Ballistic bosonic noise suppression with hybrid qumode-qubit rotation gates

TL;DR

This paper introduces a hybrid qumode-qubit interferometric scheme that suppresses thermal noise in bosonic quantum codes without active error correction, leveraging parity monitoring and a single controlled Fourier gate for improved resilience.

Contribution

The authors propose a novel hybrid CV-DV interferometer using a single qubit ancilla and controlled Fourier gates to suppress thermal noise effects in bosonic codes without active correction.

Findings

Suppresses noise to order η^2 without active correction

Achieves success probability >0.5 under certain conditions

Simplifies to a single qumode rotation and CF gate for parity codes

Abstract

Noise suppression is of paramount importance for reliable quantum information processing and computation. We show that for any single-mode bosonic code (qumode) corrupted by thermal~noise at rate~$\eta$ and mean \mbox{excitation}~$\bar{n}$, a hybrid continuous-discrete-variable~(CV-DV) interferometer using only a single qubit ancilla~(DV) and two controlled~Fourier~(CF) gates sandwiching the noise channel suppresses its effects to $\mathcal{O}(\eta^2)$ \emph{without} any active error correction or destructive measurements of the encoded state and with high success probabilities~$>0.5$ if~$\eta(1+\bar{n})<0.5$. This suppression scheme works by conditionally monitoring the photon-number parities after the interferometer. Bosonic codes with two logical states of the same photon-number parity (like-parity codes) are \emph{completely resilient} to DV amplitude- and phase-damping ancilla noise. For such codes, the interferometer simplifies to the use of a qumode rotation gate and a \emph{single} CF~gate. This presents a clear advantage of our CF-gate-based error suppression scheme over previously-proposed ``bypass'' protocols, where qubit information transferred to the DV mode is readily corrupted by damping~noise. Finally, we present a simple extension to direct communication of qumode states between two parties over a noisy channel using a preshared DV entangled state, by implementing a CF gate in the first laboratory and its inverse in the other. Such a communication protocol achieves a similar fidelity performance at the same success rate as the single-party case, but with greater resilience to the ancilla noise than DV~teleportation. Resource-efficient multi-qubit codes that depend on a few essential long-range interactions can benefit from it.