Wolfram H.P. Pernice

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).

Quantum key distribution (QKD) promises information-theoretic security, yet practical implementations remain vulnerable to device-level attacks such as detector blinding. In such attacks, an eavesdropper can apply optical pulses of increased optical power to manipulate single-photon detectors, gaining control over the receiver’s electrical output and compromising the key. While superconducting nanowire single-photon detectors (SNSPDs) offer high efficiency, low dark counts, and excellent timing, they are not immune to these attacks. Existing countermeasures either require additional hardware or complex protocols, often reducing performance. A key challenge is therefore to design intrinsically secure detectors that provide direct detection of blinding attempts without adding system complexity or relying on statistical analysis. In this work, we demonstrate that waveguide-integrated SNSPDs (WI-SNSPDs) with engineered series inductances act as an intrinsic countermeasure. By varying the series inductance of an SNSPD between ≈38 nH and 442 nH, we show that detectors with low kinetic inductance (<172 nH) enter a latched resistive state immediately when exposed to optical pulses of elevated optical power, preventing blinding entirely. This approach embeds hardware-level security directly into the detector design and leverages the flexibility of waveguide integration for low-inductance configurations. Beyond attack prevention, such designs can be integrated into scalable photonic circuits, combining high-performance quantum detection with robust security features for practical QKD systems.

Advancements in quantum computing pose significant threats to the security of conventional encryption standards. Although quantum key distribution systems offer inherent security, they face challenges in achieving practical secret-key rates over long transmission distances (2). Multiplexing schemes can substantially enhance key rates, and integrated photonic chip technologies provide the necessary scalability and system efficiency for high key rates using wavelength-division time-bin protocols. We propose a hybrid sender module that employs on-chip lasers and fast modulators on the indium-phosphide platform for qubit state preparation. The multiplexer is fabricated on a low-loss silicon nitride chip, utilizing ring resonators and Bragg reflectors. We report progress in developing a sender module designed to achieve secret key rates in the gigabit per second range. Additionally, the system is compatible with a receiver that integrates waveguide-based superconducting nanowire single-photon detectors with on-chip de-multiplexing, ensuring high detection efficiency and key rates (1). This design supports the implementation of protocols such as the one-decoy state time-bin BB84 protocol and promises further scalability.

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.