arXiv subjects

Ting Yu

Publications and source records attributed to Ting Yu.

At least 127 records · Page 7Linked to original sources

arXiv2014

Extracting work from the magnetic field coupled Brownian particles

Thermodynamics of the magnetic field coupled Brownian particles is studied. We show that in the presence of the magnetic field, work can be extracted from the reservoir even when the measurement operation and the potential change operation are applied in different spatial directions. In particular, we show that more work can be extracted if the measurements are applied in two different directions simultaneously. In all these cases, we show that the generalized second law involving the measurement information and potential change is satisfied. In addition, we show how the continuous potential change and measurement position affect the work extraction.

cond-mat.stat-mech↗

arXiv2014

Exact non-Markovian master equations for multiple qubit systems: quantum trajectory approach

A wide class of exact master equations for a multiple qubit system can be explicitly constructed by using the corresponding exact non-Markovian quantum state diffusion equations. These exact master equations arise naturally from the quantum decoherence dynamics of qubit system as a quantum memory coupled to a collective colored noisy source. The exact master equations are also important in optimal quantum control, quantum dissipation and quantum thermodynamics. In this paper, we show that the exact non-Markovian master equation for a dissipative N-qubit system can be derived explicitly from the statistical average of the corresponding non-Markovian quantum trajectories. We illustrated our general formulation by an explicit construction of a three-qubit system coupled to a non-Markovian bosonic environment. This multiple qubit master equation offers an accurate time evolution of quantum systems in various domains, and paves a way to investigate the memory effect of an open system in a non-Markovian regime without any approximation.

quant-ph↗

arXiv2014

Fault-tolerant breathing pattern in optical lattices as a dynamical quantum memory

Proposals for quantum information processing often require the development of new quantum tech- nologies. However, here we build quantum memory by ultracold atoms in one-dimensional optical lattices with existing state-of-the-art technology. Under a parabolic external field, we demonstrate that an arbitrary initial state at an end of the optical lattices can time-evolve and revive, with very high fidelity, at predictable discrete time intervals. Physically, the parabolic field, can catalyze a breathing pattern. The initial state is memorized by the pattern and can be retrieved at any of the revival time moments. In comparison with usual time-independent memory, we call this a dynamical memory. Furthermore, we show that the high fidelity of the quantum state at revival time moments is fault-tolerant against the fabrication defects and even time-dependent noise.

quant-ph↗

arXiv2014

Evolution of Raman G and G'(2D) Modes in Folded Graphene Layers

Bernal- and non-Bernal-stacked graphene layers have been systematically studied by Raman imaging and spectroscopy. Two dominant Raman modes, G and G' (or 2D) of folded graphene layers exhibit three types of spectral features when interlayer lattice mismatches, defined by a rotational angle varies. Among these folded graphene layers, the most interesting one is the folded graphene layers that present an extremely strong G mode enhanced by a twist-induced Van Hove singularity. The evolution of Raman G and G' modes of such folded graphene layers are probed by changing the excitation photon energies. For the first time, doublet splitting of the G' mode in folded double-layer (1 + 1) and of the G mode in folded tetra-layer (2 + 2) graphene are clearly observed and discussed. The G' mode splitting in folded double-layer graphene is attributed to the coexistence of inner and outer scattering processes and the trigonal warping effect as well as further downwards bending of the inner dispersion branch at visible excitation energy. While the two peaks of the G mode in folded tetra-layer graphene are assigned to Raman-active mode (E2g) and lattice mismatch activated infrared-active mode (E1u), which is further verified by the temperature-dependent Raman measurements. Our study provides a summary and thorough understanding of Raman spectra of Bernal- and non-Bernal-stacked graphene layers and further demonstrates the versatility of Raman spectroscopy for exploiting electronic band structures of graphene layers.

physics.optics↗

arXiv2014

Environment-Assisted Quantum State Restoration Via Weak Measurement

In this paper, a new quantum state restoration scheme is proposed based on the environment-assisted error correction (EAEC) scheme. By introducing a weak measurement reversal (WMR) operation, we shall show how to recover an initial state of a quantum open system without invoking random unitary decompositions which are known to be absent in many important physical systems. We illustrate our new scheme and compare it with a scheme purely based on the WMR operation in the case of dissipative channel. In the proposed new scheme, the successful probability of recovering an unknown initial state can be significantly improved when the information obtained from an environment measurement is taken into account. Moreover, we show that the applicable range of our proposed scheme is wider than the EAEC scheme. Finally, the optimization of the successful probability for possible Kraus decompositions is discussed.

quant-ph↗

arXiv2014

Obtaining Information about Queries behind Views and Dependencies

We consider the problems of finding and determining certain query answers and of determining containment between queries; each problem is formulated in presence of materialized views and dependencies under the closed-world assumption. We show a tight relationship between the problems in this setting. Further, we introduce algorithms for solving each problem for those inputs where all the queries and views are conjunctive, and the dependencies are embedded weakly acyclic. We also determine the complexity of each problem under the security-relevant complexity measure introduced by Zhang and Mendelzon in 2005. The problems studied in this paper are fundamental in ensuring correct specification of database access-control policies, in particular in case of fine-grained access control. Our approaches can also be applied in the areas of inference control, secure data publishing, and database auditing.

