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arXiv:2607.13105

Biased-Noise Quantum Reed--Solomon Codes and a Tornado Concatenation for Cat Qubits

Cheng-You Ho, Justin Luo, Henry Ng, Simba Shi, Daniel Wang

quant-phcs.ITSubmitted Jul 14, 2026• Announced Jul 15, 2026• 0 citations• Revised Jul 18, 2026 (v2)
v1 · Jul 14, 2026v2 · Jul 18, 2026v3 · Jul 21, 2026
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Abstract

Dissipative cat qubits exponentially suppress one Pauli error channel with the mean photon number, leaving the conjugate bit-flip error as the dominant failure mode. This strong noise bias makes the full machinery of general quantum error correction unnecessary: a code need only protect against a single error type, and any classical linear code can be promoted to a Clifford stabilizer code that does exactly this. We use this observation to build a \emph{bit-flip-only} quantum Reed--Solomon (RS) code. Starting from the maximum-distance-separable RS code $[7,3,5]$ over $\GF(2^3)$, we binary-expand it to the linear code $[21,9,6]$ over $\GF(2)$ and realize it as a $[[21,9,d_X=6,d_Z=1]]$ bit-flip code whose stabilizers are products of $Z$ operators. Because no phase-flip correction is attempted, the construction discards the redundancy that standard quantum RS codes spend on correcting $Z$ errors -- which a strongly biased cat qubit renders unnecessary -- and yields a shallow Clifford circuit that samples directly in Stim. Errors are decoded by an optimal bounded-distance syndrome-lookup table. We then introduce a \emph{Tornado} architecture: a two-layer concatenation that wraps every position of the outer RS code in an inner distance-three repetition code, yielding a $[[63,9,18]]$ code decoded by a two-stage inner majority vote and outer lookup decoder. Monte-Carlo simulation shows that at a physical bit-flip rate $p=0.1$ the Tornado code reaches a logical error rate $\pL \approx 5.3\times10^{-3}$, below both parent codes, and that its logical error rate scales as $\pL \propto p^{6}$ at low $p$, in contrast to $p^{2}$ for the repetition code and $p^{3}$ for the standalone RS code. We give the exact construction, the error and circuit model, an asymptotic scaling analysis, and an honest account of the overhead cost and single-shot assumptions.

Comments
All authors contributed equally. 12 pages, 3 figures, 2 tables. Winning project of MIT iQuHACK 2026, Alice & Bob Track

