John Bell’s theorem of 1964 states that local elements of physical reality, existing independent of measurement, are inconsistent with the predictions of quantum mechanics (Bell, J. S. (1964), Physics (College. Park. Md). Specifically, correlations between measurement results from distant entangled systems would be smaller than predicted by quantum physics. This is expressed in Bell’s inequalities. Employing modifications of Bell’s inequalities, many experiments have been performed that convincingly support the quantum predictions. Yet, all experiments rely on assumptions, which provide loopholes for a local realist explanation of the measurement. Here we report an experiment with polarization-entangled photons that simultaneously closes the most significant of these loopholes. We use a highly efficient source of entangled photons, distributed these over a distance of 58.5 meters, and implemented rapid random setting generation and high-efficiency detection to observe a violation of a Bell inequality with high statistical significance. The merely statistical probability of our results to occur under local realism is less than 3.74×10-31, corresponding to an 11.5 standard deviation effect.
Quantum Key Distribution (QKD - also referred to as Quantum
Cryptography) is a technique for secret key agreement. It has been shown that QKD rigged with Information-Theoretic Secure (ITS) authentication (using secret key) of the classical messages transmitted during the key distribution protocol is also ITS.
Note, QKD without any authentication can trivially be broken by
Here, we study an authentication method that was originally proposed
because of its low key consumption; a two-step authentication that uses a publicly known hash function, followed by a secret strongly universal2 hash function, which is exchanged each round. This two-step authentication is not information-theoretically secure but it was argued that nevertheless it does not compromise the security of QKD.
In the current contribution we study intrinsic weaknesses of this approach under the common assumption that the QKD adversary has
access to unlimited resources including quantum memories. We consider one implementation of Quantum Cryptographic protocols that use such authentication and demonstrate an attack that fully extract the
secret key. Even including the final key from the protocol in the authentication does not rule out the possibility of these attacks.
To rectify the situation, we propose a countermeasure that,
while not information-theoretically secure, restores the need for very large computing power for the attack to work. Finally, we specify conditions that must be satisfied by the two-step authentication in order to restore information-theoretic security.
Unconditionally secure message authentication is an important part of Quantum Cryptography (QC).We analyze
security effects of using a key obtained from QC in later rounds of QC. It has been determined earlier that partial
knowledge of the key in itself does not incur a security problem. However, by accessing the quantum channel
used in QC, the attacker can change the message to be authenticated. This, together with partial knowledge of
the key does incur a security weakness of the authentication. We suggest a simple solution to this problem, and
stress usage of this or an equivalent extra security measure in QC.