Design for a quantum cryptographic entangling probe
Howard E. Brandt · Journal of Modern Optics · 2005
I present the design for an optimized entangling probe attacking the BB84 protocol of quantum key distribution and yielding maximum information to the probe. Probe photon polarization states become entangled with the signal states on their way between the legitimate transmitter and receiver. Based on an earlier optimization of the unitary transformation representing the probe for a set error rate induced by the probe, I derive the simplest quantum circuit which faithfully yields the correct entangled state produced by the probe. The quantum circuit consists of a single CNOT gate in which the control qubit is a photon polarization-basis state of the signal, the target is a probe photon polarization basis state, and the initial state of the probe is set by the error rate. A method is determined for measuring the appropriate correlated state of the probe, yielding maximum information to the probe. Finally, a possible implementation of the entangling probe is given.