Controlling Quantum States using Phononic Crystals

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Repetition. There is power and beauty in repetition. The brilliant colours floating on the wings of certain butterfly species are not produced by dyes or pigments, but rather from repetitious features in the scales of its wings. Consider a diamond—the repetitive arrangement of atoms in its crystal determines why diamond is so beautifully transparent, but a single flaw can trap light so that it cannot escape. Repetition is not only aesthetically pleasing, but so captivating in art and nature that each individual, identical cell can be analyzed with unwavering interest. However, a small flaw or interruption of the repetition is powerful enough to trap one's gaze.The research program proposed here will harness that same foundation of repetition used to reflect light producing brilliant colours and instead apply it to sound. The sound comes in the form of surfaces acoustic waves (SAWs), which are vibrations of a solid skimming across its surface. In fact, they are smaller scaled versions (by nine orders of magnitude) of the most damaging components of an earthquake: the Rayleigh waves. The SAWs can then be thought of as “nano-earthquakes” that can fit on a microchip and be electrically generated. To create our butterfly wing on a chip, a repetitive, periodic array of nanofabricated holes are drilled, called a phononic crystal (PnC). Like the wings of a butterfly that only reflect light at certain wavelengths, the periodicity of the PnC array and the size of its holes dictate what wavelengths of SAWs will be affected. More interestingly, the PnC can be interrupted by omitting a row of holes in the array. This interruption can then trap SAWs and force them to travel along it inside the phononic crystal.In past work, we have shown that nano-earthquakes are able to move and manipulate electrons while preserving their quantum mechanical spin state. In a manner similar to a surfer riding an ocean wave, the SAW creates a moving electric field that can push electronic charges across the microchip. To transport quantum information, the electron can be initialized in a quantum “up” or “down” spin state, correlating to binary “0” and “1”, which can then be moved along by the SAW.The goals of this proposed program are twofold. The first is to design, fabricate, and test new PnC designs to control the path of a SAW. In particular, PnCs in GaAs-based systems will be targeted so that our acoustic systems can eventually couple to semiconductor nanostructures. As SAW devices are widely used in mobile communications today, continued research into complex PnC-based geometries may extend to develop new types of SAW devices for mobile technologies based on PnCs. The second and primary goal of the research proposal is to use SAWs to coherently transport electrons and their spin information along a PnC waveguide. Ultimately, the waveguides would become more complex, thus creating a novel phononic circuit through which quantum information can flow and be computed. The potential for this platform is unique to Queen's University, where Dr. Stotz and his group have expertise in both modelling, designing, and fabricating PnC structures as well as using SAWs to coherently transport electron spins.Through the study of PnCs and spintronics, students will gain practical experience in a wide variety of areas such as semiconductor device fabrication, device modelling, rf-electronics, spectroscopy, and photonics. Furthermore, the students will be involved with project planning and assessment while working in a team-oriented environment. In addition to learning about power and beauty, HQP will develop a strong background for a broad range of careers in the photonic and IT segments that are important for economic expansion in Canada.