Landauer bound in a regular Bardeen black hole: extremality and evaporation

Abstract

Does the vanishing Hawking temperature of an extremal black hole imply that information can be erased without energy transfer? We investigate this question in the regular Bardeen geometry using a two-level quantum system, representing a quantum memory, coupled to a scalar bosonic field confined within an exterior cavity. The Hawking temperature tends to zero at extremality, as does the corresponding thermal Landauer scale. The field in an exterior cavity behaves differently, provided both walls remain away from the horizon, its lowest excitation energy stays finite. For a maximally mixed memory coupled to a reservoir initially in its unique ground state, we find $Q_\infty\geqΔ_{R,\infty}(1/2-ε)$, with $Δ_{R,\infty}$ the reservoir gap and $ε$ the reset error. For a resonant single-mode interaction, we give a reset protocol that attains the bound with no work supplied by the external control. In this example, the memory itself supplies the energy gained by the field. In the multimode calculation, counter-rotating terms are retained. Keeping the pulse parameters unchanged, we find a larger residual reset error closer to extremality. For reservoirs prepared thermally, we evaluate the Reeb--Wolf balance and find that the relative-entropy contribution, expressed in energy units, can have a finite low-temperature limit. These numerical preparations are externally imposed and remain hotter than the Hawking temperature. For comparison, an unconfined scalar field has near-extremal luminosity $P_\infty\propto y^4$, with $y=r_H/|g|-\sqrt2$. Assuming a particular quasistatic energy-loss prescription gives $y\propto t^{-1/2}$; this does not establish the existence of a stable remnant. For this particular reset operation, vanishing Hawking temperature therefore does not force the transferred heat to vanish.