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Tao Li

Publications and source records attributed to Tao Li.

At least 559 records · Page 31Linked to original sources
arXiv2013

Photonic spatial Bell-state analysis for robust quantum secure direct communication using quantum dot-cavity systems

Recently, experiments showed that the spatial-mode states of entangled photons are more robust than their polarization-mode states in quantum communications. Here we construct a complete and deterministic protocol for analyzing the spatial Bell states using the interaction between a photon and an electron spin in a charged quantum dot inside a one-side micropillar microcavity. A quantum nondemolition detector (QND) for checking the parity of a two-photon system can be constructed with the giant optical Faraday rotation in this solid state system. With this parity-check QND, we present a complete and deterministic proposal for the analysis of the four spatial-mode Bell states. Moreover, we present a robust two-step quantum secure direct communication protocol based on the spatial-mode Bell states and the photonic spatial Bell-state analysis. Our analysis shows that our BSA proposal works in both the strong and the weak coupling regimes if the side leakage and cavity loss rate is small.

quant-ph↗
arXiv2013

High-efficiency multipartite entanglement purification of electron-spin states with charge detection

We present a high-efficiency multipartite entanglement purification protocol (MEPP) for electron-spin systems in a Greenberger-Horne-Zeilinger state based on their spins and their charges. Our MEPP contains two parts. The first part is our normal MEPP with which the parties can obtain a high-fidelity N-electron ensemble directly, similar to the MEPP with controlled-not gates. The second one is our recycling MEPP with entanglement link from N'-electron subsystems (2 < N' < N). It is interesting to show that the N'-electron subsystems can be obtained efficiently by measuring the electrons with potential bit-flip errors from the instances which are useless and are just discarded in all existing conventional MEPPs. Combining these two parts, our MEPP has the advantage of the efficiency higher than other MEPPs largely for electron-spin systems.

quant-ph↗
arXiv2013

Rank and genus of 3-manifolds

We construct a counterexample to the Rank versus Genus Conjecture, i.e. a closed orientable hyperbolic 3-manifold with rank of its fundamental group smaller than its Heegaard genus. Moreover, we show that the discrepancy between rank and Heegaard genus can be arbitrarily large for hyperbolic 3-manifolds. We also construct toroidal such examples containing hyperbolic JSJ pieces.

math.GT↗
arXiv2013

Taut foliations in knot complements

We show that for any nontrivial knot in $S^3$, there is an open interval containing zero such that a Dehn surgery on any slope in this interval yields a 3-manifold with taut foliations. This generalizes a theorem of Gabai on zero frame surgery.

math.GT↗
arXiv2012

Topological entanglement entropy in Gutzwiller projected spin liquids

The topological entanglement entropy(TEE) of Gutzwiller projected RVB state is studied with Monte Carlo simulation. New tricks are proposed to improve the convergence of TEE, which enable us to show that the spin liquid state studied in Ref.\cite{Vishwanath} actually does not support $Z_{2}$ topological order, a conclusion that is consistent with the information drawn from the inspection of the topological degeneracy on the same state. We find both a long ranged RVB amplitude and an approximate Marshall sign structure are at the origin of the suppression of vison gap in this spin liquid state. On the other hand, robust signature of $Z_{2}$ topological order, i.e., a TEE of $\ln2$, is clearly demonstrated for a Gutzwiiler projected RVB state on triangular lattice which is evolved from the RVB state proposed originally by Anderson\cite{Sorella}. We also find that it is the sign, rather than the amplitude of the RVB wave function, that dominates the TEE and is responsible for a positive value of TEE, which implies that the nonlocal entanglement in the RVB state is mainly encoded in the sign of the RVB wave function. Our results indicate that some information that is important for the topological property of a RVB state is missed in the effective field theory description and that a $Z_{2}$ gauge structure in the saddle point action is not enough for the RVB state to exhibit topological order, even if its spin correlation is extremely short ranged.

cond-mat.str-el↗
arXiv2012

An efficient Monte Carlo algorithm for the evaluation of Renyi entanglement entropy of a general quantum dimer model at the R-K point

A highly efficient and simple to implement Monte Carlo algorithm is proposed for the evaluation of the Renyi entanglement entropy(REE) of quantum dimer model(QDM) at the Rokhsar-Kivelson(R-K) point. It makes possible the evaluation of REE at the R-K point to the thermodynamic limit for a general QDM. We apply the algorithm to QDM on both triangular and square lattice as demonstrations and find the REE on both lattices follow perfect linear scaling in the thermodynamic limit, apart from an even-odd oscillation in the latter case. We also evaluate the topological entanglement entropy(TEE) on both lattices with a subtraction procedure. While the expected TEE of $\ln2$ is clearly demonstrated for QDM on triangular lattice, a strong oscillation of the result is found for QDM on square lattice, which implies the relevance of boundary perturbation in such a critical system.

cond-mat.str-el↗
arXiv2012

Complete hyperentangled-Bell-state analysis for photon systems assisted by quantum-dot spins in optical microcavities

