Beating the limits with initial correlations

Abstract

Fast and reliable reset of a qubit is a key prerequisite for any quantum technology. 
For real world open quantum systems undergoing non-Markovian dynamics, reset implies 
not only purification, but in particular erasure of initial correlations between qubit
and environment. Here, we derive optimal reset protocols using a combination of 
geometric and numerical control theory. For factorizing initial states, we find a 
lower limit for the entropy reduction of the qubit as well as a speed limit. 
The time-optimal solution is determined by the maximum coupling strength. Initial 
correlations, remarkably, allow for faster reset and smaller errors. Entanglement is 
not necessary.