Noisy Logic

Kento Tsubouchi, Hyukgun Kwon, Liang Jiang +1 more

Recent progress in fault-tolerant quantum computing suggests that leveraging error-syndrome information at the logical layer can substantially improve performance, including the estimation of logical observables from noisy states. In this work, based on quantum estimation theory, we develop an information-theoretic framework to quantify the utility of error syndromes for noisy logical observable estimation. We distinguish two operational regimes of such syndrome-aware protocols: classical protocols, in which the logical measurement basis is fixed and syndrome information is used only in classical post-processing, and quantum protocols, in which the logical quantum control can be tailored to depend on the observed error syndrome. For classical syndrome-aware protocols, we prove a universal limitation: on average, syndrome information can improve the effective logical error rate by at most a factor of two, implying at most a quadratic reduction in sampling overhead. In contrast, once syndrome-conditioned quantum control is permitted, we demonstrate that the effective logical error rate decays exponentially with the number of code blocks. These findings provide fundamental guidance for designing future fault-tolerant architectures that actively exploit syndrome records rather than discarding them after decoding.

Valerie Jamieson

AUGUST STERN

Christiane L. Kameni Ngassa, Valentin Savin, Elsa Dupraz +1 more

This paper investigates the behavior of the Min-Sum decoder running on noisy devices. Our aim is to evaluate the robustness of the decoder to computation noise caused by the faulty logic in the processing units. This type of noise represents a new source of errors that may occur during the decoding process. To this end, we first introduce probabilistic models for the arithmetic and logic units of the finite-precision min-sum decoder and then carry out the density evolution analysis of the noisy min-sum decoder. We show that, in some particular cases, the noise introduced by the device can help the min-sum decoder to escape from fixed points attractors and may actually result in an increased correction capacity with respect to the noiseless decoder. We also point out a specific threshold phenomenon, referred to as functional threshold, which accurately describes the convergence behavior of noisy decoders. The behavior of the noisy MS is demonstrated in the asymptotic limit of the code length through a noisy version of density evolution and is also verified in the finite-length case by Monte Carlo simulations.

Himanshu Chaudhary, Biplab Mahato, Lakshya Priyadarshi +3 more

We have developed a software library that simulates noisy quantum logic circuits. We represent quantum states by their density matrices in the Pauli basis, and incorporate possible errors in initialization, logic gates, memory and measurement using simple models. Our quantum simulator is implemented as a new backend on IBM’s open-source Qiskit platform. In this paper, we provide its description, and illustrate it with some simple examples.