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.
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.
AbstractSuperconducting digital technology based on Rapid Single Flux Quantum (RSFQ) logic offers a 50 times faster clock rate and 100 times less on chip power consumption than today's mainstream semiconductor electronics. This technology is attractive for many applications and in particular for Digital Signal Processing (DSP). The applications of the RSFQ DSPs are numerous and include multi-user detection in spread spectrum system, channel equalization for Multi-in, Multi-out (MIMO) systems, and blind detection and signal tracking for signal intelligence.At Chalmers we are developing a hybrid DSP with target performance of 30 giga operations per second. It consists of an RSFQ Multiply Accumulate unit (MAC), RSFQ fast cache memory, and an external semiconductor memory. The development of the complex high speed cryogenic digital system requires interface, packaging and testing solutions. The main difficulties are due to the 4 K operational temperature, the high magnetic field sensitivity of the RSFQ circuits, the multi-channel high bandwidth interface required in the hybrid architecture, and the high speed RSFQ digital core. The key steps towards addressing each of these problems are the subject of this thesis. The main emphasis is put on the experimental testing and interfacing of superconductor electronics. A large part of the work is connected to the experimental verification of DSP components and their incremental improvement towards functionality. The RSFQ circuits tested were fabricated using the four-metal layer VLSI processes available from NGST(8 kA/cm^2 Josephson junction critical current) and HYPRES (both 1 kA/cm^2 and HYPRES 4.5 kA/cm^2. The main results of the work are demonstrations of the RSFQ cache memory with more than 2,000 Josephson junctions, 33.5 GHz operation of the digital clock controller, and 20.48 GHz operation of the autocorrelator. In addition, various experiments have been done for investigation of parasitic effects in RSFQ circuits such as flux trapping and influence of the high current bias lines.At the final stage of the work, a unique high bandwidth 4 K cryocooler based experimental setup has been developed. The system, developed in close cooperation with Hypres Inc. USA, has 40 high speed I/Os with bandwidth of 2 Gbps, 64 DC bias channels, 4 high frequency clock lines, and 4 auxiliary control lines. It supports exchangeable test units for 10 mm x 10 mm and 20 mm x 20 mm Multi Chip Module (MCM) carriers. The availability of such a system is critical to demonstration of the hybrid DSP prototype and future commercialization.
Superconducting devices have intrinsically superior characteristics to those of semiconductor devices. Presently, we can fabricate more than twenty thousand junctions on one chip using niobium technology. We have demonstrated the operation of a network system with a superconducting interconnection chip using voltage-state logic. Single flux quantum devices are promising for future superconducting applications because the clock frequency of SFQ logic is higher than that of voltage-state. We have proposed a high-end switch based on hybrid architecture using optical devices, semiconductors and SFQ devices. To demonstrate the high-speed operation of SFQ circuits, we developed an arbiter circuit that uses SFQ components and the arbiter circuit operates at 60 GHz. We also have developed a high Tc superconducting (HTS) SFQ sampler system for observing ultra-fast signal waveforms. In addition, we will discuss the prospects of future superconducting devices based on the fabrication technologies we developed.
In the quest to build a fault-tolerant quantum computer, superconducting circuits based on Josephson junctions have emerged as a leading architecture. Coherence times have increased significantly over the last two decades, and processors with ∼ 50 qubits have been experimentally demonstrated. These systems traditionally utilize microwave frequency control signals, and heterodyne based detection schemes for measurement. Both of these techniques rely heavily on room temperature microwave generators, high-bandwidth lines from room temperature to millikelvin temperatures, and bulky non-reciprocal elements such as cryogenic microwave isolators. Reliance on these elements makes it impractical to scale existing devices up a single order of magnitude, let alone the 5-6 orders of magnitude needed for performing fault-tolerant quantum algorithms. Here, I present results that suggesting superconducting digital logic, namely Single Flux Quantum (SFQ) logic, can replace analog control and measurement techniques, avoiding the significant overhead involved. I describe a scheme for measuring qubits with a device known as a Josephson Photomultiplier (JPM), which crucially stores the result of a qubit measurement in a classical circulating supercurrent within the device and allows for integration with SFQ detection circuitry. This technique is experimentally demonstrated, with single-shot measurement fidelity of 92%. Two methods for accessing this measurement result are presented, one utilizing ballistic fluxons, and another utilizing flux comparison. Initial experimental results of the latter are presented. In addition, I describe a scheme for controlling qubits with sequences of digital SFQ pulses. This method is then used to control a qubit without a microwave signal generator, with results of an average single-qubit gate fidelity of around 95%. When combined, these techniques form a nearly fully digital interface to superconducting qubits, which could allow these systems to scale much more easily.
Superconducting Rapid Single Flux Quantum (RSFQ) circuits have the ability to perform digital operation with clock signals in the microwave frequency range. The clock signal can be either generated on-chip via Josephson oscillators, or injected externally through microwave lines. We have investigated several configurations to digitize and process on-chip microwave signals for space-based applications, based on the European FLUXONICS Foundry with a niobium-based technology that can opearate at clock frequencies in the 30-40 GHz range. This presentation will focus on the main features of the technology, the expected performances for specific applications and the current status of experimental results of several circuits measured at 4K. It will also feature prospects and expectations, based on circuits already designed and in preparation.
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.
We have designed and experimentally verified a test bench for high frequency testing of Rapid Single Flux Quantum (RSFQ) circuits. This test bench uses an external tuneable clock signal that is stable in amplitude, phase and frequency. The high frequency external clock reads out the clock pattern stored in a long shift-register. The clock pattern is consequently shifted out at high speed and splitted to feed both the circuit under test and an additional shift register in the test bench for later verification at low speed. This method can be employed for reliable high speed verification of RSFQ circuit operation, with use of only low speed readout electronics. The test bench consists of 158 Josephson junctions and occupied area is 3300 * 660 (mikro meter)^2. It was\nexperimentally verified up to 33 GHz with +- 21.7 % margins on the global bias supply current.