Somayyeh Nemati, Fatemeh Khastehdel Fumani, Saeed Mahdavifar
Here, we show that, although quantum fidelity can truly identify two quantum phase transitions of a one-dimensional spin-1/2 quantum Ising model with competing nearest and next-nearest neighbour interactions in a transverse magnetic field, it may not be a suitable approach for analyzing its ground-state phase diagram.
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Using the Wherl entropy, we study the delocalization in phase-space of energy eigenstates in the vicinity of avoided crossing in the Lipkin-Meshkov-Glick model. These avoided crossing, appearing at intermediate energies in a certain parameter region of the model, originate classically from pairs of trajectories lying in different phase space regions, which contrary to the low energy regime, are not connected by the discrete parity symmetry of the model. As coupling parameters are varied, a sudden increase of the Wherl entropy is observed for eigenstates close to the critical energy of the exci…
Crossing The Gap Using Variational Quantum Eigensolver: A Comparative Study
本文比较 VQD 与 SSVQE 计算 LiH 低激发态的表现，并提出结合 FS-VQE 的 FS-VQD 与 FS-SSVQE 以探索 H₄ 高激发态。
We have proposed the energy landscape transformation of Ising problems (ELTIP), which changes the combination of the state and eigenvalue without changing all the original eigenvalues [arXiv:2202.05927]. We study how the ELTIP affects the anti-crossing between two levels of the ground and first excited states during quantum annealing. We use a 5-spin maximum-weighted independent set for the problem to numerically investigate the anticrossing. For comparison, we introduce a non-stoquastic Hamiltonian that adds antiferromagnetic interaction to the normal transverse magnetic field. Annealing with…
A theoretical approach is proposed to treat the spin dynamics in defect color centers. The method explicitly takes into account the spin dynamics in the ground state and excited state of the defect center as well as spin state dependent transitions involving the ground state and excited state, as well as an additional intermediate state. The proposed theory is applied to treat spin-dependent phenomena is silicon carbide, namely, in spin-3/2 silicon-vacancy centers, VSi or V2 centers. Theoretical predictions of magnetic field dependent photoluminescence intensity and optically detected magnetic…
The Landau-Zener formula provides an analytical expression for the final excitation of a quantum system after passage of an avoided crossing of two energy levels. If the two levels correspond to a ground state, and to an excited state which is subject to radiative decay, the probability of exciting the system by adiabatic passage of the level crossing is reduced. In this article we use a stochastic master equation to study the level crossing dynamics when the system is subject to continuous probing of the emitted radiation. The measurement backaction on the system associated with the fluctuati…
Equivariant Variational Quantum Eigensolver to detect Phase Transitions through Energy Level Crossings
本研究提出等变量子电路保持自旋和平移对称性，精确描述链上J1-J2海森堡模型的单重态和三重态激发态，通过能级交叉检测相变点，并验证零噪声外推可恢复噪声下的物理性质。
Turbulence is an intriguing non-equilibrium state, which originates from fluid mechanics and has far-reaching consequences in the description of climate physics, the characterization of quantum hydrodynamics, and the understanding of cosmic evolution. The concept of turbulent cascade describing the energy redistribution across different length scales offers one profound route to reconcile fundamental conservative forces with observational energy non-conservation of accelerating expansion of the universe bypassing the cosmological constant. Here, we observe a dimension crossing turbulent energy…
Challenging Excited States from Adaptive Quantum Eigensolvers: Subspace Expansions vs. State-Averaged Strategies
本文提出态平均的 MORE-ADAPT-VQE，用于在近期量子计算机上计算强关联分子激发态，并在 H$_4$ 与 BeH$_2$ 上证明其优于 q-sc-EOM。
We report direct measurement of population dynamics in the excited state manifold of a nitrogen-vacancy (NV) center in diamond. We quantify the phonon-induced mixing rate and demonstrate that it can be completely suppressed at low temperatures. Further, we measure the intersystem crossing (ISC) rate for different excited states and develop a theoretical model that unifies the phonon-induced mixing and ISC mechanisms. We find that our model is in excellent agreement with experiment and that it can be used to predict unknown elements of the NV center's electronic structure. We discuss the model'…
