Nathan A. Tomlin

Nanosecond-scale timing jitter for single photon detection in transition edge sensors

Transition edge sensors (TES) have the highest reported efficiencies (>98%) for single photon detection in the visible and near infrared. Experiments in quantum information and foundations of physics that rely on this efficiency have started incorporating these detectors. However, their range of applicability has been hindered by slow operation both in recovery time and timing jitter. We show how a conventional tungsten-TES can be operated with jitter times of ≈4 ns, providing a practical simplification for experiments that rely on simultaneous high efficiency and low timing uncertainty, such as loophole free Bell inequalities and device independent quantum cryptography.Transition edge sensors (TES) have the highest reported efficiencies (>98%) for detection of single photons in the visible and near infrared. Experiments in quantum information and foundations of physics that rely critically on this efficiency have started incorporating these detectors into con- ventional quantum optics setups. However, their range of applicability has been hindered by slow operation both in recovery time and timing jitter. We show here how a conventional tungsten-TES can be operated with jitter times of < 4 ns, well within the timing resolution necessary for MHz clocking of experiments, and providing an important practical simplification for experiments that rely on the simultaneous closing of both efficiency and locality loopholes.

Extending single-photon optimized superconducting transition edge sensors beyond the single-photon counting regime.

We illuminate a photon-number-resolving transition edge sensor with strong pulses of light containing up to 6.7 million photons (0.85 pJ per pulse). These bright pulses heat the sensor far beyond its transition edge into the normal resistance regime. We show that the sensor operates from the single-photon-counting regime to picowatt levels of light and that the detection noise is below shot-noise for up to 1000 photons.

Planar electrical-substitution carbon nanotube cryogenic radiometer*

We have developed a fully-lithographic electrical-substitution planar bolometric-radiometer (PBR) that employs multiwall vertically-aligned carbon nanotubes (VACNT) as the absorber and thermistor, micro-machined Si as the weak thermal link and thin-film Mo as the electrical heater. The near-unity absorption of the VACNT over a broad wavelength range permits a planar geometry, compatible with lithographic fabrication. We present performance results on a PBR with an absorption of 0.999 35 at 1550 nm, a thermal conductance of 456 µW K−1 at 4 K and a time constant (1/e) of 7.7 ms. A single measurement of approximately 100 µW optical power at 1550 nm achieved in less than 100 s yields an expanded uncertainty of 0.14% (k = 2). We also observe an elevated superconducting transition temperature of 3.884 K for the Mo heater, which opens the possibility of future devices incorporating more sensitive thermistors and superconducting thin-film wiring.

A detector combining quantum and thermal primary radiometric standards in the same artefact

We present the concept of a dual-mode primary standard cryogenic detector, utilizing a predictable quantum efficient silicon photodiode, and demonstrate the behaviour of the detector from room temperature down to 30 K. The detector absorbs visible radiation generating either heat or photocurrent, dependent on the selected mode of operation. In effect, this detector links optical power to fundamental constants through the two different routes of operation in the one artefact. Forward biasing of the photodiode is used in lieu of resistive heating to provide the electrical substitution power. The detector has a thermal time constant of 50 s and a sensitivity of 1.39 K mW−1. Using an LED source, we measure equivalence between the two modes of operation of 1.5% at 50 K, limited principally by our knowledge of the wavelength of the emitted radiation of the source.

Carbon nanotube electrical-substitution cryogenic radiometer: initial results.

A carbon nanotube cryogenic radiometer (CNCR) has been fabricated for electrical-substitution optical power measurements. The CNCR employs vertically aligned multiwall carbon nanotube arrays (VANTAs) as the absorber, heater, and thermistor, with a micromachined silicon substrate as the weak thermal link. Compared to conventional cryogenic radiometers, the CNCR is simpler, more easily reproduced and disseminated, orders of magnitude faster, and can operate over a wide range of wavelengths without the need for a receiver cavity. We describe initial characterization results of the radiometer at 3.9 K, comparing electrical measurements and fiber-coupled optical measurements from 50 μW to 1.5 mW at the wavelength of 1550 nm. We find the response to input electrical and optical power is equivalent to within our measurement uncertainty, which is currently limited by the experimental setup (large temperature fluctuations of the cold stage) rather than the device itself. With improvements in the temperature stability, the performance of the CNCR should be limited only by our ability to measure the reflectance of the optical absorber VANTA.

Towards a fiber-coupled picowatt cryogenic radiometer.

A picowatt cryogenic radiometer (PCR) has been fabricated at the microscale level for electrical substitution optical fiber power measurements. The absorber, electrical heater, and thermometer are all on a micromachined membrane less than 1 mm on a side. Initial measurements with input powers from 50 fW to 20 nW show a response inequivalence between electrical and optical power of 8%. A comparison of the response to electrical and optical input powers between 15 pW to 70 pW yields a repeatability better than ±0.3% (k=2). From our first optical tests, the system has a noise equivalent power of ≈5×10(-15) W/√Hz at 2 Hz, but simple changes to the measurement scheme should yield an NEP 2 orders of magnitude lower.

