Science

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...

Magnetic memories and sensors are part of our every day's life. Every computer possesses a hard disk drive on which digital information is stored in a nanometer thick magnetic thin film. Magnetic sensors are used e.g. in hard disks to read out the stored information but also in other fields like the automotive sector. Like in all other fields of information technology such magnetic memories and sensors will have to work faster and faster to meet the requirements of future applications. Consequently also the test and measurement capabilities must be improved to test e.g. the ultra fast writing of hard disks and other magnetic memories or the response time of magnetic sensor elements.A research team of PTB Braunschweig and University of Bielefeld has now developed a new and versatile technique to study the ultra fast magnetization dynamics of magnetic nanostructures simply by measuring their electrical resistance. When a magnetic...

Within a cooperation with the university of Göttingen, the PTB is developing a technique that allows for a quantitative and traceable determination of the magnetization and the stray field of nanostructures. This technique combines magneto-optic measurements and magnetic force microscopy. Applying newly developed mathematical techniques the sample magnetization is calculated from the stray field data measured by magneto-optics.
By means of this method calibration samples for magnetic force microscopy can be characterized and thus, a traceable magnetic measurement with this extremely high resolution technique is made possible.The application of a magneto optical indicator film (MOIF) allows for a quantitative imaging of the stray field Hz on length scales between several mm to 300 nm. The signal can be calibrated by using well known external fields. However, inverse magneto-static calculation techniques are necessary for the determination...

As the quantum Hall resistance can now be precisely measured also with alternating current, its application as a quantum standard of capacitance is possible. To this end a cryogenic system with two quantum Hall devices has been set up. It allows to realize a quadrature bridge employing two independent ac quantum Hall resistors. In effect, this shortens the measurement-chain to a 10 pF capacitor. Also the resistors' thermal noise is much smaller than that of conventional room-temperature resistors. This permits to operate the quadrature bridge at a lower voltage which allows to dispense with the laborious determination of the voltage-dependence of a capacitor. Altogether, a lower uncertainty can be expected. In any case, the measurement-chain requires reliable ac bridges. Such bridges allow a high precision but are susceptible to systematic effects. It is therefore planned to carefully scrutinize and improve the bridges. Eventually this...

Terahertz-Strahlung mit Frequenzen zwischen 100 GHz und einigen THz ist nichts anderes als sanfte Wärmestrahlung mit besonderen Eigenschaften und wird schon seit vielen Jahren in wissenschaftlichen Anwendungen wie der Radioastronomie, der Atmosphärenforschung und der Laborspektroskopie genutzt. Die Erzeugung nutzbarer Terahertz-Wellen gestaltet sich allerdings schwierig. Durch das Aufkommen neuartiger optoelektronischer Strahlungsquellen hat das Gebiet jedoch mittlerweile eine technologische Reife erzielt, die kommerzielle Anwendungen ermöglicht. Erste Messsysteme zur Detektion von Hautkrebs, für die Personenkontrolle an Flughäfen oder zur Qualitätskontrolle von Industrieprodukten wurden bereits entwickelt. Aufgrund des zunehmenden Bedarfs an freier Bandbreite wird über kurzreichweitige Kommuni kationssysteme bei Frequenzen von 300 GHz und darüber nachgedacht. Wie breiten sich diese Wellen aber in Innenräumen aus und welche...