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Sensors

Aligning Superconducting Transition-Edge Sensors by Reflected Wave Intensity Measurement

Pei-Sa Ma, Hong-Fan Zhang, Xingxiang Zhou

Published Mar 27, 2023• 3 citations
Publisher

Abstract

It is critical to accurately align a quantum photon detector such as a superconducting transition-edge sensor (TES) to an optical fiber in order to optimize its detection efficiency. Conventionally, such alignment requires advanced infrared imaging equipment or sophisticated microfabrication. We introduce a novel technique based on the simple idea of reflected wave intensity measurement which allows to determine the boundary of the sensor and align it accurately with the fiber. By routing a light wave through an optical fiber for normal incidence on the surface of the sensor chip, and separating the reflected wave coupled back into the fiber from the input signal with a circulator, we can observe the variation in the reflected wave intensity when the beam spot of the fiber crosses the boundary between the sensor and substrate that have different reflectivity, and adjust the position of the fiber such that its output falls on the sensor. We evaluate quantitatively the precision of our alignment method, as well as the conditions that must be met to avoid photon loss caused by light beam divergence. After demonstrating the working principle of our scheme and verifying the alignment result experimentally, we employ it for efficient input signal coupling to a TES device, which is used for photon-number-resolving measurement to showcase the successful application of our alignment method in practice. Relying on only ordinary and widely used optical elements that are easy to operate and low in cost, our solution is much less demanding than conventional methods. Dramatically easier to implement and not restricted by the detection mechanism of the sensor, it is accessible to a much broader community.

