Spectrum of quantum KdV hierarchy in the semiclassical limit

Abstract

We employ semiclassical quantization to calculate spectrum of quantum KdV charges in the limit of large central charge c. Classically, KdV charges Q2n−1 generate completely integrable dynamics on the co-adjoint orbit of the Virasoro algebra. They can be expressed in terms of action variables Ik, e.g. as a power series expansion. Quantum-mechanically this series becomes the expansion in 1/c, while action variables become integer-valued quantum numbers ni. Crucially, classical expression, which is homogeneous in Ik, acquires quantum corrections that include terms of subleading powers in nk. At first two non-trivial orders in 1/c expansion these “quantum” terms can be fixed from the analytic form of Q2n−1 acting on the primary states. In this way we find explicit expression for the spectrum of Q2n−1 up to first three orders in 1/c expansion. We apply this result to study thermal expectation values of Q2n−1 and free energy of the KdV Generalized Gibbs Ensemble.