High resolution measurements of the switching current in a Josephson tunnel junction: Thermal activation and macroscopic quantum tunneling

We have developed a scheme for a high resolution measurement of the switching current distribution of a current biased Josephson tunnel junction using a timing technique. The measurement setup is implemented such that the digital control and read-out electronics are optically decoupled from the analog bias electronics attached to the sample. We have successfully used this technique to measure the thermal activation and the macroscopic quantum tunneling of the phase in a small Josephson tunnel junction with a high experimental resolution. This technique may be employed to characterize current-biased Josephson tunnel junctions for applications in quantum information processing.Comment: 10 pages, 8 figures, 1 tabl

Wideband digital phase comparator for high current shunts

A wideband phase comparator for precise measurements of phase difference of high current shunts has been developed at INRIM. The two-input digital phase detector is realized with a precision wideband digitizer connected through a pair of symmetric active guarded transformers to the outputs of the shunts under comparison. Data are first acquired asynchronously, and then transferred from on-board memory to host memory. Because of the large amount of data collected the filtering process and the analysis algorithms are performed outside the acquisition routine. Most of the systematic errors can be compensated by a proper inversion procedure. The system is suitable for comparing shunts in a wide range of currents, from several hundred of milliampere up to 100 A, and frequencies ranging between 500 Hz and 100 kHz. Expanded uncertainty (k=2) less than 0.05 mrad, for frequency up to 100 kHz, is obtained in the measurement of the phase difference of a group of 10 A shunts, provided by some European NMIs, using a digitizer with sampling frequency up to 1 MHz. An enhanced version of the phase comparator employs a new digital phase detector with higher sampling frequency and vertical resolution. This permits to decrease the contribution to the uncertainty budget of the phase detector of a factor two from 20 kHz to 100 kHz. Theories and experiments show that the phase difference between two high precision wideband digitizers, coupled as phase detector, depends on multiple factors derived from both analog and digital imprint of each sampling system.Comment: 20 pages, 9 figure

Highly-sensitive superconducting quantum interference proximity transistor

We report the design and implementation of a high-performance superconducting quantum interference proximity transistor (SQUIPT) based on aluminum-copper (Al-Cu) technology. With the adoption of a thin and short copper nanowire we demostrate full phase-driven modulation of the proximity-induced minigap in the normal metal density of states. Under optimal bias we record unprecedently high flux-to-voltage (up to 3 mV/$\Phi_0$) and flux-to-current (exceeding 100 nA/$\Phi_0$) transfer function values at sub-Kelvin temperatures, where $\Phi_0$ is the flux quantum. The best magnetic flux resolution (as low as 500 n$\Phi_0/\sqrt{Hz}$ at 240 mK, being limited by the room temperature pre-amplification stage) is reached under fixed current bias. These figures of merit combined with ultra-low power dissipation and micrometer-size dimensions make this mesoscopic interferometer attractive for low-temperature applications such as the investigation of the magnetization of small spin populations.Comment: 7 pages, 5 color figure

Chapter Development of Josephson voltage standards

Neurology & clinical neurophysiolog

Development of Josephson voltage standards

Neurology & clinical neurophysiolog

Josephson Wellenform Charakterisierung eines Sigma-Delta Analog/Digital Wandlers zur Datenerfassung in der Metrologie

A sampling system based on a 24-bits sigma-delta analog-to-digital converter (ADC) was built and characterized in order to study the feasibility of using this type of ADCs in electrical metrology. The non-linearities of the sampling system have been studied and a model for postcorrecting the measured data points established. The Hammerstein model, consisting of a static non-linear part and a linear system, was employed. A 4-th order polynomial accounts for the non-linearities of the analog electronics and the input stages of the sigma delta ADC. The linear part corresponds to the transfer function of the decimation filters internal to the ADC. The parameters for the model of the system were determined using noiseless and drift-free waveforms from a Josephson waveform synthesizer. The performance of the sampling system was verified experimentally by comparing the measured root-mean-square (rms) value of sinusoidal signals with the results from an established method. The results obtained using the post-corrected samples from the sampling system at 125 Hz agreed to within 2 μV/V with the de facto standard in metrology laboratories, which uses a high accuracy digital voltmeter. Precision measurements are limited by the decimation filters inside the ADC and can only be carried out for frequencies below 1/24-th of the equivalent sampling rate. The characterization results have shown that the non-linearities have been compensated to 5 μV/V or better and the effective resolution exceeds 20 bits, over an input range of 1 V at the equivalent sampling rate of 32 kHz. The experimental validation has proved that it is possible to measure rms values of sinusoidal signals with 1 V peak amplitudes for frequencies up to 1.3 kHz with uncertainty of 8 μV/V, significantly improving the uncertainty achievable with de facto standard which reaches 8 μV/V at 500 Hz.Ein Abtastsystem basierend auf einem 24-Bit Sigma-Delta Analog-DigitalWandler (ADC) wurde gebaut und charakterisiert, um die Möglichkeiten eines solchen ADC-Typs für Anwendungen in der elektrischen Metrologie zu untersuchen. Die Nichtlinearitäten des Abtastsystems wurden bestimmt und ein Modell für die nachträgliche Korrektur der erfassten Abtastwerte entwickelt. Dafür wurde das Hammerstein Modell verwendet, das zur Charakterisierung eines statisch, nichtlinearen Blocks gefolgt von einem linearen Teil geeignet ist. Ein Polynom vierter Ordnung wurde zur Beschreibung der statischen Nichtlinearität in der analogen Elektronik und der Eingangsstufe des Sigma-Delta ADC verwendet. Der lineare Teil des Modells umfasst die Transferfunktion des Dezimationsfilters im ADC Chip. Die Parameter für das Modell wurden mithilfe rausch- und driftloser Signale von einem Josephson Wellenform Synthesizer ermittelt. Die Leistungsfähigkeit des Abtastsystems wurde experimentell durch Effektivwertmessungen (rms) von sinusförmigen Signalen mit einem etablierten Messverfahren überprüft. Als Ergebnis wurde eine Übereinstimmung innerhalb von 2 μV/V bei 125 Hz mit dem de facto Normal der metrologischen Kalibrierlabore gefunden, das auf einem hochpräzisen Digitalvoltmeter basiert. Präzisionsmessungen haben ergeben, dass die Dezimationsfilter im ADC die maximale Frequenz auf 1/24stel der äquivalenten Abtastrate begrenzen, wenn die bestmöglichen Unsicherheiten erreicht werden sollen. Die Ergebnisse der Systemcharakterisierung haben bestätigt, dass Nichtlinearitäten auf 5 μV/V oder besser kompensiert werden. Die effektive Auflösung überschreitet 20 Bit über einen Eingangsbereich von 1 V und mit einer äquivalenten Abtastrate von 32 kHz. Die experimentelle Überprüfung hat gezeigt, dass es mit dem neuen System möglich ist, den Effektivwert sinusförmiger Signale und 1 V Amplitude für Frequenzen bis 1,3 kHz mit einer Messunsicherheit von 8 μV/V zu bestimmen, und somit die erreichbare Messunsicherheit des de facto Normals, das 8 μV/V bei 500 Hz erreicht, deutlich zu verbessern

A Josephson junction bridge track and hold circuit

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1994.Includes bibliographical references (leaves 133-135).by Kimo Y.F. Tam.Sc.D