Physics – Quantum Physics
Scientific paper
2010-02-15
Quantum Information Processing 10: 325-341 (2011)
Physics
Quantum Physics
13 pages, 5 figures
Scientific paper
10.1007/s11128-010-0197-7
We consider the reduced density matrix of a large block of consecutive spins in the ground states of the XY spin chain on an infinite lattice. We derive the spectrum of the density matrix using the expression of the Renyi entropy in terms of modular functions. The eigenvalues \lambda_n form an exact geometric sequence. For example, for strong magnetic field \lambda_n = C \exp{(-\pi \tau_0 n)}, here \tau_0>0 and C > 0 depend on the anisotropy and the magnetic field. Different eigenvalues are degenerated differently. The largest eigenvalue is unique, but the degeneracy g_n increases sub-exponentially as eigenvalues diminish: g_n \sim \exp{(\pi \sqrt{n/3})}. For weak magnetic field expressions are similar.
Franchini Fabio
Its Alexander R.
Korepin Vladimir E.
Takhtajan Leon A.
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