arXiv subjects

Xiangjun Tan

Publications and source records attributed to Xiangjun Tan.

7 recordsLinked to original sources

arXiv2026

High-Fidelity Hole Spin Qubits Reveal Quadrupolar Nuclear-Bath Dynamics in Isotopically Purified Planar Germanium

Planar Germanium has emerged as a promising platform to build spin-based large scale quantum computers. By exploiting the anisotropic hyperfine interaction of holes in Ge, qubits with long T2* have been recently realized. While the performance of single qubits is still more or less limited by 73Ge nuclear spin fluctuations, the site-to-site variation of qubit sweet spot becomes obstacles to maintaining high fidelity of each qubit across the whole wafer. To achieve high performance Ge-based quantum circuit, it is therefore essential to eliminate the origin source of hyperfine noise. In its Silicon counterparts, reduction of 29Si abundance enables exceptional high-fidelity operation. In contrast, hole qubits based on isotopically purified Ge have not been demonstrated. Here, we report the synthesis of high quality 2-dimensional hole gas (2DHG) with enriched 70GeH4 precursor. Due to the suppression of nonzero spin nucleus, the qubits' T2* on the sweet spot is moderately extended beyond 20 us, surpassing the previous best reported Ge hole qubits. More importantly, the qubits' T2* off the sweet spot is enhanced to above 3 us, enabling single qubit gate fidelity exceeding 99.9% in both operating regimes. Hahn-echo spectroscopy further resolves a finite-frequency nuclear-noise channel that is distinct from the conventional Larmor-linked hyperfine response. We associate this channel with quadrupole-modified dynamics of residual 73Ge nuclei sampling local electric-field gradients near the Ge/SiGe interface. Its field scaling and angle-dependent visibility are consistent with a qubit-visible quadrupolar nuclear-noise component transduced through the anisotropic hyperfine interaction of Ge holes. These results establish isotopically purified planar Ge as a high-coherence scalable platform for hole spin qubits and provide a spectroscopic probe of interfacial quadrupolar nuclear dynamics.

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arXiv2026

Quantum Error Correction Assisted Axion Search in CMOS Spin Qubit Arrays

Searches for axion and axionlike dark matter based on solid-state spin qubits are fundamentally limited by strong longitudinal dephasing, which rapidly suppresses the sensitivity gains offered by entanglement. Here we show that quantum error correction (QEC) can substantially enhance axion search sensitivity in realistic semiconductor spin qubit platforms by mitigating this dominant noise source. By integrating an optimally chosen repetition code QEC with logical GHZ block entanglement, we derive closed-form expressions for the quantum Fisher information that explicitly incorporate the finite coherence time of the axion field. Our analysis demonstrates that modest QEC cycle frequencies are sufficient to significantly reduce the effective dephasing rate, thereby restoring a broad parameter regime in which entanglement-enhanced sensing surpasses the standard quantum limit. Projecting these results onto CMOS-compatible device parameters, we find that QEC-protected entangled sensing can revive otherwise inaccessible quantum advantages, yielding up to order-of-magnitude improvements in sensitivity to the axion-electron coupling $g_{ae}$. These results establish a practical and theoretically controlled pathway for using QEC to improve qubit array searches for physics beyond the Standard Model.

quant-ph↗

arXiv2026

Cancelling second order frequency shifts in Ge hole spin qubits via bichromatic control

Germanium quantum dot hole spin qubits are compatible with fully electrical control and are progressing toward multi-qubit operations. However, their coherence is limited by charge noise and driving field induced frequency shifts, and the resulting ensemble $1/f$ dephasing. Here we theoretically demonstrate that a bichromatic driving scheme cancels the second order frequency shift from the control field without sacrificing the electric dipole spin resonance (EDSR) rate, and without additional gate design or microwave engineering. Based on this property, we further demonstrate that bichromatic control creates a wide operating window that reduces sensitivity to quasi-static charge noise and thus enhances single qubit gate fidelity. This method provides a low-power route to a stabler frequency operation in germanium hole spin qubits and is readily transferable to other semiconductor spin qubit platforms.

quant-ph↗

arXiv2026

Axion Signal Search Using Hybrid Nuclear-Electronic Spin Systems

Conventional nuclear magnetic resonance searches for the galactic axion wind lose sensitivity at low frequencies due to the unfavourable scaling of inductive readout. Here, we propose a hybrid architecture where the hyperfine interaction transduces axion-driven nuclear precession into a high-bandwidth electron-spin readout channel. We demonstrate analytically that this dispersive upconversion preserves the specific sidereal and annual modulation signatures required to distinguish dark matter signals from instrumental backgrounds. When instantiated in a silicon ${ }^{209} \text{Bi}$ donor platform, the hybrid sensor is projected to outperform direct nuclear detection by more than an order of magnitude over the $10^{-16}-10^{-6} \text{eV}$ wide mass range. With collective enhancement, the design reaches a $5 \sigma$ sensitivity to DFSZ axion-nucleon couplings within one year, establishing hyperfine-mediated sensing as a competitive path for compact, solid-state dark matter searches.