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Dissipative cat qubits exponentially suppress one Pauli error channel with the mean photon number, leaving the conjugate bit-flip error as the dominant failure mode. This strong noise bias makes the full machinery of general quantum error correction unnecessary: a code need only protect against a single error type, and any classical linear code can be promoted to a Clifford stabilizer code that does exactly this. We use this observation to build a \emph{bit-flip-only} quantum Reed--Solomon (RS) code. Starting from the maximum-distance-separable RS code [7,3,5][7,3,5][7,3,5] over \GF(23)\GF(2^3)\GF(23), we binary-expand it to the linear code [21,9,6][21,9,6][21,9,6] over \GF(2)\GF(2)\GF(2) and realize it as a [[21,9,dX=6,dZ=1]][[21,9,d_X=6,d_Z=1]][[21,9,dX​=6,dZ​=1]] bit-flip code whose stabilizers are products of ZZZ operators. Because no phase-flip correction is attempted, the construction discards the redundancy that standard quantum RS codes spend on correcting ZZZ errors -- which a strongly biased cat qubit renders unnecessary -- and yields a shallow Clifford circuit that samples directly in Stim. Errors are decoded by an optimal bounded-distance syndrome-lookup table. We then introduce a \emph{Tornado} architecture: a two-layer concatenation that wraps every position of the outer RS code in an inner distance-three repetition code, yielding a [[63,9,18]][[63,9,18]][[63,9,18]] code decoded by a two-stage inner majority vote and outer lookup decoder. Monte-Carlo simulation shows that at a physical bit-flip rate p=0.1p=0.1p=0.1 the Tornado code reaches a logical error rate \pL≈5.3×10−3\pL \approx 5.3\times10^{-3}\pL≈5.3×10−3, below both parent codes, and that its logical error rate scales as \pL∝p6\pL \propto p^{6}\pL∝p6 at low ppp, in contrast to p2p^{2}p2 for the repetition code and p3p^{3}p3 for the standalone RS code. We give the exact construction, the error and circuit model, an asymptotic scaling analysis, and an honest account of the overhead cost and single-shot assumptions.
Biased-noise qubits, in which one type of error (e.g. XXX- and YYY-type errors) is significantly suppressed relative to the other (e.g. ZZZ-type errors), can significantly reduce the overhead of quantum error correction. Codes such as the rectangular surface code or XZZX code substantially reduce the qubit overhead under biased noise, but they still face challenges. The rectangular surface code suffers from a relatively low threshold, while the XZZX code requires twice as many physical qubits to maintain the same code distance as the surface code. In this work, we introduce a 2D local code construction that outperforms these codes for noise biases η≥7×104η\ge 7\times10^{4}η≥7×104, reducing the qubit overhead by over 50% at pZ=10−3p_Z=10^{-3}pZ​=10−3 and η=2×106η= 2 \times 10^6η=2×106 to achieve a logical error rate of 10−1210^{-12}10−12. Our construction relies on the concatenation of two classical codes. The inner codes are repetition phase-flip codes while the outer codes are high-rate bit-flip codes enabled by their implementation at the logical level, which circumvents device connectivity constraints. These results indicate that under sufficiently biased noise, it is advantageous to address phase-flip and bit-flip errors at different layers of the coding scheme. The inner code should prioritize a high threshold for phase-flip errors, while the bit-flip outer code should optimize for encoding rate efficiency. In the strong biased-noise regime, high-rate outer codes keep the overhead for correcting residual bit-flip errors comparable to that of the repetition code itself, meaningfully lower than that required by earlier approaches.
To solve problems of practical importance1,2, quantum computers probably need to incorporate quantum error correction, in which a logical qubit is redundantly encoded in many noisy physical qubits3–5. The large physical-qubit overhead associated with error correction motivates the search for more hardware-efficient approaches6–18. Here, using a superconducting quantum circuit19, we realize a logical qubit memory formed from the concatenation of encoded bosonic cat qubits with an outer repetition code of distance d = 5 (ref. 10). A stabilizing circuit passively protects cat qubits against bit flips20–24. The repetition code, using ancilla transmons for syndrome measurement, corrects cat qubit phase flips. We study the performance and scaling of the logical qubit memory, finding that the phase-flip correcting repetition code operates below the threshold. The logical bit-flip error is suppressed with increasing cat qubit mean photon number, enabled by our realization of a cat-transmon noise-biased CX gate. The minimum measured logical error per cycle is on average 1.75(2)% for the distance-3 code sections, and 1.65(3)% for the distance-5 code. Despite the increased number of fault locations of the distance-5 code, the high degree of noise bias preserved during error correction enables comparable performance. These results, where the intrinsic error suppression of the bosonic encodings enables us to use a hardware-efficient outer error-correcting code, indicate that concatenated bosonic codes can be a compelling model for reaching fault-tolerant quantum computation. Bosonic qubits can be engineered to feature intrinsic protection against certain kinds of errors, which makes quantum error correction across many bosonic qubits possible with less overhead.
