## 5 Improved Setup with a Fully Active Base Selection  
Following the discussion above, I propose an improved receiving station, shown in Figure 4. It consists of two physically distinct detection bases and a mirror which directs _all_ incoming light to _one and only one_ base under the control of a local and private QRNG. This switching mechanism is allowed by the laws of quantum mechanics and could be implemented, for example, via the use of a motorized mirror, a MEMS router [25; 26], a mechanical switch inside optical fibers [27], or an electro-optically controlled Mach-Zehnder interferometer (MZI) 2 \(\times\) 2 port switch [28]. While micromechanical mirror switches offer switching times as low as a microsecond, an MZI offers sub-nanosecond  
Figure 3: A receiver for the BB84 protocol with an active receiver with a phase electro-modulator (PEM); the measurement basis is determined by the quantum random number generator (QRNG) controlling the PEM. switching times and is suitable for photonic integration. Whichever technique is used, it is imperative that it is insensitive to polarization and does not introduce high losses. For all of these techniques, insertion losses of a fraction of a dB have been demonstrated, which is quite acceptable given the total loss budget of a typical quantum channel. For simplicity without a loss of generality, I assume that the four detectors operate in a free-running mode, meaning that they are ready to detect at all times except during the dead time. In particular, this means that gating is assumed to be implemented at the level of data analysis, not at the level of disabling the optical sensors. If the latter is preferred, for example, to reduce noise, a modification using double-gating can be implemented that would preserve sensitivity to out-of-gate thermal blinding. I will now analyze how this setup operates under a DBA attack. A DBA attack requires the careful tailoring of the characteristics of the blinding light (e.g., its power, polarization, and whether it should be a CW or a pulsed light) as well as the precise modeling of single-photon detectors in order to arrive at a reliable blinding technique for a specific receiving station. The novelty of the proposed receiver (shown in Figure 4) with respect to the two previous receivers (shown in Figures 2 and 3) is that here, only one (randomly selected) base receives light, while the other base is in darkness. Following the previous discussion, I will differentiate two cases: continuous blinding light and pulsed blinding light. In the case of a **continuous blinding light** attack on the new receiver, [PERSON] cannot keep both bases blinded at all times, nor is her attack invisible. Namely, whenever the QRNG changes the receiving base, the two detectors that were previously in the dark are both simultaneously hit by a strong blinding light. One is able to predict what happens after without modeling any a particular detector. When a single-photon sensitive detector which was previously in the dark and ready to detect is hit by a strong light, it will detect, that is, produce a \"click\", an output pulse signifying a detection, at least once. This is because the sensor will perform exactly its intended function when a single photon is detected; for example, an avalanche photodiode will become highly conductive, while a superconductive nanowire will become highly resistant. These transitions will necessarily generate a detection signal. The detector(s) in question may become blinded, but that will only happen _after_ the initial detection. As explained before, due to an inability to decide which state was sent, [PERSON] will have to discard such an event, and this will happen in 50% of gates.