Quantum Nyquist temperature fluctuations

Using the Nyquist approach, temperature fluctuations of an object, in thermal contact with a reservoir, are studied. We argue that, upon decreasing the size of the object, one necessarily reaches the quantum regime. The crossover temperature between the classical and quantum regimes is given by T* ∼ ℏ/k_Bτ, where τ is the thermal relaxation time of the system. For a nano-scale metallic particle in a good thermal contact with a reservoir, T* can be on a scale of a few Kelvin.