The Economy

Mit dem Eintritt Bulgariens in die EU ergab sich die Notwendigkeit, die korrekte Rück- führung der dortigen nationalen Normale für Wechselspannung durch einen interna- tionalen Vergleich sicherzustellen. In einem bilateralen Vergleich zwischen dem BIM in Bulgarien und der PTB wurden 3 Volt Wechselspannungen bei Frequenzen von 55 Hz, 1 kHz, 20 kHz und 100 kHz gemessen. Die Messwerte zeigen eine sehr gute Überein- stimmung (degree of equivalence) von besser als 8 µV/V. Es wurde eine Verbindung zu dem Key Comparison CCEM-K6.a hergestellt, an dem die PTB früher teilge- nommen hatte. Der bilaterale Vergleich wurde als EUROMET.EM-K6.a in die Daten- bank des BIPM aufgenommen.

The novel method is based on a single airline one-port measurement which is performed with the pre-calibrated vector network analyzer. By applying a sophisticated numerical analyzing scheme to the measured broadband data set, the complexvalued residual system error parameter of the analyzer can be extracted over the entire frequency range. From the error terms, a second-order calibration of the analyzer can be performed. The method enables an easy performance check of mechanical calibration kits as well as of electronic calibration units. In collaboration with PTB WG 8.42 und in cooperation with Rohde & Schwarz, the method is currently extended to 2-port measurements. A corresponding publication has been awarded at the 69. ARFTG conference 2008 for best paper.

The Josephson junctions with Al2O3-double barriers (SINIS junctions) used at PTB for programmable voltage standards can be damaged during the rather complex fabrication process. The reason why some critical currents strongly deviate from the average value is not known. A technological alternative are SNS junctions with only one normal conductor as effective tunnel barrier. They may reduce the problem of process-induced damages to a minimum and would, moreover, simplify the fabrication process. However, due to the low resistivity of the normal conductors used up to now, programmable voltage standards on the basis of SNS junctions could only be driven at frequencies up to 20 GHz. A new technology recently developed at NIST has the potential to overcome the frequency limit set by the IcRn-product (Ic: critical current, Rn: normal resistance of the junction). This technology is using a co-sputtered high-resistive NbxSi1-x alloy embedded...

DC-SQUIDs (SQUID: Superconducting QUantum Interference Device) are investigated for their usability as phase quantum bits (qubits) within the EU-project "EuroSQIP" in cooperation with the CNRS in Grenoble. In collaboration with our partners at the CNRS an optimized SQUID circuit was designed, including a high frequency supply and filter structures. The SQUID circuits were fabricated from Nb/AlO_x/Nb trilayers at PTB, using electron beam lithography, reactive ion etching and chemical-mechanical polishing of an SiO₂ insulating layer. After a first examination of the SQUID structures at 4,2 K, some appropriate circuits were selected and characterized in detail in a dilution refrigerator at mK temperatures in Grenoble. The measurements demonstrated that the DC-SQUID can be operated as a qubit, i.e. as a two level quantum system. The SQUIDs showed a coherence time of Rabi-oscillations which is typical for phase qubits. The results demonstrate...

In the frame of the EU-funded research project RSFQubit we develop special single flux quanta (RSFQ logic) circuits for control of the Josephson qubits. A serious problem of such circuits having traditional design are significant fluctuations of the output magnetic flux due to Johnson current noise generated by low-ohmic shunting resistors. Such resistors ensure the damping necessary for the correct processing of SFQ pulses. The low-frequency components of this noise (below ca. 10 GHz) cause strong decoherence of a coupled qubit and must therefore be suppressed. We solve this problem by replacing the resistors by frequency-dependent shunts presented by distributed dissipative transmission RC-lines with appropriate cut-off frequency. In our technology these elements are implemented as resistive thin-film micro-strips (this normal-metal layer is normally used for the shunting ohmic resistors). A correct operation of the basic RSFQ...

One of the problems of integration of Josephson digital circuits operating on single flux quanta (SFQ) logic with Josephson qubits is to fabricate large enough values L of superconducting inductances forming the loops used for storing and processing SFQ. The large values of L are dictated by the necessity of having the product I_cL close to the value of the flux quantum Ф₀, where is I_c (presumably small) is the critical current of a Josephson junction. The large values of L are, however, difficult to realize in a compact configuration which is less sensitive to external noise. Moreover the ground plane ensuring shielding of the circuit from spurious electromagnetic fields substantially reduces the resulting inductance. We solve this problem by replacing large-size geometric inductances by arrays of several Josephson junctions, i.e. active elements, having a nonlinear-inductance characteristic. An array of such junctions can be compact...

High-ohmic miniature metal strips are remarkable building elements for single electron (SET) circuits. These elements enhance the Coulomb blockade in SET transistors and suppress cotunneling in SET pumps. Recently, in cooperation with the group of Moscow State University we investigated the low-temperature behavior of Cr films (see Fig. 1a,b) having very high room-temperature resistance per square of the film, i.e. > 5 kΩ. The IV-curves of these samples showed large blockade voltage and very pronounced hysteresis near the edge of the blockade. To understand the nature of such behavior we model the system by a two-dimensional array of small metallic grains with neighbor-neighbor coupling via oxide tunnel junctions. Our simulation showed that the observed behavior of the Cr film can be caused by inclusion in the film of few relatively large islands (grains). These islands operate as SET traps which in the charged state can block the SET...

Advanced lithographic methods allow the fabrication of semiconducting and metallic structures in which controlled transport of single electrons is possible. So called "pumping" of single electrons has been studied at PTB with regard to the development of a future quantum standard for electrical current, which links the ampere to the elementary charge and frequency. Apart from the requirement of high accuracy, such a standard should also produce currents in the nanoampere range or higher in order to be usable in practice. We have implemented a device a in low-dimensional semiconducting material, and demonstrated the controlled transfer of single electrons between two reservoirs using only one modulated gate voltage. A novel nonadiabatic pumping mechanism has been employed. Theoretical investigations predict a metrologically relevant accuracy for this mechanism. Since it only requires a single modulated gate voltage, the complexity of the...