  The d-wave symmetry of the order parameter with zero energy gap in nodal
directions stands in the way of using high-temperature superconductors for
quantum applications. We investigate the symmetry of the order parameter in
ultra-thin YBa2Cu3O7-x (YBCO) films by measuring the electrical transport
properties of nanowires and nanoconstrictions aligned at different angles
relative to the main crystallographic axes. The anisotropy of the nanowire
critical current in the nodal and antinodal directions reduces with the
decrease in the film thickness. The Andreev reflection spectroscopy shows the
presence of a thickness-dependent energy gap that doesn't exist in bulk YBCO.
We find that the thickness-dependent energy gap appears due to the quantum size
effects in ultra-thin YBCO films that open the superconducting energy gap along
the entire Fermi surface. The fully gapped state of the ultra-thin YBCO films
makes them a very promising platform for quantum applications, including
quantum computing and quantum communications.
