Answer
Zero.
Work Step by Step
Apply Kirchhoff’s loop rule to the circuit
Begin at point a, and proceed counterclockwise. We see that the current flows counterclockwise.
$$-IR+1.5V-IR-IR+1.5V-IR=0$$
$$I=\frac{1.5V}{2R}$$
Now find the potential difference between points a and b by walking the path between them and adding up the changes in voltage.
$$V_{ab}=-IR+1.5V-IR=1.5V-2R\frac{1.5V}{2R}=0$$
As expected, the symmetry of the situation leads us to conclude that there is no potential difference between points a and b.
