arXiv subjects

L. Tian

Publications and source records attributed to L. Tian.

At least 19 recordsLinked to original sources

arXiv2026

Electroluminescence in dopant-free GaAs/AlGaAs single heterojunctions: 2D free excitons, H-band, and the tidal effect

Bright electroluminescence (EL) from dopant-free ambipolar lateral p-n junctions in GaAs/AlGaAs single heterointerface (SH) heterostructures is used to probe neutral free excitons arising from two-dimensional electron and hole gases (2DEGs and 2DHGs). The EL spectra reveal both the heavy-hole neutral free exciton (X$^0$) and the high-energy free exciton of the H band (HE). A combination of transition energies, lifetimes, spatial emission profiles, and temperature dependences points to a predominantly two-dimensional character for these excitons at the SH. For X$^0$, the EL peak energies (1515.5-1515.7 meV) lie slightly above the corresponding bulk GaAs photoluminescence (PL) line at 1515.3 meV, while time-resolved measurements yield markedly shorter lifetimes for EL than for PL (337 ps vs. 1610 ps), consistent with recombination in a confined interfacial layer. The HE exciton exhibits a Stark blueshift under forward bias below threshold, and its energies and lifetimes (down to 575 ps) are tuned by the topgate voltage; above threshold, HE emission is quenched in favor of X$^0$. Finally, the tidal effect $-$ a form of pulsed EL generated by swapping the topgate voltage polarity in ambipolar field-effect transistors $-$ produces an X$^0$ line at the same energy as in the lateral p-n junction and reproduces the characteristic nonmonotonic frequency dependence of the brightness previously observed in quantum-well heterostructures, again indicating a 2D-like origin. Taken together, these results show electrically generated and controllable 2D-like excitons (HE and X$^0$), thereby bridging 2D exciton physics and 2DEG/2DHG platforms in dopant-free GaAs/AlGaAs SH devices.

cond-mat.mes-hall↗

arXiv2025

Pulsed Generation of Continuous-Variable Cluster States in a Phononic Quantum Network

Cluster states are multipartite entangled states that are maximally connected and resilient to decoherence, making them valuable resources for quantum information processing. Continuous-variable (CV) cluster states have been extensively investigated for such applications. Here we present a pulsed protocol for generating CV cluster states in a phononic quantum network composed of phonon waveguides, mechanical resonators, and optical cavities. A key feature of this architecture is its modular design, where pairs of mechanical modes serve as building blocks with only local, tunable interactions between mechanical and cavity modes. We characterize the generated cluster states through the average nullifier of the CV modes. Our numerical results show that high-quality CV cluster states can be generated with moderate squeezing parameters, whereas strong squeezing and the resulting large phonon occupations can degrade the cluster states in the presence of finite dissipation. We further show that, under moderate squeezing and dissipation, the average nullifier can decrease with increasing system size $N$, demonstrating the scalability of the proposed scheme. As a direct application, we demonstrate that distant mechanical modes can be entangled through local measurements.

quant-ph↗

arXiv2024

Local Counterdiabatic Driving for Jaynes-Cummings Lattices

Jaynes-Cummings (JC) lattices can be constructed by connecting quantum two-level systems with cavities and have been widely studied for polariton many-body states and multipartite entanglement. Although adiabatic evolution has been studied for the generation of many-body states in this system, its reliance on long timescales can lead to serious decoherence. Here we present a scheme that utilizes local counterdiabatic (CD) driving to provide fast and high-fidelity state preparation in JC lattices. The exact CD Hamiltonian for this system contains nonlocal couplings between qubits and cavities at different and distant sites, which causes a challenge in the implementation. Leveraging the symmetries of the eigenstates under both periodic and open boundary conditions, we derive a local CD Hamiltonian that generates the same dynamics as the exact CD Hamiltonian and our numerical simulations confirm this result. We also show that a multipartite W-state can be prepared with high fidelity using this method. The implementation and decoherence of this scheme with superconducting quantum devices are also discussed.

quant-ph↗

arXiv2024

Pulsed Electroluminescence in a Dopant-free Gateable Semiconductor

We report on a stable form of pulsed electroluminescence in a dopant-free direct bandgap semiconductor heterostructure which we coin the tidal effect. Swapping an inducing gate voltage in an ambipolar field effect transistor allows incoming and outgoing carriers of opposite charge to meet and recombine radiatively. We develop a model to explain the carrier dynamics that underpins the frequency response of the pulsed electroluminescence intensity. Higher mobilities enable larger active emission areas than previous reports, as well as stable emission over long timescales.

cond-mat.mes-hall↗

arXiv2022

A Method to Load Tellurium in Liquid Scintillator for the Study of Neutrinoless Double Beta Decay

