arXiv subjects

Zi-Han Chen

Publications and source records attributed to Zi-Han Chen.

8 recordsLinked to original sources

arXiv2026

Hierarchical Logical Processor on the Rotated Surface Code with Shuttle Buses

Quantum platforms with beyond-planar connectivity provide new opportunities for fault-tolerant quantum computation (FTQC). While quantum low-density parity-check (qLDPC) codes offer high encoding efficiency, their direct implementation requires non-local couplings in every round of syndrome extraction, incurring additional physical error and implementation complexity. To reduce the frequency of such couplings, we propose the Hierarchical Logical Processor (HLP), which concatenates a high-rate quantum CSS code with the rotated surface code (RSC). HLPs can achieve beyond-RSC encoding efficiency while requiring long-range connectivity only once every $\Theta(d_0)$ rounds of level-0 error correction, where $d_0$ denotes the base-code distance, substantially reducing the frequency of non-local couplings relative to direct implementations of qLDPC codes. HLPs introduce elongated RSC patches called shuttle buses. Using transversal hybrid-unit CNOT gates, a single shuttle bus can simultaneously couple to multiple standard RSC patches. This capability enables efficient level-1 syndrome extraction with suppressed level-1 error correlations and supports highly parallel logical Pauli measurements. We perform circuit-level simulations of several concrete HLP constructions and benchmark both logical memory and logical Pauli measurement performance. At a physical error rate of $10^{-3}$, an HLP based on the [[256,194,4]] code achieves 3-4 times higher qubit efficiency than the standard RSC. Compared with the yoked surface code on the same level-1 code, this HLP reduces the space overhead per logical qubit by 100-200 physical qubits and shortens the logical error-correction cycle time by a factor of 20-30.

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arXiv2026

Squeezed-slit Bohr-Einstein Interferometer

The Einstein-Bohr recoiling-slit gedankenexperiment, a cornerstone of quantum complementarity, has long been constrained by the zero-point fluctuations of the atomic slit -- the spatial Standard Quantum Limit (SQL). Here we transcend this fundamental boundary through active quantum state engineering of a single-atom slit. By implementing a non-adiabatic quench-evolve-quench protocol, we prepare the atomic motion in a squeezed state, dynamically redistributing phase-space uncertainty to suppress which-path information and restore high-visibility interference beyond the static vacuum limit. We report an intrinsic visibility of $0.938_{-0.008}^{+0.004}$, violating the SQL ($0.819$) by over 10 standard deviations, corresponding to $7.6(2)$ dB of effective squeezing. Our work reveals Kerr-induced non-Gaussian dynamics and reinterprets the traditional interferometer as a powerful tool for continuous-variable Wigner tomography, bridging the gap between quantum foundations and advanced metrology.

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arXiv2025

Efficient Magic State Cultivation on $\mathbb{RP}^2$

Preparing high-fidelity logical magic states is crucial for fault-tolerant quantum computation. Among prior attempts to reduce the substantial cost of magic state preparation, magic state cultivation (MSC), a recently proposed protocol for preparing $\mathrm{T}$ states without magic state distillation, achieves state-of-the-art efficiency. Inspired by this work, we propose a new MSC procedure that would produce a logical $\mathrm{T}$ state on a rotated surface code at a further reduced cost. For our MSC protocol, we define a new code family, the $\mathbb {RP}^2$ code, by putting the rotated surface code on $\mathbb{RP}^2$ (a two-dimensional manifold), as well as two self-dual CSS codes named SRP-3 and SRP-5 respectively. Small $\mathbb{RP}^2$ codes are used to hold logical information and checked by syndrome extraction (SE) circuits. We design fast morphing circuits that enable switching between a distance 3 (5) $\mathbb{RP}^2$ code and an SRP-3 (SRP-5) code on which we can efficiently check the correctness of the logical state. To preserve the high accuracy of the cultivated logical $\mathrm{T}$ state, we design an efficient and easy-to-decode expansion stage that grows a small $\mathbb{RP}^2$ code to a large rotated surface code in one round. Our MSC protocol utilizes non-local connectivity, available on both neutral atom array and ion trap platforms. According to our Monte Carlo sampling results, our MSC protocol requires about an order of magnitude smaller space-time volume to reach a target logical error rate around $10^{-9}$ compared to the original MSC protocol.

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arXiv2024

Transversal Logical Clifford gates on rotated surface codes with reconfigurable neutral atom arrays

We propose hardware-efficient schemes for implementing logical H and S gates transversally on rotated surface codes with reconfigurable neutral atom arrays. For logical H gates, we develop a simple strategy to rotate code patches efficiently with two sets of 2D-acousto-optic deflectors (2D-AODs). Our protocol for logical S gates utilizes the time-dynamics of the data and ancilla qubits during syndrome extraction (SE). In particular, we break away from traditional schemes where transversal logical gates take place between two SE rounds and instead embed our fold-transversal logical operation inside a single SE round, leveraging the fact that data and ancilla qubits can be morphed to an unrotated surface code state at half-cycle. Under circuit noise, we observe the performance of our S gate protocol is on par with the quantum memory. Together with transversal logical CNOT gates, our protocols complete a transversal logical Clifford gate set on rotated surface codes and admit efficient implementation on neutral atom array platforms.

