arXiv subjects

Asif Equbal

Publications and source records attributed to Asif Equbal.

5 recordsLinked to original sources

arXiv2026

Quadrature-Symmetric PulsePol for Robust Quantum Control Beyond the Ideal Pulse Approximation

PulsePol is an elegantly designed pulse-sequence-based quantum control scheme that enables polarization transfer between electron and nuclear spins, for example, in nitrogen-vacancy (NV) centers. However, previous analyses of PulsePol assumed very strong, near-ideal, instantaneous microwave pulses, which is rarely achievable at higher magnetic fields. We revisit the PulsePol scheme under finite-pulse constraints and show that its performance significantly degrades due to finite-pulse effects. Using bimodal Floquet theory, we identify the symmetry-breaking mechanism responsible for this deterioration in fidelity. By phase adjustment, we reestablish the proper symmetry of the interaction-frame spin Hamiltonian, leading to a sequence called Q-PulsePol, where "Q" reflects the restored quadrature symmetry. Our results demonstrate robustness to finite-pulse effects and improved polarization transfer efficiency, establishing Q-PulsePol as a practical and reliable scheme for bulk hyperpolarization of nuclear spins in solids using a single-mode (zero-quantum or double-quantum) transfer. This work bridges idealized quantum control with realistic pulse engineering, establishing design rules for spin-based quantum control protocols.

quant-ph↗

arXiv2025

Scalable quantum error mitigation for dynamical decoupling

Quantum coherence remains a fundamental challenge for advancing quantum technologies. Although dynamical decoupling can suppress decoherence noise, it frequently misestimates decoherence times due to control errors -- a previously underappreciated issue. Here, we present Hadamard phase cycling, a scalable non-Markovian quantum error mitigation method using group-structured phase configurations to filter spurious dynamics. Validated across molecular electron spins, nitrogen-vacancy centers in diamond, nuclear spins, trapped ions, and superconducting qubits, this technique enables accurate decoherence time characterization and enhanced state fidelity with linear complexity. Our results indicate that many reported ultralong decoherence times stem from artifacts like coherence-population mixing rather than genuine noise suppression. By ensuring dynamical authenticity, Hadamard phase cycling establishes a robust framework for reliable quantum control, paving the way for reassessment and advancement of coherence benchmarks in the NISQ era.

quant-ph↗

arXiv2024

Role of Quantum Coherence in Chirped Dynamic Nuclear Polarization

Dynamic Nuclear Polarization (DNP) is transforming NMR and MRI by significantly enhancing sensitivity through the transfer of polarization from electron spins to nuclear spins via microwave irradiation. However, the use of monochromatic continuous-wave (CW) irradiation limits the efficiency of DNP for systems with heterogeneous broad EPR lines. Broad-band techniques such as chirp irradiation offer a solution, particularly for Solid Effect (SE) DNP in such cases. Despite its widespread use, the role of quantum coherence generated during chirp irradiation remains unclear, even though it is a key factor in determining the maximum achievable DNP efficiency. In this work, we use density matrix formalism to provide a comprehensive understanding of the quantum coherence generated during electron-nucleus double-quantum (DQ) and zero-quantum (ZQ) SE transitions and their impact on Integrated Solid Effect (ISE) DNP under chirp irradiation. Our analysis employs fictitious product-operator bases to trace the evolution of electron-nucleus coherence leading to integrated or differentiated SE. We also explore the previously unexamined role of decoherence in optimizing chirped DNP. These findings provide new insights into low-temperature DNP and triplet DNP using photoexcited electrons.

quant-ph↗

arXiv2023

P1 center electron spin clusters are prevalent in type Ib diamond

Understanding the spatial distribution of P1 centers is crucial for diamond-based sensors and quantum devices. P1 centers serve as a polarization source for DNP quantum sensing and play a significant role in the relaxation of NV centers. Additionally, the distribution of NV centers correlates with the distribution of P1 centers, as NV centers are formed through the conversion of P1 centers. We utilized dynamic nuclear polarization (DNP) and pulsed electron paramagnetic resonance (EPR) techniques that revealed strong clustering of a significant population of P1 centers that exhibit exchange coupling and produce asymmetric lineshapes. The $^{13}$C DNP frequency profile at high magnetic field revealed a pattern that requires an asymmetric EPR lineshape of the P1 clusters with electron-electron (e-e) coupling strengths exceeding the $^{13}$C nuclear Larmor frequency. EPR and DNP characterization at high magnetic fields was necessary to resolve energy contributions from different e-e couplings. We employed a two-frequency pump-probe pulsed Electron Double Resonance (ELDOR) technique to show crosstalk between the isolated and clustered P1 centers. This finding implies that the clustered P1 centers affect all P1 populations. Direct observation of clustered P1 centers and their asymmetric lineshape is a novel and crucial insight into understanding magnetic noise sources for quantum information applications of diamonds and for designing diamond-based polarizing agents with optimized DNP efficiency for $^{13}$C and other nuclear spins of analytes. We propose that room temperature $^{13}$C DNP at high field, achievable through straightforward modifications to existing solution-state NMR systems, is a potent tool for evaluating and controlling diamond defects.

physics.chem-ph↗

arXiv2023

Direct Sensing of Remote Nuclei: Expanding the Reach of Cross-Effect Dynamic Nuclear Polarization

Dynamic Nuclear Polarization (DNP) has revolutionized the field of solid-state NMR spectroscopy by significantly enhancing the sensitivity of nuclear magnetic resonance experiments. Conventionally, cross effect DNP relies on biradicals to transfer polarization from coupled electron spins to nearby nuclear spins and subsequent relay to target nuclei via spin diffusion mechanism. However, the direct transfer of polarization to distant nuclei remains a significant challenge, limiting its applicability in various contexts. In this work, we propose a novel biradical design concept that involves a very strong electron-electron coupling, with a magnitude of hundreds of MHz, which enables efficient direct polarization transfer from electron spins to nuclear spins over much longer distances, exceeding 2.0 nm. We discuss the potential of this tailored biradicals in scenarios where conventional spin diffusion mechanisms are inefficient or when direct nuclear spin sensing through electron spin interactions is desired. Our study presents a promising avenue for expanding the scope of cross effect DNP in solid-state NMR spectroscopy and opens new opportunities for investigating a wide range of biological and material systems. Our research also provides insight into the DNP buildup time of commercially available biradicals.

physics.chem-ph↗