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Applied Physics Letters

Direct observation of subpicosecond single-flux-quantum generation in pulse-driven Y–Ba–Cu–O Josephson junctions

Roman Adam, Marc Currie, Carlo Kosik Williams, Roman Sobolewski, Oliver Harnack, Marian Darula

Published Jan 24, 2000• 14 citations
Publisher

Abstract

We report our time-resolved measurements of switching dynamics of grain-boundary, 5-μm-wide Y–Ba–Cu–O (YBCO) Josephson junctions, excited by 2-ps-wide electrical pulses optically generated by a voltage-biased, 5-μm-wide, 10-μm-long YBCO microbridge. The time-resolved transients were recorded at 20 K using a subpicosecond cryogenic electro-optic sampling system. The intrinsic junction response was separated from the circuit-related feedthrough, and we observed both experimentally and in numerical simulations a 0.8-ps-wide, single-flux-quantum (SFQ) pulse, generated by the switching of the junction with the IcRn product equal to ∼2 mV. At the same time, the measured turn-on delay time was almost three times shorter than that obtained from simulations, questioning applicability of the resistively shunted tunnel junction model to nonhysteretic, high-temperature superconducting weak links. Our test structure's power consumption associated with the single SFQ pulse generation was ∼0.1 μW, leading to a (switching time)×(dissipated power) product equal to 0.08 aJ.

DOI
10.1063/1.125790

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We report our time-resolved measurements of switching dynamics of grain-boundary, 5-μm-wide Y–Ba–Cu–O (YBCO) Josephson junctions, excited by 2-ps-wide electrical pulses optically generated by a voltage-biased, 5-μm-wide, 10-μm-long YBCO microbridge. The time-resolved transients were recorded at 20 K using a subpicosecond cryogenic electro-optic sampling system. The intrinsic junction response was separated from the circuit-related feedthrough, and we observed both experimentally and in numerical simulations a 0.8-ps-wide, single-flux-quantum (SFQ) pulse, generated by the switching of the junction with the IcRn product equal to ∼2 mV. At the same time, the measured turn-on delay time was almost three times shorter than that obtained from simulations, questioning applicability of the resistively shunted tunnel junction model to nonhysteretic, high-temperature superconducting weak links. Our test structure's power consumption associated with the single SFQ pulse generation was ∼0.1 μW, leading to a (switching time)×(dissipated power) product equal to 0.08 aJ.
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.
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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.>
A high-resolution Josephson sampler using a single fluxoid is fabricated. The sampling pulse generator comprised a two-junction interferometer (DC–SQUID) with a damping resistance resistively coupled to a single junction as the sampling gate to reduce a turn-on delay. An ultra-fast sampling pulse was generated at a vortex-to-vortex transition in this sampler. The fabricated sampler had less than a 5ps time resolution and about a 5 µA current sensitivity. The dynamic waveform associated with two kinds of transitions was observed by the use of this sampler: a vortex-to-vortex transition and a vortex-to-voltage transition. The results show that the pulse corresponding to a single fluxoid has a nearly constant pulse height that is independent of the bias current and that setting the bias current near the critical point results in the long rise time of a DC–SQUID.