Electronic and photonic devices that operate at the quantum limit will enable many new technologies, including ultrafast photonic switches, fundamentally secure communication, and quantum information processing. Efforts to engineer single-spin optoelectronic devices based on single quantum dots have faced significant challenges because ensembles of quantum dots always have a large inhomogeneous distribution in energy levels. The distribution in energy levels prevents spin bits based on single quantum dots from being integrated into multi-bit devices where each bit must be tuned into resonance with a discrete number of fixed laser or optical cavity wavelengths. Recent discoveries have demonstrated that quantum dot molecules have optical transitions whose wavelength can be tuned in situ over a range ten times larger than available in present single-spin bit designs. Moreover, these quantum dot molecules have other tunable optoelectronic and spin properties that can be engineered at the single-spin quantum level. This program supports development of a prototype bit, based on quantum dot molecules, that can isolate and control a single spin. Coherent and time-resolved magneto-optical techniques will be used to develop and demonstrate operation of this prototype bit and quantify the wavelength tunability that can be achieved. The results will provide a direct path to the production of scalable spin-based optoelectronic devices.
Intellectual Merit Single quantum dots are being actively pursued for integration into photonic and spin-based optoelectronic devices, but the inhomogeneous distribution of energy levels in ensembles of single quantum dots provides a fundamental barrier to scalability. This limitation can be overcome with the proposed new single-spin bit design based on quantum dot molecules. The key element of the proposed bit architecture is the use of indirect optical transitions whose wavelength is an order of magnitude more sensitive to applied electric field than the transitions of single quantum dots. Experiments have shown that these indirect transitions can have dipole matrix elements only a few times weaker than direct transitions. Spin initialization, manipulation and readout methods that utilize the indirect transitions will be developed. The range of wavelength tunability that can be achieved while maintaining spin initialization and readout will be measured. The bit design and spin-control protocols take advantage of recently discovered tunable spin interactions in quantum dot molecules to eliminate the need for transverse magnetic fields and incorporate nondestructive readout. The proposed work will develop and demonstrate a spin-bit design with at least an order of magnitude more wavelength tunability than existing spin-bit designs and consequently eliminate one of the largest obstacles to the scalable production of single-spin-based optoelectronic devices.
Broader Impacts The proposed work will enrich new courses already under development and provide crucial training for graduate students who will lead the next generation of electronic and photonic device research. The work will further broaden the exposure and opportunities for undergraduates and teacher-scholars participating in summer research programs. Funds from this program will provide local K-12 teachers with equipment to bring cutting-edge scientific concepts into their classrooms and inspire the next generation of students to pursue careers in STEM fields.
Intellectual Merits Quantum dots are tiny semiconductors, typically about 10,000 times smaller than the diameter of a human hair. Because of their extremely small size, quantum dots can contain only a few electrons and these electrons can only have specific energies. This "ladder" of allowed energy states is analogous to the energy levels of electrons bound to atoms. As a result, quantum dots are often called "artificial atoms." Unlike natural atoms, the size, shape and composition of quantum dots can be changed to tune the "atomic" properties. When two or more "artificial atoms" are brought together and allowed to interact, they can form quantum dot "molecules" that have unique and tunable properties. This is quite similar to the way natural atoms form many different molecules with different properties, but the formation of "molecular" states in quantum dot molecules can be tuned by applied stimuli like electric or magnetic fields. In addition to their charge, electrons also have a property called spin that has two possible directions: parallel or anti-parallel to an applied magnetic field. The two allowed spin projections could serve as the "bit" states for new optoelectronic devices that take advantage of unique quantum mechanical properties to create new "quantum computers" that could significantly outperform traditional computers. The idea of spin-based quantum computers has been around for well over a decade and there have been many proposals for devices that use light to initialize, manipulate and readout the bit states of single charges confined in single quantum dots. One of the biggest challenges that has hampered the development of scalable optically-driven spin-based quantum computing is the fact that the exact energy level of each quantum dot can not be precisely controlled. As a result, it is very hard to build a device that reliably interacts with each bit. With support from this project we have developed a new quantum computing device design that uses bit states composed of quantum dot molecules instead of single quantum dots. The advantage of this new design is that each quantum dot molecule can be tuned, in situ, to a specific energy level. This approach allows us to overcome the uncertainty in quantum dot energy levels that has plagued previous device designs. Moreover, this design provides a mechanism for turning on and off individual bit states, which significantly enhances device functionality. The approach also leverages some of the unique properties of the quantum dot molecules to overcome other problems that have plagued the development of scalable devices. We have designed a new device built around quantum dot molecules, performed extensive calculations of the required size and shape of the quantum dot molecule, computationally identified the best operational parameters for the device and simulated the bit performance that can be expected. We have taken several important steps toward realizing this bit design in practice by designing and growing a first generation of quantum dot molecules tailored for this application and characterizing charge interactions in quantum dot molecules. These charge interactions provide an important source of noise that must be managed to improve device performance, but also offer new opportunities for improved readout mechanisms. Broader Impacts In parallel with our research efforts, we have supported a program that disseminates educational modules for hands-on K-12 education. The original modules were prepared by teachers who spent the summer working with our research group. This program seeded a college-wide effort that has created 10 different modules that have been used in more than 10 schools and at informal science education events reaching in excess of 10,000 students. Funds from this program supported the acquisition of additional modules and science demonstration equipment that expands our ability to disseminate science content to the local community. All modules, including those funded by this award, are available for free loan through the UD College of Engineering Lending Library.
