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The strong interaction of individual quantum emitters with resonant cavities is of fundamental interest for understanding light–matter interactions. Plasmonic cavities hold the promise of attaining the strong coupling regime even under ambient conditions and within subdiffraction volumes. Recent experiments revealed strong coupling between individual plasmonic structures and multiple organic molecules; however, strong coupling at the limit of a single quantum emitter has not been reported so far. Here we demonstrate vacuum Rabi splitting, a manifestation of strong coupling, using silver bowtie plasmonic cavities loaded with semiconductor quantum dots (QDs). A transparency dip is observed in the scattering spectra of individual bowties with one to a few QDs, which are directly counted in their gaps. A coupling rate as high as 120 meV is registered even with a single QD, placing the bowtie-QD constructs close to the strong coupling regime. These observations are verified by polarization-dependent experiments and validated by electromagnetic calculations. Strong coupling at the limit of a single quantum emitter has not been reported. Here, Santhosh et al. show a transparency dip is observed in the scattering spectra of individual silver bowties with one to a few quantum dots, placing the plasmonic bowtie-quantum dot constructs close to the strong coupling regime.

Recent years have seen a growing interest in strong coupling between plasmons and excitons, as a way to generate new quantum optical testbeds and influence chemical dynamics and reactivity. Strong coupling to bright plasmonic modes has been achieved even with single quantum emitters. Dark plasmonic modes fare better in some applications due to longer lifetimes, but are difficult to probe as they are subradiant. Here, we apply electron energy loss (EEL) spectroscopy to demonstrate that a dark mode of an individual plasmonic bowtie can interact with a small number of quantum emitters, as evidenced by Rabi-split spectra. Coupling strengths of up to 85 meV place the bowtie-emitter devices at the onset of the strong coupling regime. Remarkably, the coupling occurs at the periphery of the bowtie gaps, even while the electron beam probes their center. Our findings pave the way for using EEL spectroscopy to study exciton-plasmon interactions involving non-emissive photonic modes. Dark plasmonic modes fare better in some applications due to longer lifetimes but, being subradiant, are difficult to probe. The authors apply electron energy loss spectroscopy to demonstrate that a dark mode of a plasmonic cavity can couple with a few quantum emitters to exhibit vacuum Rabi splitting.

The complementary optical properties of surface plasmon excitations of metal nanostructures and long-lived excitations of semiconductor quantum dots (QDs) make them excellent candidates for studies of optical coupling at the nanoscale level. Plasmonic devices confine light to nanometer-sized regions of space, which turns them into effective cavities for quantum emitters. QDs possess large oscillator strengths and high photostability, making them useful for studies down to the single-particle level. Depending on structure and energy scales, QD excitons and surface plasmons (SPs) can couple either weakly or strongly, resulting in different unique optical properties. While in the weak coupling regime plasmonic cavities (PCs) mostly enhance the radiative rate of an emitter, in the strong coupling regime the energy level of the two systems mix together, forming coupled matter-light states. The interaction of QD excitons with PCs has been widely investigated experimentally as well as theoretically, with an eye on potential applications ranging from sensing to quantum information technology. In this review we provide a comprehensive introduction to this exciting field of current research, and an overview of studies of QD-plasmon systems in the weak and strong coupling regimes.

In recent years, it has been established that electron transport through a chiral system depends on spin. In several studies, it has further been established that charge polarization in proteins may affect their activity and, specifically, that this polarization is electron spin-dependent. Here, we review experimental methods that enable the spin dependence of protein polarizability to be recorded and describe results from several studies that indicate the importance of spin in controlling the reactivity of proteins. We conclude by suggesting why this spin dependence may be of importance and discussing how future studies might explore pathways within proteins by which polarizability affects protein reactive sites.

Plasmonic cavities can confine electromagnetic radiation to deep sub-wavelength regimes. This facilitates strong coupling phenomena to be observed at the limit of individual quantum emitters. Here, we report an extensive set of measurements of plasmonic cavities hosting one to a few semiconductor quantum dots. Scattering spectra show Rabi splitting, demonstrating that these devices are close to the strong coupling regime. Using Hanbury Brown and Twiss interferometry, we observe non-classical emission, allowing us to directly determine the number of emitters in each device. Surprising features in photoluminescence spectra point to the contribution of multiple excited states. Using model simulations based on an extended Jaynes-Cummings Hamiltonian, we find that the involvement of a dark state of the quantum dots explains the experimental findings. The coupling of quantum emitters to plasmonic cavities thus exposes complex relaxation pathways and emerges as an unconventional means to control dynamics of quantum states. Strong coupling of quantum emitters to plasmonic cavities allows exploring rich phenomenologies of light-matter interaction. Here, the authors report experiments on single colloidal quantum dots coupled to plasmonic metal nanostructures, revealing complex interactions between bright and dark states.

