Thread by @fgrosshans

Seated for a talk by Mathieu Bozzio, on "Towards trusting your local Franprix's payment terminal..." a.k.a. “Money money money, must be funny, in a quantum world !”.

Joint work with Eleni Diamanti and myself, arXiv:1812.09256 arxiv.org/abs/1812.09256
#LTQI

Mathieu Bozzio: in the 1970s, Stephen Wiesner invented a quantum banknotes scheme, with security based on the no-cloning theorem.
Mint→Client→Bank

For quantum credit cards, there is classical communication between the Merchant and Bank:
Mint→Client→Merchant→Bank
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Mathieu Bozzio’s approach : the attack cooresponds ti minimizing losses & error using convex optimization. To be practical, we use coherent states, with the following mapping
|0⟩→|α⟩⊗|vac⟩ ; |1⟩→|vac⟩⊗|α⟩
|±⟩→|α/√2⟩⊗|±α/√2⟩
|±i⟩→|α/√2⟩⊗|±iα/√2⟩
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Mathieu Bozzio uses squashing to reduces the detection to the 3 dimensional space span{|0⟩, |1⟩, |∅⟩}.
The Choi–Jamiołkowski isomorphism to maps any channel Λ (made by the advesary) to the density matrix J(Λ)=1/d ∑_ij Λ(|i⟩⟨j|)⊗⟨j|.

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Mathieu Bozzio also defines Error operators and Losses operators for any cloning attack Λ.
Form this, we find an SDP to minimize, minimizing the losses with fixed error. (One coud also minimize Errors at fixed losses)
This work both for trusted and untrusted terminals #LTQI

Mathieu Bozzio: For untrusted terminals, were the security region is *really* narrow (essentially, zero error allowed).
If one uses phase randomization, this really improves the security, including for untrusted terminals.
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