Modular variable laser cooling for efficient entropy extraction

Abstract

We propose and experimentally demonstrate a method for laser cooling an oscillator based on sequences of spin-state-dependent displacements followed by spin repumping. For a thermal state with mean occupation $\bar{n}\gg 1$ the method attains a reduction to 0.632 of the initial thermal oscillator occupation for two repumps of the two-level spin state. This is within a factor of 2.53 of the optimum that might be expected due to the reduction of the oscillator entropy by $2 \ln(2)$. We show that the method, which is based on encoding the value of the modular-variable of the oscillator into the spin, has a simple semi-classical description in terms of a Bayesian update. We demonstrate the method experimentally using the internal and motional states of a single trapped ion.