We present results on ultra low noise YBa$_2$Cu$_3$O$_{7-delta}$ nano Superconducting QUantum Interference Devices (nanoSQUIDs). To realize such devices, we implemented high quality YBCO nanowires, working as weak links between two electrodes. We observe critical current modulation as a function of an externally applied magnetic field in the full temperature range below the transition temperature $T_C$. The white flux noise below 1 $mu Phi_0/sqrt{mathrm{Hz}}$ at T = 8 K makes our nanoSQUIDs very attractive for the detection of small spin systems.
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