Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Entanglement between distant qubits in cyclic XX chains   We evaluate the exact concurrence between any two spins in a cyclic XX chain
of n spins placed in a uniform transverse magnetic field, both at zero and
finite temperature, by means of the Jordan-Wigner transformation plus a number
parity projected statistics. It is shown that while at T=0 there is always
entanglement between any two spins in a narrow field interval before the
transition to the aligned state, at low but non-zero temperatures the
entanglement remains non-zero for arbitrarily high fields, for any pair
separation L, although its magnitude decreases exponentially with the field. It
is also demonstrated that all associated limit temperatures approach a constant
non-zero value in this limit, which decreases as 1/L^2 for L<<n but exhibit
special finite size effects for distant qubits (L approx. n/2). Related aspects
such as the different behavior of even and odd antiferromagnetic chains, the
existence of n ground state transitions and the thermodynamic limit are also
discussed.

4
The categories of this paper are: cond-mat
23
The categories of this paper are: solv-int
18
The categories of this paper are: econ
29
The categories of this paper are: funct-an
3
The categories of this paper are: physics
12
The categories of this paper are: nucl-th
9
The categories of this paper are: nlin
37
The categories of this paper are: supr-con
34
The categories of this paper are: chem-ph
22
The categories of this paper are: chao-dyn
28
The categories of this paper are: alg-geom
0
The categories of this paper are: hep-ph
8
The categories of this paper are: hep-ex
15
The categories of this paper are: nucl-ex
7
The categories of this paper are: hep-th
30
The categories of this paper are: q-alg