Bell's inequality, random sequence, and quantum key distribution (3 pages)

Fumiaki Morikoshi

Gan Gao

Won-Young Hwang

The Ekert 1991 quantum key distribution (QKD) protocol appears to be secure regardless of whatever devices legitimate users adopt for the protocol, as long as the devices give a result that violates Bell's inequality. However, this is not the case if they ignore nondetection events because Eve can make use of the detection loophole, as Larrson showed [Quantum Inf. Comput. 2, 434 (2002)]. We show that even when legitimate users take into account nondetection events Eve can successfully eavesdrop if the QKD system has been appropriately designed by the manufacturer. A loophole utilized here is that of ``free choice'' (or ``real randomness''). Local QKD devices with a pseudorandom sequence generator installed in them can apparently violate Bell's inequality.

Mate Jagnjić, Josip Vuković, Dario Bojanjac

Antonio Acín, Nicolas Gisin, Lluís Masanes

The first step in any quantum key distribution (QKD) protocol consists of sequences of measurements that produce correlated classical data. We show that these correlation data must violate some Bell inequality in order to contain distillable secrecy, if not they could be produced by quantum measurements performed on a separable state of larger dimension. We introduce a new QKD protocol and prove its security against any individual attack by an adversary only limited by the no-signaling condition.

Huawang Qin, Hao Xu, K.S. Tang