arXiv subjects

D. X. Li

Publications and source records attributed to D. X. Li.

13 recordsLinked to original sources

arXiv2024

Generation of complete graph states in a spin-$1/2$ Heisenberg chain with a globally optimized magnetic field

Graph states possess significant practical value in measurement-based quantum computation, with complete graph states that exhibit exceptional performance in quantum metrology. In this work, we introduce a method for generating multiparticle complete graph states using a spin-$1/2$ Heisenberg $XX$ chain subjected to a time-varying magnetic field, which applies to a wide range of systems. Our scheme relies exclusively on nearest-neighbor interactions between atoms, with real-time magnetic field formation facilitated by quantum optimal control theory. We focus specifically on neutral-atom systems, finding that multiparticle complete graph states with $N=3\sim6$ can be achieved in less than $0.25~μ{\rm s}$, utilizing a hopping amplitude of ${J}/{(2π)} = -2.443~{\rm MHz}$. This assumes an initial state provided by an equal-weight superposition of all spin states that are encoded by the dipolar interacting Rydberg states. Additionally, we thoroughly address various experimental imperfections and showcase the robustness of our approach against atomic vibrations, fluctuations in pulse amplitude, and spontaneous emission of Rydberg states. Considering the common occurrence of disturbances in experimental setups of neutral-atom systems, our one-step strategy for achieving such graph states emerges as a more empirically viable alternative to techniques based on controlled-Z gates.

quant-ph↗

arXiv2023

Change of superconducting character in UTe2induced by magnetic field

UTe2 is a recently discovered spin-triplet superconductor. One of the characteristic features of UTe2 is a magnetic field (H)-boosted superconductivity above 16 T when H is applied exactly parallel to the b axis. To date, this superconducting (SC) state has not been thoroughly investigated, and the SC properties as well as the spin state of this high-H SC (HHSC) phase are not well understood. In this study, we performed AC magnetic susceptibility and nuclear magnetic resonance (NMR) measurements and found that, up to 24.8 T, the HHSC state is intrinsic to UTe2 and quite sensitive to the H angle, and that its SC character is different from that in the low-H SC (LHSC) state. The dominant spin component of the spin-triplet pair is along the a axis in the LHSC state but is changed in the HHSC state along the b axis. Our results indicate that H-induced multiple SC states originate from the remaining spin degrees of freedom.

cond-mat.supr-con↗

arXiv2021

Disappearance of spin glass behavior in ThCr2Si2-type intermetallic PrAu2Si2

It is unexpected that a spin-glass transition, which generally occurs only in the system with some form of disorder, was observed in the ThCr2Si2-type compound PrAu2Si2 at a temperature of ~3 K. This puzzling phenomenon was later explained based on a novel dynamic frustration model that does not involve static disorder. We present the results of re-verification of the reported spin-glass behaviors by measuring the physical properties of three polycrystalline PrAu2Si2 samples annealed under different conditions. Indeed, in the sample annealed at 827 C for one week, a spin-glass transition does occur at a temperature of Tf=2.8 K as that reported previously in the literature. However, it is newly found that the spin-glass effect is actually more pronounced in the as-cast sample, and almost completely disappears in the well-annealed (at 850 C for 4 weeks) sample. The apparent sample dependence of the magnetic characteristics of PrAu2Si2 is discussed by comparing it with similar phenomena observed in the isomorphic compounds URh2Ge2 and CeAu2Si2. Our experimental results strongly suggest that the spin-glass behavior observed in the as cast and insufficient annealed samples is most likely due to the presence of small amount of crystalline impurities and/or partial site disorder on the Au and Si sublattices, and thus is not the inherent characteristic of ideal ThCr2Si2-type PrAu2Si2. The perfectly ordered PrAu2Si2 should be regarded as a paramagnetic system with obvious crystal-field effects.

cond-mat.str-el↗

arXiv2020

Dissipative engineering a tripartite Greenberger-Horne-Zeilinger state for neutral atoms

