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IEEE Transactions on Applied Superconductivity

Digital SQUIDs: New definitions and results

Vasili K. Semenov

Published Jun 1, 2003• 29 citations
Publisher

Abstract

Recently we found that several simple SFQ circuits (normally used as ADC comparators) dramatically outperform conventional SQUIDs in time resolution and slew rate. For example, it is possible to reach a resolution of about a few percent of flux quantum by using a single-shot measurement that takes only a few picoseconds. All these SFQ circuits produce during each clock period only one bit of information that indicates whether the applied flux is larger or smaller the threshold. As a result it is natural to call them digital (or logic) SQUIDs. The new sensors can be used to detect a direction of the electrical current induced in a superconductor loop by the external field and digitally compensated by another SFQ devices. This composite device contains two kinds of digital components and also could be called a Digital SQUID. It has a limited practical value with a hundreds MHz clock rate. Fortunately, the clock frequency could be increased to tens of GHz. But in this case the third digital device (a decimation filter) should be added to filter out off-band noise and reduce the output clock frequency. The last device is rather similar to an ADC and also could be called a Digital SQUID. It is easy to show that the last Digital SQUID has a practically unlimited slew rate and dynamic range without suffering significant loss of energy sensitivity of the conventional two-junction SQUID.

DOI
10.1109/tasc.2003.814027

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Recently we found that several simple SFQ circuits (normally used as ADC comparators) dramatically outperform conventional SQUIDs in time resolution and slew rate. For example, it is possible to reach a resolution of about a few percent of flux quantum by using a single-shot measurement that takes only a few picoseconds. All these SFQ circuits produce during each clock period only one bit of information that indicates whether the applied flux is larger or smaller the threshold. As a result it is natural to call them digital (or logic) SQUIDs. The new sensors can be used to detect a direction of the electrical current induced in a superconductor loop by the external field and digitally compensated by another SFQ devices. This composite device contains two kinds of digital components and also could be called a Digital SQUID. It has a limited practical value with a hundreds MHz clock rate. Fortunately, the clock frequency could be increased to tens of GHz. But in this case the third digital device (a decimation filter) should be added to filter out off-band noise and reduce the output clock frequency. The last device is rather similar to an ADC and also could be called a Digital SQUID. It is easy to show that the last Digital SQUID has a practically unlimited slew rate and dynamic range without suffering significant loss of energy sensitivity of the conventional two-junction SQUID.
Digital SQUIDs based on the single flux quantum (SFQ) logic have attractive characteristics, such as wide dynamic range and high slew rate. However, the resolution of digital SQUIDs is limited to SFQ. To realize sub-flux quantum resolution for these SQUIDs, we have adopted two types of feedback combinations: one with the SFQ input/output for a main loop and the other with a mutually coupled loop. The main loop receives direct Φ0feedback and sub-flux quantum feedback Φ0/n, where n is an integer, from the mutually coupled loop. We confirmed the correct operation of our device at a clock frequency of 1.25 GHz for a digital SQUID with a sub-Φ0flux feedback resolution of Φ0/8 from circuit simulation. The slew rate is found to be 1.56×108Φ0/s in this simulation (fc=1.25 GHz, n=8). The flux resolution for the digital SQUID shows improvement with increasing n and reached below 40 mΦ0with n=210; however the slew rate is decreased with increasing n and reached 1.22×106Φ0/swith fc=1.25 GHz and n=210.
