# Werner state

A Werner state is a $d^{2}$ × $d^{2}$-dimensional bipartite quantum state density matrix that is invariant under all unitary operators of the form $U\otimes U$. That is, it is a bipartite quantum state $\rho _{AB}$ that satisfies
    $\rho _{AB}=(U\otimes U)\rho _{AB}(U^{\dagger }\otimes U^{\dagger })$
for all unitary operators U acting on d-dimensional Hilbert space. These states were first developed by Reinhard F. Werner in 1989.

## General definition

Every Werner state $W_{AB}^{(p,d)}$ is a mixture of projectors onto the symmetric and antisymmetric subspaces, with the relative weight $p\in [0,1]$ being the main parameter that defines the state, in addition to the dimension $d\geq 2$:
    $W_{AB}^{(p,d)}=p{\frac {2}{d(d+1)}}P_{AB}^{\text{sym}}+(1-p){\frac {2}{d(d-1)}}P_{AB}^{\text{as}},$
where
    $P_{AB}^{\text{sym}}={\frac {1}{2}}(I_{AB}+F_{AB}),$
    $P_{AB}^{\text{as}}={\frac {1}{2}}(I_{AB}-F_{AB}),$
are the projectors and
    $F_{AB}=\sum _{ij}|i\rangle \langle j|_{A}\otimes |j\rangle \langle i|_{B}$
is the permutation or flip operator that exchanges the two subsystems A and B.
Werner states are separable for p ≥ 1⁄2 and entangled for p < 1⁄2. All entangled Werner states violate the PPT separability criterion, but for d ≥ 3 no Werner state violates the weaker reduction criterion. Werner states can be parametrized in different ways. One way of writing them is
    $\rho _{AB}={\frac {1}{d^{2}-d\alpha }}(I_{AB}-\alpha F_{AB}),$
where the new parameter α varies between −1 and 1 and relates to p as
    $\alpha =((1-2p)d+1)/(1-2p+d).$

## Two-qubit example

Two-qubit Werner states, corresponding to $d=2$ above, can be written explicitly in matrix form as$W_{AB}^{(p,2)}={\frac {p}{6}}{\begin{pmatrix}2&0&0&0\\0&1&1&0\\0&1&1&0\\0&0&0&2\end{pmatrix}}+{\frac {(1-p)}{2}}{\begin{pmatrix}0&0&0&0\\0&1&-1&0\\0&-1&1&0\\0&0&0&0\end{pmatrix}}={\begin{pmatrix}{\frac {p}{3}}&0&0&0\\0&{\frac {3-2p}{6}}&{\frac {-3+4p}{6}}&0\\0&{\frac {-3+4p}{6}}&{\frac {3-2p}{6}}&0\\0&0&0&{\frac {p}{3}}\end{pmatrix}}.$Equivalently, these can be written as a convex combination of the totally mixed state with (the projection onto) a Bell state: $W_{AB}^{(\lambda ,2)}=\lambda |\Psi ^{-}\rangle \!\langle \Psi ^{-}|+{\frac {1-\lambda }{4}}I_{AB},\qquad |\Psi ^{-}\rangle \equiv {\frac {1}{\sqrt {2}}}(|01\rangle -|10\rangle ),$ where $\lambda \in [-1/3,1]$ (or, confining oneself to positive values, $\lambda \in [0,1]$) is related to $p$ by $\lambda =(3-4p)/3$. Then, two-qubit Werner states are separable for $\lambda \leq 1/3$ and entangled for $\lambda >1/3$.

## Werner-Holevo channels

A Werner-Holevo quantum channel ${\mathcal {W}}_{A\rightarrow B}^{\left(p,d\right)}$ with parameters $p\in \left[0,1\right]$ and integer $d\geq 2$ is defined as
    ${\mathcal {W}}_{A\rightarrow B}^{\left(p,d\right)}=p{\mathcal {W}}_{A\rightarrow B}^{\text{sym}}+\left(1-p\right){\mathcal {W}}_{A\rightarrow B}^{\text{as}},$
where the quantum channels ${\mathcal {W}}_{A\rightarrow B}^{\text{sym}}$ and ${\mathcal {W}}_{A\rightarrow B}^{\text{as}}$ are defined as
    ${\mathcal {W}}_{A\rightarrow B}^{\text{sym}}(X_{A})={\frac {1}{d+1}}\left[\operatorname {Tr} [X_{A}]I_{B}+\operatorname {id} _{A\rightarrow B}(T_{A}(X_{A}))\right],$
    ${\mathcal {W}}_{A\rightarrow B}^{\text{as}}(X_{A})={\frac {1}{d-1}}\left[\operatorname {Tr} [X_{A}]I_{B}-\operatorname {id} _{A\rightarrow B}(T_{A}(X_{A}))\right],$
and $T_{A}$ denotes the partial transpose map on system A. Note that the Choi state of the Werner-Holevo channel ${\mathcal {W}}_{A\rightarrow B}^{p,d}$ is a Werner state:
    ${\mathcal {W}}_{A\rightarrow B}^{\left(p,d\right)}(\Phi _{RA})=p{\frac {2}{d\left(d+1\right)}}P_{RB}^{\text{sym}}+\left(1-p\right){\frac {2}{d\left(d-1\right)}}P_{RB}^{\text{as}},$
where $\Phi _{RA}={\frac {1}{d}}\sum _{i,j}|i\rangle \langle j|_{R}\otimes |i\rangle \langle j|_{A}$.

## Multipartite Werner states

Werner states can be generalized to the multipartite case. An N-party Werner state is a state that is invariant under $U\otimes U\otimes \cdots \otimes U$ for any unitary U on a single subsystem. The Werner state is no longer described by a single parameter, but by N! − 1 parameters, and is a linear combination of the N! different permutations on N systems.