Recently, photon number resolving detectors, which are based on micro-calorimeters with superconducting transition edge sensors (TESs) [1], have shown excellent performances such as high quantum efficiency of 89 %, and high energy resolution of 0.2 eV [2]. To extend their applications to quantum telecommunication fields, however, the detectors should have high timing properties of the low timing jitters and the fast recovering time. To achieve these properties, we have developed new TES detectors with a titanium-based superconductor. Titanium has the transition temperature (Tc) at 0.39 K in bulk. This will improve the timing properties of TES devices because the operating temperature at Tc is strongly related to the intrinsic time constant of the device and the thermal diffusions of the incident photon energy.
Recently, photon number resolving detectors, which are based on micro-calorimeters with superconducting transition edge sensors (TESs) [1], have shown excellent performances such as high quantum efficiency of 89 %, and high energy resolution of 0.2 eV [2]. To extend their applications to quantum telecommunication fields, however, the detectors should have high timing properties of the low timing jitters and the fast recovering time. To achieve these properties, we have developed new TES detectors with a titanium-based superconductor. Titanium has the transition temperature (Tc) at 0.39 K in bulk. This will improve the timing properties of TES devices because the operating temperature at Tc is strongly related to the intrinsic time constant of the device and the thermal diffusions of the incident photon energy.
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.
Transition Edge Sensors (TESs) are characterized by the intrinsic figures of merit to resolve both the energy and the statistical distribution of the incident photons. These properties lead TES devices to become the best single photon detectors for quantum technology experiments. For a TES based on titanium and gold, has been reached, at telecommunication wavelength, an unprecedented intrinsic energy resolution (0.113 eV). The uncertainty analysis of both energy resolution and photon state assignment has been discussed. The thermal properties of the superconductive device have been studied by fitting the bias curves to evaluate the theoretical limit of the energy resolution.
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.
Superconducting transition-edge sensors (TESs), working as single-photon detectors with high quantum efficiency, low dark count rate, and photon number resolving capability, have been successfully used in quantum information experiments as well as astronomical transient object observations. Aiming to achieve high count rate, we are developing titanium-based superconducting TES single-photon detectors with a critical temperature of ~300 mK. Ti-based TESs with active area from 10 × 10 μm to 40 × 40 μm are first fabricated on silicon substrate coated with SiOx. The fabricated TES devices show a thermal conductance scaling with the active area and an effective time constant of ~4 μs from pulsed laser diode illumination. In order to get high quantum efficiency, we further design and fabricate a 1550-nm optical cavity based on dielectric mirror and antireflection layer, and study its absorption with an IR spectrometer. Ti TES devices on the dielectric mirror are then fabricated and the superconducting transition is measured.
Strong interests on optical quantum based metrology, quantum information and particularly in quantum cryptography are continuously growing. The main limitations to the developments in these fields are due to non-ideal devices: both single photon sources and single photon detectors. In these field of applications, detectors require to be able to resolve the number of photons in a light pulse. Presently state of the art indicates that classical semiconductor light detectors (i.e. avalanche photodiode or single photon avalanche diode) are not able to discriminate the number of photon arriving at the same time. In the meanwhile, superconducting devices have shown the possibility to resolve single photon pulses. One of the most promising superconducting detectors is the Transition-Edge Sensor (TES): a microcalorimeter that takes advantage of the sharp transition (few millikelvin) from the superconducting to the normal phase; for this reason it is sometimes called Superconductive Phase Thermometer (SPT). In the ultraviolet (UV) to infrared (IR) wavelength range, the photons are absorbed directly by the superconductive thin film and the absorbed energy induces an increase of the TES resistance. Thanks to the applied bias voltage, which maintains the device in the transition region the photon absorption induces a decrease of the TES current, measured by a dc-SQUID amplifier, and the pulse integral of the bias power reduction corresponds to the absorbed energy. This means that TESs have the very interesting properties to be able to detect single photons with an intrinsic energy resolution, without filters or gratings, that limit the quantum efficiency. By contrary of classical detectors, if monochromatic light irradiates TESs, as usually happens in communication systems, they show the photon-number resolving (PNR) capability and due to the good signal to noise ratio TESs are almost free from dark counts. Moreover, in the superconducting detector family, TESs are the only true photon-number resolving detectors operating in the VIS-NIR range. Together with quantum information science, the PNR property results useful even for optical radiometry too. In the optical community, the candela - the International System (SI) unit for the luminous intensity - has not a common consensus whether its present definition fully satisfies the current and future needs of growing associated technology. Furthermore, actually there are substantial efforts directed toward a new definitions of four base SI units: the proposal wants to link the SI units to fundamental constants, leaving f.i. material artifact. Considering the recent advances in optical radiometry and in quantum technologies, for the candela world it means redefine its unit linking to the Planck constant and consequently expressing the luminous intensity unit in terms of photon number rather than optical power. This challenge has been accepted by several national metrology institutes to demonstrate the feasibility of redefining the candela. Inside this research project called `qu-candela', the TES PNR capability has been considered to build the bridge between the quantum and classical world of radiometry: i.e. the detector possibility to measure optical powers from one single photon per second to the lower limits of cryogenic radiometry, 104 photons per second. The theme of this work of thesis is to investigate both optical and electrical characterization of different kind of TESs based on a titanium/gold multilayer film, produced and developed at the National Institute of Metrological Research (INRIM) of Torino. Thanks to the proximity effect, the multilayer allows to lower with continuity the critical temperature from that of the Ti bulk (Tc ~ 390 mK) to those of interest: ~ 300 mK and ~ 100 mK. Detectors with higher Tc have shown a faster response pulse with a relaxing time constant of the order of 200 ns, while for the lower Tc sensors, the time constant is about 10 µs. By contrary to the response time, the detector intrinsic energy resolution is proportional to its film critical temperature. Our sensors work to discriminate incident photon from UV wavelengths to those typical of the telecommunications, 1310 nm and 1550 nm. Irradiating a TES with an active area of 10x10 µm^2 by incident photons of 0.79 eV (corresponds to a wavelength 1570 nm), the best energy resolution obtained has been 0.18 eV. Detectors with higher active area 20x20 µm^2 have a worse energy resolution, because it is also proportional to the material film heat capacity. In the meanwhile due to the same reason these kind of sensors present a bigger saturation energy. This has allowed to investigate on the TES capability to discriminate up to 29 incident photons simultaneously. Until now, such count represents the bigger amount of photons discriminated by single photon detectors, without reaching the device saturation, with a linear behaviour. From this count it has been estimated 12 photons on average, per pulse, at 9 kHz repetition rate; this results in a photon flux of about 105 photons/s, demonstrating the possibility of having a detector able to work from low flux regime to 1 photon/s to flux measurable by conventional semiconductor device (f.i. single photon avalanche detector SPAD). An innovative absolute calibration technique for PNR detector has been demonstrated. The absolute technique is based on the Klyshko's efficient solution to measure detection efficiency in photon counting rate and well know for common click-no-click detector. In fact, exploiting the recent developments in quantum state world, it is possible to work with quasi single photon state, by using a parametric down conversion heralded single photon source, and calibrate PNR detectors without requiring reference standards. The best detection efficiency, of ca. 50%, has been reached by coupling the smaller active area detectors with a 9 µm core optical fiber, single mode at telecom wavelengths
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%.