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The nitrogen-vacancy center in diamond has been broadly applied in quantum sensing since it is sensitive to different physical quantities. Meanwhile, it is difficult to isolate disturbances from unwanted physical quantities in practical applications. Here, we present a fiber-based quantum thermometer by tracking the sharp-dip in the zero-field optically detected magnetic resonance spectrum in a high-density nitrogen-vacancy ensemble. Such a scheme can not only significantly isolate the magnetic field and microwave power drift but also improve the temperature sensitivity. Thanks to its simplicity and compatibility in implementation and robustness, this quantum thermometer is then applied to the surface temperature imaging of an electronic chip with a sensitivity of 18mK/Hz. It thus paves the way to high sensitive temperature measurements in ambiguous environments.

Nitrogen-vacancy centers in diamond are attractive as quantum sensors owing to their remarkable optical and spin properties under ambient conditions. Here we experimentally demonstrated a hybrid fiber-based thermometer coupled with nitrogen-vacancy center ensemble and a permanent magnet, where the temperature sensitivity was improved by converting the temperature variation to the magnetic field change based on the thermal-demagnetization of the permanent magnet. We have achieved both large temperature working range (room temperature to 373 K) and millikelvin sensitivity (1.6 mK/ √ Hz), nearly 6-fold improvement compared with conventional technique. This stable and compact hybrid thermometer will enable a wide range of applications for large-area detection and imaging with high temperature sensitivity.A stable and compact thermometer capable of millikelvin resolution over a large temperature range could provide a powerful tool in many areas of physical, chemical, and biological researches 1 . Lots of promising approaches to local temperature sensing are being explored at present, including Raman spectroscopy 2 , scanning probe microscopy 3 , and fluorescence-based measuremnets 4 using nanoparticles and organicdyes 5 . However, many of these methods are limited by drawbacks such as low sensitivity and systematic errors due to fluctuations in the fluorescence rate and the local environment 1,6 .In recent years, the negatively charged nitrogen-vacancy (NV) center, a point defect in diamond, provides a promising system to realize practical quantum devices which have been successfully applied to a wide range of applications in quantum information processing and sensing in both physical and life sciences 7 . These applications of the NV center are based upon its remarkable optical and spin properties: bright optical fluorescence, long-lived spin coherence, and mature optical polarization and readout at room temperature 8 . For the NV-based temperature sensing, the techniques with modified spin-echo sequence 1 and high-order Carr-Purcell-Meiboom-Gill method 9,10 have achieved a sensitivity of 10 mK/ √ Hz. A nano-thermometer composed of NV centers and a magnetic nanoparticle has been experimentally demonstrated 11 , where an optimal temperature sensitivity of 3 mK/ √ Hz has been obtained by the critical magnetization of the magnetic nanoparticle near Cuire temperature. Moreover, the recently developed fiber-optic probes coupled with NV centers were shown to enable a temperature measurement with a 20 mK accuracy using optically detected magnetic resonance (ODMR) [12][13][14] .In order to further enhance the sensitivity of the NV thera) Electronic mometer for practical application, here, we proposed a hybrid fiber-based thermometer coupled with NV center ensembles and a permanent magnet. By converting the temperature variation to a magnetic field change 11,15 of the permanent magnet, this thermometer can achieve a high sensitivity of 1.6 mK/ √ Hz and a large temperature working range, where the permanent magnet is served as...

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