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arXiv2024

Taming Rydberg Decay with Measurement-based Quantum Computation

Programmable neutral atom arrays show great promise for fault-tolerant quantum computing. A dominant physical error on this platform is qubit leakage and loss, notably decay errors from the Rydberg state during two-qubit gates. Such leakage events are particularly detrimental as they propagate, generating correlated errors that severely degrade the effective error distance of quantum error correction codes. Here, we present a novel approach to address Rydberg decay errors leveraging measurement-based quantum computation (MBQC). Our scheme strategically exploits the inherent geometric structure of topological cluster states and only uses final leakage detection information to locate propagated errors originating from Rydberg decay. This eliminates the need for complex and atom-species-specific mid-circuit leakage detection, offering broader applicability, e.g., to the well-established Rb atom platform. We demonstrate a high error threshold of 3.65\% per CZ gate for pure Rydberg decay and achieve a favorable error distance $d_e \approx d$. Our method compares favorably with state-of-the-art erasure conversion protocols in the sub-threshold performance, offering comparable or marginally larger logical error rates while significantly reducing experimental overhead.

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arXiv2024

Unconditionally decoherence-free quantum error mitigation by density matrix vectorization

Fighting against noise is crucial for NISQ devices to demonstrate practical quantum applications. In this work, we give a new paradigm of quantum error mitigation based on the vectorization of density matrices. Different from the ideas of existing quantum error mitigation methods that try to distill noiseless information from noisy quantum states, our proposal directly changes the way of encoding information and maps the density matrices of noisy quantum states to noiseless pure states, which is realized by a novel and NISQ-friendly measurement protocol and a classical post-processing procedure. Our protocol requires no knowledge of the noise model, no ability to tune the noise strength, and no ancilla qubits for complicated controlled unitaries. Under our encoding, NISQ devices are always preparing pure quantum states which are highly desired resources for variational quantum algorithms to have good performance in many tasks. We show how this protocol can be well-fitted into variational quantum algorithms. We give several concrete ansatz constructions that are suitable for our proposal and do theoretical analysis on the sampling complexity, the expressibility, and the trainability. We also give a discussion on how this protocol is influenced by large noise and how it can be well combined with other quantum error mitigation protocols. The effectiveness of our proposal is demonstrated by various numerical experiments.

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arXiv2024

Leapfrogging Sycamore: Harnessing 1432 GPUs for 7$\times$ Faster Quantum Random Circuit Sampling

Random quantum circuit sampling serves as a benchmark to demonstrate quantum computational advantage. Recent progress in classical algorithms, especially those based on tensor network methods, has significantly reduced the classical simulation time and challenged the claim of the first-generation quantum advantage experiments. However, in terms of generating uncorrelated samples, time-to-solution, and energy consumption, previous classical simulation experiments still underperform the \textit{Sycamore} processor. Here we report an energy-efficient classical simulation algorithm, using 1432 GPUs to simulate quantum random circuit sampling which generates uncorrelated samples with higher linear cross entropy score and is 7 times faster than \textit{Sycamore} 53 qubits experiment. We propose a post-processing algorithm to reduce the overall complexity, and integrated state-of-the-art high-performance general-purpose GPU to achieve two orders of lower energy consumption compared to previous works. Our work provides the first unambiguous experimental evidence to refute \textit{Sycamore}'s claim of quantum advantage, and redefines the boundary of quantum computational advantage using random circuit sampling.

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arXiv2024

A polynomial-time dissipation-based quantum algorithm for solving the ground states of a class of classically hard Hamiltonians

In this work, we give a polynomial-time quantum algorithm for solving the ground states of a class of classically hard Hamiltonians. The mechanism of the exponential speedup that appeared in our algorithm comes from dissipation in open quantum systems. To utilize the dissipation, we introduce a new idea of treating vectorized density matrices as pure states, which we call the vectorization picture. By doing so, the Lindblad master equation (LME) becomes a Schr\"odinger equation with non-Hermitian Hamiltonian. The steady state of the LME, therefore, corresponds to the ground states of a special class of Hamiltonians. The runtime of the LME has no dependence on the overlap between the initial state and the ground state. For the input part, given a Hamiltonian, under plausible assumptions, we give a polynomial-time classical procedure to judge and solve whether there exists LME with the desired steady state. For the output part, we propose a novel measurement strategy to extract information about the ground state from the original steady density matrix. We show that the Hamiltonians that can be efficiently solved by our algorithms contain classically hard instances assuming $\text{P}\neq \text{BQP}$. We also discuss possible exponential complexity separations between our algorithm and previous quantum algorithms without using the vectorization picture.

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