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IEEE Transactions on Applied Superconductivity

Subpicosecond measurements of the response of Josephson transmission lines to large current pulses

M. Currie, Roman Sobolewski, T.Y. Hsiang

Published Jun 1, 1999• 2 citations
Publisher

Abstract

Josephson transmission lines (JTLs) are commonly used as signal interconnects in single-flux quantum (SFQ) circuits. They are also used, however, to transform arbitrary waveforms into SFQ pulses. Here we use a low-temperature electro-optic sampling system with subpicosecond resolution to observe the response of a JTL to a picosecond input pulse. Previously, this system has been used to directly observe SFQ pulses on niobium transmission lines. In our experiment, niobium metal semiconductor-metal (MSM) photodiodes were connected to 3, 4, 19, and 20-stage JTLs. The MSM photodiodes provided picosecond pulses to excite the JTL. The JTL bias and input pulse amplitude were varied to observe fluxon interaction and time delay of the JTL stages. Observed JTL behavior is dominated by dipolar oscillations radiated from the first stages of the JTL. By varying the values of line inductance and junction capacitance, thus changing the oscillation periods, these oscillations were traced to the coupling of the junction capacitance and line inductance. This is a new feature of the JTL that has thus far not been observed or incorporated into simulations. Our results could have significant implications on the limit of SFQ circuits.

