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Spatially Resolved Quantum Nano-Optics of Single Photons Using an Electron Microscope


LPS researchers fabricated a single photon source by exciting a color center in a nanodiamond using a nanometer-wide electron beam. This new excitation method, having a much better spatial resolution than the optical excitation, paves the way for the study of quantum emitters that are closely packed.

Many developments in the field of quantum information require working with individual photons. A solution to create photons one by one is to use a single quantum emitter. Just after the emission of a photon, the emitter gets back to its ground state. It emits a new photon only following a minimum period of time, necessary to get back to the excited state and to decay again. To prevent multiple sources to be activated simultaneously, it is necessary that potential emitters are separated by a distance larger than the size of the excitation area. LPS physicists fabricated for the first time a single photon source with nanometer spatial resolution, that is to say, much finer than with an optical excitation.

By placing a nanodiamond in an electron microscope, the researchers excited color centers with an intense nanometer sized electron beam. This work, published in the journal Physical Review Letters, opens new perspectives in the study of quantum properties of light-matter interaction at the nanometer scale.
The " color centers " are defects in crystals which are transparent in their absence. One of the best known color center in the diamond is a defect resulting from the presence of a vacancy and a nitrogen atom replacing a carbon atom. It gives, when present, a yellow color to the diamond. The excitation by a laser beam of such an individual color center shows that it is an effective source of single photons. In a nanodiamond whose size is a few tens of nanometers, the color centers are much closer than the wavelength of the excitation light. Thus, optical methods do not generally allow you to select each of them independently.

To overcome the limitations of purely optical methods, physicists in LPS used an electron microscope. A very fine and intense beam of electrons is focused to a size of a nanometer and excites color centers present in a very small region. An efficient light collection system can gather over 50 % of the emitted light. Detectors that are ultrafast and sensitive enough to count photons one by one are used to measure the correlations between the emission of two separate photons. This determines whether or not the emitter is a single photon source. When a large number of quantum emitters are present, the emitted photons are not correlated and all delays between two photons are equally likely. However, with one unique emitter, two photons cannot be emitted simultaneously and must wait a minimum period after the detection of a photon to detect a second. By analyzing the time statistics depending on the position of the electron beam excitation, physicists have located individual color centers in a diamond nanocrystal. In particular, they distinguished the presence of a single photon emitter separated by an hundred nanometer distance from an area with a larger number of centers within a nanodiamond. This work opens new perspectives in the study of quantum emitters at the nanoscale .

Figure 1 : a. scanning electron microscopy image (HADF-STEM) of a diamond nanoparticle. The electrons in a STEM can be focused on areas smaller than a nanometer. The light emitted upon excitation by electrons (cathodoluminescence) can be detected according to the position of the electron beam. b. Autocorrelation function of the two light beams corresponding to positions of the electron beam (red and blue frames on a)) separated by 130 nm. The hollow shape indicates a purely quantum state, for two different positions.

Reference :

Spatially Resolved Quantum Nano-Optics of Single Photons Using an Electron Microscope, L. H. G. Tizei et M. Kociak, Physical Review Letters, 110, 153604 (2013)

Contact :

Mathieu Kociak (mathieu.kociak@u-psud.fr)