  It has been a long sought goal of Quantum Simulation to find answers to long
standing questions in condensed matter physics. A famous example is the ground
and the excitations of 2D Hubbard model with strong repulsion below half
filling. The system is a doped antiferromagnet. It is of great interests
because of its possible relation to high Tc superconductor. Theoretically, the
fermion excitations of this model are believed to split up into holons and
spinions, and a moving holon is believed to leave behind it a string of "wrong"
spins that mismatch with the antiferromagnet background. Here, we show that the
properties of the ground state wavefunction and the holon excitation of the 2D
Hubbard model can be revealed in unprecedented detail using the technique of
quantum interference in atomic physics. This is achieved by using quantum
interference to measure the Marshall sign of the doped antiferromanget. The
region of wrong Marshall sign directly reflects the spatial extent of
fluctuating string attached to the holon.
