Martin A. Wolff

We present an on-chip receiver for time-based quantum key distribution (QKD) protocols such as the three-state time-bin protocol. The device features fully integrated superconducting nanowire single-photon detectors (SNSPD), low-loss delay lines and broadband 3D fiber-to-chip couplers with a total footprint of 800x800µm^2 on a single chip. By using waveguide-integrated SNSPDs featuring small dead times and low dark-count rates we are able to operate at 2.5 GHz clock rates and achieve high performance without saturating the detector at short distances. The device is demonstrated to work for wavelengths from 1480 nm to 1610 nm, but can be easily adapted to also work at visible light (on the same chip).

Emerging quantum technologies, such as quantum key distribution, increase the demand for reliable tools that enable single-photon generation, manipulation and sensing on an increasingly large scale. In the framework of integrated photonics, these needs can be fulfilled by patterning highly stable photonic devices on monolithic silicon chips in CMOS compatible processes. In this work we show how advanced single-photon detection capabilities are achieved on a silicon chip, realizing a 4x4 array of waveguide-integrated superconducting nanowire single-photon detectors (SNSPDs). Our detectors are fabricated from highly uniform superconducting NbTiN thin films in a U-shape geometry atop of silicon nitride strip waveguides [1]. The nanophotonic circuitry is interfaced with scalable 3D polymeric out-of-plane fiber-to-chip couplers [2] featuring high broadband transmission in the telecom regime. In order to address each detector individually via a separate fiber-optic channel, we precisely align a 16-channel 2D fiber array to the 4x4 coupler matrix. We evaluate the performance of our waveguide-integrated SNSPD array in a cost-efficient closed-cycle cryostat at 3.5 K. We find dark count rates below 10 Hz combined with a superior system detection efficiency of up to 50 % at 30 MHz count rate for telecom-wavelength photons. In addition, we achieve 120 ps timing jitter with a simple two-stage room-temperature amplification approach. Our detector arrays pave the way for parallelized multi-channel single-photon detection and will therewith enable ultra-fast quantum key distribution. Furthermore, our approach allows for integrating sophisticated nanophotonic devices with waveguide-coupled single-photon detectors providing additional variability and functionality.