Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Four-level systems and a universal quantum gate   We discuss a possibility of implementing a universal quantum XOR gate, by
using two coupled quantum dots subjected to external magnetic fields that are
parallel and slightly different. We consider this system in two different field
configurations. In the first case, parallel external fields with the intensity
difference at each spin being proportional to the time-dependent interaction
between the spins. General exact solution describing this system is presented
and analyzed to adjust parameters of the field. Then we consider parallel
fields with intensity difference at each spin being constant and the
interaction between the spins switching on and off adiabatically. In both cases
we adjust characteristics of the external fields (their intensities and
duration) in order to have the parallel pulse adequate for constructing the XOR
gate. In order to provide a complete theoretical description of all the cases,
we derive relations between the spin interaction, the inter-dot distance, and
the external field.

20
The categories of this paper are: acc-phys
14
The categories of this paper are: math-ph
30
The categories of this paper are: q-alg
37
The categories of this paper are: supr-con
27
The categories of this paper are: comp-gas
18
The categories of this paper are: econ
16
The categories of this paper are: stat
12
The categories of this paper are: nucl-th
33
The categories of this paper are: plasm-ph
7
The categories of this paper are: hep-th
28
The categories of this paper are: alg-geom
35
The categories of this paper are: mtrl-th
0
The categories of this paper are: hep-ph
10
The categories of this paper are: q-bio
3
The categories of this paper are: physics
34
The categories of this paper are: chem-ph