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Quantum Information Theory with Gaussian Systems

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4 <strong>Gaussian</strong> quantum cellular automata<br />

After carrying out the reduction, this becomes<br />

ˆγ(k) ↦→ ÂT (k) · ˆγ(k) · Â(k) + ĈT (k) · ˆγ ′ (k) · Ĉ(k). (4.40)<br />

The output state of an irreversible <strong>Gaussian</strong> qca <strong>with</strong> transformation Γ and noise<br />

form g is given in (4.33) and corresponds to a transformation of the correlation<br />

function as<br />

ˆγ(k) ↦→ Γ T (k) · ˆγ(k) · Γ(k) + ˆg(k). (4.41)<br />

Comparing (4.40) and (4.41) yields for the irreversible qca T:<br />

Γ(k) = Â(k) and ˆg(k) = ĈT (k) ˆγ ′ (k) Ĉ(k).<br />

Indeed, g is real and symmetric as required, i.e. ˆg(k) = −ˆg(−k) = ˆg(k), because C<br />

is real and γ ′ is real and symmetric. <br />

So, while reversible <strong>Gaussian</strong> qcas <strong>with</strong> local ancillas can implement irreversible<br />

<strong>Gaussian</strong> qcas of type IV, the converse is unfortunately not clear: are all irreversible<br />

<strong>Gaussian</strong> qcas of type IV? The answer to this question is an important step towards<br />

the definition of <strong>Gaussian</strong> as well as general irreversible qcas.<br />

98

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