We studied the dynamic nuclear spin polarization of nitrogen in negatively charged nitrogen-vacancy (NV) centers in diamond both experimentally and theoretically over a wide range of magnetic fields from 0 to 1100 G covering both the excited-state level anti-crossing and the ground-state level anti-crossing magnetic field regions. Special attention was paid to the less studied ground-state level anti-crossing region. The nuclear spin polarization was inferred from measurements of the optically detected magnetic resonance signal. These measurements show that a very large (up to $96 \pm 2\%$) nu…
The intersystem crossing (ISC) is an important process in many solid-state atomlike impurities. For example, it allows the electronic spin state of the nitrogen-vacancy (NV) center in diamond to be initialized and read out using optical fields at ambient temperatures. This capability has enabled a wide array of applications in metrology and quantum information science. Here, we develop a microscopic model of the state-selective ISC from the optical excited state manifold of the NV center. By correlating the electron-phonon interactions that mediate the ISC with those that induce population dyn…
Vibrationally highly excited trilobite molecules stabilized by non-adiabatic coupling
本文首次在铷-87中观测到高激发三叶虫里德伯分子的振动态，通过非绝热耦合抑制衰变，实现稳定，并首次直接测量超越玻恩-奥本海默物理。
We propose an excited-state molecular dynamics simulation method based on variational quantum algorithms at a computational cost comparable to that of ground-state simulations. We utilize the feature that excited states can be obtained as metastable states in the restricted variational quantum eigensolver calculation with a hardware-efficient ansatz. To demonstrate the effectiveness of the method, molecular dynamics simulations are performed for the S1 excited states of H2 and CH2NH molecules. The results are consistent with those of the exact adiabatic simulations in the S1 states, except for…
We investigate crossing behavior of ground state entanglement Renyi entropies of quantum critical systems. We find a novel property that the ground state in one quantum phase cannot be locally transferred to that of another phase, that means a global transformation is necessary. This also provides a clear evidence to confirm the long standing expectation that entanglement Renyi entropy contains more information than entanglement von Neumann entropy. The method of studying crossing behavior of entanglement Renyi entropies can distinguish different quantum phases well. We also study the excited …
In this work, the anisotropic variant of the quantum Rabi model with different coupling strengths of the rotating and counter-rotating wave terms is studied by the Bogoliubov operator approach. The anisotropy preserves the parity symmetry of the original model. We derive the corresponding $G$-function, which yields both the regular and exceptional eigenvalues. The exceptional eigenvalues correspond to the crossing points of two energy levels with different parities and are doubly degenerate. We find analytically that the ground-state and the first excited state can cross several times, indicat…
We study the spontaneous emission of a partially excited Bose-Einstein condensate composed of two-level atoms. The formation of polaritons induced by the ground-state part of the condensate leads to an avoided crossing in the photon spectrum. This avoided crossing acts similarly to a photonic band gap and modifies the spontaneous emission rate.
Symmetry-Checking in Band Structure Calculations on a Noisy Quantum Computer
本研究提出了一种在噪声量子计算机上识别双层石墨烯带结构交叉点对称性的方法，通过特征检查量子电路和去极化噪声模型评估其鲁棒性。
The importance of triplet states in the photorelaxation dynamics of SO2 is studied by mixed quantum-classical dynamics simulations. Using the Surface Hopping including ARbitrary Couplings (Sharc) method, intersystem crossing processes caused by spin-orbit coupling are found occuring on an ultrafast time scale (few 100 fs) and thus competing with internal conversion. While in the singlet-only dynamics only oscillatory population transfer between the 1B1 and 1A2 states is observed, in the dynamics including singlet and triplet states we find additionally continuous ISC to the 3B2 state and to a …
Effects of XX-catalysts on quantum annealing spectra with perturbative crossings
研究XX催化剂对量子退火谱的影响，发现其效果对问题编码极敏感，并揭示基态矢量负分量在能隙响应中的关键作用，探讨了闭合能隙在非绝热协议中的潜在利用。
We show that excited-state quantum phase transitions (ESQPTs) in a system in which the parity symmetry is broken can be used to engineer an energy-cat state -- a Schrödinger cat state involving a quantum superposition of both different positions and energies. By means of a generalization of the Rabi model, we show that adding a parity-breaking term annihilates the ground-state quantum phase transition between normal and superradiant phases, and induces the formation of three excited-state phases, all of them identified by means of an observable with two eigenvalues. In one of these phases, lev…