cs.DB↗

arXiv2013

Non-Markovian Quantum Trajectories of Many-Body Quantum Open Systems

A long-standing open problem in non-Markovian quantum state diffusion (QSD) approach to open quantum systems is to establish the non-Markovian QSD equations for multiple qubit systems. In this paper, we settle this important question by explicitly constructing a set of exact time-local QSD equations for $N$-qubit systems. Our exact time-local (convolutionless) QSD equations have paved the way towards simulating quantum dynamics of many-body open systems interacting with a common bosonic environment. The applicability of this multiple-qubit stochastic equation is exemplified by numerically solving several quantum open many-body systems concerning quantum coherence dynamics and dynamical control.

quant-ph↗

arXiv2013

Resonant Low Frequency Interlayer Shear Modes in Folded Graphene Layers

Naturally or artificially stacking extra layers on single layer graphene (SLG) forms few-layer graphene (FLG), which has attracted tremendous attention owing to its exceptional properties inherited from SLG and new features generated by introducing extra freedom. In FLG, shear modes play a critical role in understanding its distinctive properties. Unfortunately, energies of shear modes are so close to excitation lasers to be fully blocked by a Rayleigh rejecter. This greatly hinders investigations of shear modes in FLG. Here, we demonstrate dramatically enhanced shear modes in properly folded FLG. Benefiting from the extremely strong signals, for the first time, enhancement mechanism, vibrational symmetry, anharmonicity and electron-phonon coupling (EPC) of shear modes are uncovered through studies of two-dimensional (2D) Raman mapping, polarization- and temperature-dependent Raman spectroscopy. This work complements Raman studies of graphene layers, and paves an efficient way to exploit low frequency shear modes of FLG and other 2D layered materials.

cond-mat.mes-hall↗

arXiv2013

Synthesis and optical properties of large-scale single-crystalline two-dimensional semiconductor WS2 monolayer from chemical vapor deposition

Two-dimensional (2D) transition metal dichalcogenides (TMDs), especially MoS2 and WS2 recently attract extensive attentions due to their rich physics and great potential applications. Superior to graphene, MS2 (M = Mo/W) monolayers have a native direct energy gap in visible frequency range. This promises great future of MS2 for optoelectronics. To exploit properties and further develop more applications, producing large-scale single crystals of MS2 by a facile method is highly demanded. Here, we report the synthesis of large-scale triangular single crystals of WS2 monolayer from a chemical vapor deposition process and systematic optical studies of such WS2 monolayers. The observations of high yield of light emission and valley-selective circular dichroism experimentally evidence the high optical quality of the WS2 monolayers. This work paves the road to fabricate large-scale single crystalline 2D semiconductors and study their fundamentals. It must be very meaningful for exploiting great potentials of WS2 for future optoelectronics.

cond-mat.mtrl-sci↗

arXiv2013

Detector-induced backaction on the counting statistics of a double quantum dot

Full counting statistics of electron transport is of fundamental importance for a deeper understanding of the underlying physical processes in quantum transport in nanoscale devices. The backaction effect from a detector on the nanoscale devices is also essential due to its inevitable presence in experiments. Here we investigate the backaction of a charge detector in the form of a quantum point contact (QPC) on the counting statistics of a biased double quantum dot (DQD). We show that this inevitable QPC-induced backaction can have profound effects on the counting statistics under certain conditions, e.g., changing the shot noise from being sub-Poissonian to super-Poissonian, and changing the skewness from being positive to negative. Also, we show that both Fano factor and skewness can be either enhanced or suppressed by increasing the energy difference between two single-dot levels of the DQD under the detector-induced backaction.

cond-mat.mes-hall↗

arXiv2013

Dynamics of coupled cavity arrays embedded in a non-Markovian bath

In this paper, the non-Markovian quantum dynamics of a coupled $N$-cavity model is studied based on the quantum state diffusion (QSD) approach. The time-local Diósi-Gisin-Strunz equation and the corresponding exact master equation are derived for the model consisting of a coupled cavity array. The resulting QSD equation serves as a stochastic solution to a genuine $N$-partite continuous-variable (CV) system. Several non-Markovian effects are studied in two interesting examples -- two-cavity and three-cavity, under different boundary conditions. We have shown that the environment-memory can facilitate the cat-like state transfer from one cavity to another in the case of a strongly non-Markovian environment.

quant-ph↗

arXiv2013

Non-Perturbative Quantum Dynamical Decoupling

Current dynamical control based on the bang-bang control mechanism involving various types of pulse sequences is essentially a perturbative theory. This paper presents a non-perturbative dynamical control approach based on the exact stochastic Schrödinger equation. We report our findings on the pulse parameter regions in which the effective dynamical control can be exercised. The onset of the effective control zones reflects the non-perturbative feature of our approach. The non-perturbative methods offer possible new implementations when several different parameter regions are available.

