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On-chip microlaser driven sources of indistinguishable photons for quantum networks

Currently, photons are the most promising candidate as a resource for quantum communication protocols. They naturally propagate at the speed of light and their states can be transferred robustly over dozens of kilometers in fibers and even larger distances via satellite communication. In fact, modern telecommunication networks already rely on the exchange of information via optical fiber networks and could be adapted and re-used for quantum communication. However, such networks require not only the emerge of quantum repeaters, but a scalable, and size- and cost-efficient source of single photons at the telecom wavelength range.

In many of our projects we have investigated Bragg-reflection waveguides as a probable candidate for such a source. The large effective optical nonlinearity of AlGaAs semiconductor devices allows for bright emission of photon pairs at a wavelength around 1550nm via parametric down-conversion. This process, however, has a large disadvantage in its probabilistic nature. As such, it can never be a true on-demand single photon source. While this can be overcome by heralding of one of the photons of a pair and clever algorithms, the overall device efficiency will still suffer.  

In contrast, quantum dots act as artificial atoms within a semiconductor structure. Hence, their emission consists of single photons (aside from loss and background mechanisms). Furthermore, by incorporating a micro-laser on the same device as the quantum dots, it is possible to tune and excite them electrically. However, typically quantum dots in this material platform emit photons at a wavelength around 800-900nm, which is not compatible with the current telecommunication standard. A lot of effort is being put into tuning their emission wavelength via strain or electricity, which typically decreases the quality of the emitted photons.

In this project, we want to utilize the high effective nonlinearity of the AlGaAs Bragg-reflection waveguides grown by our collaborators at the University of Würzburg to convert photons emitted from an embedded quantum dot from 900nm to 1550nm using an external 2150nm laser via difference frequency generation on-chip. Utilizing in-situ EBL established by our collaborators at the University of Berlin, we can place a single quantum dot in the center of a waveguide crossing, which will allow us to separate the on-chip pump laser from the fields involved in the three-wave process, which are coupled to the waveguide via grating couplers.

 

Funding

The Austrian Science Fund funds our D-A-CH project "On-chip microlaser driven sources of indistinguishable photons for quantum networks" (I-5061, together with Tobias Huber, University of Würzburg and Stephan Reizenstein, University of Berlin)