A method has been developed to load tellurium into liquid scintillator so as to permit searches for neutrinoless double beta decay with high sensitivity. The approach involves the synthesis of an oil-soluble tellurium compound from telluric acid and an organic diol. The process utilises distillable chemicals that can be safely handled underground and affords low radioactive backgrounds, low optical absorption and high light yields at loading levels of at least several percent Te by weight.

physics.ins-det↗

arXiv2015

Quantum coherence in ultrastrong optomechanics

Ultrastrong light-matter interaction in an optomechanical system can result in nonlinear optical effects such as photon blockade. The system-bath couplings in such systems play an essential role in observing these effects. Here we study the quantum coherence of an optomechanical system with a dressed-state master equation approach. Our master equation includes photon-number-dependent terms that induce dephasing in this system. Cavity dephasing, second-order photon correlation, and two-cavity entanglement are studied with the dressed-state master equation.

quant-ph↗

arXiv2011

Cavity cooling of a mechanical resonator in the presence of two-level-system defects

Cavity cooling via quantum backaction force can extract thermal fluctuations from a mechanical resonator to reach the quantum ground state. Surface or bulk two-level-system (TLS) defects in a mechanical resonator can couple with the mechanical mode via deformation potential and affect the cooling process significantly. Here, we develop a theory to study the cavity cooling of a mechanical mode in the presence of a TLS defect using the adiabatic elimination technique. Our result shows that the cooling process depends strongly on the resonance and the damping rate of the TLS.

quant-ph↗

arXiv2011

Deterministic Generation of Entangled Photons in Superconducting Resonator Arrays

We present a scheme for the deterministic generation of entangled photon pairs in a superconducting resonator array. The resonators form a Jaynes-Cummings lattice via the coupling to superconducting qubits, and the Kerr-like nonlinearity arises due to the coupling.We show that entangled photons can be generated on demand by applying spectroscopic techniques and exploiting the nonlinearity and symmetry in the resonators. The scheme is robust against small parameter spreads due to fabrication errors. Our findings can be used as a key element for quantum information processing in superconducting quantum circuits.

quant-ph↗

arXiv2010

Optical wavelength conversion of quantum states with optomechanics

An optomechanical interface that converts quantum states between optical fields with distinct wavelengths is proposed. A mechanical mode couples to two optical modes via radiation pressure and mediates the quantum state mapping between the two optical modes. A sequence of optomechanical $π/2$ pulses enables state-swapping between optical and mechanical states as well as the cooling of the mechanical mode. Theoretical analysis shows that high fidelity conversion can be realized for states with small photon numbers in systems with experimentally achievable parameters. The pulsed conversion process also makes it possible to maintain high conversion fidelity at elevated bath temperatures.

quant-ph↗

arXiv2010

Circuit QED and sudden phase switching in a superconducting qubit array

Superconducting qubits connected in an array can form quantum many-body systems such as the quantum Ising model. By coupling the qubits to a superconducting resonator, the combined system forms a circuit QED system. Here, we study the nonlinear behavior in the many-body state of the qubit array using a semiclassical approach. We show that sudden switchings as well as a bistable regime between the ferromagnetic phase and the paramagnetic phase can be observed in the qubit array. A superconducting circuit to implement this system is presented with realistic parameters .

quant-ph↗

arXiv2009

A Controllable Interaction between Two-Level Systems inside a Josephson Junction

Two-level system fluctuators (TLS's) in the tunnel barrier of a Josephson junction have recently been demonstrated to cause novel energy splittings in spectroscopic measurements of superconducting phase qubits. With their strong coupling to the Josephson junction and relatively long decoherence times, TLS's can be considered as potential qubits and demonstrate coherent quantum effects. Here, we study the effective interaction between the TLS qubits that is mediated by a Josephson junction resonator driven by an external microwave source. This effective interaction can enable controlled quantum logic gates between the TLS's. Our study can be extended to other superconducting resonators coupling with TLS's.

quant-ph↗

arXiv2009

Realistic quantum manipulation of two-level system fluctuators

Two-level system fluctuators in superconducting devices have demonstrated coherent coupling with superconducting qubits. Here, we show that universal quantum logic gates can be realized in these two-level systems solely by tuning a superconducting resonator in which they are imbedded. Because of the large energy separation between the fluctuators, conventional gate schemes in the cavity QED approach that are widely used for solid-state qubits cannot be directly applied to the fluctuators. We study a scheme to perform the gate operations by exploiting the controllability of the superconducting resonator with realistic parameters. Numerical simulation that takes into account the decay of the resonator mode shows that the quantum logic gates can be realized with high fidelity at moderate resonator decay rate. The quantum logic gates can also be realized between fluctuators inside different Josephson junctions that are connected by a superconducting loop. Our scheme can be applied to explore the coupling between two-level system fluctuators and superconducting resonators as well as the coherent properties of the fluctuators.

