Efficient room-temperature molecular single-photon sources for quantum key distribution.

Quantum key distribution (QKD) allows the distribution of cryptographic keys between multiple users in an information-theoretic secure way, exploiting quantum physics. While current QKD systems are mainly based on attenuated laser pulses, deterministic single-photon sources could give concrete advantages in terms of secret key rate (SKR) and security owing to the negligible probability of multi-photon events. Here, we introduce and demonstrate a proof-of-concept QKD system exploiting a molecule-based single-photon source operating at room temperature and emitting at 785 nm. With an estimated maximum SKR of 0.5 Mbps, our solution paves the way for room-temperature single-photon sources for quantum communication protocols.

Coherent characterisation of a single molecule in a photonic black box.

Extinction spectroscopy is a powerful tool for demonstrating the coupling of a single quantum emitter to a photonic structure. However, it can be challenging in all but the simplest of geometries to deduce an accurate value of the coupling efficiency from the measured spectrum. Here we develop a theoretical framework to deduce the coupling efficiency from the measured transmission and reflection spectra without precise knowledge of the photonic environment. We then consider the case of a waveguide interrupted by a transverse cut in which an emitter is placed. We apply that theory to a silicon nitride waveguide interrupted by a gap filled with anthracene that is doped with dibenzoterrylene molecules. We describe the fabrication of these devices, and experimentally characterise the waveguide coupling of a single molecule in the gap.

Laser-Induced Frequency Tuning of Fourier-Limited Single-Molecule Emitters.

The local interaction of charges and light in organic solids is the basis of distinct and fundamental effects. We here observe, at the single-molecule scale, how a focused laser beam can locally shift by hundreds of times their natural line width and, in a persistent way, the transition frequency of organic chromophores cooled at liquid helium temperature in different host matrices. Supported by quantum chemistry calculations, the results can be interpreted as effects of a photoionization cascade, leading to a stable electric field, which Stark-shifts the molecular electronic levels. The experimental observation is then applied to a common challenge in quantum photonics, i.e., the independent tuning and synchronization of close-by quantum emitters, which is desirable for multiphoton experiments. Five molecules that are spatially separated by about 50 µm and originally 20 GHz apart are brought into resonance within twice their line width. This tuning method, which does not require additional fabrication steps, is here independently applied to multiple emitters, with an emission line width that is only limited by the spontaneous decay and an inhomogeneous broadening limited to 1 nm. The system hence shows promise for photonic quantum technologies.

Cold and Hot Spots: From Inhibition to Enhancement by Nanoscale Phase Tuning of Optical Nanoantennas.

Optical nanoantennas are well-known for the confinement of light into nanoscale hot spots, suitable for emission enhancement and sensing applications. Here, we show how control of the antenna dimensions allows tuning the local optical phase, hence turning a hot spot into a cold spot. We manipulate the local intensity exploiting the interference between driving and scattered field. Using single molecules as local detectors, we experimentally show the creation of subwavelength pockets with full suppression of the driving field. Remarkably, together with the cold excitation spots, we observe inhibition of emission by the phase-tuned nanoantenna. The fluorescence lifetime of a molecule scanned in such volumes becomes longer, showing slow down of spontaneous decay. In conclusion, the spatial phase of a nanoantenna is a powerful knob to tune between enhancement and inhibition in a 3-dimensional subwavelength volume.

Electrical Control of Lifetime-Limited Quantum Emitters Using 2D Materials.

Solid-state quantum emitters are a mainstay of quantum nanophotonics as integrated single-photon sources (SPS) and optical nanoprobes. Integrating such emitters with active nanophotonic elements is desirable in order to attain efficient control of their optical properties, but it typically degrades the photostability of the emitter itself. Here, we demonstrate a tunable hybrid device that integrates state of the art lifetime-limited single emitters (line width ∼40 MHz) and 2D materials at subwavelength separation without degradation of the emission properties. Our device's nanoscale dimensions enable ultrabroadband tuning (tuning range >400 GHz) and fast modulation (frequency ∼100 MHz) of the emission energy, which renders it an integrated, ultracompact tunable SPS. Conversely, this offers a novel approach to optical sensing of 2D material properties using a single emitter as a nanoprobe.

Self-Assembled Nanocrystals of Polycyclic Aromatic Hydrocarbons Show Photostable Single-Photon Emission.

Quantum technologies could largely benefit from the control of quantum emitters in sub-micrometric size crystals. These are naturally prone to integration in hybrid devices, including heterostructures and complex photonic devices. Currently available quantum emitters in nanocrystals suffer from spectral instability, preventing their use as single-photon sources for most quantum optics operations. In this work we report on the performances of single-photon emission from organic nanocrystals (average size of hundreds of nm), made of anthracene (Ac) and doped with dibenzoterrylene (DBT) molecules. The source has hours-long photostability with respect to frequency and intensity, both at room and at cryogenic temperature. When cooled to 3 K, the 00-zero phonon line shows linewidth values (50 MHz) close to the lifetime limit. Such optical properties in a nanocrystalline environment recommend the proposed organic nanocrystals as single-photon sources for integrated photonic quantum technologies.

