In quantum computing, a graph state is a special type of multi-qubit state that can be represented by a graph. Each qubit is represented by a vertex of the graph, and there is an edge between every interacting pair of qubits. In particular, they are a convenient way of representing certain types of entangled states.
Graph states are useful in quantum error-correcting codes, entanglement measurement and purification and for characterization of computational resources in measurement based quantum computing models. A graph state is a particular case of a 2-uniform hypergraph state, a generalization where the edges have N cardinality.
Quantum graph states can be defined in two equivalent ways: through the notion of quantum circuits and stabilizer formalism.
Quantum circuit definition
where and the operator is the controlled-Z interaction between the two vertices (corresponding to two qubits) and
Stabilizer formalism definition
An alternative and equivalent definition is the following, which makes use of the stabilizer formalism.
Define an operator for each vertex of :
where are the Pauli matrices and is the set of vertices adjacent to . The operators commute. The graph state is defined as the simultaneous -eigenvalue eigenstate of the operators :
Equivalence between the two definitions
A proof of the equivalence of the two definitions can be found in.
- If is a three-vertex path, then the stabilizers are
The corresponding quantum state is
- If is a triangle on three vertices, then the stabilizers are
The corresponding quantum state is
Observe that and are locally equivalent to each other, i.e., can be mapped to each other by applying one-qubit unitary transformations. Indeed, switching and on the first and last qubits, while switching and on the middle qubit, maps the stabilizer group of one into that of the other.
More generally, two graph states are locally equivalent if and only if the corresponding graphs are related by a sequence of so-called "local complementation" steps, as shown by Van den Nest et al. (2005).
- M. Hein; J. Eisert; H. J. Briegel (2004). "Multiparty entanglement in graph states". Physical Review A. 69 (6): 062311. arXiv:quant-ph/0307130. Bibcode:2004PhRvA..69f2311H. doi:10.1103/PhysRevA.69.062311. S2CID 108290803.
- S. Anders; H. J. Briegel (2006). "Fast simulation of stabilizer circuits using a graph-state representation". Physical Review A. 73 (2): 022334. arXiv:quant-ph/0504117. Bibcode:2006PhRvA..73b2334A. doi:10.1103/PhysRevA.73.022334. S2CID 12763101.
- M. Van den Nest; J. Dehaene; B. De Moor (2005). "Local unitary versus local Clifford equivalence of stabilizer states". Physical Review A. 71 (6): 062323. arXiv:quant-ph/0411115. Bibcode:2005PhRvA..71f2323V. doi:10.1103/PhysRevA.71.062323. S2CID 119466090.
- Hein M.; Dür W.; Eisert J.; Raussendorf R.; Van den Nest M.; Briegel H.-J. (2006). "Entanglement in graph states and its applications". Proceedings of the International School of Physics "Enrico Fermi". 162 (Quantum Computers, Algorithms and Chaos): 115–218. arXiv:quant-ph/0602096. Bibcode:2006quant.ph..2096H. doi:10.3254/978-1-61499-018-5-115. ISSN 0074-784X.
- Van den Nest, Maarten; Dehaene, Jeroen; De Moor, Bart (2004-09-17). "Efficient algorithm to recognize the local Clifford equivalence of graph states". Physical Review A. 70 (3): 034302. arXiv:quant-ph/0405023. Bibcode:2004PhRvA..70c4302V. doi:10.1103/PhysRevA.70.034302. ISSN 1050-2947. S2CID 35190821.