Qubits with strongly biased noise, in which phase-flip errors are orders of magnitude more frequent than bit-flips, arise both naturally, as in electron and nuclear spins, and by engineering, as in stabilized cat qubits. This noise structure holds the promise of reducing the daunting hardware overhead of fault-tolerant quantum computing, but exploiting it requires physical operations that do not convert frequent phase-flips into rare bit-flips. In this review, we analyze the most prominent fault-tolerant protocols for biased-noise qubits, organized according to the available set of such bias-preserving operations. When this set is restricted to the CZ gate together with preparation and measurement in the X basis, we show that the complexity of the required syndrome extraction gadgets essentially cancels the benefit of the noise bias: at experimentally relevant error rates, one may as well ignore the bias and rely on standard error correction designed for depolarizing noise. The situation changes drastically when a bias-preserving CX gate is available: the hierarchy of errors can then be reflected in the structure of the code, with frequent phase-flips corrected by a dedicated high-threshold code and rare bit-flips by concatenation with a high-rate code. The same hierarchy also enables hardware-efficient preparation of magic states. Finally, as a bias-preserving CX is forbidden in naturally biased platforms and challenging in engineered ones, we present a measurement-based architecture in which a high-fidelity quantum non-demolition readout of multi-qubit Pauli Z operators takes its place, extending these overhead reductions to a much broader range of physical platforms.
We estimate and analyze the error rates and the resource overheads of the repetition cat qubit approach to universal and fault-tolerant quantum computation. The cat qubits stabilized by two-photon dissipation exhibit an extremely biased noise where the bit-flip error rate is exponentially suppressed with the mean number of photons. In a recent work we suggested that the remaining phase-flip error channel could be suppressed using a one-dimensional repetition code. Indeed, using only bias-preserving gates on the cat qubits, it is possible to build a universal set of fault-tolerant logical gates at the level of the repetition cat qubit. In this paper we perform Monte Carlo simulations of all the circuits implementing the protected logical gates, using a circuit-level error model. Furthermore, we analyze two different approaches to implement a fault-tolerant Toffoli gate on repetition cat qubits. These numerical simulations indicate that very low logical error rates could be achieved with a reasonable resource overhead, and with parameters that are within the reach of near-term circuit QED experiments.
Bosonic "cat qubits" encoded in superpositions of well-separated coherent states |+a⟩ and |-a⟩ are attractive for quantum error correction because single-photon loss acts as a phase-flip error while the dominant, hardware-limiting bit-flip error is suppressed exponentially in the mean photon number |a|². We ask whether an analogous noise bias can be engineered using the orbital angular momentum (OAM) of light instead of its amplitude, encoding a qubit in a superposition of two well-separated OAM eigenstates |+L⟩ and |-L⟩. We model the dominant OAM channel noise—local mode-mixing induced by fiber imperfections or wavefront aberrations, which couples only adjacent topological charges (Δl = ±1)—as a continuous-time random walk on the integers. We define an operational experimental protocol to extract the channel hopping rate directly from the linear spatial growth of the OAM variance Var(l) = γz. We show analytically and numerically that this local, nearest-neighbor character converts almost all population leakage out of the code space into a detectable erasure, while the probability of an undetected logical bit-flip is suppressed as exp(-2L² / γz), a direct structural analog of the exp(-2|a|²) bias of amplitude-phase cat qubits. This revised version (v2) corrects the erasure-probability expression using an exact closed form (Skellam/Bessel identity), adds an explicit resource-overhead analysis quantifying the repetition cost of the scheme, and discusses the impact of a mode-dependent hopping rate γ(l) on the achievable suppression. We argue that periodic syndrome extraction using existing phase-locked OAM mode sorters could realize the required error-detection step without a dedicated nonlinear dissipative stabilizer. ---Keywords: Quantum Error Correction, Cat Qubits, Orbital Angular Momentum (OAM), Noise Bias, Erasure Channel, Random Walk, Frugal Quantum Hardware, Open Science.Repository contents: Includes manuscript PDF and full Python simulation package (NumPy/SciPy/Matplotlib).
Cat qubits, a type of bosonic qubit encoded in a harmonic oscillator, can exhibit an exponential noise bias against bit-flip errors with increasing mean photon number. Here, we focus on cat qubits stabilized by two-photon dissipation, where pairs of photons are added and removed from a harmonic oscillator by an auxiliary, lossy buffer mode. This process requires a large loss rate and strong nonlinearities of the buffer mode that must not degrade the coherence and linearity of the oscillator. In this work, we show how to overcome this challenge by coloring the loss environment of the buffer mode with a multipole filter and optimizing the circuit to take into account additional inductances in the buffer mode. Using these techniques, we achieve near-ideal enhancement of cat-qubit bit-flip times with increasing photon number, reaching over 0.1 s with a mean photon number of only 4. Concurrently, our cat qubit remains highly phase coherent, with phase-flip times corresponding to an effective lifetime of T 1 , eff ≃ 70 μ s , comparable with the bare oscillator lifetime. We achieve this performance even in the presence of an ancilla transmon, used for reading out the cat-qubit states, by engineering a tunable oscillator-ancilla dispersive coupling. Furthermore, the low nonlinearity of the harmonic oscillator mode allows us to perform pulsed cat-qubit stabilization, an important control primitive, where the stabilization can remain off for a significant fraction (e.g., two-thirds) of a 3 μ s cycle without degrading bit-flip times. These advances are important for the realization of scalable error correction with cat qubits, where large noise bias and low phase-flip error rate enable the use of hardware-efficient outer error-correcting codes.