Quantum-cryptographic entangling probe

Attacking quantum key distribution with single-photon two-qubit quantum logic

“…Slutsky et al [8] demonstrated that the Fuchs-Peres constructwith the appropriate choice of probe state, interaction, and measurement-affords Eve the maximum amount of Rényi information about the error-free sifted bits that Bob receives for a given level of disturbance, i.e., for a given probability that a sifted bit will be received in er-ror. Brandt [5] extended the Slutsky et al treatment by showing that the optimal probe could be realized with a single CNOT gate. Figure 1 shows an abstract diagram of the resulting Fuchs-Peres-Brandt probe.…”

“…Our attention, however, will be directed toward attacking BB84 QKD, as to our knowledge no such experiments have been performed, although a variety of potentially practical ap- * Electronic address: jhs@mit.edu proaches have been discussed [4]. Our particular objective will be to show that current technology permits physical simulation of the Fuchs-Peres-Brandt (FPB) probe [5], i.e., the most powerful individual attack on singlephoton BB84, and that developments underway in quantum nondemolition (QND) detection may soon turn this physical simulation into a full implementation of the attack. Thus we believe it is of interest to construct the physical simulation and put BB84's security to the test: how much information can Eve really derive about the key that Alice and Bob have distilled while keeping Alice and Bob oblivious to her presence.…”

Entangled eavesdropping in quantum key distribution

“…(Note that E = 1/3 corresponds to complete information gain by the quantum cryptographic entangling probe.) Also the probe and measurement implementations remain the same (as in [1], [2], [3]) with the initial state of the probe now given by Eq. (6) or (21) below.…”

“…To accomplish this, the following sign choices must be made for the probe parameter µ in Eqs. (26) and (27) of [1]:…”

“…A design was recently given for an optimized entangling probe attacking the BB84 (Bennett-Brassard 1984) protocol of quantum key distribution and yielding maximum Renyi information to the probe [1], [2]. Probe photon polarization states become optimally entangled with the signal states on their way between the legitimate transmitter and receiver.…”