Carbon nanotube-based black coatings

Coatings comprised of carbon nanotubes are very black; that is, characterized by low reflectance over a broad wavelength range from the visible to far infrared. Arguably there is no other material that is comparable. This is attributable to the intrinsic properties of graphene as well as the morphology (density, thickness, disorder, tube size) of the coating. The need for black coatings is persistent for a variety of applications such as baffles and traps for space instruments. Because of the thermal properties, nanotube coatings are also well suited for thermal detectors, blackbodies and other applications where light is trapped and converted to heat. We briefly describe a history of other coatings such as nickel phosphorous, gold black and carbon-based paints and the comparable structural morphology that we associate with very black coatings. In many cases, it is a significant challenge to put the blackest coating on something useful. We describe the growth of carbon nanotube forests on substrates such as metals and silicon along with the catalyst requirements and temperature limitations. We also describe coatings derived from carbon nanotubes and applied like paint. Another significant challenge is that of building the measurement apparatus and determining the optical properties of something having negligible reflectance. There exists information in the literature for effective media approximations to model the dielectric function of vertically aligned arrays. We summarize this as well as other approaches that are useful for predicting the coating behavior along with the refractive index of graphite from the literature that is necessary for the models we know of. We provide an appendix of our best recipes for making as-grown, sprayed or other coatings for the blackest and most robust coating for a chosen substrate and a description of reflectance measurements.

Planar hyperblack absolute radiometer

The absolute responsivity of a planar cryogenic radiometer fabricated from micromachined silicon and having carbon nanotubes, as the absorber and thermistor were measured in the visible and far infrared (free-field terahertz) wavelength range by means of detector-based radiometry. The temperature coefficient of the thermistor near 4.8 K and noise equivalent power were evaluated along with independent characterization of the window transmittance and specular reflectance of the nanotube absorber. Measurements of absolute power by means of electrical substitution are compared to the German national standard and the uncertainty of the radiometer responsivity as a function of wavelength is summarized.

Method to determine the absorptance of thin films for photovoltaic technology

We have demonstrated a novel method to determine optical properties of opaque or semi-transparent films for photovoltaic (PV) applications. Such films may be the basis of transparent conductors or photoconductive material. As an example, we measure the absolute absorptance (at visible and near infrared wavelengths) of an optically thick single-wall carbon nanotube (SWCNT) film by using a pyroelectric detector. This novel method obviates the need for analysis with respect to polarization and associated difficulties of ellipsometry. The Kramers-Kronig relation is used to determine the thick film index of refraction, which we use to calculate the optical properties of thin films as a function of thickness. A transmittance measurement obtained from a thin SWCNT film shows excellent agreement with results from our model.

Carbon nanotube planar absorber cryogenic bolometer as a novel primary absolute standard detector for optical power measurements for fibre optics and photonics (Conference Presentation)

Primary standards of optical radiation total radiant flux are traditionally realized by absolute cryogenic radiometers [1] working on the principle of electrical substitution with a relative total uncertainty of 1e-4 in the power measurement. The current cryogenic radiometers though operate over a limited spectral range, usually from 350 nm to 800 nm and working with free space beam. For fibre optics telecom spectral range 1300 nm - 1650 nm this scale is then extended in several steps, typically via application of other standard detector systems such as spectrally flat room temperature pyro detectors [2] and spectrally dependent temperature stabilized solid state detectors [3], which adversely affects the scale accuracy by a factor of approximately one order of magnitude. The typical relative total uncertainty of state-of-the-art transfer standard fibre coupled detectors reaches 0.5 %. Recently published results on planar electrical-substitution carbon nanotube cryogenic radiometer (PCBR) [4] brought the opportunities for using these systems as new absolute primary standards in telecom spectral range directly in fibre coupled configuration. This shortens the traceability chain, with a potential improvement in the total uncertainty to below 0.1 %. CMI in collaboration with NIST are developing the first prototypes of fibre coupled PCBR systems. First both free space and fibre coupled measurements have confirmed radiometric The paper will present both the core physical parameters of these PCBR electrical-substitution systems and initial results including the currently achieved agreement of traditional transfer standards with the PCBR at the level of 0.2 %. The work reported in this abstract was partially funded by project EMPIR 14IND13 PhotInd. This project has received funding from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme. References: [1] Martin J E, Fox N P and Key P J 1985 Metrologia 21, 147 [2] Lehman J., Theocharous E., Eppeldauer G., and Pannel C., “Gold-black coatings for freestanding pyroelectric detectors” Measurement Science and Technology, 14, 916-922, 2003 [3] E. Theocharous, M. Smid, T. Ward, N. Fox, “The establishment of an absolute infrared scale using cavity pyroelectric detectors”, in preparation. [4] N A Tomlin, M White, I Vayshenker, S I Woods and J H Lehman, Planar electrical-substitution carbon nanotube cryogenic radiometer 2015 Metrologia 52 376