# Mapping graph state orbits under local complementation

1Quantum Engineering Technology (QET) Labs, H. H. Wills Physics Laboratory & Department of Electrical & Electronic Engineering, University of Bristol, Merchant Venturers Building, Woodland Road, Bristol BS8 1UB, UK
2QuTech - TU Delft, Lorentzweg 1, 2628CJ Delft, The Netherlands

### Abstract

Graph states, and the entanglement they posses, are central to modern quantum computing and communications architectures. Local complementation – the graph operation that links all local-Clifford equivalent graph states – allows us to classify all stabiliser states by their entanglement. Here, we study the structure of the orbits generated by local complementation, mapping them up to 9 qubits and revealing a rich hidden structure. We provide programs to compute these orbits, along with our data for each of the $587$ orbits up to $9$ qubits and a means to visualise them. We find direct links between the connectivity of certain orbits with the entanglement properties of their component graph states. Furthermore, we observe the correlations between graph-theoretical orbit properties, such as diameter and colourability, with Schmidt measure and preparation complexity and suggest potential applications. It is well known that graph theory and quantum entanglement have strong interplay – our exploration deepens this relationship, providing new tools with which to probe the nature of entanglement.

Graph states are ubiquitous representations of entanglement in quantum information science, and classify the most studied set of quantum states—clifford states—by the entanglement they possess.

However, many graph states are locally equivalent to one another, that is, they possess the same type of entanglement. Graph states which are locally equivalent can be transformed into one another by successive applications of the graph operation local complementation (example shown above). Using this operation, we can analyse only graph structure of the state, which is much simpler than analysing the exponentially large quantum state vector. This equivalence of graph states has been studied previously, with all graph states up to 12 qubits classified.

However, local complementation gives us more than sets of locally equivalent graphs: it also gives us an orbit (example shown above) which tells us how different graphs are related via local complementation. In this work we study these orbits, and relate their properties to properties of the entangled quantum states they contain. We find that orbit properties, such as colourability, correlate with entanglement properties, such as schmidt measure, and discuss applications of local complementation in quantum technology.

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