# Modeling of entangled states in qubit clusters

E. Andre, A.N. Tsirulev

^{} Tver State University

**DOI:** 10.26456/pcascnn/2022.14.342

* Original article*

**Abstract: ** The model of universal quantum computation, which uses quantum circuits consisting of one-qubit and two-qubit logic elements, is implemented in several existing quantum-computing devices. In the last decade, the idea of using multiqubit gates has become very relevant, since this, in the future, will reduce the noise level of quantum circuits. The main resource of quantum computing is the entanglement of individual qubits that form a cluster. Despite the actuality of this issue, so far only a few examples of the simplest logic elements with entanglement are considered in theory for a system of three qubits (Toffoli element and double controlled NOT). This work is devoted to mathematical modeling of the entangled states of quantum systems composed of several qubits. A mathematical method is proposed for the exact or approximate construction of Hamiltonians generating the required unitary transformations. It turns out that the approach based on the representation of Hamiltonians and unitary transformations in the Pauli basis is the most suitable in this context for two reasons: firstly, the Pauli basis forms the Lie algebra of the corresponding unitary group; secondly, there are only real coefficients in the decompositions of Hamiltonians and state density operators in this basis. The method is considered in detail on the example of a three-qubit cluster driven by a ternary Hamiltonian to obtain the Greenberger-Horn-Zeilinger entangled state. For this system, the thermal state is also studied and the corresponding density operator is obtained.

*Keywords: quantum gate, quantum entangled state, unitary transformation, decomposition of Hamiltonian, Pauli basis, Greenberger-Horn-Zeilinger state *

- Eduardo . Andre – 3rd year postgraduate student, Applied Physics Department, Tver State University
- Alexander N. Tsirulev – Dr. Sc., Professor, General Mathematics and Mathematical Physics Department, Tver State University

**Reference: **

**Andre, E.. ** Modeling of entangled states in qubit clusters /
E.. Andre, A.N. Tsirulev //
Physical and chemical aspects of the study of clusters, nanostructures and nanomaterials. — 2022. — I. 14. — P. 342-351. DOI: 10.26456/pcascnn/2022.14.342. (In Russian).

**Full article (in Russian): **
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