In a quantum network, information is transmitted and processed using quantum mechanical objects called qubits. This revolutionary computational paradigm enables unprecedented information processing capabilities such as unbreakable cryptographic codes and exponential speedup of computational tasks. To achieve these remarkable capabilities requires the ability to both store qubits and create qubit-qubit interactions over long distances. Trapped spins in solids offer a remarkable system for storing quantum information, but these spins cannot easily interact with each other unless they are in close proximity. Photons provide a promising solution to this problem because they can be transmitted over long distances to create effective interactions between spins that are separated by long distances. However, long distance communication requires photons at optical frequencies while spins usually have resonances in the microwave frequency ranges. Because of this large frequency mismatch, photons typically don't interact with spins. In this project, the group will use optical cavities strongly coupled to a single spin trapped in a quantum dot to solve this problem. Cavities can create a strong effective spin-photon interface by enhancing light-matter interactions. These enhanced interactions open up the possibility for optical frequency photons to couple spins separated by long distances for quantum networks. The group will demonstrate photon mediated spin interactions using quantum dots coupled to optical cavities. Quantum dots are nanoscale structures that behave as artificial atoms. A quantum dot can capture an additional charge that behaves as a trapped spin qubit. By strongly coupling the quantum dot to a cavity, the group will develop a device called a quantum transistor, which forms the basic building block for complex quantum networks. Methods to utilize this device to implement quantum logic operations over long distances will be explored. These results could ultimately enable chip-integrated solid-state quantum devices that form the building blocks for long distance quantum networks.
A novel approach to spin-based quantum information processing where photons mediate effective spin-spin interactions will be developed. The fundamental building block for this approach is the spin-photon quantum transistor, which enables a single spin quantum bit (qubit) to apply quantum logic operations on a photon. This spin-photon transistor will be realized using a charged indium arsenide (InAs) quantum dot in a photonic crystal cavity. The charged dot contains an additional electron or hole that provides a spin degree of freedom with long coherence times. By coupling the quantum dot to a photonic crystal cavity, it is possible to attain a strong light-matter interface where the state of the spin modulates the cavity spectrum. This work will attain a better scientific understanding of the system and underlying decoherence mechanisms, and address practical device design and fabrication challenges for creating a scalable quantum architecture. This will provide a unique approach to spin-based quantum information processing that have many important advantages including the ability to couple arbitrary spins, implement gate operations on ultra-fast timescales, and create effective interactions over long distances for quantum networking. Novel devices that could enable quantum information processing in a chip-integrated device that is compact and scalable will be investigated. Major device design and fabrication challenges will be addressed that are crucial for scalable implementation including optimizing light-matter interactions in photonic crystals and aligning quantum dots spatially and spectrally with resonator modes. This could provide a direct pathway for developing highly compact and scalable quantum information processing on a semiconductor chip. This capability would have a revolutionary impact on information technology, enabling exponential faster computation, unconditionally secure communication, and high precision sensors that operate far below the classical noise limit. The devices developed could have major impact in other fields such as opto-electronics, nonlinear optics, and spintronics. In addition to the proposed research effort, the research program will support training of graduate and undergraduate students, and develop an outreach program to create interdisciplinary research opportunities for local high school students.
