# Difference between revisions of "SymmetricExtension"

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|name=SymmetricExtension | |name=SymmetricExtension | ||

|desc=Determines whether or not an operator has a [[symmetric extension]] | |desc=Determines whether or not an operator has a [[symmetric extension]] | ||

− | |req=[http://cvxr.com/cvx/ cvx] | + | |req=[http://cvxr.com/cvx/ cvx] |

− | | | + | |rel=[[IsPPT]]<br />[[IsSeparable]]<br />[[SymmetricInnerExtension]]<br />[[SymmetricProjection]] |

+ | |cat=[[List of functions#Entanglement_and_separability|Entanglement and separability]] | ||

|upd=February 21, 2014 | |upd=February 21, 2014 | ||

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<tt>'''SymmetricExtension'''</tt> is a [[List of functions|function]] that determines whether or not a given [[positive semidefinite]] operator has a symmetric extension. This function is extremely useful for showing that [[quantum state]]s are [[entangled]] (see the [[#Examples|Examples]] section). Various types of symmetric extensions can be looked for – by default, the extension that is looked for must be [[Bosonic]] and have [[positive partial transpose]], but both of these restrictions can be removed. | <tt>'''SymmetricExtension'''</tt> is a [[List of functions|function]] that determines whether or not a given [[positive semidefinite]] operator has a symmetric extension. This function is extremely useful for showing that [[quantum state]]s are [[entangled]] (see the [[#Examples|Examples]] section). Various types of symmetric extensions can be looked for – by default, the extension that is looked for must be [[Bosonic]] and have [[positive partial transpose]], but both of these restrictions can be removed. | ||

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===2-qubit symmetric extension=== | ===2-qubit symmetric extension=== | ||

It is known<ref>J. Chen, Z. Ji, D. Kribs, N. Lütkenhaus, and B. Zeng. ''Symmetric extension of two-qubit states''. E-print: [http://arxiv.org/abs/1310.3530 arXiv:1310.3530] [quant-ph]</ref> that a 2-qubit state $\rho_{AB}$ has a (not necessarily PPT) symmetric extension if and only if ${\rm Tr}(\rho_B^2) \geq {\rm Tr}(\rho_{AB}^2) - 4\sqrt{\det(\rho_{AB})}$. The following code verifies that one such state does indeed have a symmetric extension. | It is known<ref>J. Chen, Z. Ji, D. Kribs, N. Lütkenhaus, and B. Zeng. ''Symmetric extension of two-qubit states''. E-print: [http://arxiv.org/abs/1310.3530 arXiv:1310.3530] [quant-ph]</ref> that a 2-qubit state $\rho_{AB}$ has a (not necessarily PPT) symmetric extension if and only if ${\rm Tr}(\rho_B^2) \geq {\rm Tr}(\rho_{AB}^2) - 4\sqrt{\det(\rho_{AB})}$. The following code verifies that one such state does indeed have a symmetric extension. | ||

− | < | + | <syntaxhighlight> |

>> rho = [1 0 0 -1;0 1 1/2 0;0 1/2 1 0;-1 0 0 1]; | >> rho = [1 0 0 -1;0 1 1/2 0;0 1/2 1 0;-1 0 0 1]; | ||

− | >> [trace( | + | >> [trace(PartialTrace(rho)^2), trace(rho^2) - 4*sqrt(det(rho))] % if the first number is >= the second number, rho has a symmetric extension |

ans = | ans = | ||

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1 | 1 | ||

− | </ | + | </syntaxhighlight> |

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+ | {{SourceCode|name=SymmetricExtension}} | ||

==References== | ==References== | ||

<references /> | <references /> |

## Revision as of 15:27, 22 September 2014

SymmetricExtension | |

Determines whether or not an operator has a symmetric extension | |

Other toolboxes required | cvx |
---|---|

Related functions | IsPPT IsSeparable SymmetricInnerExtension SymmetricProjection |

Function category | Entanglement and separability |

` SymmetricExtension` is a function that determines whether or not a given positive semidefinite operator has a symmetric extension. This function is extremely useful for showing that quantum states are entangled (see the Examples section). Various types of symmetric extensions can be looked for – by default, the extension that is looked for must be Bosonic and have positive partial transpose, but both of these restrictions can be removed.

## Syntax

`EX = SymmetricExtension(X)``EX = SymmetricExtension(X,K)``EX = SymmetricExtension(X,K,DIM)``EX = SymmetricExtension(X,K,DIM,PPT)``EX = SymmetricExtension(X,K,DIM,PPT,BOS)``EX = SymmetricExtension(X,K,DIM,PPT,BOS,TOL)``[EX,WIT] = SymmetricExtension(X,K,DIM,PPT,BOS,TOL)`

## Argument descriptions

### Input arguments

`X`: A positive semidefinite operator.`K`(optional, default 2): The number of copies of the second subsystem in the desired symmetric extension.`DIM`(optional, by default has both subsystems of equal dimension): A 1-by-2 vector containing the dimensions of the two subsystems that`X`acts on.`PPT`(optional, default 1): A flag (either 1 or 0) that indicates whether or not the desired symmetric extension must have positive partial transpose.`BOS`(optional, default 1): A flag (either 1 or 0) that indicates whether or not the desired symmetric extension must be Bosonic (i.e., have its range contained within the symmetric subspace).`TOL`(optional, default`eps^(1/4)`): The numerical tolerance used throughout this script. It is recommended that this is left at the default value unless numerical problems arise and the script has difficulty determining whether or not`X`has a symmetric extension.

### Output arguments

`EX`: A flag (either 1 or 0) indicating that`X`does or does not have a symmetric extension of the desired type.`WIT`(optional): A witness that verifies that the answer provided by`EX`is correct. If`EX = 1`(i.e.,`X`has a symmetric extension) then`WIT`is such a symmetric extension. If`EX = 0`(i.e., no symmetric extension exists) then WIT is an entanglement witness with`trace(WIT*X) = -1`but`trace(WIT*Y) >= 0`for all symmetrically extendable`Y`.

## Examples

### 2-qubit symmetric extension

It is known^{[1]} that a 2-qubit state $\rho_{AB}$ has a (not necessarily PPT) symmetric extension if and only if ${\rm Tr}(\rho_B^2) \geq {\rm Tr}(\rho_{AB}^2) - 4\sqrt{\det(\rho_{AB})}$. The following code verifies that one such state does indeed have a symmetric extension.

```
>> rho = [1 0 0 -1;0 1 1/2 0;0 1/2 1 0;-1 0 0 1];
>> [trace(PartialTrace(rho)^2), trace(rho^2) - 4*sqrt(det(rho))] % if the first number is >= the second number, rho has a symmetric extension
ans =
8.0000 6.5000
>> SymmetricExtension(rho,2,[2,2],0) % verify that rho has a symmetric extension
ans =
1
```

## Source code

Click here to view this function's source code on github.

## References

- ↑ J. Chen, Z. Ji, D. Kribs, N. Lütkenhaus, and B. Zeng.
*Symmetric extension of two-qubit states*. E-print: arXiv:1310.3530 [quant-ph]