arXiv subjects

Aman Ullah

Publications and source records attributed to Aman Ullah.

11 recordsLinked to original sources

arXiv2026

Cavity-Free Distributed Quantum Computing with Rydberg Ensembles via Collective Enhancement

We present a complete protocol for cavity-free quantum networking based on collective enhancement in Rydberg atom ensembles. The scheme combines Rydberg blockade, collectively enhanced light--matter coupling, and phase-matched directional emission to remove the need for optical cavities while retaining efficiencies comparable to cavity-assisted interfaces. The protocol proceeds in three steps: (i)~local control--ensemble entanglement generated by Rydberg blockade with gate fidelity $F_{\mathrm{gate}}\approx 99.93\%$; (ii)~atom--photon conversion through Raman emission from an oblate spheroidal ensemble, yielding directional emission efficiency $\eta_{\mathrm{dir}}\approx 73\%$ and single-node efficiency $\eta_{\mathrm{node}}\approx 40\%$; and (iii)~remote atom--atom entanglement via Hong--Ou--Mandel interference, producing Bell states with fidelity $F>97.5\%$. Incorporating quantum memories allows up to $M\approx 100$ retry attempts within a coherence time $T_2>100\,\mu\mathrm{s}$, enabling entanglement generation rates of approximately $4\,\mathrm{kHz}$ over a 20~km separation. Collectively enhanced Rydberg ensembles thus provide a practical, cavity-free interface for scalable distributed quantum computing and secure quantum communication.

quant-ph↗

arXiv2025

Effects of Coupling Between Chiral Vibrations and Spins in Molecular Magnets

In single molecular magnets, chiral vibrations carrying vibrational angular momentum ($\hat{L}^{\text{vib}}$) emerge due to the splitting of a doubly degenerate vibrational mode. Here, we identify a new type of effective spin-vibrational coupling responsible for lifting this degeneracy, which can facilitate optically selective excitations. In the presence of an external Zeeman field, this coupling breaks both inversion (in-plane parity) $\mathcal{P}$ and time-reversal $\mathcal{T}$ symmetries, imparting distinct geometric phases to the resulting dressed spin-vibronic states. The wave function of the spin-vibronic state is characterized by a $\pi$-Berry phase, which results in magneto-optical circular dichroism. This framework is validated using density functional theory and multi-reference \emph{ab initio} calculations on the Ce(trenovan) molecular magnet.

cond-mat.mes-hall↗

arXiv2025

Harnessing Chiral Spin States in Molecular Nanomagnets for Quantum Technologies

We present a theoretical framework to investigate spin chirality in molecular quantum systems. Focusing on a minimal three-spin-center model with antiferromagnetic exchange and symmetry breaking driven by an electric-field-induced Dzyaloshinskii-Moriya interaction and applied magnetic fields-give rise to chiral ground states characterized by nonzero scalar spin chirality, $\chi = \textbf{S}_1\cdot(\textbf{S}_r\times \textbf{S}_2)$. The emergent chiral qubits naturally suppress always-on interactions that can not be switched off in weakly coupled qubits, as demonstrated through Liouville-von Neumann dynamics, which reveal phase difference in superposition states that form chiral qubits. To validate this framework, we examine realistic lanthanide complexes with radical-bridged magnetic centers, where spin-orbit coupling and asymmetric exchange facilitate chirality. Our findings establish spin chirality engineering as a promising strategy for mitigating always-on interaction in entangling two chiral qubits in molecular quantum technologies.

