Decoding Multimode Gottesman-Kitaev-Preskill Codes with Noisy Auxiliary States

In order to achieve fault-tolerant quantum computing, we make use of quantum error correction schemes designed to protect the logical information of the system from decoherence. A promising way to preserve such information is to use the multimode Gottesman-Kitaev-Preskill (GKP) encoding, which encodes logical qubits into several harmonic oscillators. In this work, we focus on decoding the measurements obtained from Steane-type quantum error correction protocols for multimode GKP codes. We propose a decoder that considers the noise present on the auxiliary states, more specifically by tracking the correlations between errors on different modes spreading throughout the error-correction circuit. We show that leveraging the correlations between measurement results and the actual error affecting the multimode GKP state can decrease the logical error probability by at least an order of magnitude, yielding more robust quantum computation.