Quantum Flows for Secret Key Distribution

In practice, the security of a Quantum Key Distribution (QKD) system relies not only on quantum mechanical principles, but it also significantly relies on the physical implementation of the protocol. Nowadays, technological detector loopholes of QKD systems have been demonstrated, and some successful attacks that exploit vulnerabilities of the Avalanche Photo Diodes (APD’s) have been performed. The ideal quantum scenario has been continuously reduced, thus requiring to enhance QKD to more general protocols that can be device-independent. Recently, it has been shown that faked states attack works successfully on widely used QKD protocols, namely BB84, SARG04, phase-time, DPSK, Ekert protocol and the decoy method. In the QKD protocols published so far, it is defined a single photonic gain because there is an unique qubit class that is used to construct the photonic quantum flow that goes from Alice to Bob. Frequently, the security of a protocol is based on its capability to detect deviations in the photonic gain of the quantum flow in presence of Eve. In contrast, in the ack − QKD protocol, we use two different quantum flows, each one using a multi-qubit state. The multi-qubit is formed by two qubits, so we call it a bi-qubit state. These bi-qubits are chosen in such a way that they are non − orthogonal or parallel states. Let us summarize briefly the ack − QKD protocol: Consider a BB84 based protocol that encodes a classical bit using the four non-orthogonal quantum states: X0, X1, Z0 and Z1. In contrast, in the ack − QKD protocol Alice codifies one classical bit using two quantum states. To distill one secret bit Alice sends two consecutive pulses to Bob who measures them using the same basis measurement (X or Z). In the protocol, such pair of pulses can be non − orthogonal states: (X0, Z0), (X0, Z1), (X1, Z0), (X1, Z1) or parallel states: (X0, X0), (Z0, Z0), (X1, X1), (Z1, Z1). Alice chooses randomly between sending parallel or non − orthogonal states. We argue that the following statements are true: