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Inquantum information andquantum computing, acluster state^[1] is a type of highly entangled state of multiplequbits. Cluster states are generated inlattices of qubits withIsing type interactions. A clusterC is a connected subset of ad-dimensional lattice, and a cluster state is a pure state of the qubits located onC. They are different from other types of entangled states such asGHZ states orW states in that it is more difficult to eliminatequantum entanglement (viaprojective measurements) in the case of cluster states. Another way of thinking of cluster states is as a particular instance ofgraph states, where the underlying graph is a connected subset of ad-dimensional lattice. Cluster states are especially useful in the context of theone-way quantum computer. For a comprehensible introduction to the topic seeBriegel (2009).

Formally, cluster states${\displaystyle |{\varphi }_{\{\kappa \}}{⟩}_C}$ are states which obey the set eigenvalue equations:

${\displaystyle K^{(a)}{|{\varphi }_{\{\kappa \}}⟩}_C=(-1{)}^{{\kappa }_a}{|{\varphi }_{\{\kappa \}}⟩}_C}$

where${\displaystyle K^{(a)}}$ are the correlation operators

${\displaystyle K^{(a)}={\sigma }_x^{(a)}\underset{b\in \mathrm{N}(a)}{⨂}{\sigma }_z^{(b)}}$

with${\displaystyle {\sigma }_x}$ and${\displaystyle {\sigma }_z}$ beingPauli matrices,${\displaystyle N(a)}$ denoting theneighbourhood of${\displaystyle a}$ and${\displaystyle \{{\kappa }_a\in \{0,1\}|a\in C\}}$ being a set of binary parameters specifying the particular instance of a cluster state.

Here are some examples of one-dimensional cluster states (d=1), for${\displaystyle n=2,3,4}$, where${\displaystyle n}$ is the number of qubits. We take${\displaystyle {\kappa }_a=0}$ for all${\displaystyle a}$, which means the cluster state is the unique simultaneous eigenstate that has corresponding eigenvalue 1 under all correlation operators. In each example the set of correlation operators${\displaystyle \{K^{(a)}{\}}_a}$and the corresponding cluster state is listed.

• ${\displaystyle n=2}$
  ${\displaystyle \{{\sigma }_x{\sigma }_z,{\sigma }_z{\sigma }_x\}}$

${\displaystyle |\varphi ⟩=\frac{1}{\sqrt{2}}(|0+⟩+|1-⟩)}$
This is an EPR-pair (up to local transformations).

• ${\displaystyle n=3}$

${\displaystyle \{{\sigma }_x{\sigma }_{z}I,{\sigma }_z{\sigma }_x{\sigma }_z,I{\sigma }_z{\sigma }_x\}}$

${\displaystyle |\varphi ⟩=\frac{1}{\sqrt{2}}(|+0+⟩+|-1-⟩)}$
This is the GHZ-state (up to local transformations).

• ${\displaystyle n=4}$

${\displaystyle \{{\sigma }_x{\sigma }_{z}II,{\sigma }_z{\sigma }_x{\sigma }_{z}I,I{\sigma }_z{\sigma }_x{\sigma }_z,II{\sigma }_z{\sigma }_x\}}$

${\displaystyle |\varphi ⟩=\frac{1}{2}(|+0+0⟩+|+0-1⟩+|-1+0⟩-|-1-1⟩)}$.

This is not a GHZ-state andcan not be converted to a GHZ-state with local operations.

In all examples${\displaystyle I}$ is the identity operator, and tensor products are omitted. The states above can be obtained from the all zero state${\displaystyle |0\dots 0⟩}$ by first applying a Hadamard gate to every qubit, and then a controlled-Z gate between all qubits that are adjacent to each other.

Cluster states can be realized experimentally. One way to create a cluster state is by encodinglogical qubits into the polarization of photons, one common encoding is the following:

${\displaystyle \{\begin{array}{l}|0{⟩}_{\mathrm{L}}⟷|\mathrm{H}⟩\\ |1{⟩}_{\mathrm{L}}⟷|\mathrm{V}⟩\end{array}}$

This is not the only possible encoding, however it is one of the simplest: with this encoding entangled pairs can be created experimentally throughspontaneous parametric down-conversion.^[2][3] The entangled pairs that can be generated this way have the form

${\displaystyle |\psi ⟩=\frac{1}{\sqrt{2}}(|\mathrm{H}⟩|\mathrm{H}⟩+{\mathrm{e}}^{\mathrm{i}\varphi }|\mathrm{V}⟩|\mathrm{V}⟩)}$

equivalent to the logical state

${\displaystyle |\psi ⟩=\frac{1}{\sqrt{2}}(|0⟩|0⟩+e^{i\varphi }|1⟩|1⟩)}$

for the two choices of the phase${\displaystyle \varphi =0,\pi }$ the twoBell states${\displaystyle |{\mathrm{\Phi }}^{+}⟩,|{\mathrm{\Phi }}^{-}⟩}$ are obtained: these are themselves two examples of two-qubits cluster states. Through the use of linear optic devices asbeam-splitters orwave-plates these Bell states can interact and form more complex cluster states.^[4] Cluster states have been created also inoptical lattices ofcold atoms.^[5]

After a cluster state was created in an experiment, it is important to verify that indeed, an entangled quantum state has been created. Thefidelity with respect to the${\displaystyle N}$-qubit cluster state${\displaystyle |C_N⟩}$ is given by

${\displaystyle F_{CN}=\mathrm{T}\mathrm{r}(\rho |C_N⟩⟨C_N|),}$

It has been shown that if${\displaystyle F_{CN}>1/2}$, then the state${\displaystyle \rho }$ has genuine multiparticle entanglement.^[6] Thus, one can obtain anentanglement witness detecting entanglement close the cluster states as

${\displaystyle W_{CN}=\frac{1}{2}\mathrm{I}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{i}\mathrm{t}\mathrm{y}-|C_N⟩⟨C_N|.}$

where signals genuine multiparticle entanglement.

Such a witness cannot be measured directly. It has to be decomposed to a sum of correlations terms, which can then be measured. However, for large systems this approach can be difficult.

There are also entanglement witnesses that work in very large systems, and they also detect genuine multipartite entanglement close to cluster states. They need only the minimal two local measurement settings.^[6] Similar conditions can also be used to put a lower bound on the fidelity with respect to an ideal cluster state.^[7] These criteria have been used first in an experiment realizing four-qubit cluster states with photons.^[3] These approaches have also been used to propose methods for detecting entanglement in a smaller part of a large cluster state or graph state realized in optical lattices.^[8]

Bell inequalities have also been developed for cluster states.^[9][10][11] All these entanglement conditions and Bell inequalities are based on the stabilizer formalism.^[12]

• Bell state
• Graph state
• Optical cluster state
• Dicke state

^[13]

• Quantum information science
• Quantum states

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