Bell-state analysis (BSA) is essential in quantum communication, but it is impossible to distinguish unambiguously the four Bell states in the polarization degree of freedom (DOF) of two-photon systems with only linear optical elements, except for the case in which the BSA is assisted with hyperentangled states, the simultaneous entanglement in more than one DOF. Here, we propose a scheme to distinguish completely the 16 hyperentangled Bell states in both the polarization and the spatial-mode DOFs of two-photon systems, by using the giant nonlinear optics in quantum dot-cavity systems. This scheme can be applied to increase the channel capacity of long-distance quantum communication based on hyperentanglement, such as entanglement swapping, teleportation, and superdense coding. We use hyperentanglement swapping as an example to show the application of this HBSA.

quant-ph↗
arXiv2012

Complete deterministic multi-electron Greenberger-Horne-Zeilinger state analyzer for quantum communication

We present a scheme for the multi-electron Greenberger-Horne-Zeilinger (GHZ) state analyzer, resorting to an interface between the polarization of a probe photon and the spin of an electron in a quantum dot embedded in a microcavity. All the multi-spin GHZ states can be completely discriminated by using single-photon detectors and linear optical elements. Our scheme has some features. First, it is a complete GHZ-state analyzer for multi-electron spin systems. Second, the initial entangled states remain after being identified and they can be used for a successive task. Third, the electron qubits are static and the photons play a role of a medium for information transfer, which has a good application in quantum repeater in which the electron qubits are used to store the information and the photon qubits are used to transfer the information between others.

quant-ph↗
arXiv2012

Single-photon-assisted entanglement concentration of a multi-photon system in a partially entangled W state with weak cross-Kerr nonlinearity

We propose a nonlocal entanglement concentration protocol (ECP) for $N$-photon systems in a partially entangled W state, resorting to some ancillary single photons and the parity-check measurement based on cross-Kerr nonlinearity. One party in quantum communication first performs a parity-check measurement on her photon in an $N$-photon system and an ancillary photon, and then she picks up the even-parity instance for obtaining the standard W state. When she obtains an odd-parity instance, the system is in a less-entanglement state and it is the resource in the next round of entanglement concentration. By iterating the entanglement concentration process several times, the present ECP has the total success probability approaching to the limit in theory. The present ECP has the advantage of a high success probability. Moreover, the present ECP requires only the $N$-photon system itself and some ancillary single photons, not two copies of the systems, which decreases the difficulty of its implementation largely in experiment. It maybe have good applications in quantum communication in future.

quant-ph↗
arXiv2012

Dynamics of entanglement for a two-parameter class of states in a qubit-qutrit system

We investigate the dynamics of entanglement for a two-parameter class of states in a hybrid qubit-qutrit system under the influence of various dissipative channels. Our results show that entanglement sudden death (ESD) is a general phenomenon and it usually takes place in a qubit-qutrit system interacting with various noisy channels, not only the case with dephasing and depolarizing channels observed by others. ESD can only be avoided for some initially entangled states under some particular noisy channels. Moreover, the environment affects the entanglement and the coherence of the system in very different ways.

quant-ph↗
arXiv2012

Gapped spin liquid phase in the J1-J2 Heisenberg model by a Bosonic resonating valence-bond ansatz

We study the ground-state phase diagram of the spin-1/2 J1-J2 Heisenberg model on the square lattice with an accurate Bosonic resonating valence-bond (RVB) wave function. In contrast to the RVB ansatz based on Schwinger Fermions, the representation based on Schwinger Bosons, supplemented by a variational Monte Carlo technique enforcing the exact projection onto the physical subspace, is able to describe a fully gapped spin liquid in the strongly frustrated regime. In particular, a fully symmetric Z2 spin liquid is stable between two antiferromagnetic phases; a continuous transition at J2=0.4J1, when the Marshall sign rule begins to be essentially violated, and a first-order transition around J2=0.6J1 are present. Most importantly, the triplet gap is found to have a non-monotonic behavior, reaching a maximum around J2=0.51J1, when the lowest spinon excitation moves from the Γto the M point, i.e., k=(π,0).

cond-mat.str-el↗
arXiv2011

Spontaneous quantum Hall effect in quarter-doped Hubbard model on honeycomb lattice and its possible realization in quarter-doped graphene system

We show that as the result of the nesting property of the Fermi surface, the quarter-doped Hubbard model on honeycomb lattice is unstable with respect to the formation of a magnetic insulating state with nonzero spin chirality for infinitesimally small strength of electron correlation. The insulating state is found to be topological nontrivial and to have a quantized Hall conductance of $σ_{xy}=\frac{e^{2}}{h}$. We find the Fermi surface nesting is robust for arbitrary value of next-nearest-neighbor hopping integral. It is thus very possible that the quarter-doped graphene system will realize such an exotic ground state. We also show that the quarter-doped Hubbard model on honeycomb lattice is in exact equivalence in the weak coupling limit with the 3/4-filled Hubbard model on triangular lattice, in which similar effect is also observed.