The multipartite Greenberger-Horne-Zeilinger (GHZ) states are indispensable elements for various quantum information processing tasks. Here we put forward two deterministic proposals to dissipatively prepare tripartite GHZ states in a neutral atom system. The first scheme can be considered as an extension of a recent work [T. M. Wintermantel, Y. Wang, G. Lochead, \textit{et al}, {Phys. Rev. Lett. \textbf{124}, 070503 (2020)}]. By virtue of the polychromatic driving fields and the engineered spontaneous emission, a multipartite GHZ state with odd numbers of atoms are generated with a high efficiency. This scheme effectively overcomes the problem of dependence on the initial state but sensitive to the decay of Rydberg state. In the second scenario, we exploit the spontaneous emission of the Rydberg states as a resource, thence a steady tripartite GHZ state with fidelity around $98\%$ can be obtained by simultaneously integrating the switching driving of unconventional Rydberg pumping and the Rydberg antiblockade effect.

quant-ph↗

arXiv2019

Deterministic generation of maximally discordant mixed states by dissipation

Entanglement can be considered as a special quantum correlation, but not the only kind. Even for a separable quantum system, it is allowed to exist non-classical correlations. Here we propose two dissipative schemes for generating a maximally correlated state of two qubits in the absence of quantum entanglement, which was raised by [F. Galve, G. L. Giorgi, and R. Zambrini, {\color{blue}Phys. Rev. A {\bf 83}, 012102 (2011)}]. These protocols take full advantages of the interaction between four-level atoms and strongly lossy optical cavities. In the first scenario, we alternatively change the phases of two classical driving fields, while the second proposal introduces a strongly lossy coupled-cavity system. Both schemes can realize all Lindblad terms required by the dissipative dynamics, guaranteeing the maximally quantum dissonant state to be the unique steady state for a certain subspace of system. Moreover, since the target state is a mixed state, the performance of our method is evaluated by the definition of super-fidelity $G(ρ_{1},ρ_{2})$, and the strictly numerical simulations indicate that fidelity outstripping $99\%$ of the quantum dissonant state is achievable with the current cavity quantum electrodynamics parameters.

quant-ph↗

arXiv2018

Rapid population transfer of a two-level system by a polychromatically driving field

We propose a simple exact analytical solution for a model consisting of a two-level system and a polychromatically driving field. It helps us to realize a rapid complete population transfer from the ground state to the excited state, and the system can be stable at the excited state for an extremely long time. A combination of the mechanism and the Rydberg atoms successfully prepares the Bell state and multipartite $W$ state, and the experimental feasibility is discussed via the current experimental parameters. Finally, the simple exact analytical solution is generalized into a three-level system, which leads to a significant enhancement of the robustness against dissipation.

quant-ph↗

arXiv2018

Unconventional Rydberg pumping and applications in quantum information processing

We propose a mechanism of unconventional Rydberg pumping (URP) via simultaneously driving each Rydberg atom by two classical fields with different strengths of Rabi frequencies. This mechanism differs from the general Rydberg blockade or Rydberg antiblockade since it is closely related to the ground states of atoms, i.e. two atoms in the same ground state are stable while two atoms in different ground states are resonantly excited. Furthermore, we find the URP can be employed to simplify some special quantum information processing tasks, such as implementation of a three-qubit controlled phase gate with only a single Rabi oscillation, preparation of two- and three-dimensional steady-state entanglement with two identical atoms, and realization of the autonomous quantum error correction in a Rydberg-atom-cavity system. The feasibility of the above applications is certified explicitly by the state-of-the-art technology.

quant-ph↗

arXiv2018

Engineering steady-state entanglement via dissipation and quantum Zeno dynamics in optical cavity

A new mechanism is proposed for dissipatively preparing maximal Bell entangled state of two atoms in an optical cavity. This scheme integrates the spontaneous emission, the light shift of atoms in the presence of dispersive microwave field, and the quantum Zeno dynamics induced by continuous coupling, to obtain a unique steady state irrespective of initial state. Even for a large cavity decay, a high-fidelity entangled state is achievable at a short convergence time, since the occupation of cavity mode is inhibited by the Zeno requirement. Therefore, a low single-atom cooperativity $C=g^2/(κγ)$ is good enough for realizing a high fidelity of entanglement in a wide range of decoherence parameters. As a straightforward extension, the feasibility for preparation of two-atom Knill-Laflamme-Milburn state with the same mechanism is also discussed.