We propose that a broad class of excited-state quantum phase transitions (ESQPTs) gives rise to two different excited-state quantum phases. These phases are identified by means of an operator, $\hat{\mathcal{C}}$, which is a constant of motion only in one of them. Hence, the ESQPT critical energy splits the spectrum into one phase where the equilibirium expectation values of physical observables crucially depend on this constant of motion, and another phase where the energy is the only relevant thermodynamic magnitude. The trademark feature of this operator is that it has two different eigenva…
Robust Nuclear Spin Polarization via Ground-State Level Anti-Crossing of Boron Vacancy Defects in Hexagonal Boron Nitride
本研究提出利用基态能级反交叉（GSLAC）技术在六方氮化硼的硼空位缺陷中实现核自旋极化，该方法所需激光功率显著低于激发态方法，并实现了核自旋的直接光学读出，为精确操控核自 spin提供了可行方案。
Unified adiabatic and diabatic excited-state description via the ensemble-variational quantum eigensolver
本文提出了一种基于集合变分量子特征求解器的方法，用于统一处理绝热和非绝热激发态问题，并展示了如何扩展该方法以处理三个或更多耦合电子态的模型。
The nonequilibrium ultracold bosonic quantum dynamics in finite optical lattices of unit filling following a linear interaction quench from a superfluid to a Mott insulator state and vice versa is investigated. The resulting dynamical response consists of various inter and intraband tunneling modes. We find that the competition between the quench rate and the interparticle repulsion leads to a resonant dynamical response, at moderate ramp times, being related to avoided crossings in the many-body eigenspectrum with varying interaction strength. Crossing the regime of weak to strong interaction…
A main distinguishing feature of non-Hermitian quantum mechanics is the presence of exceptional points (EPs). They correspond to the coalescence of two energy levels and their respective eigenvectors. Here, we use the Lipkin-Meshkov-Glick (LMG) model as a testbed to explore the strong connection between EPs and the onset of excited state quantum phase transitions (ESQPTs). We show that for finite systems, the exact degeneracies (EPs) obtained with the non-Hermitian LMG Hamiltonian continued into the complex plane are directly linked with the avoided crossings that characterize the ESQPTs for t…
Two recent preprints [B. Altshuler, H. Krovi, and J. Roland, "Quantum adiabatic optimization fails for random instances of NP-complete problems", arXiv:0908.2782 and "Anderson localization casts clouds over adiabatic quantum optimization", arXiv:0912.0746] argue that random 4th order perturbative corrections to the energies of local minima of random instances of NP-complete problem lead to avoided crossings that cause the failure of quantum adiabatic algorithm (due to exponentially small gap) close to the end, for very small transverse field that scales as an inverse power of instance size N. …
State-Averaged Orbital-Optimized VQE: A quantum algorithm for the democratic description of ground and excited electronic states
本文提出SA-OO-VQE混合量子-经典算法，通过态平均轨道优化VQE方法平等处理简并态，在含噪声中等规模量子计算机上同时求解基态与激发态，避免特定态方法在避免交叉和锥形交叉处的数值优化困难。
Role of spectral structure in adiabatic ground-state preparation of the XXZ model
研究XXZ模型中谱结构对绝热基态制备的影响，通过三种策略比较发现，谱退化是限制因素，简单优化初始哈密顿量可有效抑制临界交叉，而反迪布里克项需在消除退化后才有效。
Non-Adiabatic Quantum Optimization for Crossing Quantum Phase Transitions
针对有限时间内多体量子系统基态穿越量子相变时绝热性破坏引发的激发问题，受Kibble-Zurek机制启发分析现有最优控制程序的绝热性时间尺度，提出非绝热量子优化（NAQO）框架，利用Landau-Zener公式并考虑高激发态作用，在多种模型中性能优于现有方法。
The quantum photodynamics of a simple diatomic molecule with a permanent dipole immersed within an optical cavity containing a quantized radiation field is studied in detail. The chosen molecule under study, lithium fluoride (LiF), is characterized by the presence of an avoided crossing between the two lowest $^1Σ$ potential energy curves (covalent-ionic diabatic crossing). Without field, after prompt excitation from the ground state $1\; ^1Σ$, the excited nuclear wave packet moves back and forth in the upper $2\; ^1Σ$ state, but in the proximity of the avoided crossing, the non-adiabatic coup…
Atomistic electronic structure calculations are performed to study the coherent inter-dot couplings of the electronic states in a single InGaAs quantum dot molecule. The experimentally observed excitonic spectrum [12] is quantitatively reproduced, and the correct energy states are identified based on a previously validated atomistic tight binding model. The extended devices are represented explicitly in space with 15 million atom structures. An excited state spectroscopy technique is presented in which the externally applied electric field is swept to probe the ladder of the electronic energy …
Dynamical Consequences of Nontrivial Topology of Molecular Conical Intersections
该研究通过线性Jahn-Teller模型推导半整数拓扑不变量，结合表面跳跃模拟发现，分子锥形交叉的非平凡拓扑会改变相同本征面的跳跃速率，拓展了拓扑对分子激发态动力学的影响范围。
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