For quantum applications, it is important to generate quantum states of light and detect them with extremely high efficiency. For future applications, it also important to do this at scale. This presents many engineering and metrology challenges. This paper discusses some of the open challenges and opportunities in single photon detector efficiency measurements, including the challenges ofmetrology for waveguide-integrated detectors on photonic circuits.
For quantum applications, it is important to generate quantum states of light and detect them with extremely high efficiency. For future applications, it also important to do this at scale. This presents many engineering and metrology challenges. This paper discusses some of the open challenges and opportunities in single photon detector efficiency measurements, including the challenges ofmetrology for waveguide-integrated detectors on photonic circuits.
Photonics is an effective platform for engineering quantum states of many photons that can be harnessed for applications in information processing tasks, ranging from secure communications to enhanced sensors to efficient simulation of physical or other systems. I will discuss recent developments and some challenges to achieving this promise. Building and measuring multi-photon quantum states suitable for these applications demands a means for them to interfere and for detection at the level of individual photons. Photonic integrated circuits are well suited to these tasks at a scale that is not possible by other means. Advances in the design and utilization of circuits and detectors enable quantum advantage, and indeed provide new opportunities for classical optical systems.
We will present our current efforts on single photon quantum metrology using high-efficiency superconducting detectors and high-efficiency single-photon sources based on spontaneous parametric downconversion. Optical power measurements based on single photon counting could establish a quantum standard for optical power calibration in the future. At NIST we are pursuing the development of single photon sources and single photon detectors for metrology, quantum and classical applications. As part of these efforts, we are pursuing the establishment of a measurement service for the calibration of single photon detectors. We present how our calibration is tied to the calibration of our transfer standard optical fiber power meters. Using the beamsplitter method, we have implemented a fiber-coupled and free-space measurement system. Also, we have developed testbeds and measurement protocols for the characterization of single photon sources and single photon detectors. We will review several methods, which allow for the characterization of the spatial and spectral degree of freedom in spontaneous parametric downconversion.
We develop quantum devices for the generation, manipulation and detection of light at the single photon level. Quantum dot devices allow for the generation of single and entangled photons at various frequencies including telecom wavelengths. We also develop single photon detectors with high efficiency based on NbTIN superconducting nanostructures, low noise and high time resolution to enable a range of applications including quantum communication, quantum sensing and integrated photonics. To allow for complex systems, integrated quantum optics circuits where we combine quantum sources and superconducting detectors are under development.
The generation, measurement, and manipulation of light at the single- and few-photon levels underpin a rapidly expanding range of applications. These range from applications moving into the few-photon regime in order to achieve improved sensitivity and/or energy efficiency, as well as new applications that operate solely in this regime, such as quantum key distribution and physical quantum random number generation. There is intensive research to develop new quantum optical technologies, for example, quantum sensing, simulation, and computing. These applications rely on the performance of the single-photon sources and detectors they employ; this review article gives an overview of the methods, both conventional and recently developed, that are available for measuring the performance of these devices, with traceability to the SI system.
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.
Two important challenges in quantum photonics are to generate useful states with high fidelity, and to detect them and verify their properties. Particularly valuable states are single photons and entangled photon pairs in well-defined optical modes, as they can be used in many quantum information protocols or used to build up more complex states. For sources, we employ integrated nonlinear optics (waveguides in lithium niobite and potassium titanyl phosphate) to maximize brightness and go beyond what is possible in bulk optics, showing simultaneously high state fidelity, heralding efficiency, and spectral purity across three experiments: first we show record heralding efficiency in a fully-fibered heralded single-photon source, and use it to probe the tradeoff between spectral purity and heralding efficiency in non-engineered sources. With an engineered source, we then herald up to 50 photons in a nonclassical state. The last source is for polarization-entangled photon pairs, with brightness of 3.5 million pairs/s·mW, fidelity to a Bell state of 96%, heralding efficiency of 43%, and HOM interference visibility of 82%. Once a complex state is constructed, it must also be verified. For this we employ a time-multiplexed detector consisting of a fibre loop and a single-photon detector. Surprisingly, we are able to extract information even in the saturation regime of the detector. We use the click statistics of the time-multiplexed detector to verify the non-classicality of quantum light, and we use its extremely high dynamic range (123 dB) to measure a macroscopic power level with a single-photon detector. Eliminating calibrated attenuators with this approach will allow direct standardization of quantum and classical optical power levels.