Draft:Petz recovery map

• Comment: Need more inline citations to denote notability and verifiability, preferably online sources. The Herald (Benison) (talk) 09:57, 16 February 2024 (UTC)
• Comment: I think the declination by The Herald (Benison) was a bit harsh, but it illustrates what I mentioned when I draftified the article -- it's notability needs to be apparent to a non-specialist. I suggest you ask for help by posting to WT:PHYSICS where I know there are people who are somewhat experts in these areas. I have published in other areas of physics, but not this area so I am not the person to help.Ldm1954 (talk) 14:24, 16 February 2024 (UTC)

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In quantum information theory, the Petz recovery map is a quantum map that proposed by Dénes Petz. The Petz recovery map is a quantum channel associated with a given quantum channel and quantum state. This recovery map is designed in a manner that, when applied to an output state resulting from the given quantum channel acting on an input state, it enables the inference of the original input state. In essence, the Petz recovery map serves as a tool for reconstructing information about the initial quantum state from its transformed counterpart under the influence of the specified quantum channel.

The Petz recovery map finds applications in various domains, including quantum retrodiction, quantum error correction, and entanglement wedge reconstruction for black hole physics.

Definition

Suppose we have a quantum state which is described by a density operator $\sigma$ and a quantum channel ${\mathcal {E}}$, the Petz recovery map is defined as

${\mathcal {P}}_{\sigma ,{\mathcal {E}}}(\rho )=\sigma ^{1/2}{\mathcal {E}}^{\dagger }({\mathcal {E}}(\sigma )^{-1/2}\rho {\mathcal {E}}(\sigma )^{-1/2})\sigma ^{1/2}.$

Notice that ${\mathcal {E}}^{\dagger }$is the Hilbert-Schmidt adjoint of ${\mathcal {E}}$.

The Petz map has been generalized in various ways in the field of quantum information theory.

Properties of the Petz recovery map

1. The Petz recovery map is a completely positive map, since (i) sandwiching by the positive semi-definite operator ${\mathcal {E}}(\sigma )^{-1/2}(\cdot ){\mathcal {E}}(\sigma )^{-1/2}$is completely positive; (ii) ${\mathcal {E}}^{\dagger }$is also completely positive when ${\mathcal {E}}$ is completely positive; and (iii) sandwiching by the positive semi-definite operator $\sigma ^{1/2}(\cdot )\sigma ^{1/2}$ is completely positive.
2. It's also clear that ${\mathcal {P}}_{\sigma ,{\mathcal {E}}}$ is is trace non-increasing

${\begin{aligned}\operatorname {Tr} \left[{\mathcal {P}}_{\sigma ,{\mathcal {N}}}(X)\right]&=\operatorname {Tr} \left[\sigma ^{\frac {1}{2}}{\mathcal {E}}^{\dagger }\left({\mathcal {E}}(\sigma )^{-{\frac {1}{2}}}X{\mathcal {E}}(\sigma )^{-{\frac {1}{2}}}\right)\sigma ^{\frac {1}{2}}\right]\\&=\operatorname {Tr} \left[\sigma {\mathcal {E}}^{\dagger }\left({\mathcal {E}}(\sigma )^{-{\frac {1}{2}}}X{\mathcal {E}}(\sigma )^{-{\frac {1}{2}}}\right)\right]\\&=\operatorname {Tr} \left[{\mathcal {E}}(\sigma ){\mathcal {E}}(\sigma )^{-{\frac {1}{2}}}X{\mathcal {E}}(\sigma )^{-{\frac {1}{2}}}\right]\\&=\operatorname {Tr} \left[{\mathcal {E}}(\sigma )^{-{\frac {1}{2}}}{\mathcal {E}}(\sigma ){\mathcal {E}}(\sigma )^{-{\frac {1}{2}}}X\right]\\&=\operatorname {Tr} \left[\Pi _{{\mathcal {E}}(\sigma )}X\right]\\&\leq \operatorname {Tr} [X]\end{aligned}}$

1. From 1 and 2, when ${\mathcal {E}}(\sigma )$ is invertable, the Petz recovery map ${\mathcal {P}}_{\sigma ,{\mathcal {E}}}$ is a quantum channel, viz., a completely positive trace-preserving (CPTP) map.

References

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Categories:

• Quantum information theory
• Eponymous equations of physics
• Von Neumann algebras