Super-Cool Engine

One of the first engines to gain popularity in many sentient species is dubbed Super-Cool.  Essentially quite simple in operation but cleverly bringing together two main technologies into one overall design.

The first stage of the engine is almost conventional by any standards.  Hydrogen is stripped of its electron and the resulting proton cooled to about one nanokelvin.  At this temperature it is bowling along at a leisurely one milli meter per second.

The second stage entangles these into multiple pairs by passing them through what is effectively a very old but very large double slit experiment.  These entangled protons are chunked up into batches by lasers and programmed. Each batch now forms a large quantum computer and each spend a short time calculating a specific large prime number. This calculation adjusts their final state into just the right space time orientation so that they instantly form what is effectively a new particle made up of 239 protons (which is itself a prime number).

Still moving at a leisurely pace these newly formed particles enter an expansion chamber where they are given a small kick by a little laser light designed to break the entangled states.  At this point, at an almost infinitesimal time later, these highly unstable particles explode into many sub-particles with a certain loss of mass providing a very useful net energy gain. The mass to energy conversion is maximised due to the initial size and state of the new proton particles.

This resultant energy is concentrated and tuned by super-cooled magnets and lasers and fires the resulting highly radioactive mess at near light speed out the back of the engine. 

This results in very high rates of thrust for small amounts of matter and thus a nicely efficient engine design for early space travel.

(Patent Applied for! - CST )