Collaborative Research: CIF: Medium: QODED: Quantum codes Optimized for the Dynamics between Encoded Computation and Decoding using Classical Coding Techniques

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Leveraging quantum phenomena in nature for processing information promises exciting gains and advantages in several tasks such as computing and communications. At the minute scale where such phenomena occur, however, the information carriers, such as atoms, are fragile and highly susceptible to noise. In order to build scalable and reliable quantum systems, one needs to mitigate noise through error-correction techniques. A quantum error correcting code (QECC) encodes data in a larger mathematical space so that the redundancy can be use to detect and correct errors. For quantum computation, one needs to regularly do such error correction, which tries to preserve data as it is, while also performing active computation to process the data, which keeps altering it. In order to be fault-tolerant, such a quantum computer needs to find ways of carefully balancing computation and decoding (error correction) while keeping resource requirements at a minimum. This is a very challenging task, and this project develops methods to study modern QECCs that are optimized for the dynamics between encoded computation and decoding using techniques from classical error-correction theory. The team will also nurture young talent in these areas and inspire underrepresented groups to join the growing quantum workforce.The celebrated threshold theorem of QEC established that quantum information can be protected indefinitely as long as each hardware component meets a fidelity threshold that is a function of the specific QECC. This project will provide a concrete understanding of desirable code structure, motivated by practical constraints, and hence address both thresholds and computational overhead as a function of this structure. Specifically, the intellectual contributions of the proposed research plan can be summarized as follows: (1) Recent results of investigators have produced systematic methods to synthesize logical operations on stabilizer codes. The team of researchers will begin by applying such methods to quantum low-density parity-check (QLDPC) codes in order to understand their utility for logical computation. (2) The team will explore strategies such as concatenation and lifting algebraic protographs to combine the best aspects of QLDPC codes and algebraic codes such as quantum Reed-Muller codes. (3) For QEC, it has been observed that iterative decoders with symmetric message updates fail on QLDPC codes due to cycles and "quantum" trapping sets related to error degeneracy. The team will leverage their classical expertise to develop single-shot algorithms based on message-passing with noisy syndromes, understand the effect of non-linear message updates, and analyze error floors. (4) The investigators will also determine how the desirable graph structure for such methods interplay with realizing logical operations on these hybrid QECCs. Finally, the team will combine these insights with other promising approaches such as measurement-based quantum computation, which will make the results apply across a wide array of technologies.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.