cond-mat.mes-hall↗

arXiv2024

A call for frugal modelling: two case studies involving molecular spin dynamics

As scientists living through a climate emergency, we have a responsibility to lead by example, or to at least be consistent with our understanding of the problem. This common goal of reducing the carbon footprint of our work can be approached through a variety of strategies. For theoreticians, this includes not only optimizing algorithms and improving computational efficiency but also adopting a frugal approach to modeling. Here we present and critically illustrate this principle. First, we compare two models of very different level of sophistication which nevertheless yield the same qualitative agreement with an experiment involving electric manipulation of molecular spin qubits while presenting a difference in cost of $>4$ orders of magnitude. As a second stage, an already minimalistic model of the potential use of single-ion magnets to implement a network of probabilistic p-bits, programmed in two different programming languages, is shown to present a difference in cost of a factor of $\simeq 50$. In both examples, the computationally expensive version of the model was the one that was published. As a community, we still have a lot of room for improvement in this direction.

physics.soc-ph↗

arXiv2022

Electrical two-qubit gates within a pair of clock-qubit magnetic molecules

Enhanced coherence in HoW$_{10}$ molecular spin qubits has been demonstrated by use of Clock Transitions (CTs). More recently it was shown that, while operating at the CTs, it was possible to use an electrical field to selectively address HoW$_{10}$ molecules pointing in a given direction, within a crystal that contains two kinds of identical but inversion-related molecules. Herein we theoretically explore the possibility of employing the electric field to effect entangling two-qubit quantum gates among two neighbouring CT-protected HoW$_{10}$ qubits within a diluted crystal. We estimate the thermal evolution of $T_1$, $T_2$, find that CTs are also optimal operating points from the point of view of phonons, and lay out how to combine a sequence of microwave and electric field pulses to achieve coherent control within a 2-qubit operating space that is protected both from spin-bath and from phonon-bath decoherence. Finally, we found a highly protected 1-qubit subspace resulting from the interaction between two clock molecules.

quant-ph↗

arXiv2021

Quantum coherent spin-electric control in a molecular nanomagnet at clock transitions

Electrical control of spins at the nanoscale offers significant architectural advantages in spintronics, because electric fields can be confined over shorter length scales than magnetic fields. Thus, recent demonstrations of electric-field (E-field) sensitivities in molecular spin materials are tantalising, raising the viability of the quantum analogues of macroscopic magneto-electric devices.However, the E-field sensitivities reported so far are rather weak, prompting the question of how to design molecules with stronger spin-electric couplings. Here we show that one path is to identify an energy scale in the spin spectrum that is associated with a structural degree of freedom with a significant electrical polarisability. We study an example of a molecular nanomagnet in which a small structural distortion establishes clock transitions (i.e. transitions whose energy is to first order independent of magnetic field) in the spin spectrum; the fact that this distortion is associated with an electric dipole allows us to control the clock transition energy to an unprecedented degree. We demonstrate coherent electrical control of the quantum spin state and exploit it to manipulate independently the two magnetically-identical but inversion-related molecules in the unit cell of the crystal. Our findings pave the way for the use of molecular spins in quantum technologies and spintronics.

cond-mat.mes-hall↗

arXiv2021

Binding Sites, Vibrations and Spin-Lattice Relaxation Times in Europium(II)-based Metallofullerene Spin Qubits

To design molecular spin qubits with enhanced quantum coherence, a control of the coupling between the local vibrations and the spin states is crucial, which could be realized in principle by engineering molecular structures via coordination chemistry. To this end, understanding the underlying structural factors that govern the spin relaxation is a central topic. Here, we report the investigation of the spin dynamics in a series of chemically-designed europium(II)-based endohedral metallofullerenes (EMFs). By introducing a unique structural difference, i.e. metal-cage binding site, while keeping other molecular parameters constant between different complexes, these manifest the key role of the three low energy metal-based vibrations in mediating the spin-lattice relaxation times (T1). The temperature dependence of T1 can thus be normalized by the frequencies of these low energy vibrations to show an unprecedentedly universal behavior for EMFs in frozen CS2 solution. Our theoretical analysis indicates that this structural difference determines not only the vibrational rigidity but also spin-vibration coupling in these EMF-based qubit candidates.

physics.chem-ph↗

arXiv2021

Insights on the coupling between vibronically active molecular vibrations and lattice phonons in molecular nanomagnets

