  An unconventional method of continuous solid-state cryogenic cooling
utilizing the electron subband degeneracy of semiconductor heterostructures is
proposed in this Letter. An electrostatic heat pump is modeled, which employs
subband "expansion" and "compression" to reach sub-dilution refrigeration
temperatures with the fundamental limit set by electron-phonon interaction.
Using an ultra-wide GaAs quantum well as an example, the cooling power per unit
volume is estimated to reach $4.5\ \rm mW/cm^3$ with a hot-side temperature of
$300\ \rm mK$, suitable for applications such as quantum computers or infrared
detectors.

Mesoscale and Nanoscale Physics;Quantum Physics