quant-ph↗

arXiv2018

Effect of spin relaxations on the spin mixing conductances for a bilayer structure

The spin current can result in a spin-transfer torque in the normal-metal(NM)|ferromagnetic-insulator(FMI) or normal-metal(NM)|ferromagnetic-metal(FMM) bilayer. In the earlier study on this issue, the spin relaxations were ignored or introduced phenomenologically. In this paper, considering the FMM or FMI with spin relaxations described by a non-Hermitian Hamiltonian, we derive an effective spin-transfer torque and an effective spin mixing conductance in the non-Hermitian bilayer. The dependence of the effective spin mixing conductance on the system parameters (such as insulating gap, \textit{s-d} coupling, and layer thickness) as well as the relations between the real part and the imaginary part of the effective spin mixing conductance are given and discussed. We find that the effective spin mixing conductance can be enhanced in the non-Hermitian system. This provides us with the possibility to enhance the spin mixing conductance.

cond-mat.mes-hall↗

arXiv2017

Ground-state blockade of Rydberg atoms and application in entanglement generation

We propose a mechanism of ground-state blockade between two $N$-type Rydberg atoms in virtue of Rydberg-antiblockade effect and Raman transition. Inspired by the quantum Zeno effect, the strong Rydberg antiblockade interaction plays a role in frequently measuring one ground state of two, leading to a blockade effect for double occupation of the corresponding quantum state. By encoding the logic qubits into the ground states, we efficiently avoid the spontaneous emission of the excited Rydberg state, and maintain the nonlinear Rydberg-Rydberg interaction at the same time. As applications, we discuss in detail the feasibility of preparing two-atom and three-atom entanglement with ground-state blockade in closed system and open system, respectively, which shows that a high fidelity of entangled state can be obtained with current experimental parameters.

quant-ph↗

arXiv2003

In-Plane Magnetic Anisotropy In RF Sputtered Fe-N Thin Films

We have fabricated Fe(N) thin films with varied N2 partial pressure and studied the microstructure, morphology, magnetic properties and resistivity by using X-ray diffraction, atomic force microscopy, transmission electron microscopy, vibrating-sample magnetometer and angle-resolved M-H hysteresis Loop tracer and standard four-point probe method. In the presence of low N2 partial pressure, Fe(N) films showed a basic bcc a-Fe structure with a preferred (110) texture. A variation of in-plane magnetic anisotropy of the Fe(N) films was observed with the changing of N component. The evolution of in-plane anisotropy in the films was attributed to the directional order mechanism. Nitrogen atoms play an important role in refining the a-Fe grains and inducing uniaxial anisotropy.

cond-mat.mtrl-sci↗

arXiv2002

Thermal Properties of Heavy Fermion Compound YbP

Low-temperature specific heat and its field-dependence up to 16 T was measured in a stoichiometric single crystal of YbP. A sharp peak was observed at {\it T}$_{\rm N}$ = 0.53 K in zero magnetic field. Application of external field seems to induce a new magnetic phase above 11 T. The field dependence of the transition temperature in the high-field phase is different from that of the low field phase. The linear coefficient of the electronic specific heat is estimated as 120 mJ/mole K$^{2}$ from low temperature specfic heat, suggesting heavy Fermion state in YbP.

cond-mat.str-el↗

arXiv2001

Reversible Structural Transition in Epitaxial Manganite Film

A reversible structural transition of an epitaxial La2/3Sr1/3MnO3 film deposited on the LaAlO3 substrate has been investigated by means of in situ high-resolution transmission electron microscopy (HREM) and electron diffraction, combined with image and diffraction calculations. We observe that the crystallographic symmetry of the film can be lowered via electron beam irradiation, leading to a rhombohedral-monoclinic transition. This transition can be attributed to the cooperating effect of the mismatch stress and the irradiation-induced thermal-stress.

cond-mat.str-el↗