Quantum coherence control usually requires low temperature environments. Even for nitrogen-vacancy center spins in diamond, a remarkable exception, the coherence signal is limited to about 700 K due to the quench of the spin-dependent fluorescence at a higher temperature. Here researchers overcome this limit and demonstrate quantum coherence control of the electron spins of nitrogen-vacancy centers in nanodiamonds at temperatures near 1000 K. The scheme is based on initialization and readout of the spins at room temperature and control at high temperature, which is enabled by pulse laser heating and rapid diffusion cooling of nanodiamonds on amorphous carbon films. Using the diamond magnetometry based on optically detected magnetic resonance up to 800 K, they observe the magnetic phase transition of a single nickel nanoparticle at about 615 K. This work enables nano-thermometry and nano-magnetometry in the high-temperature regime.
The nitrogen-vacancy (NV) center in diamond has been demonstrated to have robust quantum coherence at room temperature and even up to 700 K, which has stimulated many studies for quantum information processing and quantum sensing.
nd nano-magnetometry. Compared with other quantum sensors1 like SQUIDs, trapped ions, atomic vapors or scanning probes, NV spins in diamond combine the merits of good sensitivity, nanoscale spatial resolution, and a wide range of working conditions30 (including cryogenic temperature to about 1000 K, from vacuum to ambient conditions, and high pressure). Using the photon counts (8 Mps), the ODMR width (10 MHz, mainly due to the internal local charge and strain-induced broadening32), and the ODMR contrast (5%) of the NDs we have measured at 1000 K, we estimate the temperature sensitivity to be about 250 mK Hz−1/2 based on the ZFS shift dD/dT ≈ 240 kHz K−1 and the magnetic field sensitivity to be 2.5 μT Hz−1/2. The temperature sensitivity can be further improved by adopting advanced sensing protocols like D-Ramsey and magnetic criticality enhanced thermometer29 and by using NV centers of good spin coherence and fast temperature control.
The current scheme is limited by the speed of laser heating and diffusion cooling as compared with the relaxation time of NV center electron spins at high temperature. To enable the fast heating and cooling, the applications are restricted to samples of small sizes. However, such sample sizes do not impose a severe constraint on many applications in materials sciences and engineering studies or in device physics. The temperature range of the spin coherence control in diamond can be pushed to even higher. In current experiment, the spin relaxation time at about 1000 K is comparable to the laser heating time in our setup, making it challenging to observe the ODMR above 1000 K. This limit can be overcome by either increasing the heating and cooling speed or by increasing the spin relaxation time. The latter can be realized, e.g., by resorting to diamond samples with better coherence (such as nano-pillars with single NV centers).