# RIKEN cuts qubits for [[30,6,5]] code state preparation from 180 to 84 in simulation

- Published: 2026-11-15 07:51 UTC
- Section: Digital & Computing › Quantum technology
- Tags: [[30,6,5]] code, symplectic double code, fault-tolerant quantum computing, quantum error correction, logical qubits, state preparation, RIKEN, CSS code, magic state distillation
- URL: https://realtech.media/news/rt-fc3b815088cf8b76d19e1cb8_riken_cuts_qubits/

**RIKEN researchers designed a fault-tolerant encoder for a code with 6 logical qubits in 30 physical ones. Results come from circuit-level numerical simulation, not hardware.**

![RIKEN cuts qubits for 30,6,5 code state preparation from 180 to 84 in simulation](https://realtech.media/media/2026/11/eb632f13e11dcc25.webp)
*xenospectrum.com*

Researchers at RIKEN and the Japan Science and Technology Agency (JST) proposed a fault-tolerant method for preparing logical states of a [[30,6,5]] symplectic double code, according to a paper on arxiv.org [1]. The code stores 6 logical qubits in 30 physical qubits with code distance 5, an encoding rate of 20% [1]. The paper was published in Science Advances on November 13 (US time), as announced by RIKEN.

## Code and encoder

The [[30,6,5]] code is a Calderbank-Shor-Steane (CSS) code derived from a [[15,3,5]] non-CSS stabilizer code via the symplectic double construction [1]. Its encoding rate is higher than that of the bivariate bicycle code [[72,12,6]] (about 17%) and the trivariate bicycle code [[30,4,5]] (about 13%), while the code distance is kept [1]. Earlier studies of fault-tolerant computing with symplectic double codes covered distances up to 3 [1].

The proposed zero-state encoder extends Goto's minimum-overhead verification method for the Steane seven-qubit code [1]. It detects crucial errors during encoding with as few auxiliary qubits as possible [1]. Per RIKEN, the preparation circuit including verification needs 84 physical qubits, versus 180 in the conventional six-copy scheme. The 84 includes auxiliary qubits, so it is not comparable with the 30 qubits of the code itself. The number of two-qubit gates in the circuit for the plus state was reduced from 108 to 67.

## Simulation and limits

The authors evaluated the encoder with a circuit-level noise model and report logical error rates as low as those of the six-copy scheme [1]. After each physical CNOT, one of 15 two-qubit Pauli errors occurs with probability pcirc/15, and initialization and measurement carry a bit-flip error with probability pcirc. Memory errors and single-qubit gate errors are not included, and the results are not hardware measurements.

The authors also developed an arbitrary-state encoder that injects arbitrary quantum states into the code space [1]. Combined with basic fault-tolerant operations, such as magic state distillation, it supports universal quantum computation [1]. As next steps the authors name experimental implementation on trapped ions and neutral atoms, and optimization of circuit compilation.

## Sources

1. [arxiv.org](https://arxiv.org/html/2509.15457v1?utm_source=chatgpt.com)
2. [xenospectrum.com](https://xenospectrum.com/riken-quantum-code-state-preparation)

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Source: RealTech, realtech.media — https://realtech.media/news/rt-fc3b815088cf8b76d19e1cb8_riken_cuts_qubits/