The observation of quantum-dot resonance fluorescence enabled a new solid-state approach to generating single photons with a bandwidth approaching the natural linewidth of a quantum-dot transition. Here, we operate in the small Rabi frequency limit of resonance fluorescence--the Heitler regime--to generate subnatural linewidth and high-coherence quantum light from a single quantum dot. The measured single-photon coherence is 30 times longer than the lifetime of the quantum-dot transition, and the single photons exhibit a linewidth which is inherited from the excitation laser. In contrast, intensity-correlation measurements reveal that this photon source maintains a high degree of antibunching behavior on the order of the transition lifetime with vanishing two-photon scattering probability. Generating decoherence-free phase-locked single photons from multiple quantum systems will be feasible with our approach.

Reliable preparation, manipulation and measurement protocols are necessary to exploit a physical system as a quantum bit. Spins in optically active quantum dots offer one potential realization and recent demonstrations have shown high-fidelity preparation and ultrafast coherent manipulation. The final challenge-that is, single-shot measurement of the electron spin-has proved to be the most difficult of the three and so far only time-averaged optical measurements have been reported. The main obstacle to optical spin readout in single quantum dots is that the same laser that probes the spin state also flips the spin being measured. Here, by using a gate-controlled quantum dot molecule, we present the ability to measure the spin state of a single electron in real time via the intermittency of quantum dot resonance fluorescence. The quantum dot molecule, unlike its single quantum dot counterpart, allows separate and independent optical transitions for state preparation, manipulation and measurement, avoiding the dilemma of relying on the same transition to address the spin state of an electron.

Resonant Raman spectroscopy of single carbon nanotubes suspended across trenches displays red shifts of up to 30 meV of the electronic transition energies as a function of the surrounding dielectric environment. We develop a simple scaling relationship between the exciton binding energy and the external dielectric function and thus quantify the effect of screening. Our results imply that the underlying particle interaction energies change by hundreds of meV.The long predicted presence of excitons with large binding energies in carbon nanotubes (CNT) 1,2,3,4,5 has been experimentally confirmed by recent two-photon experiments 6,7,8 . With binding energies of hundreds of meV and Coulomb energies highly sensitive to screening due to the one dimensional nature of CNTs, one expects that the measured optical transition energies should change significantly with changes in the dielectric environment. Yet experiments report variations on a scale of just a few tens of meV across dielectric environments as different as CNT bundles in solution, micelle encapsulated CNTs, and individual nanotubes suspended in air 9,10 . Lefebvre et al. measured the photoluminescence (PL) emission from CNTs freely suspended in air 9 and compared the results with the PL from micelle encapsulated nanotubes published by Bachilo et al. 11 . By using family structure to correlate CNT species between the two data sets, they were able to show average red shifts of only 28 meV and 16 meV in E 11 and E 22 , respectively (where E ii is the optical transition energy associated with the i th subband), upon micelle encapsulation, a surprisingly small change given the different environments.In this work, we investigate the underlying reasons for this small variation of the observed optical transition energies. We follow the shift of the electronic energy levels as we control the screening of the Coulomb interaction in single CNTs suspended across trenches. Specifically, we use resonant Raman spectroscopy (RRS) to probe the optical transition energy E 22 of a given CNT as we change the dielectric environment from dry N 2 to high humidity N 2 to water. We present experimental evidence of dramatic underlying changes in those particle interaction energies that largely cancel each other, leading to the small variations in observed optical transition energies, in accordance with the picture described by Ando and Kane and * Corresponding author. E-mail: swan@bu.edu. Mele 1,2 .Typical spectra from resonant Raman scattering of single CVD grown CNTs suspended across trenches 12 are shown in Figure 1a. Details of the experiment have been presented elsewhere. 13,14 The Stokes scattering peak intensities for each Raman active phonon mode appearing in the spectra are plotted against laser excitation energy resulting in the resonance excitation profile (REP) for a given phonon mode. In general, the REP will be double peaked from the combined resonances of the incoming and outgoing photons with the electronic structure of the nanotube, which are resolvable when ...