The basic concepts of the ultrafast optoelectronic interface for superconducting digital electronics are presented. Several implementations of both the input optical-to-electrical and the output electrical-to-optical transducers are discussed. As an example, an optoelectronic telecommunication router is used that is capable of switching signals at a >100 Gbit s −1 rate on a single wavelength. We demonstrate that superconducting optoelectronics emerges as the ultimate solution to the ultrafast input/output (I/O) interface of single-flux-quantum (SFQ)-based, superconducting digital electronics.
The basic concepts of the ultrafast optoelectronic interface for superconducting digital electronics are presented. Several implementations of both the input optical-to-electrical and the output electrical-to-optical transducers are discussed. As an example, an optoelectronic telecommunication router is used that is capable of switching signals at a >100 Gbit s −1 rate on a single wavelength. We demonstrate that superconducting optoelectronics emerges as the ultimate solution to the ultrafast input/output (I/O) interface of single-flux-quantum (SFQ)-based, superconducting digital electronics.
Ultrafast phenomena, optoelectronics, and superconductivity are acknowledged fields of technological importance and a very large amount of research has been performed in these areas in recent years. In modern, highly-advanced digital electronic circuits, the primary function of high-performance, high-speed signal processing is given to superconducting single-flux-quantum (SFQ) logical devices, based on resistively-shunted Josephson tunnel junctions. The development of this branch of digital electronics requires ultrafast, and ultra sensitive optical-to-electrical transducers for fiber optic interconnects, which must be able to transform to the electrical domain the input information coded in the form of a train of ultrafast optical pulses and, subsequently, feed it into the SFQ processor.
Interface circuits are the key components that enable the hybrid integration of superconductor and semiconductor digital electronics. The design requirements of superconductor-semiconductor interface circuits vary depending on the application, such as high-performance classical computing, superconducting quantum computing, and digital signal processing. In this survey, various interface circuits are categorized based on the working principle and structure. The superconducting output drivers are explored, which are capable of converting and amplifying, e.g., single flux quantum (SFQ) voltage pulses, to voltage levels that semiconductor circuits can process. Several trade-offs between circuit- and system-level design parameters are examined. Accordingly, parameters such as the data rate, output voltage, power dissipation, layout area, thermal/heat load of cryogenic cables, and bit-error rate are considered.
The main goal of this work is to evaluate theoretically and experimentally the properties of ultrafast opto-superconducting interfaces used to trigger superconducting digital RSFQ circuits externally and to read the outcoming signal from the rapid single-flux-quantum (RSFQ) circuits. To do this, optoelectronics sampling techniques based on a Ti-Sa femtosecond pulsed laser are used. In a first step, miscellaneous Josephson junction-based devices (series of junctions, SQUIDS, simple RSFQ circuits) have been measured to evaluate the properties of the superconductive devices. In particular, I-V curves and magnetic diffraction patterns curves have been obtained. Since the picosecond pulses carried by the RSFQ circuits are weak, of the order of 1 mV or less, the interfaces with the external world have to be carefully designed. Therefore, the potentialities of different photoconductive switches for ultrafast and sensitive detection of picosecond pulses emitted by superconducting digital circuits are evaluated. To do so, a study of the influence of different electrical parameters associated to the geometry of photoswitches electrodes deposited on LT-GaAs materials is performed. The electrical response of dc-biased photoswitches illuminated by femtosecond pulses from a pulsed 800 nm Ti-Sa laser has been calculated for different geometries of electrodes. Using electro-optical sampling techniques, the response of some photoswitches are currently being measured.
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.
A superconductive single flux quantum (SFQ) digital integrated circuit with an optical data input has been demonstrated at the system level of 40 Gbit/s for the first time. A uni travelling carrier photodiode (UTC-PD) and an SFQ circuit chip, on which 40 Gbit/s I/O interfaces were integrated, were packaged on a cold stage (4–6 K) of a cryocooler system. The UTC-PD's output was connected to an input port of the SFQ circuit with a Cu coaxial cable. From room temperature, a 40 GHz clock was supplied to the SFQ circuit while 40 Gbit/s optical data were supplied to the UTC-PD. The SFQ circuit operated correctly with the optical 40 Gbit/s data. The measured bit error rate was less than 10−13 for 1.2 mW optical PRBS 7 input at 40 Gbit/s.
Josephson-junction-based low temperature rapid single flux quantum (RSFQ) superconducting electronics is already a mature technology, which is expected to perform at speeds exceeding 100 GHz [1]. However, the RSFQ technology still lacks a fast enough output interface between its operating circuit temperature T = 4.2 K and the room temperature electronic environment. RSFQ circuits work at liquid helium temperatures and the direct interconnection of these circuits and room temperature circuits using, e.g., copper transmission lines would consume too much of the cooling power [2]. To solve this problem a magneto-optical (MO) output interface has been proposed [3], in which the output current pulse from the RSFQ circuit is converted into an optical pulse and delivered to room temperature readout circuitry via an optical fiber.