As the clock speeds of experimental digital electronic circuits increase beyond the measurement ranges of commercial oscilloscopes, the testing and verification thereof becomes a significant problem. Superconducting circuits add further complications, such as 4 Kelvin operating temperatures, output signal levels of around 300 muV, and the need for comprehensive magnetic field screening. This article discusses the testing of such circuits with minimal equipment and very low budgets
As the clock speeds of experimental digital electronic circuits increase beyond the measurement ranges of commercial oscilloscopes, the testing and verification thereof becomes a significant problem. Superconducting circuits add further complications, such as 4 Kelvin operating temperatures, output signal levels of around 300 muV, and the need for comprehensive magnetic field screening. This article discusses the testing of such circuits with minimal equipment and very low budgets
Superconductive electronics offers a unique combination of high bandwidth and low power for mission critical hardware such as digital processors at 20 to 100 GHz, microwatt per bit analog to digital converters, large digital switches at multi-Gb/s data rates, and low noise parametric amplifiers. This cryogenic technology, typically operating at a few degrees or a few tens of degrees Kelvin, depends upon the unique physics of superconducting materials such as Nb, NbN, and YBaCuO, and the fundamental circuit element, the Josephson junction (JJ) to create unique architectures of unprecedented design efficiency and performance. These circuits are based on the single flux quantum (SFQ) logic family, a technology that has no parallel in semiconductor electronics. Digital data is transmitted by voltage pulses with amplitude less than 1 mV, but at extremely high speeds. Individual gates have been operated as high as 370 GHz. In this paper, we present a brief overview of superconductive digital technology, describe some recent circuits, and discuss how the challenges to superconductive technology are being met, including the use of multi-chip module (MCM) technology, and the potential use of III-V HBT and HEMT circuits to provide interface drivers and amplifiers, possibly operated cryogenically.
Software-Defined Radio (SDR), as explained in [1], is a promising concept for the wireless communication industry since the entire transceiver function can be implemented in software, avoiding the replacement of hardware each time the system has to be upgraded. However, the practical implementation of such a system is depended on the hardware, which consists of precise, ultra highspeed electronic devices such as Analog-to-Digital Converters (ADCs). The present state-of-the-art semiconductor ADCs cannot be used for the implementation of SDR in the near future [2]. Some of the factors that hamper the implementation are the requirements of very high speed of a few tens of GHz and the high resolution (e.g. 16 bits) at these speeds. On the other hand, using the Low-Temperature Superconducting (LTS) technology [3], which relays on the quantum mechanical property of superconductors, very high-speed electronic devices with the accuracy of a flux quantum, can be constructed. In a closed superconducting loop, the magnetic flux can exist only in discrete quantized amounts equal to the multiples of the magnetic flux quantum, φ0 = h/2e ~ 2.07x10 Wb. The theoretical speed limit of such a Rapid Single Flux Quantum (RSFQ) device is close to 1 THz [4]. These RSFQ circuits are the fastest of all devices built in any known technology. The basic building block of an RSFQ circuit is the Josephson Junction (JJ), which is formed when two superconductors are separated by an interface. Since RSFQ circuits work at extremely high speeds, testing and verification of such a device is a difficult and challenging task. Direct testing at these speeds is not possible at this moment. Furthermore, the costs of the equipment required for such tests would be extremely high. Hence methods have to be developed to reduce the requirements of external
Prime motivators in the evolution of increasingly sophisticated communication and detection systems are the needs for handling ever wider signal bandwidths and higher data processing speeds. These same needs drive the development of electronic device technology. Until recently the superconductive community has been tightly focused on digital devices for high speed computers. The purpose of this paper is to describe opportunities and challenges which exist for both analog and digital devices in a less familiar area, that of wideband signal processing. The function and purpose of analog signal-processing components, including matched filters, correlators and Fourier transformers, will be described and examples of superconductive implementations given. A canonic signal-processing system is then configured using these components in combination with analog/digital converters and digital output circuits to highlight the important issues of dynamic range, accuracy and equivalent computation rate. Superconductive circuits hold promise for processing signals of 10-GHz bandwidth. Signal processing systems, however, can be properly designed and implemented only through a synergistic combination of the talents of device physicists, circuit designers, algorithm architects and system engineers. An immediate challenge to the applied superconductivity community is to begin sharing ideas with these other researchers.
A test setup bench is being implemented in the laboratory to test digital superconducting circuits under various conditions of operation. I will present in a first step the computer-controlled test environment developed to extract the main parameters of Josephson devices used in digital circuits and to test the circuits at low frequencies. Some results obtained on some Josephson junction arrays and circuits will be shown to illustrate this part. In a second step, I will review the microwave and optical signal feeds and readout that are under development in order to trigger and diagnose digital circuits at higher frequencies. Finally, a study of the expected performance of an on-chip diagnostic system aimed at discriminating different signal waveforms generated by superconducting digital circuits will be presented.
