arXiv subjects

Guanjie He

Publications and source records attributed to Guanjie He.

4 recordsLinked to original sources

arXiv2025

Jenga-Krotov algorithm: Efficient compilation of multi-qubit gates for exchange-only qubits

Exchange-only (EO) qubits, implemented in triple-quantum-dot systems, offer a compelling platform for scalable semiconductor-based quantum computing by enabling universal control through purely exchange interactions. While high-fidelity single- and two-qubit gates have been demonstrated, the synthesis of efficient multi-qubit operations-such as the Toffoli gate-remains a key bottleneck. Conventional gate decompositions into elementary operations lead to prohibitively long and error-prone pulse sequences, limiting practical deployment. In this work, we introduce a gradient-based optimization algorithm, Jenga-Krotov (JK), tailored to discover compact, high-fidelity EO gate sequences. Applying JK to the Toffoli gate, we reduce the number of required exchange unitaries from 216 (in direct decomposition) to 92, and compress the time steps required from 162 to 50, all while maintaining target fidelity. Under realistic noise, the accumulated gate error from our optimized sequence is an order of magnitude lower than that of conventional approaches. We have also applied the JK algorithm to other multi-qubit gates and algorithm. For the Fredkin gate, it reduces the number of time steps from 200 to 104 and the number of exchange unitaries from 276 to 172. For the quantum Fourier transform, it compresses the sequence from 180 to 80 time steps and from 237 to 202 exchange unitaries. These results demonstrate that the JK algorithm is a general and scalable strategy for multi-qubit gate synthesis in EO architectures, potentially facilitating realization of multi-qubit algorithms on semiconductor platforms.

quant-ph↗

arXiv2024

Phonon Dephasing, Entanglement and Exchange-Only Toffoli Gate Sequence in Quantum Dot Spin Chains

The quantum dot spin chain system is vital for quantum simulation and studying collective electron behaviors, necessitating an understanding of its mechanisms and control protocols. Chapter 1 introduces key concepts, focusing on the extended Hubbard model, double quantum dot systems, and electron-phonon coupling. Chapter 2 explores electron-phonon coupling in multielectron double quantum dots under unbiased and biased scenarios via detuning variations. In the unbiased case, dephasing due to electron-phonon coupling generally increases with more electrons in the right dot; this trend is inconsistent in the biased case, suggesting potential advantages of multielectron quantum dots under certain conditions. Chapter 3 investigates entanglement entropy in a multielectron quantum dot spin chain described by the extended Hubbard model. Local and pairwise entanglement are influenced by Coulomb interactions, tunneling strengths, electronic configurations, and site potential energies. The entanglement diagram reveals phase transitions significantly impacted by coupling strength ratios and potential energy variations; adjusting the potential energy of a specific dot critically influences ground state configurations and entanglement entropy. Chapter 4, inspired by the decoherence-free subspace concept, explores operation sequences in a nine-spin, nine-quantum-dot system defined by the Heisenberg model, with bases determined by total angular momentum quantum numbers. Employing the Krotov method of quantum optimal control, we identify a more efficient pulse-level operation sequence for an exchange-only quantum dot spin chain, offering a superior alternative to conventional quantum gate decomposition and potentially enhancing the development of more concise quantum algorithm representations.

quant-ph↗

arXiv2024

Exploring Entanglement Spectrum and Phase Diagram in multi-electron Quantum Dot Chains

We investigate the entanglement properties in semiconductor quantum dot systems modeled by extended Hubbard model, focusing on the impact of potential energy variations and electron interactions within a four-site quantum dot spin chain. Our study explores local and pairwise entanglement across configurations with electron counts N=4 and N=6, under different potential energy settings. By adjusting the potential energy in specific dots and examining the entanglement across various interaction regimes, we identify significant variations in the ground states of quantum dots. Our results reveal that local potential modifications lead to notable redistributions of electron configurations, significantly affecting the entanglement properties. These changes are depicted in phase diagrams that show entanglement dependencies on interaction strengths and potential energy adjustments, highlighting complex entanglement dynamics and phase transitions triggered by inter-dot interactions.

cond-mat.mes-hall↗

arXiv2022

Theory on electron-phonon spin dehphasing in GaAs multi-electron double quantum dots

Recent studies reveal that a double-quantum-dot system hosting more than two electrons may be superior in certain aspects as compared to the traditional case in which only two electrons are confined (a singlet-triplet qubit). We study the electron-phonon dephasing occurring in a GaAs multi-electron double-quantum-dot system, in a biased case in which the singlet state is hybridized, as well as in an unbiased case in which the hybridization is absent. We have found that while the electron-phonon dephasing rate increases with the number of electrons confined in the unbiased case, this does not hold in the biased case. We define a merit figure as a ratio between the exchange energy and the dephasing rate, and have shown that in experimentally relevant range of the exchange energy, the merit figure actually increases with the number of electrons in the biased case. Our results show that the multi-electron quantum-dot system has another advantage in mitigating the effect of electron-phonon dephasing, which is previously under-appreciated in the literature.

cond-mat.mes-hall↗