On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 − 8 + 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M ⊙ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 Mpc ) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

Gravitational waves were discovered with the detection of binary black-hole mergers and they should also be detectable from lower-mass neutron-star mergers. These are predicted to eject material rich in heavy radioactive isotopes that can power an electromagnetic signal. This signal is luminous at optical and infrared wavelengths and is called a kilonova. The gravitational-wave source GW170817 arose from a binary neutron-star merger in the nearby Universe with a relatively well confined sky position and distance estimate. Here we report observations and physical modelling of a rapidly fading electromagnetic transient in the galaxy NGC 4993, which is spatially coincident with GW170817 and with a weak, short γ-ray burst. The transient has physical parameters that broadly match the theoretical predictions of blue kilonovae from neutron-star mergers. The emitted electromagnetic radiation can be explained with an ejected mass of 0.04 ± 0.01 solar masses, with an opacity of less than 0.5 square centimetres per gram, at a velocity of 0.2 ± 0.1 times light speed. The power source is constrained to have a power-law slope of -1.2 ± 0.3, consistent with radioactive powering from r-process nuclides. (The r-process is a series of neutron capture reactions that synthesise many of the elements heavier than iron.) We identify line features in the spectra that are consistent with light r-process elements (atomic masses of 90-140). As it fades, the transient rapidly becomes red, and a higher-opacity, lanthanide-rich ejecta component may contribute to the emission. This indicates that neutron-star mergers produce gravitational waves and radioactively powered kilonovae, and are a nucleosynthetic source of the r-process elements.

In this paper we investigate the coherence properties of a quantum dot under two-photon resonant excitation in combination with an additional photo-neutralization laser. The photo-neutralization increases the efficiency of the excitation process and thus, the brightness of the source, by a factor of approximately 1.5 for biexciton-exciton pairs. This enhancement does not degrade the relevant coherences in the system; neither the single photon coherence time, nor the coherence of the excitation process.

We show that resonance fluorescence, i.e. the resonant emission of a coherently driven two-level system, can be realized with a semiconductor quantum dot. The dot is embedded in a planar optical micro-cavity and excited in a wave-guide mode so as to discriminate its emission from residual laser scattering. The transition from the weak to the strong excitation regime is characterized by the emergence of oscillations in the first-order correlation function of the fluorescence, g(τ ), as measured by interferometry. The measurements correspond to a Mollow triplet with a Rabi splitting of up to 13.3 µeV. Second-order-correlation measurements further confirm non-classical light emission. [3,4,5,6,7], as well as photon anti-bunching [8], and were previously only observable in isolated atoms or ions. In addition, QDs can be readily integrated into optical micro-cavities making them attractive for a number of applications, particularly quantum information processing and high efficiency light sources. For example, QDs could be used to realize deterministic solid-state single photon sources [9,10,11] and qubit-photon interfaces [12]. Advances in high-Q cavities have shown that not only can the spontaneous emission rate be dramatically increased by the Purcell effect [13,14], but emission can be reversed in the strong coupling regime [15,16,17]. Despite these efforts, however, quantum dot-based cavity quantum electrodynamics (QED) lacks an ingredient essential to the success of atomic cavity QED, namely the ability to truly resonantly manipulate the two-level system [9,10,11]. Current approaches can at best populate the dot in one of its excited states, which subsequently relaxes in some way to the emitting ground state. This incoherent relaxation has been addressed theoretically [18,19], and experimentally [20] but direct resonant excitation and collection in the ground state has so far not been reported as it is very challenging to differentiate the resonance fluorescence from same-frequency laser scattering off defects, contaminants, etc. In quantum dots without cavities, coherent manipulation of ground-state excitons has nonetheless been achieved with a number of techniques including differential transmission [6], differential * Electronic address: shih@physics.utexas.edu reflectivity [21], four-wave mixing [22], photodiode spectroscopy [7], and Stark-shift modulation absorption spectroscopy [23]. However, none of these is able to collect and use the actual photon emission which limits their use in many potential applications of QDs. This report presents the first measurement of resonance fluorescence in a single self-assembled quantum dot. Described by Mollow in 1969 [24], the resonant emission of a two-state quantum system under strong coherent excitation is distinguished by an oscillatory first-order correlation function, g(τ ), that we observe with interferometry. We use a planar optical micro-cavity to guide the excitation laser between the cavity mirrors and simultaneously enhance the single photon emission in th...

We demonstrate and characterize interference between discrete photons emitted by two separate semiconductor quantum dot states in different samples excited by a pulsed laser. Their energies are tuned into resonance using strain. The photons have a total coalescence probability of 18.1% and the coincidence rate is below the classical limit. Postselection of coincidences within a narrow time window increases the coalescence probability to 47%. The probabilities are reduced from unity because of dephasing and the postselection value is also reduced by the detector time response.