We approximate the noisy Burgers equation by the single-step model, alias the asymmetric simple exclusion process. The generator of the corresponding master equation is identical to the ferromagnetic Heisenberg spin chain with a purely imaginary XY coupling. We Bethe diagonalize this nonsymmetric Hamiltonian. We show that the gap between the ground state and first excited state scales as N -3/2 for large system size N. The gap between the largest and next-largest eigenvalue scales as N -1 . This property hints at conformal invariance. We also explain the connection to the six-vertex model