Measurements of extremely weak magnetic fields in unshielded environments require sensors providing a high dynamic range and slew rate while maintaining a high resolution. At present, analog Superconducting Quantum Interference Devices (SQUIDs) are the most sensitive sensors for magnetic flux. Due to their room-temperature feedback electronics necessary to maintain a stable point of operation, slew rates of those sensors are limited. The dynamic range is usually restricted by the resolution of classical analog to digital converters. The digital SQUID follows a different approach: a change of the magnetic field penetrating the pickup loop is translated by the sensor into a stream of voltage pulses. Each pulse represents the change of the magnetic flux inside the pickup loop by one flux quantum Φ0. By fast counting of these flux quanta, it is possible to outperform the analog SQUID in terms of dynamic range and maximum slew rate. We present our latest results with respect to the internal mechanisms of the digital SQUID and present a method to compensate the main source for nonlinear behavior of the sensor. The analyzed circuit was fabricated by FLUXONICS Foundry (1) in a 1 kA/cm² niobium fabrication process.
Abstract Digital superconducting quantum interference device (SQUID with Single Flux Quantum (SFQ) feedback operates as a Δ-type oversampling A/D converter. Magnetic flux resolution at Nyquist frequency can be improved by taking sub SFQ feedback resolution and large oversampling ratio. Using high critical current density Nb process for fabricating SFQ circuits, we expected more higher operation speed for the digital SQUID and resulting to higher magnetic flux resolutions. We expected a performance of the digital SQUID magnetometer fabricated using 10 kA/cm2 Nb process based on CAD layout, with an up/down counter using T2FF cells and parallel SFQ feedback. For the clock frequency of 13 GHz and the Nyquist frequency of 1.587 MHz, the magnetic field noise was lower than 1.5 fT / Hz and the slew rate was exceeded 500 mT/s.
Digital superconducting quantum interference devices (SQUIDs) based on single-flux quantum (SFQ) logic have attractive characteristics such as wide dynamic ranges and high slew rates. However, the resolution of a typical digital SQUID is limited by SFQ feedback. To realize lower magnetic field noise by magnetometers using digital SQUIDs, we have adopted two feedback combinations: one, utilizing the SFQ input/output for a main loop and the other, using a mutually coupled loop. The main loop receives direct Φ0feedback and subflux quantum feedback Φ0/n, where n is an integer from the mutually coupled loop. The flux resolution for the digital SQUID shows improvement with increasing n and reaches below 40 mΦ0at n = 210; however, the slew rate decreases with increasing n and reaches 1.22 × 106Φ0/s at fc= 1.25 GHz and n = 210. For a magnetometer using a digital SQUID with a pickup loop, we confirmed a magnetic field noise of less than 100 fT/√Hz with a signal bandwidth of 1 MHz for operation frequency fcof 20 GHz.
Superconductive microwave rapid single-flux-quantum (RSFQ) digital circuits offer the possibility to digitally process signals in the frequency range of tens of GHz. An example of application to bring the advantages of this technology deals with digital SQUIDs, used for sensitive magnetometry with a broadband dynamics connected to a slew-rate in the GHz range. First recently obtained results of digital SQUID operation, based on Nb/Al-Al2O3/Nb shunted Josephson junctions, are presented in this article. In order to obtain the best performance of RSFQ circuits, and especially to push their speed at its limits, a precise diagnostic of such circuits is needed. We are developing a time-resolved diagnostic system based on sensitive photoconductive switches to sample the weak signals coming out of RSFQ circuits at cryogenic temperature. This system includes two different sorts of opto-RSFQ interfaces, based on superconducting and semiconducting materials, respectively. Some simulation results in the microwave range, along with measurements giving the potentialities of different photoswitches are presented.
Superconducting Quantum Interference Devices (SQUIDs) are known to be the most sensitive magnetometers. We present a new type of low-Tc digital SQUID that is based on Rapid-Single-Flux-Quantum (RSFQ) Logic. It allows the on-chip conversion of the detected magnetic field into a digital form, suitable for noiseless processing, very high dynamic range and high slew rate. The sensitivity, limited to one magnetic single flux quantum, and a dynamic range of 76 dB, that corresponds to an upper limit of the magnetic field amplitude higher than 5 μT, have been measured along with overnight stability. First signatures of magnetic fields have been observed simultaneously with the ones of analogue SQUIDs in the ultra low noise environment of the Laboratoire Souterrain à Bas Bruit (LSBB) located in Rustrel, Provence, France.