Figure (b)
The variation of the potential energy of the SQUID
(y-axis) with current at three different values of the external magnetic flux
through the ring. The two lowest energy levels in the system, |1> and |2>, are
shown in red. When the potential-energy curve is asymmetric (left and right
curves), these states are essentially localized in one or other of the wells,
although there is a small "leakage" due to tunnelling through the barrier. When
the potential is symmetric (middle curve), quantum mechanics predicts that
tunnelling will result in the energy eigenstates of the system becoming linear
superpositions of the localized states, |1> and |2>. The so-called antisymmetric
state, ( |1> |2> ) / 21/2, will have a slightly higher energy than the
symmetric state, ( |1> + |2> ) / 21/2. In the absence of tunnelling, and hence of
superposition, the states would be localized with the same energy (yellow
lines).