A single defect center in diamond periodically excited by a laser is shown to provide a simple realization for a system obeying a fluctuation theorem for nonthermal noise. The distribution of these fluctuations is distinctly non-Gaussian, which has also been verified by numerical calculation. For driving protocols symmetric under time reversal a more restricted form of the theorem holds, which is also known from entropy fluctuations caused by thermal noise.

Using fluorescence spectroscopy we directly measure entropy production of a single two-level system realized experimentally as an optically driven defect center in diamond. We exploit a recent suggestion to define entropy on the level of a single stochastic trajectory (Seifert, Phys. Rev. Lett. 95, 040602 (2005)). Entropy production can then be split into one of the system itself and one of the surrounding medium. We demonstrate that the total entropy production obeys various exact relations for finite time trajectories.Entropy as the central concept in statistical physics pervades many branches of science. Well-defined and uncontested only in equilibrium, its extension to timedependent non-equilibrium phenomena has been debated since the days of Boltzmann mostly in relation to an explanation of irreversibility and a foundation of the second law of thermodynamics [1]. Major progress arose with the formulation of the fluctuation theorem, which quantifies in the long time limit the probability of entropy annihilating trajectories in small systems constantly driven in a steady state [2,3,4,5,6]. Entropy production in these systems is either defined as phase space contraction rate or associated with a dissipation functional, which ultimately should describe the dissipated heat.By introducing the notion of a stochastic entropy along a single trajectory, it has become possible both to extend the validity of the fluctuation theorem to finite times and to prove an integral fluctuation theorem for the total entropy production in arbitrarily driven systems governed by stochastic dynamics [7]. In this Letter, using our previously introduced driven two-level system [8], we measure the stochastic entropy production along single trajectories and demonstrate that it obeys various exact relations for finite times.Fluctuation theorems for entropy production should be distinguished from related theorems like the Jarzynski relation [9], Crooks' theorem [10], and the Hatano-Sasa relation [11]. The first two allow to extract free energy differences from non-equilibrium work measurements. Experimental tests have been performed by using a torsional pendulum [12], by mechanically stretching RNA hairpins [13,14] as well as by driving a colloidal particle in a time-dependent harmonic [15,16] and non-harmonic potential [17]. The third one yields an exact relation for transition between different steady states from which a general Clausius inequality follows that has been tested using a driven colloidal particle [18]. All these relations address a small system embedded in a surrounding heat bath of constant temperature. In contrast, our set-up works athermally and does therefore neither involve nor require any notion of dissipated heat. While our previous work has demonstrated an exact Jarzynski-like relation for the athermal analog of "dissipated work" [8], the present paper addresses the concept of stochastic entropy production directly. The crucial difference is that the derivation of the latter requires using the actual nonequilibrium...

Spectroscopic and polarization properties of single light-harvesting complexes of higher plants (LHC-II) were studied at both room temperature and T < 5 K. Monomeric complexes emit roughly linearly polarized fluorescence light thus indicating the existence of only one emitting state. Most probably this observation is explained by efficient triplet quenching restricted to one chlorophyll a (Chl a) molecule or by rather irreversible energy transfer within the pool of Chl a molecules. LHC-II complexes in the trimeric (native) arrangement bleach in a number of steps, suggesting localization of excitations within the monomeric subunits. Interpretation of the fluorescence polarization properties of trimers requires the assumption of transition dipole moments tilted out of the symmetry plane of the complex. Low-temperature fluorescence emission of trimers is characterized by several narrow spectral lines. Even at lowest excitation intensities, we observed considerable spectral diffusion most probably due to low temperature protein dynamics. These results also indicate weak interaction between Chls belonging to different monomeric subunits within the trimer thus leading to a localization of excitations within the monomer. The experimental results demonstrate the feasibility of polarization sensitive studies on single LHC-II complexes and suggest an application for determination of the Chl transition-dipole moment orientations, a key issue in understanding the structure-function relationships.

The bleaching dynamics of reconstituted single light-harvesting chlorophyll a/b complexes (LHCIIb) was investigated. The complexes containing one histidine 6 tag per monomeric subunit were immobilised predominantly in a defined orientation with their symmetry axis perpendicular to a Ni-ion-containing surface allowing for the first time the examination of single LHCIIb in an aqueous environment. Most complexes exhibit photobleaching in one step, indicating coupling between the monomeric subunits leading to an energy transfer between adjacent subunits. Differences in bleaching behaviour between these and previous observations with single LHCIIb are discussed.

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