhlmann, "Implementation of superconductive passive phase shifters within in high-speed integrated RSFQ digital circuits", 8l European Conference on Applied Superconductivity (EUCAS'07), Brussels, Belgium, 16.-20.09.2007.
  
  
• B. Dimov, D.V. Balashov, M.I. Khabipov, Th. Ortlepp, F.-Im. Buchholz, A.B. Zorin, J. Niemeyer, F.H. Uhlmann, "RSFQ Digital Electronics with Superconductive Passive Phase Shifters - a High-Speed Test", Kryoelektronische Bauelemente, Kryo 2007, Herrsching am Ammersee, Germany, 03.-05.10.2007.
  
  
• B. Dimov, D.V. Balashov, M.I. Khabipov, Th. Ortlepp, F.-Im. Buchholz, A.B. Zorin, J. Niemeyer, F.H. Uhlmann, "Superconductive passive phase phifters for integrated RSFQ digital circuits", 11th International Superconductive Electronics Conference (ISEC 2007), Washington D.C., USA, 10.-14.06.2007.
  
  
• B. Dimov, D.V. Balashov, M.I. Khabipov, Th. Ortlepp, F.-Im. Buchholz, A.B. Zorin, J. Niemeyer, F.H. Uhlmann, "Towards the implementation of superconductive passive phase shifters in high-speed complex RSFQ digital circuits", 4th FLUXONICS RSFQ Design Workshop, Ilmenau, Germany, 23.-2S.09.2007.
  
  
• D.V. Balashov, B. Dimov, M. Khabipov, Th. Ortlepp, D. Hagedorn, A.B. Zorin, F.-Im, Buchholz, F.H. Uhlmann, J. Niemeyer, "Passive phase shifter for superconducting Josephson circuits", IEEE Trans. Appl. Supercond., vol. 17 (2007), 2,1, pp. 142-145, dx.doi.org/IO.1109/TASC.2007.897382.
  
  
• D.V. Balashov, B. Dimov, M.I. Khabipov, Th. Ortlepp, A.B. Zorin, F.-Im. Buchholz, F.H. Uhlmann, J. Niemeyer, "Frozen-flux-quanta phase shifter for integrated RSFQqubit circuit applications", Kryoelektronische Bauelemente, Kryo 2006, Jena/Gabelbach, Germany, 03.-05,10.2006.
  
  
• D.V. Balashov, M.I, Khabipov, A.B. Zorin, J. Niemeyer, B. Dimov, Th. Ortlepp, F.H. Uhlmann, "Superconductive phase shifter for RSFQ circuits", 4th FLUXONICS RSFQ Design Workshop, Ilmenau, Germany, 23.-25.09.2007.
  
  
• D.V. Balashov, M.I. Khabipov, A.B. Zorin, F.-Im. Buchholz, D. Hagedorn, J. Niemeyer, B. Dimov, Th. Ortlepp, F.H. Uhlmann, "Superconductive passive phase shifter for RSFQ-Qubit circuits", Applied Superconductivity Conference, ASC 2006, Seattle, USA, 27.08.-01.09.2006.
  
  
• D.V. Balashov, M.I. Khabipov, A.B. Zorin, S. Bogoslovsky, F.-Im. Buchholz, J. Niemeyer, "Characterization of SIN junctions as non-linear damping elements for RSFQ - qubit circuits", Extended Abstracts (2005), 10th International Superconductive Electronics Conference (ISEC 2005), P-C.06, Noordwijkerhout, NL.
  
  
• D.V. Balashov, M.I. Khabipov, A.B. Zorin, S.A. Bogoslovsky, F.-Im. Buchholz, J. Niemeyer, "Characterization of SIN junctions as non-linear damping elements for RSFQ - qubit circuits", 10th Internat. Supercond. Electron. Conf., ISEC 2005, Noordwijkerhout, Niederlande, 05.-09.09.2005.
  
  
• D.V. Balashov, M.I. Khabipov, D. Hagedorn, A.B. Zorin, F.-Im. Buchholz, J. Niemeyer, B. Dimov, Th. Ortlepp, F.H. Uhlmann, "Frozen-flux-quanta phase shifter for digital superconducting circuits", Verhandlungen der Deutschen Physikalischen Gesellschaft, Reihe 6: 41 (2006), l, 539 - 539, Bad Honnef: DPG, DPGFrühjahrstagung des Arbeitskreises Festkörperphysik zusammen mit der Condensed Matter Division der EPS, Dresden, Germany, 26.-S 1.03.2006.
  
  
• D.V. Balashov, M.I. Khabipov, S.I. Lotkhov, S.A. Bogoslovsky, F.-Im. Buchholz, A.B. Zorin, "Non-linear shunting of Josephson junctions for RSFQ-qubit experiments", Kryoelektronische Bauelemente 2005, Kryo'05, Bad Herrenalb, 09.- 11.10.2005, Germanay.
  
  
• F.-Im. Buchholz, D.V. Balashov, R. Dolata, D. Hagedorn, M.I. Khabipov, J. Kohlmann, A.B. Zorin, J. Niemeyer, "LTS junction technology for RSFQ and qubit circuit applications", 18th Int. Symp. on Supercond., ISS 2005, Tsukuba, Japan, 24.-26.10.2005.
  
  
• F.-Im. Buchholz, D.V. Balashov, R. Dolata, D. Hagedorn, M.I. Khabipov, J. Kohlmann, A.B. Zorin, J. Niemeyer, "LTS junction technology for RSFQ and qubit circuit applications", Physica C, vol. 445-448 (2006), pp. 930-936, dx.doi.org/10.1016/j.phvsc.2006.05.059.
  
  
• M.I. Khabipov, D.V. Balashov, E.M. Tolkacheva, A.B. Zorin, F.-Im. Buchholz, J. Niemeyer, "RSFQ circuits for a Josephson qubit control", 4th FLUXONICS RSFQ Design Workshop, Ilmenau, Germany, 23.-25.09.2007.
  
  
• S.V. Lotkhov, D.V. Balashov, M.I. Khabipov, F.-Im. Buchholz, A.B. Zorin, "Subgap conductivity in SIN-junctions of high barrier transparency", Physica C, vol. 449 (2) (2006), pp. 81-86.