Beaming light from a quantum emitter with a planar optical antenna.

The efficient interaction of light with quantum emitters is crucial to most applications in nano and quantum photonics, such as sensing or quantum information processing. Effective excitation and photon extraction are particularly important for the weak signals emitted by a single atom or molecule. Recent works have introduced novel collection strategies, which demonstrate that large efficiencies can be achieved by either planar dielectric antennas combined with high numerical aperture objectives or optical nanostructures that beam emission into a narrow angular distribution. However, the first approach requires the use of elaborate collection optics, while the latter is based on accurate positioning of the quantum emitter near complex nanoscale architectures; hence, sophisticated fabrication and experimental capabilities are needed. Here we present a theoretical and experimental demonstration of a planar optical antenna that beams light emitted by a single molecule, which results in increased collection efficiency at small angles without stringent requirements on the emitter position. The proposed device exhibits broadband performance and is spectrally scalable, and it is simple to fabricate and therefore applies to a wide range of quantum emitters. Our design finds immediate application in spectroscopy, quantum optics and sensing.

A realistic fabrication and design concept for quantum gates based on single emitters integrated in plasmonic-dielectric waveguide structures.

Tremendous enhancement of light-matter interaction in plasmonic-dielectric hybrid devices allows for non-linearities at the level of single emitters and few photons, such as single photon transistors. However, constructing integrated components for such devices is technologically extremely challenging. We tackle this task by lithographically fabricating an on-chip plasmonic waveguide-structure connected to far-field in- and out-coupling ports via low-loss dielectric waveguides. We precisely describe our lithographic approach and characterize the fabricated integrated chip. We find excellent agreement with rigorous numerical simulations. Based on these findings we perform a numerical optimization and calculate concrete numbers for a plasmonic single-photon transistor.

Electromechanical control of nitrogen-vacancy defect emission using graphene NEMS.

Despite recent progress in nano-optomechanics, active control of optical fields at the nanoscale has not been achieved with an on-chip nano-electromechanical system (NEMS) thus far. Here we present a new type of hybrid system, consisting of an on-chip graphene NEMS suspended a few tens of nanometres above nitrogen-vacancy centres (NVCs), which are stable single-photon emitters embedded in nanodiamonds. Electromechanical control of the photons emitted by the NVC is provided by electrostatic tuning of the graphene NEMS position, which is transduced to a modulation of NVC emission intensity. The optomechanical coupling between the graphene displacement and the NVC emission is based on near-field dipole-dipole interaction. This class of optomechanical coupling increases strongly for smaller distances, making it suitable for nanoscale devices. These achievements hold promise for selective control of emitter arrays on-chip, optical spectroscopy of individual nano-objects, integrated optomechanical information processing and open new avenues towards quantum optomechanics.

Exceptional reduction of the diffusion constant in partially disordered photonic crystals.

In real photonic crystals light is scattered by the imperfections of the periodic potential. We study experimentally the propagation of diffused light in silicon inverse opals and report an exceptionally reduced diffusion constant of 3.0+/-0.7 m(2)/s, in samples which are only partially disordered. Waves scattered both by the lattice planes and by their imperfections interfere and light is efficiently trapped in this hybrid scattering regime. Not only higher quality crystals, but also random materials present an order of magnitude bigger diffusion constant and hence weaker scattering.

Three-dimensional silicon inverse photonic quasicrystals for infrared wavelengths.

Quasicrystals are a class of lattices characterized by a lack of translational symmetry. Nevertheless, the points of the lattice are deterministically arranged, obeying rotational symmetry. Thus, we expect properties that are different from both crystals and glasses. Indeed, naturally occurring electronic quasicrystals (for example, AlPdMn metal alloys) show peculiar electronic, vibrational and physico-chemical properties. Regarding artificial quasicrystals for electromagnetic waves, three-dimensional (3D) structures have recently been realized at GHz frequencies and 2D structures have been reported for the near-infrared region. Here, we report on the first fabrication and characterization of 3D quasicrystals for infrared frequencies. Using direct laser writing combined with a silicon inversion procedure, we achieve high-quality silicon inverse icosahedral structures. Both polymeric and silicon quasicrystals are characterized by means of electron microscopy and visible-light Laue diffraction. The diffraction patterns of structures with a local five-fold real-space symmetry axis reveal a ten-fold symmetry as required by theory for 3D structures.

Zener tunneling of light waves in an optical superlattice.

We report on the observation of Zener tunneling of light waves in spectral and time-resolved transmission measurements, performed on an optical superlattice made of porous silicon. The structure was designed to have two photonic minibands, spaced by a narrow frequency gap. A gradient in the refractive index was introduced to create two optical Wannier-Stark ladders and, at a critical value of the optical gradient, tunneling between energy bands was observed in the form of an enhanced transmission peak and a characteristic time dependence of the transmission.