Single Flux Quantum Electronics Research Articles

Electrically controlled hybrid superconductor–ferromagnet cell for high density cryogenic memory

We report the fabrication and testing, at 4.2 K, of an S1IS2FS3 device, where S, F, and I denote a superconductor (Nb), a ferromagnetic material (Permalloy), and an insulator (AlOx), respectively. The F layer covers about one half of the top electrode of the S1IS2 Josephson junction and is positioned off-center. Electric current, Itr, along the S3 electrode can change the magnetization of the F layer in such a way that, for one direction of Itr, a magnetic flux penetrates the junction perpendicular to the layers, whereas for the opposite direction, the perpendicular magnetic flux can be removed. In the former state, the modulation pattern of the Josephson critical current, Ic, in the magnetic field, H, may acquire minimum near H = 0 and restores its usual shape with maximum in the second state. These states can be used for building a compact cryogenic memory compatible with single flux quantum electronics.

Coherence-limited digital control of a superconducting qubit using a Josephson pulse generator at 3 K

Compared to traditional semiconductor control electronics (TSCE) located at room temperature, cryogenic single flux quantum (SFQ) electronics can provide qubit measurement and control alternatives that address critical issues related to scalability of cryogenic quantum processors. Single-qubit control and readout have been demonstrated recently using SFQ circuits coupled to superconducting qubits. Experiments where the SFQ electronics are co-located with the qubit have suffered from excess decoherence and loss due to quasiparticle poisoning of the qubit. A previous experiment by our group showed that moving the control electronics to the 3 K stage of the dilution refrigerator avoided this source of decoherence in a high-coherence three-dimensional transmon geometry. In this paper, we also generate the pulses at the 3 K stage but have optimized the qubit design and control lines for scalable two-dimensional transmon devices. We directly compare the qubit lifetime T1, coherence time T2*, and gate fidelity when the qubit is controlled by the Josephson pulse generator (JPG) circuit vs the TSCE setup. We find agreement within the daily fluctuations for T1 and T2*, and agreement within 10% for randomized benchmarking. We also performed interleaved randomized benchmarking on individual JPG gates demonstrating an average error per gate of 0.46% showing good agreement with what is expected based on the qubit coherence and higher-state leakage. These results are an order of magnitude improvement in gate fidelity over our previous work and demonstrate that a Josephson microwave source operated at 3 K is a promising component for scalable qubit control.

Fast and accurate inductance and coupling calculation for a multi-layer Nb process

Currently, fabrication processes for superconductive integrated circuits are moving to multiple wiring and shielding layers, some of which are placed below the main ground plane (GP) and device layers. The Advanced Industrial Science and Technology advanced process (ADP2) was the first such multi-layer Nb process with planarized passive transmission line and GP layers below the junction layer, and is at the time of writing still the most developed. This process allows complex circuit designs, and accurate inductance extraction helps to push the boundaries of the layouts possible. We show that the position of ground connections between ground layers influences the inductance of structures for which these GPs act as return path, and that this needs to be accounted for in modelling. However, due to the number of wiring layers and GPs, full layout modelling of large cells causes long calculation times. In this paper we discuss methods with which to reduce model size, and calibrate InductEx calculations using these methods against measured results. We show that model reduction followed by calibration results in fast calculation times while good accuracy is maintained. We also show that InductEx correctly handles coupling between conductors in a multi-layer layout, and how to model layouts to gauge unwanted coupling between power lines and single flux quantum electronics.

Mathematical Analysis of Multiplexing Techniques for SNSPD Arrays

Superconducting nanowire single photon detectors (SNSPDs) are distinguished by a very high sensitivity, a broad spectral range and a sparse dark count rate. Therefore, arrays of SNSPDs enable novel ultrasensitive imaging applications. The goal is to realize large arrays in the near future. Hence, in order to improve detector systems, there is the necessity to develop multiplexing techniques to keep the number of wires in the cryogenic environment as low as possible. We realized a readout circuit with rapid single flux quantum electronics. All detector output signals were merged to a single output channel. This leads to a loss of spatial resolution of the information provided by the single pixels. In order to overcome this effect, this work describes a theoretical analysis of time and code multiplexing technique for arrays of SNSPDs and a comparison concerning their applicability in different cases. We developed rules for the usability of each multiplexing technique and verified the results with measurements of a 4-pixel SNSPD array.

Improved operation range of digital superconductive electronics by implementing passive phaseshifters

We implemented small superconducting loops containing a frozen single flux quantum in single flux quantum (SFQ) electronics. Such an element acts as a π-phaseshifter which is the missing complementary circuit element in SFQ electronics and can significantly improve the circuit reliability and robustness. We designed a four bit ripple counter including toggle flip-flops (TFF) with these π-phaseshifters. We analyzed the circuit stability by means of bit error rate (BER) measurements versus bias supply. The measured operation range of the counter in standard SFQ technique was ±14% and could be improved to ±24% for the new version utilizing π-phaseshifters. These results are a clear proof for a more robust circuit realization achieved without any additional power consumption. Furthermore, it supports the reduction of present power consumption in SFQ circuits by using smaller switching energy as well as the development of more complex circuits because of a reduced sensitivity against technological parameter spread.

Reduced Probability of Noise Introduced Malfunction in RSFQ Circuits by Implementing Intrinsic $\pi$-Phaseshifter

The rapid single flux quantum electronics is characterized by a low switching energy, which makes it susceptible to noise introduced bit-errors. For industrial applications a certain noise immunity is required which is still a challenge especially for circuits of higher complexity. We analysed the influence of each individual resistor in a toggle flip-flop circuit to the overall bit-error rate. The shunt resistors of comparator structures are the dominant noise sources and bias resistors have almost no influence.

The asynchronous rapid single-flux quantum electronics – a promising alternative for the development of high-performance digital circuits

Abstract. In this paper, we investigate the application of the asynchronous logic approach for the realization of ultra high-speed digital electronics with high complexity. We evaluate the possible physical, technological, and schematical origins of restrictions limiting such an application, and propose solutions for their overcoming. Although our considerations are based on the rapid single-flux quantum technique, the conclusions derived can be generalized about any type of digital information coding.

Phase engineering techniques in superconducting quantum electronics

Due to the pulse driven nature of the Rapid Single Flux Quantum electronics nearly every basic cell requires the capability of temporary data storing. Implementing phase shifting elements in this essential device leads to several advantages concerning the device characteristics. There are different concepts enabling phase shifting elements. We give a comparative overview about these approaches. The effect of this novel element on a basic cell is analyzed exampling a toggle-flip-flop. Based on the effective noise temperature determined from the experimental results of a standard flip-flop, the bit error rate for several toggle-flip-flop realizations containing different phase shifting elements was calculated. A significantly improved area of function could be shown by simulated error rates lower than 10-12 with a DC bias margin better than ±63.5%.

Quantum computing based on a superconducting quantum interference device: exploiting the flux basis

Two classes of superconducting devices have been proposed as quantum bits (qubits) for realizing quantum logic operations. The flux qubits based on a superconducting quantum interference device (SQUID) appear to be particularly promising owing to the macroscopic nature of the qubit and potential integration with high-speed control circuitry in the form of rapid single-flux quantum electronics. Recent progress is discussed and near-term challenges mentioned. The radio frequency SQUID-based qubit offers a prospect for a reliably manufacturable scalable approach.

Sub-picosecond measurement of time intervals using single flux quantum electronics

A single-flux-quantum (SFQ) pulse coincidence detector based on resistively shunted nonhysteretic Josephson junctions was designed and simulated. The coincidence detector generates an SFQ pulse when the delay between the arrival of SFQ pulses at its two inputs is less than the coincidence threshold. Simulations indicate that the minimum coincidence threshold time can be as short as 400+or-200 fs, assuming Josephson junction characteristic voltages of 1 mV, overdamped dynamics, and 4.2 K operating temperatures. Circuit architectures exploiting this gate are suggested. Estimates of the effects of thermal noise on resolution are presented, indicating the potential for various time-domain measurements with subpicosecond resolution.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>