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arXiv2025

Annual-modulation fingerprint of the axion wind induced sideband triplet in quantum dot spin qubit sensors

We propose a phase-coherent, narrowband magnetometer for searching couplings between axions or axion-like particles (ALPs) and electron spins, using gate-defined silicon quantum-dot spin qubits. With repeated Ramsey echo sequences and dispersive readout, the qubit precession response can be tracked with sub-Hz spectral resolution. The accessible axion mass window is determined using a series of filtering protocols that take into account sensing noise, including readout errors and $1/f$ noise. We demonstrate clear evidence of sidereal modulation of the signal due to Earth's rotation, while Earth's orbital motion produces an annual amplitude envelope that generates sidebands at fixed frequency spacing $\pm \Omega_\oplus$ around the sidereal component. For axion masses between $1$-$10~\mu{\rm eV}$, the proposed method covers axion-electron coupling strengths $g_{ae}$ ranging from $10^{-14}$ to $10^{-10}$. Including both daily and annual modulation patterns in the likelihood analysis enhances the rejection of stationary or instrumental noise. Our results indicate that spin-qubit magnetometry can achieve sensitivities approaching those suggested by astrophysical considerations, providing a complementary, laboratory-based probe of axion-electron interactions. Although we focus on silicon spin-qubit architectures, the approach is broadly applicable to spin-based quantum sensors.

quant-ph↗

arXiv2025

Toward Axion Signal Extraction in Semiconductor Spin Qubits Via Spectral Engineering

Recent advances in quantum sensing and computational technologies indicate the possibility of improving the precision of measurements aimed at detecting cosmological particles and weakly interacting massive particles using various qubit platforms. While recent progress has been made, mitigating environmental noise remains a challenge in extracting particle parameters with high fidelity. Addressing these challenges requires efforts on two levels. At the device level, the qubit and its array acting as a probe, must be isolated from electrical and magnetic noise through optimized device geometry. At the signal-processing level, it is necessary to develop filtering methods targeting specific noise spectra based on different qubit architectures. In this work, we explore the possibility of using semiconductor quantum dot spin qubits as a platform to search for quantum chromodynamics axions and, more broadly, axion like particles (ALPs). Starting by deriving an effective Hamiltonian for electron-axion interactions, we identify an axion-induced effective magnetic field and determine the characteristic axion oscillation frequency. To suppress charge noise in the devices and environmental noise, we first analyze the charge noise spectrum and then develop a dedicated filtering and noise-reduction protocol, paving the way for exploring feasible axion mass ranges. Our preliminary study holds promise for enhancing the screening of various axion signals using quantum technologies. We expect that our analysis and filtering protocol can help advance the use of semiconductor quantum dot spin qubit arrays in axion detection.

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arXiv2024

Quantum Computing for Phonon Scattering Effects on Thermal Conductivity

Recent investigations have demonstrated that multi-phonon scattering processes substantially influence the thermal conductivity of materials, posing significant computational challenges for classical simulations as the complexity of phonon modes escalates. This study examines the potential of quantum simulations to address these challenges, utilizing Noisy Intermediate Scale Quantum era (NISQ) quantum computational capabilities and quantum error mitigation techniques to optimize thermal conductivity calculations. Employing the Variational Quantum Eigensolver (VQE) algorithm, we simulate phonon-phonon contributions based on the Boltzmann Transport Equation (BTE). Our methodology involves mapping multi-phonon scattering systems to fermionic spin operators, necessitating the creation of a customized ansatz to balance circuit accuracy and depth. We construct the system within Fock space using bosonic operators and transform the Hamiltonian into the sum of Pauli operators suitable for quantum computation. By addressing the impact of depolarization and non-unitary noise effects, we benchmark the noise influence and implement error mitigation strategies to develop a more efficient model for quantum simulations in the NISQ era.

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