quant-ph↗

arXiv2013

One Component Dynamical Equation and Noise Induced Adiabaticity

The adiabatic theorem addresses the dynamics of a target instantaneous eigenstate of a time-dependent Hamiltonian. We use a Feshbach P-Q partitioning technique to derive a closed one-component integro-differential equation. The resultant equation properly traces the footprint of the target eigenstate. The physical significance of the derived dynamical equation is illustrated by both general analysis and concrete examples. Surprisingly, we find an anomalous phenomenon showing that a dephasing white noise can enhance and even induce adiabaticity. This new phenomenon may naturally occur in many physical systems. We also show that white noises can also shorten the total duration of dynamic processes such as adiabatic quantum computing.

quant-ph↗

arXiv2013

Restoration of Quantum State in Dephasing Channel

In this paper, we propose an explicit scheme to fully recover a multiple-qubit state subject to a phase damping noise. We establish the theoretical framework and the operational procedure to restore an unknown initial quantum state for an N-qubit model interacting with either individual baths or a common bath. We give an explicit construction of the random unitary (RU) Kraus decomposition for an N-qubit model interacting with a common bath. We also demonstrate how to use only one unitary reversal operation to restore an arbitrary state with phase damping noise. In principle, the initial state can always be recovered with a success probability of 1. Interestingly, we found that non-RU decomposition can also be used to restore some particular entangled states. This may open a new path to restore a quantum state beyond the standard RU scheme.

quant-ph↗

arXiv2013

Non-Markovian Fermionic Stochastic Schrödinger Equation for Open System Dynamics

In this paper we present an exact Grassmann stochastic Schrödinger equation for the dynamics of an open fermionic quantum system coupled to a reservoir consisting of a finite or infinite number of fermions. We use this stochastic approach to derive the exact master equation for a fermionic system strongly coupled to electronic reservoirs. The generality and applicability of this Grassmann stochastic approach is justified and exemplified by several quantum open system problems concerning quantum decoherence and quantum transport for both vacuum and finite-temperature fermionic reservoirs. We show that the quantum coherence property of the quantum dot system can be profoundly modified by the environment memory.

quant-ph↗

arXiv2013

Beta-Ag2Te: A topological insulator with strong anisotropy

We present evidence of topological surface states in beta-Ag2Te through first-principles calculations and periodic quantum interference effect in single crystalline nanoribbon. Our first-principles calculations show that beta-Ag2Te is a topological insulator with a gapless Dirac cone with strong anisotropy. To experimentally probe the topological surface state, we synthesized high quality beta-Ag2Te nanoribbons and performed electron transport measurements. The coexistence of pronounced Aharonov-Bohm oscillations and weak Altshuler-Aronov-Spivak oscillations clearly demonstrates coherent electron transport around the perimeter of beta-Ag2Te nanoribbon and therefore the existence of metallic surface states, which is further supported by the temperature dependence of resistivity for beta-Ag2Te nanoribbons with different cross section areas. Highly anisotropic topological surface state of beta-Ag2Te suggests that the material is a promising material for fundamental study and future spintronic devices.

cond-mat.mes-hall↗

arXiv2013

Mining Frequent Graph Patterns with Differential Privacy

Discovering frequent graph patterns in a graph database offers valuable information in a variety of applications. However, if the graph dataset contains sensitive data of individuals such as mobile phone-call graphs and web-click graphs, releasing discovered frequent patterns may present a threat to the privacy of individuals. {\em Differential privacy} has recently emerged as the {\em de facto} standard for private data analysis due to its provable privacy guarantee. In this paper we propose the first differentially private algorithm for mining frequent graph patterns. We first show that previous techniques on differentially private discovery of frequent {\em itemsets} cannot apply in mining frequent graph patterns due to the inherent complexity of handling structural information in graphs. We then address this challenge by proposing a Markov Chain Monte Carlo (MCMC) sampling based algorithm. Unlike previous work on frequent itemset mining, our techniques do not rely on the output of a non-private mining algorithm. Instead, we observe that both frequent graph pattern mining and the guarantee of differential privacy can be unified into an MCMC sampling framework. In addition, we establish the privacy and utility guarantee of our algorithm and propose an efficient neighboring pattern counting technique as well. Experimental results show that the proposed algorithm is able to output frequent patterns with good precision.

cs.DB↗

arXiv2013

Terahertz conductivity of twisted bilayer graphene

Using terahertz time-domain spectroscopy, the real part of optical conductivity [$σ_{1}(ω)$] of twisted bilayer graphene was obtained at different temperatures (10 -- 300 K) in the frequency range 0.3 -- 3 THz. On top of a Drude-like response, we see a strong peak in $σ_{1} (ω)$ at $\sim$2.7 THz. We analyze the overall Drude-like response using a disorder-dependent (unitary scattering) model, then attribute the peak at 2.7 THz to an enhanced density of states at that energy, that is caused by the presence of a van Hove singularity arising from a commensurate twisting of the two graphene layers.

cond-mat.mes-hall↗