Exponentially Decreasing Critical Detection Efficiency for Any Bell Inequality.

We address the problem of closing the detection efficiency loophole in Bell experiments, which is crucial for real-world applications. Every Bell inequality has a critical detection efficiency η that must be surpassed to avoid the detection loophole. Here, we propose a general method for reducing the critical detection efficiency of any Bell inequality to arbitrary low values. This is accomplished by entangling two particles in N orthogonal subspaces (e.g., N degrees of freedom) and conducting N Bell tests in parallel. Furthermore, the proposed method is based on the introduction of penalized N-product (PNP) Bell inequalities, for which the so-called simultaneous measurement loophole is closed, and the maximum value for local hidden-variable theories is simply the Nth power of the one of the Bell inequality initially considered. We show that, for the PNP Bell inequalities, the critical detection efficiency decays exponentially with N. The strength of our method is illustrated with a detailed study of the PNP Bell inequalities resulting from the Clauser-Horne-Shimony-Holt inequality.

On the security of semi-device-independent QKD protocols.

While fully device-independent security in (BB84-like) prepare-and-measure quantum key distribution (QKD) is impossible, it can be guaranteed against individual attacks in a semi-device-independent (SDI) scenario, wherein no assumptions are made on the characteristics of the hardware used except for an upper bound on the dimension of the communicated system. Studying security under such minimal assumptions is especially relevant in the context of the recent quantum hacking attacks wherein the eavesdroppers can not only construct the devices used by the communicating parties but are also able to remotely alter their behavior. In this work, we study the security of a SDIQKD protocol based on the prepare-and-measure quantum implementation of a well-known cryptographic primitive, the random access code (RAC). We consider imperfect detectors and establish the critical values of the security parameters (the observed success probability of the RAC and the detection efficiency) required for guaranteeing security against eavesdroppers with and without quantum memory. Furthermore, we suggest a minimal characterization of the preparation device in order to lower the requirements for establishing a secure key.