Photon-Number Resolving by Superconductive Devices

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 wavelength

Aluminum-Titanium Bilayer for Near-Infrared Transition Edge Sensors

Transition-edge sensors (TESs) are single photon detectors attractive for applications in quantum optics and quantum information experiments owing to their photon number resolving capability. Nowadays, high-energy resolution TESs for telecommunication are based on either W or Au/Ti films, demonstrating slow recovery time constants. We report our progress on the development of an Al/Ti TES. Since bulk aluminum has a critical temperature (Tc) of ca. 1.2 K and a sufficiently low specific heat (less than 10(-4) J/cm³K²), it can be employed to produce the sensitive material for optical TESs. Furthermore, exploiting its high Tc, Al-based TESs can be trimmed in a wider temperature range with respect to Ti or W. A first Al/Ti TES with a Tc ≈ 142 mK, investigated from a thermal and optical point of view, has shown a response time constant of about 2 μs and single photon discrimination with 0.34 eV energy resolution at telecom wavelength, demonstrating that Al/Ti films are suitable to produce TESs for visible and NIR photon counting.Transition-edge sensors (TESs) are single photon detectors attractive for applications in quantum optics and quantum information experiments owing to their photon number resolving capability. Nowadays, high-energy resolution TESs for telecommunication are based on either W or Au/Ti films, demonstrating slow recovery time constants. We report our progress on the development of an Al/Ti TES. Since bulk aluminum has a critical temperature (Tc) of ca. 1.2 K and a sufficiently low specific heat (less than 10(-4) J/cm³K²), it can be employed to produce the sensitive material for optical TESs. Furthermore, exploiting its high Tc, Al-based TESs can be trimmed in a wider temperature range with respect to Ti or W. A first Al/Ti TES with a Tc ≈ 142 mK, investigated from a thermal and optical point of view, has shown a response time constant of about 2 μs and single photon discrimination with 0.34 eV energy resolution at telecom wavelength, demonstrating that Al/Ti films are suitable to produce TESs for visible and NIR photon counting

Predictable quantum efficient detector based on n-type silicon photodiodes

Timo Dönsberg et al. -- 16 pags., 16 figs., 4 tabs. -- Open Access funded by Creative Commons Atribution Licence 3.0The predictable quantum efficient detector (PQED) consists of two custom-made induced junction photodiodes that are mounted in a wedged trap configuration for the reduction of reflectance losses. Until now, all manufactured PQED photodiodes have been based on a structure where a SiO2 layer is thermally grown on top of p-type silicon substrate. In this paper, we present the design, manufacturing, modelling and characterization of a new type of PQED, where the photodiodes have an Al2O3 layer on top of n-type silicon substrate. Atomic layer deposition is used to deposit the layer to the desired thickness. Two sets of photodiodes with varying oxide thicknesses and substrate doping concentrations were fabricated. In order to predict recombination losses of charge carriers, a 3D model of the photodiode was built into Cogenda Genius semiconductor simulation software. It is important to note that a novel experimental method was developed to obtain values for the 3D model parameters. This makes the prediction of the PQED responsivity a completely autonomous process. Detectors were characterized for temperature dependence of dark current, spatial uniformity of responsivity, reflectance, linearity and absolute responsivity at the wavelengths of 488¿nm and 532¿nm. For both sets of photodiodes, the modelled and measured responsivities were generally in agreement within the measurement and modelling uncertainties of around 100 parts per million (ppm). There is, however, an indication that the modelled internal quantum deficiency may be underestimated by a similar amount. Moreover, the responsivities of the detectors were spatially uniform within 30¿ppm peak-to-peak variation. The results obtained in this research indicate that the n-type induced junction photodiode is a very promising alternative to the existing p-type detectors, and thus give additional credibility to the concept of modelled quantum detector serving as a primary standard. Furthermore, the manufacturing of PQEDs is no longer dependent on the availability of a certain type of very lightly doped p-type silicon wafers.The research leading to these results has received funding from the European Metrology Research Programme (EMRP) project SIB57 'New Primary Standards and Traceability for Radiometry'. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. Financial support from the Academy of Finland through the Finnish Centre of Excellence in Atomic Layer Deposition is also acknowledged.Peer Reviewe

Quantum characterization of superconducting photon counters

We address the quantum characterization of photon counters based on transition-edge sensors (TESs) and present the first experimental tomography of the positive operator-valued measure (POVM) of a TES. We provide the reliable tomographic reconstruction of the POVM elements up to 11 detected photons and M=100 incoming photons, demonstrating that it is a linear detector.Comment: 3 figures, NJP (to appear

Predictable quantum efficient detector based on n-type silicon photodiodes

The predictable quantum efficient detector (PQED) consists of two custom-made induced junction photodiodes that are mounted in a wedged trap configuration for the reduction of reflectance losses. Until now, all manufactured PQED photodiodes have been based on a structure where a SiO2 layer is thermally grown on top of p-type silicon substrate. In this paper, we present the design, manufacturing, modelling and characterization of a new type of PQED, where the photodiodes have an Al2O3 layer on top of n-type silicon substrate. Atomic layer deposition is used to deposit the layer to the desired thickness. Two sets of photodiodes with varying oxide thicknesses and substrate doping concentrations were fabricated. In order to predict recombination losses of charge carriers, a 3D model of the photodiode was built into Cogenda Genius semiconductor simulation software. It is important to note that a novel experimental method was developed to obtain values for the 3D model parameters. This makes the prediction of the PQED responsivity a completely autonomous process. Detectors were characterized for temperature dependence of dark current, spatial uniformity of responsivity, reflectance, linearity and absolute responsivity at the wavelengths of 488 nm and 532 nm. For both sets of photodiodes, the modelled and measured responsivities were generally in agreement within the measurement and modelling uncertainties of around 100 parts per million (ppm). There is, however, an indication that the modelled internal quantum deficiency may be underestimated by a similar amount. Moreover, the responsivities of the detectors were spatially uniform within 30 ppm peak-to-peak variation. The results obtained in this research indicate that the n-type induced junction photodiode is a very promising alternative to the existing p-type detectors, and thus give additional credibility to the concept of modelled quantum detector serving as a primary standard. Furthermore, the manufacturing of PQEDs is no longer dependent on the availability of a certain type of very lightly doped p-type silicon wafers.Peer reviewe