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Spatial Characterization Of Two-Photon States - GAP-Optique

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1.4. The mode function in matrix form<br />

and allows to solve some of those integrals analytically. For instance, the mode<br />

function normalization requires six integrals over the modulus square of the<br />

mode function. As appendix B shows, the matrix notation provides a way to<br />

calculate the normalization constant analytically,<br />

<br />

dx exp<br />

<br />

− 1<br />

2 xt <br />

(2A)x<br />

and thus the normalized mode function reads<br />

=<br />

Φ(qs, Ωs, qi, Ωi) = [det(2A)]1/4<br />

(2π) 3/2<br />

(2π) 3<br />

, (1.38)<br />

[det(2A)] 1/2<br />

<br />

exp − 1<br />

2 xt <br />

Ax . (1.39)<br />

In the same way, many other integrals over the mode function can be solved<br />

analytically using the matrix representation of the mode function.<br />

Conclusion<br />

The two-photon mode function can be written in a matrix form after a series<br />

of approximations. The matrix notation makes it possible to calculate several<br />

features of the down-conversion photons analytically, and reduces the numerical<br />

calculation time of others.<br />

The next chapter describes the different correlations in the two-photon state<br />

using the purity as a correlation indicator. The use of the approximate mode<br />

function in matrix notation makes it possible to find an analytical expression<br />

for the purity, and to study the effect of the different parameters on it.<br />

13

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