7. Conclusions

Alice's Biqubit Eve's Basis Eve's Detection Forwarded States Eve's Result ðj0Zi; j0ZiÞ Z ð�; j0ZiÞ ðj0Zi; j0ZiÞ Hidden

ð i j1<sup>Z</sup> ; j0ZiÞ Z ð iÞ �; j0<sup>Z</sup> ð i j0<sup>Z</sup> ; j0ZiÞ Detected

However, she can use six possible states, but one of them is erroneous, so she introduces an error probability of <sup>1</sup>

Table 5. As soon as Eve detects the first state of a biqubit, she tries to fake the second state.

Figure 9. The error rate of double-detection events caused by the IRFS attack is <sup>1</sup>

quantum channel, the maximum secure distance to detect the IRFS attack is 176 km. In the presence of the IRFS attack,

perfect visibility and zero dark counts are assumed in the link between Alice and Eve and from her to Bob.

the six choices for ðj0Zi; j0ZiÞ and ðj1Zi; j0ZiÞ biqubits are shown

58 Advanced Technologies of Quantum Key Distribution

X ð�; j0XiÞ ðj0Xi; j0XiÞ Hidden

X ð iÞ �; j0<sup>X</sup> ð i j0<sup>X</sup> ; j0XiÞ Hidden

ð�; j1XiÞ ðj0Xi; j1XiÞ Hidden

ð iÞ �; j1<sup>X</sup> ð i j0<sup>X</sup> ; j1XiÞ Hidden

ðj1Zi; j0ZiÞ Detected

ðj1Xi; j0XiÞ Hidden

ð i j1<sup>X</sup> ; j1XiÞ Hidden

ð i j1<sup>Z</sup> ; j0ZiÞ Hidden

ð i j1<sup>X</sup> ; j0XiÞ Hidden

ð i j1<sup>X</sup> ; j1XiÞ Hidden

6. When it is compared to the QBER of the

6. Here,

In the quantum flows approach, the transmitter interleaves pairs of quantum states, parallel and orthogonal (non-orthogonal), while the receiver applies active basis selection to perform state measurement. The QKD protocols based on quantum flows uses the same optical hardware of the BB84 protocol, and they can be implemented in most QKD systems as a software module application.

The ack-QKD protocol can be useful to detect the PNS attack. If the eavesdropper adjusts the transmittance TAB of the channel it produces a deviation in one or in both photonic gains; thus, she will introduce a detectable QBER to the system.

On the other side the intercept resend with faked (blinding) states (IRFS) attack is detected by the nack-state protocol using the gain of single- and double-detection events where the QBER of double-detection events of the quantum channel is compared against the <sup>1</sup> <sup>6</sup> error rate caused by the eavesdropper, so the maximum secure distance results in 176 km.

Although double-detection events represent a small fraction of the total detection events, they are useful to detect the IRFS attack. In addition, the smaller QBER can be useful in future implementations to distill secret bits at longer distances.