DOI
10.1109/77.783792

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Josephson transmission lines (JTLs) are commonly used as signal interconnects in single-flux quantum (SFQ) circuits. They are also used, however, to transform arbitrary waveforms into SFQ pulses. Here we use a low-temperature electro-optic sampling system with subpicosecond resolution to observe the response of a JTL to a picosecond input pulse. Previously, this system has been used to directly observe SFQ pulses on niobium transmission lines. In our experiment, niobium metal semiconductor-metal (MSM) photodiodes were connected to 3, 4, 19, and 20-stage JTLs. The MSM photodiodes provided picosecond pulses to excite the JTL. The JTL bias and input pulse amplitude were varied to observe fluxon interaction and time delay of the JTL stages. Observed JTL behavior is dominated by dipolar oscillations radiated from the first stages of the JTL. By varying the values of line inductance and junction capacitance, thus changing the oscillation periods, these oscillations were traced to the coupling of the junction capacitance and line inductance. This is a new feature of the JTL that has thus far not been observed or incorporated into simulations. Our results could have significant implications on the limit of SFQ circuits.
Unlike coaxial cable, optical fiber has a bandwidth commensurate with the clock rates of even the fastest rapid single flux quantum (RSFQ) circuits. Employing this advantage, we have developed an optoelectronic clocking system, in which optical pulses from a picosecond laser are delivered via fiber to a superconducting chip, on which metal-semiconductor-metal (MSM) photodiodes generate fast electrical pulses with subpicosecond timing accuracy. An optical pulse splitter, constructed out of beamsplitters, mirrors, and half-wave plates, permits selection of pulse pattern and clock rate from 80.6 MHz up to 20.6 GHz. With a sampling oscilloscope, we have directly observed optical triggering of an RSFQ T flip-flop at frequencies up to 1.3 GHz with a variety of bit patterns. Optical triggering of a Josephson transmission line (JTL) at frequencies up to 20.6 GHz has been verified by precise measurement of the Josephson voltage; this result represents the highest clock rate ever reported for an optical interface for RSFQ circuits.
We present comprehensive experimental and numerical studies of the subpicosecond switching dynamics of YBa2Cu3O7-x (YBCO) grain-boundary Josephson junctions, excited by single-picosecond electrical pulses. The test structures were patterned in 100-nm- thick YBCO films grown by pulsed laser deposition on (100) MgO bicrystal substrates. Each sample consisted of a coplanar strip (CPS) transmission line, a microbridge acting as the electrical pulse generator, and a single Josephson junction positioned between the CPS lines about 100 micrometer away from the bridge. The junctions were characterized by the nonhysteretic current-voltage characteristics with the characteristic voltage approximately equal to 2.0 mV at 20 K (temperature of our experiments). A train of 100- fs-wide optical pulses from a Ti:sapphire laser photo-excited the microbridge and generated 2-ps-wide electrical pulses, which were then applied to switch the junction. In addition to the input pulse, the junction was dc-biased at +0.7 Ic, -0.7 Ic, +1.5 Ic, -1.5 Ic, and zero-Ic, where Ic is junction critical current. Time-resolved dynamics of the junction response was studied with the help of our cryogenic electro-optic sampling system, which can be regarded as a sampling oscilloscope featuring < 200-fs time resolution and < 150-(mu) V voltage sensitivity. We obtained 0.7-ps-wide single-flux-quantum (SFQ) pulses generated as a result of the junction switching process. The measurements were compared to numerical computations based on the equivalent circuit containing a resistively shunted Josephson junction, and we have found a satisfactory agreement between our simulations and experimental data. We believe our findings provide experimental confirmation of the potential of YBCO Josephson gattes as building blocks of ultrafast (sub-THz) digital electronics.
We report on the direct observation of a single-flux-quantum (SFQ) pulse. The response of a metal–semiconductor–metal photodiode to a femtosecond laser pulse was used to switch Josephson junctions and to generate an SFQ voltage pulse on a superconducting microstrip line. The detailed shape of the pulse was measured optoelectronically, using a cryogenic electro-optic sampling system. The measured SFQ pulse had a width of 3.2 ps, an amplitude of 0.67 mV, and a total pulse content of 2.1±0.2 mV×ps, corresponding to the quantum of magnetic flux h/2e. With larger excitation, multiple SFQ pulses were observed. Numerical simulations are shown to be qualitatively similar to our experimental results.
We have generated picosecond voltage pulses on a superconducting microstrip line by using a metal-semiconductor-metal photodiode as an optoelectronic switch. These pulses are fed into a two-Josephson-junction pulse shaper to generate single-flux quantum (SFQ) pulses. Using a reflective electro-optic sampling system, SFQ pulses are directly observed for the first time. This important demonstration of nonintrusively detecting electrical signals from superconducting microstrip lines at the level of rapid, single-flux quantum (RSFQ) circuits opens up a new way to test such circuits, on issues such as design verification, jitter, and failure-mode testing. Further, we propose a variable-rate optoelectronic clock for testing the functional speed of RSFQ logic circuits, with an adjustable clock rate up to 38 Gb/s.>
Characteristics of SFQ signal propagation in oxide circuits were investigated by numerical simulation. The numerical simulation for the SFQ signal propagating in the Josephson transmission line (JTL) was performed by assuming the inductor to be a passive transmission line and by taking into account the high I/sub c/R/sub n/ value of oxide junctions. The SFQ signal was found to oscillate when the inductor length was more than 20 /spl mu/m. The origin of the oscillation and the oscillation conditions were investigated with respect to the Josephson plasma frequency. A shunt resistor connected between an inductor and the groundplane was found to be effective for suppressing the oscillation. The JTL in which shunt resistors are connected may also be used as a delay line.
We theoretically investigate a readout scheme of the quantum state of a superconducting qubit based on time delay of a single flux quantum (SFQ), also known as a fluxon, propagating in a Josephson transmission line (JTL). We concretely study the time-delay readout based on capacitive coupling between a transmon qubit and a JTL, and we evaluate the time delay depending on the qubit state. We also reveal a feature of the absence of fluxon pinning and exponential suppression of nonadiabatic transitions caused by the propagating fluxon, which is advantageous for the time-delay readout. We extend the analysis to a multi-level transmon as well. Owing to the spectral gap in the JTL, the radiative decay of the qubit mediated by the JTL is exponentially suppressed, and thus the transmission line itself also serves as a filter protecting the qubit. The readout scheme requires neither complicated wiring to low-temperature stages nor bulky microwave components, which are bottlenecks for integration of a large-scale superconducting quantum computer.