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A '''transition
==History==
The first demonstrations of the superconducting transition's measurement potential appeared in the 1940s, 30 years after [[Heike Kamerlingh Onnes|Onnes]]'s discovery of [[superconductivity]]. D. H. Andrews demonstrated the first transition-edge [[bolometer]], a current-biased [[tantalum]] wire which he used to measure an infrared signal. Subsequently he demonstrated a transition-edge [[Calorimeter (particle physics)|calorimeter]] made of [[niobium nitride]] which was used to measure [[alpha particles]].<ref>D. H. Andrews ''et al.'', "Attenuated superconductors I. For measuring infra-red radiation". ''Rev. Sci. Instrum.'', '''13''', 281 (1942), {{doi|10.1063/1.1770037}}.</ref> However, the TES detector did not gain popularity for about 50 years, due primarily to the difficulty in stabilizing the temperature within the narrow superconducting transition region, especially when more than one pixel was operated at the same time, and also due to the difficulty of signal readout from such a low-[[Electrical impedance|impedance]] system. [[Joule heating]] in a current-biased TES can lead to [[thermal runaway]] that drives the detector into the normal (non-superconducting) state, a phenomenon known as positive [[electrothermal feedback]]. The thermal runaway problem was solved in 1995 by K. D. Irwin by voltage-biasing the TES, establishing stable negative [[electrothermal feedback]], and coupling them to superconducting quantum interference devices ([[SQUID]]) current amplifiers.<ref>K. D. Irwin, "An application of electrothermal feedback for high resolution cryogenic particle detection". ''Appl. Phys. Lett.'', '''66''', 1998 (1995), {{doi|10.1063/1.113674}}.</ref> This breakthrough has led to widespread adoption of TES detectors.<ref name="IrwinHilton">K. D. Irwin and G. C. Hilton, "Transition-edge sensors", ''Cryogenic Particle Detection'', ed. C. Enss, Springer (2005), {{doi|10.1007/10933596_3}}.</ref>
==Setup, operation, and readout==
==Applications==▼
[[File:TES schematic.pdf|thumb|alt=Schematic of TES-SQUID circuit|Schematic of TES-SQUID circuit]]
The TES is voltage-biased by driving a current source ''I''<sub>bias</sub> through a load resistor ''R''<sub>L</sub> (see figure). The voltage is chosen to put the TES in its so-called "self-biased region" where the power dissipated in the device is constant with the applied voltage. When a [[photon]] is absorbed by the TES, this extra power is removed by negative [[electrothermal feedback]]: the TES [[Electrical resistance|resistance]] increases, causing a drop in TES current; the [[Joule power]] in turn drops, cooling the device back to its equilibrium state in the self-biased region. In a common [[SQUID]] readout system, the TES is operated in series with the input coil ''L'', which is inductively coupled to a SQUID series-array. Thus a change in TES current manifests as a change in the input [[flux]] to the SQUID, whose output is further amplified and read by room-temperature electronics.
==
Any [[bolometer|bolometric]] sensor employs three basic components: an [[Absorption (electromagnetic radiation)|absorber]] of incident energy, a [[thermometer]] for measuring this energy, and a [[Thermal conductivity|thermal link]] to base temperature to dissipate the absorbed energy and cool the detector.<ref name="NIST">A. Lita ''et al.'', "Counting near-infrared single-photons with 95% efficiency", ''Optics Express'' '''16''', 3032 (2008), {{doi|10.1364/OE.16.003032}}.</ref>
{{reflist}}▼
===Absorber===
The simplest absorption scheme can be applied to TESs operating in the near-IR, optical, and UV regimes. These devices generally utilize a [[tungsten]] TES as its own absorber, which absorbs up to 20% of the incident radiation.<ref name = NIST2>A. J. Miller ''et al.'', "Demonstration of a low-noise near-infrared photon counter with multiphoton discrimination", ''Appl. Phys. Lett.'', '''83''', 791–793. (2003), {{doi|10.1063/1.1596723}}.</ref> If high-efficiency detection is desired, the TES may be fabricated in a multi-layer [[optical cavity]] tuned to the desired operating wavelength and employing a backside mirror and frontside anti-reflection coating. Such techniques can decrease the transmission and reflection from the detectors to negligibly low values; 95% detection efficiency has been observed.<ref name=NIST/> At higher energies, the primary obstacle to absorption is transmission, not reflection, and thus an absorber with high photon stopping power and low heat capacity is desirable; a [[bismuth]] film is often employed.<ref name=IrwinHilton/> Any absorber should have low [[heat capacity]] with respect to the TES. Higher heat capacity in the absorber will contribute to noise and decrease the sensitivity of the detector (since a given absorbed energy will not produce as large of a change in TES resistance). For far-IR radiation into the millimeter range, the absorption schemes commonly employ [[Antenna (radio)|antennas]] or [[feedhorn]]s.<ref name=IrwinHilton />
===Thermometer===
