Silicon Nanophotonic Platform Developed from Coherent Quantum Feedback Control Networks

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Coherent quantum feedback controls have recently emerged as a new paradigm for the precise manipulation of dynamics in complex quantum systems. This has now created efficient theoretical modelling and simulation tools and has allowed for more practical considerations.

A team of researchers from the United States has now developed a new real-world application, through an integrated silicon nanophotonic platform that utilises scalable coherent quantum feedback control networks.

In recent years, coherent quantum feedback controls (QCFC) have become a rapidly interesting, interdisciplinary field for quantum control and quantum engineering applications. Many recent advances, such as frameworks for input-output theories, modelling of quantum network systems, electromagnetic fields, quantum information tasks, ultra-low-power optical processing elements in optical switching applications and language software tools for photonic circuits, have helped to propel the QCFC field into a position where the design and analysis of optical systems is now possible.

QCFC applications possess much more potential when there are many optical elements interconnected into configurable networks. The implementation of bulk-scale optics has been proven to be impractical when trying to form complex networks, and an integrated multi-component approach is required.

The integration of multi-nanoscale components into a photonic device, is not only preferable over bulk-scale elements, but also improves the reproducibility, mass production capability, long-term optical path and phase stability of the device.

Currently, silicon integrated nanophotonic materials are seen as the leading platform for constructing large scale QCFC networks. The team of US researchers have not only described the current trends and a detailed analysis of QCFC silicon nanophotonics, but have also fabricated a simple on-chip QCFC network that is composed of two coupled cavities and a feedback loop.

The researchers have coined the device as a “coupled cavity device” (CCD) and was inspired by other research that implemented a disturbance rejection network into a bulk optics device. The device was fabricated using CMOS compatible silicon nanowires that allow for the guiding of light through total internal reflections and a high index contrast.

The system is composed of two thermally controlled ring resonators coupled with a silicon nanowire bus waveguide, with an integrated thermo-optic phase shifter on each waveguide. There are two ports at each cavity, creating a total of four, but more can be theoretically added for modelling purposes. In the first cavity, a tuneable input drive, produced from a telecom laser, is located within the first port of the device and the intensity of the output is measured through the second output by an off-chip power meter. Both ports within the second cavity are unmonitored with a vacuum input. A feedback signal between the ports is created from a propagating signal from cavity 2 (the plant system) to cavity 1 (the controller).

The researchers controlled the device through the thermo-optic phase shifters with externally applied voltages, where one tunes the resonance frequency and the other induces a phase shift in the feedback signal.

The coupled cavities in the device behave in a similar manner to electromagnetically induced transparencies (EIT), where two degenerate resonances interfere and create a sharp null within the transmission spectrum.

To get a disturbance rejection, similar to that in bulk optics, the researchers had to surpass the transmission in all wavelengths, whilst keeping every other parameter fixed. The waveguide surface roughness of the silicon nanowires in the device also allows for an efficient scattering of photons through a linear loss mechanism.

One of the main points that the researchers discovered, was that when the controls were only applied to a single component, the rest of the components were affected. This is known as cross-talk, and is the result of the device being small and the controls being of a physical nature. The manipulation of the local in-situ controls, not only allows for the monitoring and control of excess heating in the device, but also minimises any other side effects.

The incorporation of silicon-based materials into QCFC photonic applications is expected to expand the integrated quantum optics toolbox, and enable the construction of increasingly complex quantum optical networks.

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This information has been sourced, reviewed and adapted from materials provided by SpingerOpen.