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Optics Express

Titanium-based transition-edge photon number resolving detector with 98% detection efficiency with index-matched small-gap fiber coupling

Daiji Fukuda, Go Fujii, Takayuki Numata, K. Amemiya, Akio Yoshizawa, Hidemi Tsuchida, Hidetoshi Fujino, Hiroyuki Ishii, Taro Itatani, Shuichiro Inoue, Tatsuya Zama

Published Jan 6, 2011• 270 citations
Publisher

Abstract

We have realized a high-detection-efficiency photon number resolving detector at an operating wavelength of about 850 nm. The detector consists of a titanium superconducting transition edge sensor in an optical cavity, which is directly coupled to an optical fiber using an approximately 300-nm gap. The gap reduces the sensitive area and heat capacity of the device, leading to high photon number resolution of 0.42 eV without sacrificing detection efficiency or signal response speed. Wavelength dependent efficiency in fiber-coupled devices, which is due to optical interference between the fiber and the device, is also decreased to less than 1% in this configuration. The overall system detection efficiency is 98%±1% at wavelengths of around 850 nm, which is the highest value ever reported in this wavelength range.

DOI
10.1364/oe.19.000870

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We have realized a high-detection-efficiency photon number resolving detector at an operating wavelength of about 850 nm. The detector consists of a titanium superconducting transition edge sensor in an optical cavity, which is directly coupled to an optical fiber using an approximately 300-nm gap. The gap reduces the sensitive area and heat capacity of the device, leading to high photon number resolution of 0.42 eV without sacrificing detection efficiency or signal response speed. Wavelength dependent efficiency in fiber-coupled devices, which is due to optical interference between the fiber and the device, is also decreased to less than 1% in this configuration. The overall system detection efficiency is 98%±1% at wavelengths of around 850 nm, which is the highest value ever reported in this wavelength range.
Transition-edge sensors (TESs) have shown the capability of both energy resolution and resolving photon numbers. In this work, we present the fabrication and characterization of high detection efficiency Ti optical TESs with the critical temperatureTc= 213 mK and the photon-number resolving capability at telecommunication wavelength 1550 nm. The Ti superconducting film is prepared by a DC magnetron sputtering technique. An optical cavity comprised of 5-layers antireflection and 15-layers high-reflection Ta2O5/SiO2is enclosed with the Ti film by an ion beam assisted deposition method to improve detection efficiency, which brings the absorption of 99.74%. The energy resolution for the Ti TES with sensitive area 20 μm × 20 μm is 0.45 eV and the detection efficiency is 97% ± 1%.
A transition edge sensor (TES) is one of superconducting photon detectors, which has a photon number resolving ability in light pulses. The TES device is a kind of calorimeters operated at an extremely low temperature, and the energy of the photons is measured as a resistance change in a superconducting transition region of the TES. The advantages of the TESs are an excellent energy resolution and a high quantum efficiency. However a response speed is limited due to slow thermal recovery time. To overcome this, we fabricated new TES devices which are based on a titanium superconductor. The critical temperature of our titanium films is around 410 mK, which greatly improves the thermal recovery time. The observed decay time constant of response signals to the light pulses is around several hundreds of ns, that make it possible to operate the devices at a counting rate over 1 MHz. The photon number resolving power is 0.35 eV(FWHM) for a 5 μm size device even at the high operating temperature. The system quantum efficiency is 65 % by embedding the TES films in an optical structure with a high reflection dielectric mirror and an anti-reflection coatings fabricated by an ion beam assisted sputtering method. These features are very promising for high speed photon number resolving applications in the quantum information field.
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.
High performance optical photon detectors are indispensable for quantum communications. Superconducting detectors are very attractive because of their high quantum efficiency and intrinsic low dark count probability. We have been developing high performance optical photon detectors with a Titanium‐based transition edge sensor (Ti‐TES), which have superconducting critical temperatures around 300 mK. The relatively high transition temperature enables rapid thermal relaxation of the absorbed photon energy, resulting in excellent temporal characteristics such as fast response time below 1 μs, low timing jitter of 33 ns, and maximum counting rate over 1 MHz. Although there is a tradeoff relationship between the temporal response and energy resolution, we have simultaneously achieved high quantum efficiency and high energy resolution. We fabricated Ti‐TES devices with an optical cavity structure for photon absorption enhancement in the superconducting film. The measured quantum efficiency is over 95 % at 850 nm wavelength, and the energy resolution is 0.28 and 0.6 eV FWHM for the 5 μm and 10 μm square size devices, respectively, which is sufficient to resolve the photon number of the 850 nm light pulses. These high performance detectors are expected to be one of the powerful components in quantum information, communication and measurement technologies. We will also report and discuss the application of the Ti‐TES photon detectors to quantum communication systems.
High-efficiency optical detectors that can determine the number of photons in a pulse of monochromatic light have applications in a variety of physics studies, including post-selection-based entanglement protocols for linear optics quantum computing and experiments that simultaneously close the detection and communication loopholes of Bell's inequalities. Here we report on our demonstration of fiber-coupled, noise-free, photon-number-resolving transition-edge sensors with 88% efficiency at 1550 nm. The efficiency of these sensors could be made even higher at any wavelength in the visible and near-infrared spectrum without resulting in a higher dark-count rate or degraded photon-number resolution.