Quantum Circuits and Spin(3n) Groups

All quantum gates with one and two qubits may be described by elements of $Spin$ groups due to isomorphisms $Spin(3) \simeq SU(2)$ and $Spin(6) \simeq SU(4)$. However, the group of $n$-qubit gates $SU(2^n)$ for $n > 2$ has bigger dimension than $Spin(3n)$. A quantum circuit with one- and two-qubit gates may be used for construction of arbitrary unitary transformation $SU(2^n)$. Analogously, the `$Spin(3n)$ circuits' are introduced in this work as products of elements associated with one- and two-qubit gates with respect to the above-mentioned isomorphisms. The matrix tensor product implementation of the $Spin(3n)$ group together with relevant models by usual quantum circuits with $2n$ qubits are investigated in such a framework. A certain resemblance with well-known sets of non-universal quantum gates e.g., matchgates, noninteracting-fermion quantum circuits) related with $Spin(2n)$ may be found in presented approach. Finally, a possibility of the classical simulation of such circuits in polynomial time is discussed.