Controlling Fully Integrated Nanodiamonds

Color centers in nanodiamonds are optically excited with a laser via nanophotonic waveguides on a silicon chip and emit single photons into collection waveguides that interface with an optical fiber array. (Source: P. Schrinner, AG Schuck)

Quantum systems make a wide range of techno­logical applications possible – in magnetic field sensing, information processing, secure communi­cation or ultra-precise time keeping. The production of these micro­scopically small structures has progressed so far that they reach dimensions below the wavelength of light. In this way, it is possible to break down hitherto existent boundaries in optics and utilize the quantum properties of light. So, quantum emitters can be embedded in nanodiamonds. These special diamonds are charac­terized by their very small particle size, which can range from just a few to several hundred nanometers. Researchers at the University of Münster have now succeeded for the first time in fully integrating nanodiamonds into nano­photonic circuits and at the same time addressing several of these nano­diamonds optically.

In the process, green laser light is directed onto colour centres in the nano­diamonds, and the indi­vidual red photons generated there are emitted into a network of nano-scale optical components. As a result, the researchers can now control these quantum systems in a fully integrated state. Previously, it was necessary to set up bulky micro­scopes in order to control such quantum systems. With fabrication techno­logies similar to those for producing chips for computer processors, light can be directed in a comparable way using waveguides on a silicon chip. These optical waveguides, measuring less than a micrometer, were produced with the electron-beam litho­graphy and reactive ion etching equipment at the Münster Nano­fabrication Facility (MNF). “Here, the size of a typical experimental set-up was shrunk to a few hundred square micrometers,” explains Carsten Schuck from the Institute of Physics at the University of Münster, who led the study in collaboration with Doris Reiter from the Institute of Solid State Theory.

“This downsizing not only means that we can save space with a view to future applications involving quantum systems in large numbers,” he adds, “but it also enables us, for the first time, to control several such quantum systems simul­taneously.” In preliminary work prior to the current study, the scientists developed suitable interfaces between the nano­diamonds and nanophotonic circuits. These interfaces were used in the new experiments, imple­menting the coupling of quantum emitters with waveguides in an especially effective way. In their experiments, the physicists utilized the Purcell effect, which causes the nanodiamond to emit the indi­vidual photons with a higher probability into the waveguide, instead of in some random direction.

The researchers also succeeded in running two magnetic field sensors, based on the inte­grated nano­diamonds, in parallel on one chip. Previously, this had only been possible individually or successively. To make this possible, the researchers exposed the integrated nano­diamonds to microwaves, thus inducing changes of the spin state of the colour centres. The orientation of the spin influences the brightness of the nano­diamonds, which was subsequently read out using the on-chip optical access. The frequency of the microwave field and therewith the observable brightness variations depend on the magnetic field at the location of the nanodiamond. “The high sensi­tivity to a local magnetic field makes it possible to construct sensors with which indi­vidual bacteria and even individual atoms can be detected,” explains Philip Schrinner.

First of all, the researchers calculated the nano­photonic interface designs using elaborate 3D simu­lations, thus determining optimal geometries. They then assembled and fabricated these components into a nanophotonic circuit. After the nano­diamonds were integrated and charac­terized using adapted technology, the team of physicists carried out the quantum mechanical measure­ments by means of a set-up customized for the purpose. “Working with diamond-based quantum systems in nano­photonic circuits allows a new kind of accessibility, as we are no longer restricted by micro­scope set-ups,” says Doris Reiter. “Using the method we have presented, it will be possible in the future to simul­taneously monitor and read out a large number of these quantum systems on one chip,” she adds. The researchers’ work creates the conditions for enabling further studies to be carried out in the field of quantum optics – studies in which nanophotonics can be used to change the photophysical properties of the diamond emitters.

In addition to this there are new appli­cation possi­bilities in the field of quantum technologies, which will benefit from the properties of inte­grated nanodiamonds – in the field of quantum sensing or quantum information processing, for example. The next steps will include imple­menting quantum sensors in the field of magnetometry, as used for example in materials analysis for semi-conductor components or brain scans. “To this end”, say Carsten Schuck, “we want to integrate a large number of sensors on one chip which can then all be read out simul­taneously, and thus not only register the magnetic field at one place, but also visualize magnetic field gradients in space.” (Source: WWU Muenster)

Reference: P. P. J. Schrinner et al.: Integration of Diamond-Based Quantum Emitters with Nanophotonic Circuits, Nano. Lett. 20, 8170 (2020); DOI: 10.1021/acs.nanolett.0c03262

Link: Integrated Quantum Technology (C. Schuck), Dept. of Physics, Westfälische Wilhelms-University WWU, Muenster, Germany

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