Digital SQUIDs: New definitions and results

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.