Light polarization is a primary degree of freedom for encoding quantum information. The scaling up of photonic quantum networks and computer architecture depends crucially on its precise characterization. This is typically achieved by placing external waveplates, polarizers, moving mounts, and recently metasurfaces, on top of the detectors. All these solutions complicate integration and scaling. Here we break convention with traditional architecture and present a monolithic, self-aligned metal-superconductor nanowire single photon detector (M-SNSPD) possessing intrinsic full polarization selectivity. Gold nanowires, co-fabricated atop NbTiN superconducting nanowires within the same lithographic footprint, act as polarization-selective plasmonic metamaterials inducing resonant absorption in the NbTiN. U-shaped wires provide linear polarization selectivity, while S-shaped meanders distinguish circular polarization, while retaining the high-count rates and low dark count rates of conventional SNSPDs. By arranging them into a four-pixel array we realize simultaneous projection onto four polarizations and demonstrate continuous polarization state tomography with an ensemble average fidelity exceeding 98%. Our approach opens new avenues towards scalable detector arrays with integrated plasmonic functionalities, for single photon polarimetry, imaging and spectroscopy.
Light polarization is a primary degree of freedom for encoding quantum information. The scaling up of photonic quantum networks and computer architecture depends crucially on its precise characterization. This is typically achieved by placing external waveplates, polarizers, moving mounts, and recently metasurfaces, on top of the detectors. All these solutions complicate integration and scaling. Here we break convention with traditional architecture and present a monolithic, self-aligned metal-superconductor nanowire single photon detector (M-SNSPD) possessing intrinsic full polarization selectivity. Gold nanowires, co-fabricated atop NbTiN superconducting nanowires within the same lithographic footprint, act as polarization-selective plasmonic metamaterials inducing resonant absorption in the NbTiN. U-shaped wires provide linear polarization selectivity, while S-shaped meanders distinguish circular polarization, while retaining the high-count rates and low dark count rates of conventional SNSPDs. By arranging them into a four-pixel array we realize simultaneous projection onto four polarizations and demonstrate continuous polarization state tomography with an ensemble average fidelity exceeding 98%. Our approach opens new avenues towards scalable detector arrays with integrated plasmonic functionalities, for single photon polarimetry, imaging and spectroscopy.
Superconducting nanowire single-photon detectors (SNSPDs) have attracted remarkable interest for visible and near infrared single-photon detection, owing to their outstanding performance. Conventional SNSPDs are generally used as binary photon-counting detector. Another important characteristic of light, i.e., polarization, has not been resolved using standalone SNSPDs. In this work, we simulated, fabricated, and characterized a linear polarimeter based on a four-pixel NbN superconducting nanowire array, capable of resolving the polarization state of linearly polarized light at the single-photon level. The detector array design is based on a division of focal plane sensor, in which the orientation of each nanowire division (pixel) is offset by 45 degree. Each single nanowire pixel operates as a combination of photon detector and almost linear polarization filter, with an average polarization extinction ratio of approximately 10. The total system detection efficiency with four pixels is approximately 1% at a total dark count rate of 680 cps, when the detector array is free-space coupled and illuminated with 1550 nm photons. The Stokes parameters are extracted from polarization measurements of the four pixels. The mean errors of the measured AoP and DoLP were about -3 degree and 0.12, respectively. Our results indicate that it is possible to develop a scalable polarization polarimeter or imager based on a superconducting nanowire array. This detector array may find promising application in single-photon polarization detection and imaging.
Superconducting nanowire single-photon detectors (SNSPDs) have attracted remarkable interest for visible and near-infrared single-photon detection due to their outstanding performance. However, conventional SNSPDs are generally used as binary photon-counting detectors. Another important characteristic of light, i.e., polarization, which can provide additional information of the object, has not been resolved using the standalone SNSPD. In this work, we present a first prototype of the polarimeter based on a four-pixel superconducting nanowire array, capable of resolving the polarization state of linearly-polarized light at the single-photon level. The detector array design is based on a division of focal plane configuration in which the orientation of each nanowire division (pixel) is offset by 45°. Each single nanowire pixel operates as a combination of a photon detector and almost linear polarization filter, with an average polarization extinction ratio of ∼10. The total system detection efficiency of the array is ∼1% at a total dark count rate of 680 cps, with a timing jitter of 126 ps, when the detector array is free-space coupled and illuminated with 1550-nm photons. The mean errors of the measured angle of polarization and degree of linear polarization were about -3° and 0.12, respectively. Furthermore, we successfully demonstrated polarization imaging at low-light level using the proposed detector. Our results pave the way for the development of a single-photon sensitive, fast, and large-scale integrated polarization polarimeter or imager. Such detector may find promising application in photon-starved polarization resolving and imaging with high spatial and temporal resolution.
