IsotropicState

IsotropicState
Produces an isotropic state
Other toolboxes required	none
Related functions	WernerState
Function category	Special states, vectors, and operators

IsotropicState is a function that returns an isotropic state (i.e., a state of the following form):

\(\displaystyle\rho_\alpha := \frac{1-\alpha}{d^2}I\otimes I + \alpha|\psi_+\rangle\langle\psi_+| \in M_d \otimes M_d,\)

where $|\psi_+\rangle:=\frac{1}{\sqrt{d}}\sum_j|j\rangle\otimes|j\rangle$ is the standard maximally-entangled pure state. Note that the output of this function is a sparse matrix.

Syntax

• RHO = IsotropicState(DIM,ALPHA)

Argument descriptions

• DIM: Dimension of the local subsystems on which RHO acts.
• ALPHA: A parameter that specifies which isotropic state is to be returned. In particular, RHO = (1-ALPHA)*I/DIM^2 + ALPHA*E, where I is the identity operator and E is the projection onto the standard maximally-entangled pure state on two copies of DIM-dimensional space. In order for RHO to be positive semidefinite (and hence a valid density matrix), it must be the case that -1/(DIM^2-1) ≤ ALPHA ≤ 1.

Examples

A qutrit isotropic state

To generate the isotropic state with parameter $\alpha = 1/2$, the following code suffices:

>> full(IsotropicState(3,1/2))

ans =

    0.2222         0         0         0    0.1667         0         0         0    0.1667
         0    0.0556         0         0         0         0         0         0         0
         0         0    0.0556         0         0         0         0         0         0
         0         0         0    0.0556         0         0         0         0         0
    0.1667         0         0         0    0.2222         0         0         0    0.1667
         0         0         0         0         0    0.0556         0         0         0
         0         0         0         0         0         0    0.0556         0         0
         0         0         0         0         0         0         0    0.0556         0
    0.1667         0         0         0    0.1667         0         0         0    0.2222

Isotropic states in general have a lot of zero entries, so this function always returns a sparse matrix. If you want a full matrix (as above), use MATLAB's full function.

>> IsotropicState(3,1/2)

ans =

   (1,1)       0.2222
   (5,1)       0.1667
   (9,1)       0.1667
   (2,2)       0.0556
   (3,3)       0.0556
   (4,4)       0.0556
   (1,5)       0.1667
   (5,5)       0.2222
   (9,5)       0.1667
   (6,6)       0.0556
   (7,7)       0.0556
   (8,8)       0.0556
   (1,9)       0.1667
   (5,9)       0.1667
   (9,9)       0.2222

Source code

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