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Paper 1

Efficient magic state cultivation with lattice surgery

Yutaka Hirano, Riki Toshio, Tomohiro Itogawa, Keisuke Fujii

Year

2025

Journal

arXiv preprint

DOI

arXiv:2510.24615

arXiv

2510.24615

Magic state distillation plays a crucial role in fault-tolerant quantum computation and represents a major bottleneck. In contrast to traditional logical-level distillation, physical-level distillation offers significant overhead reduction by enabling direct implementation with physical gates. Magic state cultivation is a state-of-the-art physical-level distillation protocol that is compatible with the square-grid connectivity and yields high-fidelity magic states. However, it relies on the complex grafted code, which incurs substantial spacetime overhead and complicates practical implementation. In this work, we propose an efficient cultivation-based protocol compatible with the square-grid connectivity. We reduce the spatial overhead by avoiding the grafted code and further reduce the average spacetime overhead by utilizing code expansion and enabling early rejection. Numerical simulations show that, with a color code distance of 3 and a physical error probability of $10^{-3}$, our protocol achieves a logical error probability for the resulting magic state comparable to that of magic state cultivation ($\approx 3 \times 10^{-6}$), while requiring about half the spacetime overhead. Our work provides an efficient and simple distillation protocol suitable for megaquop use cases and early fault-tolerant devices.

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Paper 2

Absence of quantum Darwinism as a resource in secure quantum communication and computation

Bishal Kumar Das, Sourav Manna, Vaibhav Madhok

Year

2025

Journal

arXiv preprint

DOI

arXiv:2510.03225

arXiv

2510.03225

The emergence of classical world from underlying quantum mechanics is characterized by not only vanishing quantum correlations but also an unfolding of objectivity also known as quantum Darwinism. We show that the absence of this objectivity has a quantum advantage in cryptography and also provides the crucial missing link in efficient classical simulation of quantum circuits with zero discord. For this purpose, we consider a model of mixed state quantum computation where one is promised concordant states at all stages of the quantum circuit. A concordant quantum state has zero discord with respect to any part and there exists a basis made up of a tensor product of orthonormal local subsystem basis in which the density matrix is diagonal. Efficient classical simulation of concordant computation has surprisingly been an outstanding question in quantum information theory. We argue that a key ingredient of an efficient classical simulation algorithm, a knowledge of the local basis in which the multi-party state is diagonal, is made available by quantum Darwinism. Concordant states in the absence of quantum Darwinism cannot be efficiently simulated by existing methods and give a cryptographic advantage in communication. We show this by giving a protocol for secure quantum communication that exploits this insight. Our work also has implications for the quantum-classical border and we discuss how objectivity emerging out of Darwinism demarcates this border in three ways - empirical based on our observations and experience of objectivity, information theoretic due to the absence of any quantum correlations and lastly computational in the sense discussed above. Lastly, we show that the quantum-classical boundary as drawn by quantum Darwinism as well by what can be simulated efficiently in a mixed state quantum computation aligns with the boundary given by Hardy

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