DOI
10.3390/s23073495

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It is critical to accurately align a quantum photon detector such as a superconducting transition-edge sensor (TES) to an optical fiber in order to optimize its detection efficiency. Conventionally, such alignment requires advanced infrared imaging equipment or sophisticated microfabrication. We introduce a novel technique based on the simple idea of reflected wave intensity measurement which allows to determine the boundary of the sensor and align it accurately with the fiber. By routing a light wave through an optical fiber for normal incidence on the surface of the sensor chip, and separating the reflected wave coupled back into the fiber from the input signal with a circulator, we can observe the variation in the reflected wave intensity when the beam spot of the fiber crosses the boundary between the sensor and substrate that have different reflectivity, and adjust the position of the fiber such that its output falls on the sensor. We evaluate quantitatively the precision of our alignment method, as well as the conditions that must be met to avoid photon loss caused by light beam divergence. After demonstrating the working principle of our scheme and verifying the alignment result experimentally, we employ it for efficient input signal coupling to a TES device, which is used for photon-number-resolving measurement to showcase the successful application of our alignment method in practice. Relying on only ordinary and widely used optical elements that are easy to operate and low in cost, our solution is much less demanding than conventional methods. Dramatically easier to implement and not restricted by the detection mechanism of the sensor, it is accessible to a much broader community.
We report the development of the multimode fiber-coupled superconducting nanowire transition edge sensor with high system detection efficiency at optical and near-infrared wavelength. The SNTES consists of an absorber, a sensitive thermometer, and a weak thermal link to a cold bath, For a SNTES optimized to detect single photons, the temperature change caused by the absorption of a single photo is read out by the thermometer, and the system resets by cooling through the thermal link. The graded index (GRIN) lenses were spliced to the end of the multi mode fiber with a 50 µm diameter core, and the incident light spot diameter was focused to approximately 20 µm at the active area, which is 28 µm diameter, allowing a good optical coupling. We have achieved a nominal coupling efficiency of 91 % and system efficiency of 76 % by the SNTES at 1310 nm -1350 nm. The developed practical SNTES presented in this paper could open a new frontier for a wide range of applications at optical and infrared wavelength.
Transition-Edge Sensors (TESs) are superconducting photon detectors offering high quantum efficiency, energy resolution, and photon-number discrimination, making them attractive for quantum sensing applications. We report on the design, fabrication, and characterization of a Ti/Au/Ti TES optimized for multimode fiber coupling in NV-center experiments. The device integrates a bi-layer anti-reflective coating optimized for 700 nm, the emission wavelength range of NV centers in which we are interested. Optical characterization in a cryogenic setup demonstrated an energy resolution below 2 eV and photon-number resolution capability. A dedicated calibration setup was used to evaluate the system detection efficiency (SDE), yielding0.85±0.020.85 \pm 0.020.85±0.02. These results confirm the suitability of large-area TESs for efficient multimode fiber coupling and highlight their potential for improving the performance of NV-center-based quantum sensing experiments.
Optical transition-edge sensor (TES) with high quantum efficiency, low dark count rate, and high energy resolution has been used in quantum information and communications. The gap distance between the fiber tip and TES has an influence on the TES performance. Here we measured the interference pattern of our optical TES devices with and without anti-reflection coating and obtain the gap distance. We also obtain the absolute reflectivity of the Ti-TESs by correcting the effect of power splitter and circulator. Our copper block shrinks ~10 μm when cooled from room temperature down to 100 mK. The measured reflectivity is consistent with the simulation taking the dielectric mirror, Ti film and anti-reflection coating into account. In addition, we study the gap distance dependence of the system efficiency, and find that it is in good agreement with the measured reflectivity at 100 mK. These analyses are useful for further improvement of TES packaging.
Single-photon detectors operating at visible and near-infrared wavelengths with high detection efficiency and low noise are a requirement for many quantum-information applications. Superconducting transition-edge sensors (TESs) are capable of detecting visible and near-infrared light at the single-photon level and are capable of discriminating between one- and two-photon absorption events; however these capabilities place stringent design requirements on the TES heat capacity, thermometry, and optical detection efficiency. We describe the fabrication and evaluation of a fiber-coupled, photon-number-resolving TES detector optimized for absorption at 1550 and 1310 nm wavelengths. The measured system detection efficiency at 1556 nm is 95 % +/- 2 %, which to our knowledge is the highest system detection efficiency reported for a near-infrared single-photon detector.
Transition-edge sensors (TES’s) are are extremely sensitive calorimeters able to measure energies of the order of a few electronvolts. They are now well-known in the quantum technology community because of their superb combination of high-efficiency— higher than 95% for photons in the near infrared range [1]—essentially zero dark counts, and photon-number resolution. One of the main challenges when working with TES’s is to extract the photon-number information from the continuous output signal of the detectors. The usual procedure is to accumulate signals over some time—typically ~2 GB per minuite—and then use post-processing techniques to obtain photon-number resolution from the recorded signals [2] Here we introduce an FPGA circuit that analyses TES signals in real-time—recording only specific characteristics such as signal area, height, and length—allowing near realtime photon-number resolution and reducing the memory requirements by orders-of-magnitude. Using this new capability, we are able to optimise the number-resolution of the detector to the range we are interested in for each new experiment. In a preliminary study, we calibrated the TES with a weakly-pulsed 820 nm diode laser, and using just the area data from the FPGA were able to discriminate up to 15 photons. We are able to accurately discriminate an n=2 photon Fock state with parts-per-billion precision, dropping to parts-per-hundred precision at n=15 [3]. It is well known that there are physical limits to the precision with which an image can be formed. There are ways in which this limit can be circumvented, for example using super-resolution techniques that exploit the physical structure of the object, or object illumination with entangled states of light. However, in many applications—for example when the object is very far away—we cannot directly interact with the object, or illuminate it with entangled light: the quantum state of the light field is all that is accessible to the observer. Given a finite size imaging system in the far field—i.e., systems with a finite effective numerical aperture—we show the best way to extract the spatial characteristics of the light source. We implement a general imaging method by measuring the complex degree of coherence using linear optics and our photon-number-resolving detectors. In the absence of collective or entanglement-assisted measurements, our method is optimal over a large range of practically relevant values of the complex degree of coherence [4]. We measure the size and position of a small distant source of pseudo-thermal light, and show that our method outperforms the traditional imaging method by an order of magnitude in precision. Additionally, we show that a lack of photon-number resolution in the detectors has only a modest detrimental effect on measurement precision, further highlighting the practicality of this method as a way to gain significant imaging improvements in a wide range of imaging applications. References [1] A. E. Lita, A. J. Miller, and S. W. Nam, Optics Express 16, 3032 (2008). [2] G. Brida, et al., New Journal of Physics 14, 085001, (2012). [3] L. Assis, et al., preprint (2020) [4] L. A. Howard, et al., Physical Review Letters 123, 143604 (2019).