arXiv subjects

Bilal Tariq

Publications and source records attributed to Bilal Tariq.

6 recordsLinked to original sources

arXiv2026

Two-electron spectrum of a silicon quantum dot

The energy spectrum and wave functions of electrons in a single silicon quantum dot provide valuable insights into the capabilities and limitations of such a system in quantum information processing. Here we investigate the low-lying singlet and triplet configurations and spectra in a two-electron silicon quantum dot. To build toward a comprehensive understanding, we first examine the competition between Coulomb interaction and electron kinetic and confinement energy in the absence of valley-orbit coupling, as well as consequences of valley blockade in the presence of an ideal smooth interface. For realistic interfaces the variations in the magnitude and phase of valley-orbit coupling lead to inter-valley leakage, particularly when orbital splittings approach the valley splitting. In our study we particularly focus on the impact on the compositions of low-lying singlets and triplets. We find that for experimentally relevant parameter regimes the ground singlet and triplet states usually contain multiple configurations with significant weights as a result of a complicated competition among valley-orbit coupling, confinement potential, and Coulomb interaction. We further analyze the effects of an out-of-plane magnetic field on these the two-electron spectra. Our findings could have important implications for spin qubits in Si quantum dot in various contexts, such as qubit encoding and spin measurement.

cond-mat.mes-hall↗

arXiv2025

Polarization-dependent observables in $H\to \ell^{+}\ell^{-} γ$ in the SM

The rare three body decay of a Higgs boson to a lepton-anti lepton pair and a photon has begun to attract attention, after the first evidence for the $H\to Zγ$ at CMS and ATLAS, which is a sub process of $H \to \ell^+ \ell^- γ$ . To investigate some important features of this process, we suggest that the polarized forward-backward and the photon polarization asymmetries could be useful to probe its important properties, such as the behavior of Yukawa coupling, resonance, and non-resonance contributions. Our analysis introduces a comprehensive framework to evaluate the aforementioned polarization-dependent observables. By analyzing the polarization effects of the final-state photon and lepton separately on forward-backward asymmetries, we demonstrate that loop-induced contributions play a significant role to investigate these asymmetries. Unlike the unpolarized case, where the interference effects of resonance and non-resonance effects are minimal, we show that polarization dependent observables offer a powerful tool to analyze these features of this decay mode. Furthermore, these observables can provide a handy tool for probing possible signatures of physics beyond the SM.

hep-ph↗

arXiv2024

Analysis of final state lepton polarization-dependent observables in $H\to \ell^{+}\ell^{-} γ$ in the SM at loop level

Recently, the CMS and ATLAS collaborations have announced the results for $H\rightarrow Z[\rightarrow \ell^{+}\ell^{-}]γ$ with $\ell=e$ or $μ$ \cite{CMS:2022ahq,CMS:2023mku}, where $H\rightarrow Zγ$ is a sub-process of $H\rightarrow \ell^{+} \ell^{-} γ$. This semi-leptonic Higgs decay receives loop induced resonant $H\rightarrow Z[\rightarrow \ell^{+}\ell^{-}]γ$ as well as non-resonant contributions. % as discussed in \cite{Kachanovich:2021pvx}. To probe further features coming from these contributions to $H\rightarrow \ell^{+} \ell^{-} γ$, we argue that the polarization of the final state leptons is also an important parameter. We show that the contribution from the interference of resonant and non-resonant terms plays an important role when the polarization of final state lepton is taken into account, which is negligible in the case of unpolarized leptons. For this purpose, we have calculated the polarized decay rates and the longitudinal ($P_L$), normal ($P_N$) and transverse ($P_T$) polarization asymmetries. We find that these asymmetries purely come from the loop contributions and are helpful to further investigate the resonant and non-resonant nature of $H\rightarrow Z[\rightarrow \ell^{+}\ell^{-}]γ$ decay. We observe that for $\ell=e,μ$, the longitudinal decay rate is highly suppressed around $m_{\ell\ell}\approx 60$GeV when the final lepton spin is $-\frac{1}{2}$, dramatically increasing the corresponding lepton polarization asymmetries. Furthermore, we analyze another observable, the ratio of decay rates $R^{\ell\ell'}_{i\pm}$, where $\ell$ and $\ell'$ refer to different final state lepton generations. Precise measurements of these observables at the HL-LHC and the planned $e^{+}e^{-}$ can provide a fertile ground to test not only the SM but also to examine the signatures of possible NP beyond the SM.

hep-ph↗

arXiv2021

Approaches to Constrained Quantum Approximate Optimization

We study the costs and benefits of different quantum approaches to finding approximate solutions of constrained combinatorial optimization problems with a focus on Maximum Independent Set. In the Lagrange multiplier approach we analyze the dependence of the output on graph density and circuit depth. The Quantum Alternating Ansatz Approach is then analyzed and we examine the dependence on different choices of initial states. The Quantum Alternating Ansatz Approach, although powerful, is expensive in terms of quantum resources. A new algorithm based on a "Dynamic Quantum Variational Ansatz" (DQVA) is proposed that dynamically changes to ensure the maximum utilization of a fixed allocation of quantum resources. Our analysis and the new proposed algorithm can also be generalized to other related constrained combinatorial optimization problems.

quant-ph↗

arXiv2021

Impact of the valley orbit coupling on exchange gate for spin qubits in silicon quantum dots

The presence of degenerate conduction band valleys and how they are mixed by interfaces play critical roles in determining electron interaction and spectrum in a silicon nanostructure. Here we investigate how the valley phases affect the exchange interaction in a symmetric two-electron silicon double quantum dot. Through a configuration interaction calculation, we find that exchange splitting is suppressed at a finite value of valley phase difference between the two dots, and reaches its minimum value ({\sim} 0) when the phase difference is π. Such a suppression can be explained using the Hubbard model, through the valley-phase-dependent dressing by the doubly occupied states on the ground singlet and triplet states. The contributions of the higher orbital states also play a vital role in determining the value of the exchange energy in general, which is a crucial parameter for applications such as exchange gates for spin qubits.

cond-mat.mes-hall↗

arXiv2019

Effects of Interface Steps on the Valley Orbit coupling in a Si/SiGe quantum dot

Valley-orbit coupling is a key parameter for a silicon quantum dot in determining its suitability for applications in quantum information processing. In this paper we study the effect of interface steps on the magnitude and phase of valley-orbit coupling for an electron in a silicon quantum dot. Within the effective mass approximation, we find that the location of a step on the interface is important in determining both the magnitude and the phase of the valley-orbit coupling in a Si/SiGe quantum dot. Specifically, our numerical results show that the magnitude of valley orbit coupling can be suppressed up to 75\% by a step of one atomic monolayer, and its phase can change by almost $π$. When two steps are present, the minimum value of the valley-orbit coupling can even approach zero. We also clarify the effects of an applied external magnetic field and the higher orbital states on the valley-orbit coupling. Overall, our results illustrate how interface roughness affect the valley-orbit coupling in silicon, and how spin qubits in silicon may be affected.

cond-mat.mes-hall↗