The TES operates as a thermometer in the following manner: absorbed incident energy increases the resistance of the voltage-biased sensor within its transition region, and the integral of the resulting drop in current is proportional to the energy absorbed by the detector.<ref name=NIST2/> The output signal is proportional to the temperature change of the absorber, and thus for maximal sensitivity, a TES should have low heat capacity and a narrow transition. Important TES properties including not only heat capacity but also thermal conductance are strongly temperature dependent, so the choice of [[Superconductivity#Superconducting phase transition|transition temperature]] ''T''<sub>c</sub> is critical to the device design. Furthermore, ''T''<sub>c</sub> should be chosen to accommodate the available [[cryostat|cryogenic system]]. Tungsten has been a popular choice for elemental TESs as thin-film tungsten displays two phases, one with ''T''<sub>c</sub> ~15 mK and the other with ''T''<sub>c</sub> ~1–4 K, which can be combined to finely tune the overall device ''T''<sub>c</sub>.<ref>A. Lita ''et al.'', "Tuning of tungsten thin film superconducting transition temperature for fabrication of photon number resolving detectors", ''IEEE Trans. Appl. Supercond.'', '''15''', 3528 (2005), {{doi|10.1109/TASC.2005.849033}}.</ref> Bilayer and multilayer TESs are another popular fabrication approach, where [[thin film]]s of different materials are combined to achieve the desired ''T''<sub>c</sub>.<ref name=IrwinHilton />
===Thermal conductance===
Finally, it is necessary to tune the [[Thermal conductivity|thermal coupling]] between the TES and the bath of cooling liquid; a low thermal conductance is necessary to ensure that incident energy is seen by the TES rather than being lost directly to the bath. However, the thermal link must not be too weak, as it is necessary to cool the TES back to bath temperature after the energy has been absorbed. Two approaches to control the thermal link are by electron–phonon coupling and by mechanical machining. At cryogenic temperatures, the [[electron]] and [[phonon]] systems in a material can become only weakly coupled. The electron–phonon thermal conductance is strongly temperature-dependent, and hence the thermal conductance can be tuned by adjusting ''T''<sub>c</sub>.<ref name=IrwinHilton /><ref name=NIST /> Other devices use mechanical means of controlling the thermal conductance such as building the TES on a sub-micrometre membrane over a hole in the substrate or in the middle of a sparse "spiderweb" structure.<ref>J. Bock ''et al.'', "A novel bolometer for infrared and millimeter-wave astrophysics", ''Space Science Reviews'', '''74''', 229–235 (1995), {{doi|10.1007/BF00751274}}.</ref>
==Advantages and disadvantages==
TES detectors are attractive to the scientific community for a variety of reasons. Among their most striking attributes are an unprecedented high detection efficiency customizable to wavelengths from the millimeter regime to gamma rays<ref name=IrwinHilton /><ref name=NIST /> and a theoretical negligible background dark count level (less than 1 event in 1000 s from intrinsic [[Phonon noise|thermal fluctuations]] of the device<ref name=NIST2 />). (In practice, although only a real energy signal will create a current pulse, a nonzero background level may be registered by the counting algorithm or the presence of background light in the experimental setup. Even thermal [[blackbody radiation]] may be seen by a TES optimized for use in the visible regime.)
TES single-photon detectors suffer nonetheless from a few disadvantages as compared to their [[Single-photon avalanche diode|avalanche photodiode]] (APD) counterparts. APDs are manufactured in small modules, which count photons out-of-the-box with a [[dead time]] of a few nanoseconds and output a pulse corresponding to each photon with a [[jitter]] of tens of picoseconds. In contrast, TES detectors must be operated in a cryogenic environment, output a signal that must be further analyzed to identify photons, and have a jitter of approximately 100 ns.<ref name=NIST /> Furthermore, a single-photon spike on a TES detector lasts on the order of microseconds.
▲==Applications==
TES arrays are becoming increasingly common in physics and astronomy experiments such as [[James Clerk Maxwell Telescope#SCUBA-2|SCUBA-2]], the HAWC+ instrument on the [[Stratospheric Observatory for Infrared Astronomy]], the [[Atacama Cosmology Telescope]], the [[Cryogenic Dark Matter Search]], the [[Cryogenic Observatory for Signatures Seen in Next-Generation Underground Searches]], the [[Cryogenic Rare Event Search with Superconducting Thermometers]], [[the E and B Experiment]], the [[South Pole Telescope]], the [[Spider (polarimeter)|Spider polarimeter]], the X-IFU instrument of the [[Advanced Telescope for High Energy Astrophysics]] satellite, the future [https://link.springer.com/article/10.1007/s10909-019-02329-w LiteBIRD] [[Cosmic microwave background|Cosmic Microwave Background]] polarization experiment, the [[Simons Observatory]], and the CMB Stage-IV Experiment.
==See also==
{{commons category|Transition edge sensor}}
* [[Bolometer]]
* [[Cryogenic particle detectors]] ==References==
▲{{reflist}}
[[Category:Superconducting detectors]]
[[Category:Radiometry]]
[[Category:Sensors]]
[[Category:Particle detectors]]
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