Processing and Distillation of Multi-Qubit Block Codes for Fault-Tolerant Quantum Computation

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Quantum computers, which make use of quantum mechanical effects to process information, may soon be able to solve a number of important computational problems much more efficiently than any standard classical computer. However, quantum computers are difficult to build because they are very sensitive to quantum mechanical noise, or decoherence. This project will develop new ways to overcome decoherence by using novel types of quantum error-correcting codes. Efforts to circumvent decoherence are important because quantum error correction may enable fault-tolerant quantum computing with practical devices. This project will investigate ways to improve the efficiency of quantum error-correcting codes so that fewer physical resources are needed in order to implement error-correcting protocols. This would allow significant quantum computations to be done on smaller, more reasonably sized, devices. This project will also train students who participate in this research on fundamental topics in quantum information science, and this will help to prepare them for careers in academics or high technology industries.Even with fault-tolerant designs, constructing a quantum computer in the near term is a challenge because fault-tolerance requires a large amount of overhead: each logical quantum bit (qubit) needed by a perfect quantum computer could require thousands or even tens of thousands of physical qubits after being protected by codes. This project aims to dramatically reduce the overhead needed for fault-tolerant quantum computers. Instead of encoding each logical qubit separately, much more efficient codes are used that encode many logical qubits in the same codeword. Computing with such multi-qubit codes requires new techniques, which this project will study: logical teleportation, state distillation, and the development of different kinds of codes (such as quantum low-density parity-check codes), with different properties. If everything works well, the overhead could be reduced to as low as a few tens of physical qubits per logical qubit. This would allow nontrivial quantum computations by computers of a reasonable size.