  This work is originally a Cambridge Part III essay paper. Quantum complexity
arises as an alternative measure to the Fubini metric between two quantum
states. Given two states and a set of allowed gates, it is defined as the least
complex unitary operator capable of transforming one state into the other.
Starting with K qubits evolving through a k-local Hamiltonian, it is possible
to draw an analogy between the quantum system and an auxiliary classical
system. Using the definition of complexity to define a metric for the classical
system, it is possible to relate its entropy with the quantum complexity of the
K qubits, defining the Second Law of Quantum Complexity. The law states that,
if it is not already saturated, the quantum complexity of a system will
increase with an overwhelming probability towards its maximum value. In the
context of AdS/CFT duality and the ER=EPR conjecture, the growth of the volume
of the Einstein Rosen bridge interior is proportional to the quantum complexity
of the instantaneous state of the conformal field theory. Therefore, the
interior of the wormhole connecting two entangled CFT will grow as a natural
consequence of the complexification of the boundary state.
