Faking entanglement

Bell’s theorem states that quantum mechanics can be experimentally distinguished from any local hidden-variable theory: Two widely separated measurements whose correlations violate Bell’s inequalities can’t be explained without invoking the “spooky action at a distance” that comes with quantum entanglement. Tests of the inequalities typically involve photons’ polarizations or atoms’ spins, but the theorem itself doesn’t specify what is being measured or how; only the correlations matter. However, the tests are subject to several conditions, or loopholes. For example, the experiment must be set up so that no light-speed propagation of classical information can influence the outcome, and the measurements must be efficient enough to rule out the possibility that the observed events violate Bell’s inequalities but the entire ensemble does not. Bell tests in the lab give results consistent with quantum mechanics, but none has yet closed all the loopholes simultaneously. Now, Vadim Makarov (Norwegian University of Science and Technology, Trondheim, Norway), Christian Kurtsiefer (National University of Singapore), and colleagues have experimentally shown that if they set aside one loophole or another, they can violate Bell’s inequalities in a system that manifestly lacks entanglement. They used what they call faked states: classical pulses of light that trick the detectors into behaving as if they’re detecting single photons. The work is related to their previous research on hacking the quantum key distribution (QKD) of quantum cryptosystems. By intercepting photons and resending the corresponding faked states, they can eavesdrop on the QKD connection undetected. (I. Gerhardt et al., Phys. Rev. Lett., in press, available at http://arxiv.org/abs/1106.3224 .) —Johanna Miller