The objectives of this research are to study and control the spin properties of semiconductor quantum dots using simultaneous optical and microwave field excitation. The approach is to use optical fields to create atomic transitions while simultaneously using a microwave field to induce spin flips, allowing access to atomic transitions that are normally forbidden due to spin selection rules. This method will be used to achieve coherent control of dark excitons in an indium arsenide quantum dot .
Intellectual merit: The proposed research will enable the study of fundamental spin properties in semiconductors, which is of central importance to a broad range of research fields including condensed matter physics, quantum optics, quantum information, and spintronics. It will also allow the probing and control dark of exciton states whose properties are poorly understood. Control of dark excitons further provides a method for storage and re-release of single photons, and can serve as a quantum memory for future quantum computers and quantum networks, as well as new opto-electronic and magneto-optic devices that use quantum properties to achieve improved functionality
Broader impact: This research will advance scientific knowledge in a broad range of fields that include optics, microwave engineering, and atomic physics. It could pave the wave for future exponentially faster quantum computers and unconditionally secure quantum networks. It will also provide research and educational opportunities for graduate students, and promote undergraduate and high school research through participation in the Summer Research Program at the Institute for Research in Electronics and Applied Physics.
Quantum information processing relies on the ability to manipulate single quantum systems that are highly isolated from their environment. The ability to store, manipulate, and read out quantum coherence in solid-state structures is considered an essential requirement for developing scalable and compact quantum devices. Quantum dots provide an ideal material platform for performing all these capabilities. Quantum dots are nanoscale semiconductor structures that behave as artificial atoms. They can also possess spin degrees of freedom that serve as ideal candidates for quantum bits, the basic building blocks of quantum computers. In orders to use spins as quantum bits, we need a method to manipulate them. All optical manipulation methods exist, but these methods suffer from power induced broadening that decoheres the spin on rapid timescales. Microwaves are ideal for manipulating spin because they can directly implement quantum logic operation by direct electron spin resonance. However, to implement such an approach requires a method to simultaneously excite dots with optical and microwave excitation. The goal of this project is to develop methods to control quantum dots using simultaneous excitation of optical and microwave signals. We originally set out to use these fields to access dark exciton states, which are highly stable quantum excitations that cannot be excited optically. We developed a cryogenic measurement setup that can cool quantum dot samples below 4 K temperature, apply magnetic fields of up to 9 T, and excite the system with microwaves of up to 10 GHz frequency. An image of the setup has been included. We have also developed a chip-integrated device that can isolate single quantum dots optically and excite them with microwave frequency radiation (image also included) Using this system, we attempted to find signatures of dark excitons. However, we determined after some effort that the emission from the dark exciton was too weak due to the long lifetimes of these states even at high magnetic fields. Instead, we moved to using charged quantum dots that possess an additional spin carrier. We were able to identify these spin-carrying quantum dots much more easily directly from their fluorescence spectrum. We were also able to control the spin of the quantum dot using optical pulses, indicating that we are truly isolating a single spin. We are now in the process of trying to observe electron spin resonance in these systems using direct microwave excitation. Success of this effort will enable us to control the spin with microwaves, and read the spin state optically. The results we present have important applications in the area of quantum information processing. They could ultimately lead to compact and scalable quantum devices in a chip-integrated platform. Furthermore, the devices and measurement capabilities we have realized will enhance our physical understanding of the behavior of spin in solids, elucidating decoherence mechanisms and new ways to achieve control and readout. These capabilities will have broad applicability in the fields of quantum information processing, spintronics, opto-electronics, and nanophotonics.
