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abstract: 'The Fuchs-Peres-Brandt (FPB) probe realizes the most powerful individual attack on Bennett-Brassard 1984 quantum key distribution (BB84 QKD) by means of a single controlled-NOT (CNOT) gate. This paper describes a complete physical simulation of the FPB-probe attack on polarization-based BB84 QKD using a deterministic CNOT constructed from single-photon two-qubit quantum logic. Adding polarization-preserving quantum nondemolition measurements of photon number to this configuration converts the physical simulation into a true deterministic realization of the FPB attack.'
author:
- 'Jeffrey H. Shapiro'
- 'Franco N. C. Wong'
title: 'Attacking quantum key distribution with single-photon two-qubit quantum logic'
---

Introduction
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Bennett-Brassard 1984 quantum key distribution (BB84 QKD) using single-photon polarization states works as follows [@BB84]. In each time interval allotted for a bit, Alice transmits a single photon in a randomly selected polarization, chosen from horizontal ($H$), vertical ($V$), $+$45$^\circ$, or $-$45$^\circ$, while Bob randomly chooses to detect photons in either the $H$/$V$ or $\pm$45$^\circ$ bases. Bob discloses to Alice the sequence of bit intervals and associated measurement bases for which he has detections. Alice then informs Bob which detections occurred in bases coincident with the ones that she used. These are the *sift events, i.e., bit intervals in which 