Sharding blockchains enhance scalability by distributing the workload across multiple shards. Within sharding blockchains, securely and efficiently processing cross-shard user requests remains a pivotal challenge. Traditional sharding blockchain protocols execute cross-shard requests under a processing paradigm that treats each user request as a monolithic entity to preserve its atomicity, inherently introducing significant intra-shard BFT execution overhead and cross-shard communication overhead. We propose QuarkShard, a sharding protocol that breaks away from this traditional paradigm to achieve intra-shard and cross-shard overheads below the lower bounds imposed by conventional approaches. QuarkShard decomposes cross-shard requests into single-input-shard transactions, where each transaction requires consensus and processing within only a single shard, thereby substantially reducing total request processing overhead. Meanwhile, through a carefully designed request decomposition algorithm, QuarkShard guarantees fund atomicity or location atomicity during request execution. QuarkShard also enhances the security of blockchain sharding systems against MEV attacks and flooding attacks. Empirical evaluation across a wide-area network spanning four AWS regions demonstrates that QuarkShard achieves superior performance, attaining a peak throughput of $1024$k requests per second with sub-$4$-second latency. Benchmark comparisons show that QuarkShard achieves up to $210.7\%$ and $314.8\%$ of the maximum throughputs of Kronos and 2PC, respectively.

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