Quantum macroscopicity versus distillation of macroscopic superpositions

We suggest a way to quantify a type of macroscopic entanglement via distillation of Greenberger-Horne-Zeilinger states by local operations and classical communication. We analyze how this relates to an existing measure of quantum macroscopicity based on the quantum Fisher information in several examples. Both cluster states and Kitaev surface code states are found to not be macroscopically quantum but can be distilled into macroscopic superpositions. We look at these distillation protocols in more detail and ask whether they are robust to perturbations. One key result is that one-dimensional cluster states are not distilled robustly but higher-dimensional cluster states are.

Figure 1

The graph used to define the cluster state $|C_N\rangle$. Measurements are performed on the $A$ sites in order to distill the $B$ sites into a GHZ state.

Figure 2

A star $s$ and plaquette $p$ in Kitaev's model and their corresponding operators: $z_s={\sigma }_1^z{\sigma }_2^z{\sigma }_3^z{\sigma }_4^z$ and $x_p={\sigma }_5^x{\sigma }_6^x{\sigma }_7^x{\sigma }_8^x$.

Figure 3

The highlighted loop $B$ is the set of qubits to be distilled into a GHZ state. Each segment $b_i$ has neighboring Ising spins ${\sigma }_i^{\left(i\right)}$ and ${\sigma }_i^{\left(o\right)}$.

Figure 4

An example of a loop to be distilled into a size $O\left(N\right)$ GHZ state.