Modular variable laser cooling for efficient entropy extraction

TL;DR

This paper introduces a modular variable laser cooling technique that uses spin-dependent displacements and repumping to efficiently reduce the entropy of a trapped ion oscillator, approaching near-optimal cooling limits.

Contribution

The paper presents a novel laser cooling method based on modular variables, demonstrated experimentally with a trapped ion, achieving significant entropy reduction close to theoretical limits.

Findings

Reduced oscillator occupation to 0.632 of initial with two repumps

Method approaches within a factor of 2.53 of the optimal entropy reduction

Experimental validation using a single trapped ion's internal and motional states

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.