The functional cycle of many proteins involves large-scale motions of domains and subunits. The relation between conformational dynamics and the chemical steps of enzymes remains under debate. Here we show that in the presence of substrates, domain motions of an enzyme can take place on the microsecond time scale, yet exert influence on the much-slower chemical step. We study the domain closure reaction of the enzyme adenylate kinase from Escherichia coli while in action (i.e., under turnover conditions), using single-molecule FRET spectroscopy. We find that substrate binding increases dramatically domain closing and opening times, making them as short as ∼15 and ∼45 μs, respectively. These large-scale conformational dynamics are likely the fastest measured to date, and are ∼100–200 times faster than the enzymatic turnover rate. Some active-site mutants are shown to fully or partially prevent the substrate-induced increase in domain closure times, while at the same time they also reduce enzymatic activity, establishing a clear connection between the two phenomena, despite their disparate time scales. Based on these surprising observations, we propose a paradigm for the mode of action of enzymes, in which numerous cycles of conformational rearrangement are required to find a mutual orientation of substrates that is optimal for the chemical reaction.

How biological machines harness ATP to drive mechanical work remains a crucial question. Structural studies of protein-translocating AAA+ machines proposed a coupled and sequential translocation process, whereby ATP hydrolysis events lead to short threading steps. Yet, direct real-time observation of these events remains elusive. Here, we employ single-molecule FRET spectroscopy to track substrate translocation through ClpB, a quality control AAA+ machine. We isolate ClpB and its substrate within lipid vesicles and find that translocation events, while dependent on ATP, take milliseconds, much faster than ATP hydrolysis times. Surprisingly, the translocation rate depends weakly on temperature and ATP concentration. Using three-color FRET experiments, we find that translocation events can occur bidirectionally but are not always complete. Replacing ATP with the slowly hydrolysable analog ATPγS abolishes both rapid translocation and directionality. These results indicate a fast, stochastic Brownian-motor-like mechanism, redefining how ATP is coupled with mechanical action in AAA+ machines. Cells rely on AAA+ machines to move and remodel proteins. Here, authors show that ClpB threads proteins rapidly using ATP to bias diffusive motion, revealing a fast, stochastic Brownian-motor mechanism rather than stepwise power strokes.

Large protein machines are tightly regulated through allosteric communication channels. Here we demonstrate the involvement of ultrafast conformational dynamics in allosteric regulation of ClpB, a hexameric AAA+ machine that rescues aggregated proteins. Each subunit of ClpB contains a unique coiled-coil structure, the middle domain (M domain), proposed as a control element that binds the co-chaperone DnaK. Using single-molecule FRET spectroscopy, we probe the M domain during the chaperone cycle and find it to jump on the microsecond time scale between two states, whose structures are determined. The M-domain jumps are much faster than the overall activity of ClpB, making it an effectively continuous, tunable switch. Indeed, a series of allosteric interactions are found to modulate the dynamics, including binding of nucleotides, DnaK and protein substrates. This mode of dynamic control enables fast cellular adaptation and may be a general mechanism for the regulation of cellular machineries. Large protein machines are tightly regulated through allosteric communication channels. Here authors use single-molecule FRET and demonstrate the involvement of ultrafast conformational dynamics in the allosteric regulation of ClpB, a hexameric AAA+ machine that rescues aggregated proteins.

Upon transfer from strongly denaturing to native conditions, proteins undergo a collapse that either precedes folding or occurs simultaneously with it. This collapse is similar to the well known coil-globule transition of polymers. Here we employ singlemolecule fluorescence methods to fully characterize the equilibrium coil-globule transition in the denatured state of the IgGbinding domain of protein L. By using FRET measurements on freely diffusing individual molecules, we determine the radius of gyration of the protein, which shows a gradual expansion as the concentration of the denaturant, guanidinium hydrochloride, is increased all the way up to 7 M. This expansion is observed also in fluorescence correlation spectroscopy measurements of the hydrodynamic radius of the protein. We analyze the radius of gyration measurements using the theory of the coil-globule transition of Sanchez [Sanchez, I. C. (1979) Macromolecules 12, 980-988], which balances the excluded volume entropy of the chain with the average interresidue interaction energy. In particular, we calculate the solvation energy of the denatured protein, a property that is not readily accessible in other experiments. The dependence of this energy on denaturant concentration is nonlinear, contrasting with the common linear extrapolation method used to describe denaturation energy. Interestingly, a fit to the binding model of chemical denaturation suggests a single denaturant binding site per protein residue. The size of the denatured protein under native conditions can be extrapolated from the data as well, showing that the fully collapsed state of protein is only ≈10% larger than the folded state.

Lipid membranes are complex quasi–two-dimensional fluids, whose importance in biology and unique physical/materials properties have made them a major target for biophysical research. Recent single-molecule tracking experiments in membranes have caused some controversy, calling the venerable Saffman–Delbrück model into question and suggesting that, perhaps, current understanding of membrane hydrodynamics is imperfect. However, single-molecule tracking is not well suited to resolving the details of hydrodynamic flows; observations involving correlations between multiple molecules are superior for this purpose. Here dual-color molecular tracking with submillisecond time resolution and submicron spatial resolution is employed to reveal correlations in the Brownian motion of pairs of fluorescently labeled lipids in membranes. These correlations extend hundreds of nanometers in freely floating bilayers (black lipid membranes) but are severely suppressed in supported lipid bilayers. The measurements are consistent with hydrodynamic predictions based on an extended Saffman–Delbrück theory that explicitly accounts for the two-leaflet bilayer structure of lipid membranes.