Quantum photonics relies on interfacing different technologies under mutually compatible operating conditions. While integrated optics is typically optimised for room temperature operation, superconducting nanowire single photon detectors (SNSPDs) require cryogenic operating conditions. Recently, we have demonstrated integrated photonic modulators driven by the electrical output of SNSPDs, demonstrating low-power cryogenic opto-electronic signal processing and detector read out [1]. We use titanium in-diffused waveguides in lithium niobate as our nonlinear and electro-optic integration platform, which is very useful for cryogenic prototyping and informs the design of more scalable nonlinear and electro-optic integration platforms, in particular lithium niobate on insulator (LNOI).
In this talk I will discuss our latest results implementing a cryogenic feed-forward opto-electronic circuit, which can selectively manipulate a quantum state based on a desired measurement outcome [2]. The single photon detection with an SNSPD, amplification with CMOS ICs and electro-optic modulation in lithium niobate waveguides is performed entirely within a cryostat at 0.8K. The tight integration allows us to minimize the latency between the measurement result at the single photon level and the modulation.
[1] Thiele et al., Optics Express, 31(20), 32717-32726 (2023)
[2] Thiele et al., arXiv preprint arXiv:2410.08908. (2024)

Superconducting nanowire single-photon detectors (SNSPDs) became the golden standard in single-photon counting, enabling key photonic quantum applications. Recent advances further expanded their capabilities, demonstrating that, beyond their exceptional performance in single-photon detection, SNSPDs also posses intrinsic photon-number resolution (PNR) up to a few photon numbers [1]. Mastering this would allow high-fidelity quantum light detection without requiring expensive detector arrays or complex multiplexing schemes.

Intrinsic PNR in SNSPDs is governed by the dynamics of resistive domains, which manifest in the timing of voltage response pulse following photon absorption. When multiple photons are absorbed, they may generate non-overlapping resitive domains, with their combined dynamics encoding photon number information. High-frequency readout electronics can decode this information from the voltage pulse. However, domain overlap, as well as timing jitter caused by fluctuations [2], distort photon number discrimination. Here, we present an intrinsic PNR model validated against experimental results from a commercial SNSPD. We discuss the fundamental constraints on the resolvable optical pulse duration and photon number imposed by overlapping effects and thermal fluctuations[2]. Our findings provide a framework for optimizing SNSPDs for photon-number-resolving applications.

[1] JW Los et al., APL Photonics 9, 066101 (2024)
[2] AD Semenov et al., Phys. Rev. B 102 (2024)