Abstract Polarization-resolved single-photon detection is a key requirement for advanced quantum photonic applications, including polarization-encoded quantum communication and polarimetric imaging. Although various superconducting nanowire single-photon detector architectures have demonstrated high polarization extinction ratios, concurrently achieving high optical absorption efficiency remains a nontrivial challenge. We propose a compact detector design in which a superconducting nanowire is embedded within a polarization-sensitive silicon grating cavity. The cavity supports distinct longitudinal resonances depending on the incident polarization, thereby enabling selective enhancement of absorption for the desired polarization state. Notably, this configuration introduces minimal fabrication complexity compared to conventional SNSPDs, as the grating cavity can be etched concurrently with the nanowire meander. In numerical simulations, our detector design achieves an absorption efficiency of up to 89.96% and a polarization extinction ratio of 32.39 dB , with a fill factor of just 7%. These results represent a significant advancement toward realizing high-performance, polarization-discriminating single-photon detectors.
This study proposed what we believe to be a novel method for fabricating superconducting nanowire single-photon detectors (SNSPDs) with high efficiency, polarization insensitivity, and ultrafast response. To achieve these properties in niobium nitride (NbN) SNSPDs, the periodic four-split rings (PFSR) were positioned above the nanowires. This design uses the localized surface plasmon resonance to enhance the electric field around nanowires. For an incident light with a wavelength of 1550 nm, the PFSR-SNSPD structure achieved a polarization extinction ratio of 1.0064 and absorptions of 88.94% and 88.37% under TE and TM polarizations, respectively. The nanowire length was reduced by 85% using a meandering nanowire arrangement with a fill factor of 0.074.
Advancements in integrated photonics have enabled the miniaturization and scaling of complex photonic circuits, providing stability and reliability beyond the capabilities of conventional bulk setups. A central component of quantum photonic circuits is the single-photon detector, responsible for quantum state measurement. Due to the stringent performance requirements of quantum applications, superconducting nanowire single-photon detectors (SNSPDs) have emerged as the preferred detection technology, offering superior efficiency, low noise, and high timing resolution. While tabletop SNSPDs are now mature, commercially available and contributed to significant progress in quantum computing and communication, their integration into photonic circuits is still in its infancy. Integrated SNSPDs have shown the potential to not only match but surpass tabletop detectors in several performance metrics. Nevertheless, demonstrations have largely focused on individual integrated detectors, with limited investigation into more complex integrated circuits and advanced functionalities. In this thesis, we present a novel integration platform based on photonic bound states in the continuum (BICs) to address a critical limitation of conventional SNSPD designs, where nanowires placed directly atop the waveguide core are constrained by the core geometry. The etchless nature of our BIC waveguides provides a flat surface that decouples the design of the electronic readout circuitry below from the optical circuit layer above, enabling innovative integrated-nanowire geometries and facilitating large-scale integration of SNSPDs with CMOS-compatible circuits. We established a complete fabrication and characterization workflow for integrating NbTiN-based superconducting nanowires with BIC waveguides. Our integrated detectors achieved state-of-the-art performance, including detection efficiencies exceeding 70%, intrinsic jitter below 70 ps, and ultrafast recovery times (~1.7 ns). These results pave the way toward GHz-rate quantum state measurements, essential for scalable quantum photonic processors. Leveraging the design flexibility of BIC waveguides, we introduce the concept of inline detection with SNSPDs which exploits their unique ability to precisely engineer absorption through nanowire length control. By cascading multiple partially absorbing SNSPDs, it is possible to significantly reduce the number of required optical components and miniaturize complex quantum measurement setups within an extremely compact footprint. Traditional integration geometries prevent such implementations due to significant scattering losses at nanowire terminations, which limit the feasibility of cascaded designs. On the other hand, the design flexibility provided by our BIC platform makes it ideal for development of inline detection schemes. As a proof-of-concept, we use inline detection to miniaturize two fundamental quantum optics tools: an integrated Hanbury Brown and Twiss (HBT) interferometer and a photon-number-resolving (PNR) detector. We replicated the functionalities of these setups using only two nanowires integrated along a single waveguide. These devices show extremely compact footprint, highlighting the scalability potential of our approach. We validated these devices through characterization of a single-photon source and the discrimination of collinear photon pairs. The experimental results aligned well with previous literature and theoretical predictions, confirming the reliability of our integration method. Overall, this work presents a novel approach to quantum measurements within integrated quantum photonic circuits. Inline detection represents a step forward in quantum integrated photonics, moving from individual device demonstrations toward functional quantum circuits. By introducing our BIC-based integration platform, we simplify detector design and fabrication, lowering barriers to the development of inline detection circuits and opening new opportunities in multiphoton quantum state measurements, single-photon spectroscopy, and multiphoton subtraction.