quant-ph↗

arXiv2008

Ground state cooling of nanomechanical resonator via parametric linear coupling

We present a ground state cooling scheme for a nanomechanical resonator linearly coupled with a LC oscillator. The linear coupling, when periodically modulated at red detuning, up-converts the low-frequency nanomechanical mode to the high-frequency LC oscillator mode and generates backaction force that can cool the resonator to its ground state in the resolved-sideband regime. We also study the effect of the quantum backaction noise on the cooling due to the counter rotating term in the linear coupling. The scheme can be compared with laser cooling for the atomic systems and can be realized in superconducting circuits.

quant-ph↗

arXiv2008

Parametric coupling between macroscopic quantum resonators

Time-dependent linear coupling between macroscopic quantum resonator modes generates both a parametric amplification also known as a {}"squeezing operation" and a beam splitter operation, analogous to quantum optical systems. These operations, when applied properly, can robustly generate entanglement and squeezing for the quantum resonator modes. Here, we present such coupling schemes between a nanomechanical resonator and a superconducting electrical resonator using applied microwave voltages as well as between two superconducting lumped-element electrical resonators using a r.f. SQUID-mediated tunable coupler. By calculating the logarithmic negativity of the partially transposed density matrix, we quantitatively study the entanglement generated at finite temperatures. We also show that characterization of the nanomechanical resonator state after the quantum operations can be achieved by detecting the electrical resonator only. Thus, one of the electrical resonator modes can act as a probe to measure the entanglement of the coupled systems and the degree of squeezing for the other resonator mode.

cond-mat.mes-hall↗

arXiv2008

Threshold fields for antiparallel ferroelectric domain wall motion

While an ideal antiparallel ferroelectric wall is considered a unit cell in width (~0.5nm), we show using phase field modeling that the threshold field for moving this wall dramatically drops by 2-3 orders of magnitude if the wall were diffuse by only ~2-3nm. Since antiparallel domain walls are symmetry allowed in all ferroelectrics, and since domain wall broadening on nanometer scale is widely reported in literature, this mechanism is generally applicable to all ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2007

Recovery of Hidden Interference in Mott Insulators

Particle statistics plays a crucial role in a strongly interacting quantum many-body system. Here, we study the Hubbard model for distinguishable particles at unit filling. Starting from the superfluid-like state in the strong tunneling limit and gradually reducing the tunneling so that the on-site repulsive interaction dominates, the state ends up in a symmetric superposition of Mott insulator states. This result can be experimentally confirmed by the recovery of interference patterns in the density correlation functions. We also show that this state is a maximally entangled state, in contrast to the standard picture.

quant-ph↗

arXiv2007

A Josephson Junction Microscope for Low-frequency Fluctuators

The high-Q harmonic oscillator mode of a Josephson junction can be used as a novel probe of spurious two-level systems (TLSs) inside the amorphous oxide tunnel barriers of the junction. In particular, we show that spectroscopic transmission measurements of the junction resonator mode can reveal how the coupling magnitude between the junction and the TLSs varies with an external magnetic field applied in the plane of the tunnel barrier. The proposed experiments offer the possibility of clearly resolving the underlying coupling mechanism for these spurious TLSs, an important decoherence source limiting the quality of superconducting quantum devices.

cond-mat.mes-hall↗

arXiv2006

Multiferroic Domain Dynamics in Strained Strontium Titanate

Multiferroicity can be induced in strontium titanate by applying biaxial strain, resulting in the coexistence of both ferroelectric and antiferrodistortive domains. The magnitude and sign of the strain imposed on the lattice by design can be used to tune the phase transitions and interactions between these two phenomena. Using optical second harmonic generation, we report a transition from centrosymmetric 4/mmm phase to ferroelectric mm2, followed by an antiferrodistortive transition to a coupled ferroelastic-ferroelectric mm2 phase in a strontium titanate thin film strained in biaxial tension by 0.94%. The results agree well with theoretical first principles and phase-field predictions. Direct imaging of domains arising from the ferroelectric phase transition, and its switching under electric fields is demonstrated using piezoelectric force microscopy. Nonlinear optics combined with phase-field modeling is used to show that the dominant multiferroic domain switching mechanism is through coupled 90 degree ferroelectric-ferroelastic domain wall motion. More broadly, these studies of coexisting ferroelectric (polar) and antiferrodistortive rotation (axial) phenomena could have relevance to multiferroics with coexisting ferroelectric (polar) and magnetic (axial) phenomena.

cond-mat.mtrl-sci↗