We present an experimental state-independent violation of an inequality for noncontextual theories on single particles. We show that 20 different single-photon states violate an inequality which involves correlations between results of sequential compatible measurements by at least 419 standard deviations. Our results show that, for any physical system, even for a single system, and independent of its state, there is a universal set of tests whose results do not admit a noncontextual interpretation. This sheds new light on the role of quantum mechanics in quantum information processing.

No abstract

The conflict between classical and quantum physics can be identified through a series of yes-no tests on quantum systems, without it being necessary that these systems be in special quantum states. Kochen-Specker (KS) sets of yes-no tests have this property and provide a quantum-versusclassical advantage that is free of the initialization problem that affects some quantum computers. Here, we report the first experimental implementation of a complete KS set that consists of 18 yes-no tests on four-dimensional quantum systems and show how to use the KS set to obtain a state-independent quantum advantage. We first demonstrate the unique power of this KS set for solving a task while avoiding the problem of state initialization. Such a demonstration is done by showing that, for 28 different quantum states encoded in the orbital-angular-momentum and polarization degrees of freedom of single photons, the KS set provides an impossible-to-beat solution. In a second experiment, we generate maximally contextual quantum correlations by performing compatible sequential measurements of the polarization and path of single photons. In this case, state independence is demonstrated for 15 different initial states. Maximum contextuality and state independence follow from the fact that the sequences of measurements project any initial quantum state onto one of the KS set's eigenstates. Our results show that KS sets can be used for quantuminformation processing and quantum computation and pave the way for future developments. arXiv:1209.1836v2 [quant-ph]

Many proteins that bind specific DNA sequences search the genome by combining three dimensional (3D) diffusion in the cytoplasm with one dimensional (1D) sliding on nonspecific DNA [1][2][3][4][5] . Here we combine resonance energy transfer and fluorescence correlation measurements to characterize how individual lac repressor (LacI) molecules explore DNA during the 1D phase of target search. To track the rotation of sliding LacI molecules on the microsecond time scale during DNA surface search, we use real-time single-molecule confocal laser tracking combined with fluorescence correlation spectroscopy (SMCT-FCS). The fluorescence signal fluctuations are accurately described by rotation-coupled sliding, where LacI traverses ~40 base pairs (bp) per revolution. This distance substantially exceeds the 10.5-bp helical pitch of DNA, suggesting that the sliding protein frequently hops out of the DNA groove, which would result in frequent bypassing of target sequences. Indeed, we directly observe such bypassing by single-molecule fluorescence resonance energy transfer (smFRET). A combined analysis of the smFRET and SMCT-FCS data shows that LacI at most hops one to two grooves (10-20 bp) every 250 µs. Overall, our data suggest a speed-accuracy trade-off during sliding; the weak nature of non-specific protein-DNA interactions underlies operator bypassing but also facilitates rapid sliding. We anticipate that our SMCT-FCS method to monitor rotational diffusion on the microsecond time scale while tracking individual molecules with millisecond time resolution will be applicable to the real-time investigation of many other biological interactions and effectively extends the accessible time regime by two orders of magnitude.Sequence-specific binding and recognition of DNA target sites by proteins such as transcription factors, polymerases, and DNA-modifying enzymes is at the core of cellular information processing. However, the 'target search problem' of how to rapidly yet accurately find a specific target sequence remains incompletely understood. One aspect of preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.