Sang-Wook Han

Quantum photonic integrated circuits (QPICs) operating at telecommunication wavelengths hold great potential for large-scale deployment of quantum key distribution (QKD) systems. Low-propagation loss materials, such as LiNbO3, SiNx, Si, AlN, allow for high-efficiency waveguide-based optical circuits, including phase/intensity modulator, beam splitter, and polarization controller, on a semiconductor wafer. To realize a QPIC-based QKD chip, photodetectors should be integrated onto optical circuits fabricated with low propagation loss materials. In this study, we discuss the heterogeneous integration technology of III-V photodetectors on low propagation loss materials. Particularly, we demonstrated InGaA/InAs/InGaAs quantum well (InAs QW) phototransistors integrated onto Si substrates through wafer bonding and layer transfer techniques. Our InAs QW phototransistors can detect low-intensity light at a wavelength of 1.55 um owing to their photoconductive gain based on photovoltaic effects. The photovoltaic effects in phototransistor induce a threshold voltage shift because the holes accumulate in the transistor body under light illumination. The threshold voltage shift results in a higher photocurrent than the number of photo-generated carriers under light illumination at a 1.55 um wavelength. This implies that high transconductance is desirable to obtain a high photocurrent for the phototransistor. We achieved the high responsivity in our phototransistors by optimizing InAs channel structure featuring superior transconductance characteristics (2). For future work, we will integrate InAs QW photodetectors with membrane and nanowire structures onto waveguide-based optical circuits fabricated with low propagation loss materials. We believe that our proposed phototransistor will help to demonstrate a high performance power monitoring system in QPIC.

We have successfully developed the size-efficient plug and play QKD system of which the size is 43x36x16 cm 3 . This is automatically operated by the integrated controller. Our system shows that the sifted key rate is over 1 kbps and Quantum Bit Error Rate (QBER) is less than 5% in the case of that the Bob and Alice is connected by the 25 km optical fiber.