In WP1, VTT has significantly improved the reliability and minimized parameter scatter of their Josephson junction (JJ) process, which benefits multitude of applications also beyond aCryComm. At the early phases of the project, process changes toward minimized parameter scatter yielded problems with the fabrication of SFQ circuits and only very elementary SFQ-type behaviour was observed. However, VTT has also developed the fabrication of Josephson Arbitrary Waveform Synthesizer (JAWS) circuits with the same same JJ process and demonstrated Shapiro steps. These samples provide a more realistic way toward demonstrators in the near future than SFQ. Meanwhile, VTT, ETH, and Polariton have been studying possible interfaces that will allow to drive plasmonic modulators with SFQ (EO conversion) despite the large mismatch in energy per bit and operation voltage.
In WP2-3, ETH and Polariton have performed pioneering plasmonic experiments at cryogenic temperatures. Room temperature measurement data shows that ETH has developed the two fastest photodetectors in the world using graphene as the optically sensitive element. Limited by measurement, the fastest detector shows flat frequency response up to 500 GHz at room temperature and up to 100 GHz at 4 K. ETH and Polariton performed the MZM validation of Polariton's modulators with an electro-optic bandwidth above 100 GHz, low on-off and driving voltages, data transmission experiments proved data rates up to 128 Gbit/s, all at 4 K.
In parallel, TAU has been developing their alternative approach based on high efficiency microcavity light sources following two paths: vertical cavity surface emitting lasers and quantum dot nano light sources. A remarkable breakthrough is the development of quantum dots for operation in the telecom wavelength range around 1550 nm. Also TAU have been setting up a cryostat for characterization of the fabricated light sources. The development by TAU and KTH of light sources modulated by surface acoustic waves shows promising preliminary results, but further research beyond aCryComm is required. On the opto-electric conversion front, Single Quantum has developed ultra-short SNSPD devices, aiming at higher speed, and tackling the well-known latching problem.
PTB has demonstrated a optical cryogenic oscilloscope by measuring the time-domain response of a commercial photodiode. Measurements of aCryComm's novel OECs are planned. VTT has actively developed improved cryogenic optical and 3D packaging schemes for advanced demonstrators. The most ambitious demonstrator experiments will be performed after the project has ended.
