Quantum Bath Refrigeration towards Absolute Zero: Challenging the Unattainability Principle

A minimal model of a quantum refrigerator, i.e., a periodically phase-flipped two-level system permanently coupled to a finite-capacity bath (cold bath) and an infinite heat dump (hot bath), is introduced and used to investigate the cooling of the cold bath towards absolute zero ($T=0$). Remarkably, the temperature scaling of the cold-bath cooling rate reveals that it does not vanish as $T\to 0$ for certain realistic quantized baths, e.g., phonons in strongly disordered media (fractons) or quantized spin waves in ferromagnets (magnons). This result challenges Nernst’s third-law formulation known as the unattainability principle.

Figure 1

Main panel: Schematic depiction of the required bath spectra and the qubit frequency shifts due to periodic phase flips. Inset: Schematic realization of the modulated qubit and its coupling to the baths.

Figure 2

$T_C$ change with time (cooling) for three different system-bath coupling-strength dispersion laws: $\gamma =1$ (acoustic phonons), $\gamma =3/4$ (fractons), $\gamma =0$ (magnons).