Communications transmitters, receivers, radar applications, and related test equipment require precise control over generated frequencies which can be provided by digital synthesis. Superconductivity technology offers to greatly improve the operational frequency range at a tiny fraction of the power of present GaAs and Si digital frequency synthesizers, an important consideration for systems with multiple receiver elements and satellite applications. We designed, fabricated, and tested a digital superconducting frequency synthesizer on a 1-cm square substrate in niobium technology and tested at 4 Kelvin. The chip contains a 12-bit pipelined MVTL incremental phase accumulator (simple expansion to 32 bits achieves one part in 4.3/spl times/10/sup 9/ frequency resolution). The most significant 10-bits of the accumulated phase proceed to a Sine ROM which is based on SQUID cells and employs data compression to minimize circuit size. An 8-bit ROM output word proceeds to a superconducting D/A converter to construct the analog output waveform which updates each clock cycle. We have operated the entire superconducting synthesizer above 1 GHz. Our performance goal with present fabrication technology is /spl ges/4 GHz operation.
Quantum computing promises an exponential speed-up of computation compared to what is nowadays achievable with classical computers. In this way, it enables the evaluation of more complex models and the breaching of current security algorithms. For the operation of a quantum system, many questions remain to be answered. Currently, there are several quantum technologies that promise to be both reliable and scalable, two features required for large scale quantum operations. Common to all technologies is the operating temperature that needs to be close to absolute zero, i.e. below 100 mK, to suppress environment noise and allow the quantum properties to become 'visible'. In order for any quantum processor to be operated, a so-called quantum-classical interface is required for the quantum bit (qubit) read-out and control. This interface consists of various electronic blocks, such as analog-to-digital converters, digital-to-analog converters, mixers, amplifiers and a digital controller. Especially the analog blocks require effort to meet the noise and stability constraints as not to disturb the very sensitive qubits and allow reading of the tiny signals. As the qubits live in extremely deep-cryogenic temperatures, long wires interface the cryogenic with the room temperature environment, where most of the electronics is situated. However, for a scalable system, heat injection becomes a serious problem, with many wires between 300 K and sub-Kelvin. Furthermore, such amount of interconnects is challenging to mechanically place in a dilution refrigerator. Therefore, in this work, we propose to implement the electronics not at room temperature, but at a temperature much closer to the qubits, for example at 4 K. This not only reduces significantly the wiring between room temperature and the qubits, but we can also benefit from lower electronic noise at such a temperature. The operation of conventional electronics almost 200°C below its normal temperature range is not trivial as device properties alter significantly and most circuits no longer operate as intended. In CMOS processes, the main technology in the integrated electronics world, the behaviour of the transistors, required for the implementation of any circuit, deviates considerably at low temperatures. The transistor's threshold voltage goes up, the mobility increases and the subthreshold slope becomes steeper, to name just a few of these deviations. Although there are improvements in performance, there are also some counter effects, and characterization of the transistors is needed to observe the changes. Once devices are characterized at such low temperatures, new models can be built and circuits can be simulated and adapted to operate properly at cryogenic temperatures. Luckily, there are various commercially available devices that can already withstand the chills of cold. We demonstrated various commercially available devices, such as a field-programmable gate array (FPGA) implemented in a 28 nm CMOS process, to be operating without major concerns at 4~K. Its properties alter only slightly, within 5 to 10%, and all tested circuit implementations, working at 300~K, also worked at 4 K. We combined both commercially available devices, that operate 200 K below their specified temperature range, with custom designed CMOS circuits to implement a cryogenic read-out platform for spin qubits. This system comprises amplifiers, an ADC, an FPGA, voltage regulators and a clock generator. It allows to amplify the tiny signal from the qubits and digitize it directly in the FPGA in order to process the data locally at 4~K. This system is one of the first systematic attempts at operating a part of the quantum-classical interface at cryogenic temperatures and forms the basis for future systems comprising the complete electronic interface for qubits to operate at such low temperatures. One of the main problems to tackle for cryogenic electronic systems is their power consumption. Power budgets are simply limited to roughly 1 or 2 Watts at 4 K and exponentially lower at deeper cryogenic temperatures, thus limiting the size of large-scale electronic systems. Our approach of combined commercial and custom circuits will have to be steadily replaced by a single (custom) technology that meets both power and scalability constraints. One of the best candidates is CMOS, a technology that the industry has relied upon for several decades and benefits from many optimizations thanks to Moore's law.