Spin-lattice relaxation is a key open problem to understand the spin dynamics of single-molecule magnets and molecular spin qubits. While modelling the coupling between spin states and local vibrations allows to determine the more relevant molecular vibrations for spin relaxation, this is not sufficient to explain how energy is dissipated towards the thermal bath. Herein, we employ a simple and efficient model to examine the coupling of local vibrational modes with long-wavelength longitudinal and transverse phonons in the clock-like spin qubit [Ho(W$_5$O$_{18}$)$_2$]$^{9-}$. We find that in crystals of this polyoxometalate the vibrational mode previously found to be vibronically active at low temperature does not couple significantly to lattice phonons. This means that further intramolecular energy transfer via anharmonic vibrations is necessary for spin relaxation in this system. Finally, we discuss implications for the spin-phonon coupling of [Ho(W$_5$O$_{18}$)$_2$]$^{9-}$ deposited on a MgO (001) substrate, offering a simple methodology that can be extrapolated to estimate the effects on spin relaxation of different surfaces, including 2D materials.

physics.atm-clus↗

arXiv2021

Spectroscopic analysis of vibronic relaxation pathways in molecular spin qubit $[$Ho(W$_5$O$_{18}$)$_2]^{9-}$: sparse spectra are key

Molecular vibrations play a key role in magnetic relaxation processes of molecular spin qubits as they couple to spin states, leading to the loss of quantum information. Direct experimental determination of vibronic coupling is crucial to understand and control the spin dynamics of these nano-objects, which represent the limit of miniaturization for quantum devices. Herein, we measure the vibrational properties of the molecular spin qubit $[$Ho(W$_5$O$_{18}$)$_2]^{9-}$ by means of magneto-infrared spectroscopy. Our results allow us to unravel the vibrational decoherence pathways in combination with $ab$ $initio$ calculations including vibronic coupling. We observe field-induced spectral changes near 63 and 370 cm$^{-1}$ that are modeled in terms of $f$-manifold crystal field excitations activated by odd-symmetry vibrations. The overall extent of vibronic coupling in this system is limited by a transparency window in the phonon density of states that acts to keep the intramolecular vibrations and $M_J$ levels apart. These findings advance the understanding of vibronic coupling in molecular magnets, place significant constraints on the pattern of crystal field levels in these systems, and provide a strategy for designing molecular spin qubits with improved coherence lifetimes.

cond-mat.mes-hall↗

arXiv2019

Semiparametric Estimation of Correlated Random Coefficient Models without Instrumental Variables

We study a linear random coefficient model where slope parameters may be correlated with some continuous covariates. Such a model specification may occur in empirical research, for instance, when quantifying the effect of a continuous treatment observed at two time periods. We show one can carry identification and estimation without instruments. We propose a semiparametric estimator of average partial effects and of average treatment effects on the treated. We showcase the small sample properties of our estimator in an extensive simulation study. Among other things, we reveal that it compares favorably with a control function estimator. We conclude with an application to the effect of malaria eradication on economic development in Colombia.

econ.EM↗

arXiv2019

A $π$-shaped Quantum Device for Implementation of Bell States in Solid State Environment

Electronic spin-qubit is key ingredient for quantum information processing in a solid state environment. We present a $π$-shaped two-qubit entanglement device capable of measuring the resultant states in Bell basis. In our device, source spins ($\uparrow_s$ or $\downarrow_s$) are electrically generated and tunnelled to channel where they interact with channel spins ($\uparrow_c$ or $\downarrow_c$) via exchange interaction which is responsible for 2-qubit entanglement. Electrical control over spins gives rise to the Bell states. The $U_{\sqrt{SWAP}}$ and CNOT gate operations are implemented through these Bell states for universal quantum computation. $π$-shaped quantum device can be used as a solid state interconnect between different parts of the circuit in integrated chips.

quant-ph↗