  Processes such as quantum computation, or the evolution of quantum cellular
automata are typically described by a unitary operation implemented by an
external observer. In particular, an interaction is generally turned on for a
precise amount of time, using a classical clock. A fully quantum mechanical
description of such a device would include a quantum description of the clock
whose state is generally disturbed because of the back-reaction on it. Such a
description is needed if we wish to consider finite sized autonomous quantum
machines requiring no external control. The extent of the back-reaction has
implications on how small the device can be, on the length of time the device
can run, and is required if we want to understand what a fully quantum
mechanical treatment of an observer would look like. Here, we consider the
implementation of a unitary by a finite sized device which we call the
"Quasi-Ideal clock", and show that the back-reaction on it can be made
exponentially small in the device's dimension with only a linear increase in
energy. As a result, an autonomous quantum machine need only be of modest size
and or energy. We are also able to solve a long-standing open problem by using
a finite sized quantum clock to approximate the continuous evolution of an
Idealised clock. The result has implications on the equivalence of different
paradigms of quantum thermodynamics, some which allow external control and some
which only allow autonomous thermal machines.