cond-mat.str-el↗
arXiv2011

Three-dimensional Orientation Sensors by Defocused Imaging of Gold Nanorods through an Ordinary Wide-Field Microscope

Gold (Au) nanoparticles particularly nanorods are actively exploited as imaging probes because of their special nonblinking and nonbleaching absorption, scattering and emitting properties that arise from the excitation of surface plasmons. Herein, we report a novel orientation sensor at nanoscale by defocused imaging of single Au nanorods (AuNRs) through an ordinary wide-field optical microscope. By simultaneously recording defocused images and two-photon luminescence intensities for a large number of single AuNRs, we correlate their defocused images with their three-dimensional spatial orientations. As many AuNRs can be monitored in parallel, defocused imaging of single AuNRs is a high throughput sensing technique that allows us to obtain their spatial orientations within one frame in situ and real-time. The probe size can be down to several nanometers, which is highly desirable in order to minimize any potential interference from the probe itself. Furthermore, the sensing property is insensitive to the excitation polarization and the distribution of the probe aspect ratio, which allows AuNRs of any length in a proper regime to be used as orientation sensors without changing the laser frequency and polarization. These unique features make the orientation probes proposed here outstanding candidates for optical imaging and sensing in material science and biology.

physics.optics↗
arXiv2011

A Statistically Modelling Method for Performance Limits in Sensor Localization

In this paper, we study performance limits of sensor localization from a novel perspective. Specifically, we consider the Cramer-Rao Lower Bound (CRLB) in single-hop sensor localization using measurements from received signal strength (RSS), time of arrival (TOA) and bearing, respectively, but differently from the existing work, we statistically analyze the trace of the associated CRLB matrix (i.e. as a scalar metric for performance limits of sensor localization) by assuming anchor locations are random. By the Central Limit Theorems for $U$-statistics, we show that as the number of the anchors increases, this scalar metric is asymptotically normal in the RSS/bearing case, and converges to a random variable which is an affine transformation of a chi-square random variable of degree 2 in the TOA case. Moreover, we provide formulas quantitatively describing the relationship among the mean and standard deviation of the scalar metric, the number of the anchors, the parameters of communication channels, the noise statistics in measurements and the spatial distribution of the anchors. These formulas, though asymptotic in the number of the anchors, in many cases turn out to be remarkably accurate in predicting performance limits, even if the number is small. Simulations are carried out to confirm our results.

eess.SY↗
arXiv2011

Orbital magnetic response and the anisotropy of magnetic susceptibility in the Iron-based superconductors

We propose that the orbital angular momentum of the conduction electrons in the Iron-based superconductors is activated in their low energy physics. Using a five-band tight-binding model derived from fitting the LDA band structure, we find that the orbital magnetic susceptibility of the conduction electrons in such a multi-orbital system is several times larger than the Pauli spin susceptibility and is comparable in magnitude to the observed total magnetic susceptibility. The orbital magnetic susceptibility in the Fe-As plane($χ^{x}_{L}$) is found to be larger than that perpendicular to the Fe-As plane($χ^{z}_{L}$) by a factor about two and the total magnetic susceptibility in the normal state can be fitted with formula $χ(T,θ)\approx χ_{s}(T)+χ_{L}(θ)$, where $χ_{s}(T)$ is the temperature dependent isotropic part due to spin and $χ_{L}(θ)$ is the temperature independent anisotropic part due to orbital. In the superconducting state, $χ^{x}_{L}$ is found to be significantly reduced as the pairing gap develops, while $χ^{z}_{L}$ is almost not affected by the superconducting transition. We argue the large anisotropy observed in the bulk magnetic susceptibility and the Knight shift in the Iron-based superconductors should be attributed to the orbital magnetic response of their conduction electrons.

cond-mat.supr-con↗
arXiv2011

Antiferromagnetic long range order in the uniform resonating valence bond state on square lattice

With extensive variational Monte Carlo simulation, we show that the uniform resonating valence bond state(U-RVB) on square lattice is actually antiferromagnetic long range ordered. The ordered moment is estimated to be $m\approx 0.17$. Finite size scaling analysis on lattice up to lattice size of $50\times 50$ shows that the spin structure factor at the antiferromagnetic ordering wave vector follows perfectly the $\mathrm{S}_{\mathrm{q}=(π,π)} \simeq S_{0}+α(1/L)^{5/4}$ behavior, where $L$ is linear scale of the lattice. Such a behavior is quite unexpected from the slave Boson mean field treatment or the Gutzwiller approximation of the uniform RVB state.

cond-mat.str-el↗
arXiv2011

Chiral d-wave RVB state on honeycomb lattice as a generalized staggered flux phase

We show the chiral d-wave RVB state on honeycomb lattice stands as a natural generalization of the staggered flux phase on square lattice. Although the state is generated from a time reversal symmetry broken mean field ansatz, it actually represents a fully symmetric spin liquid state with a positive definite wave function in the sense of Marshall sign rule for unfrustrated antiferromagnets. The evolution of the state with the parameter $Δ/χ$ follows exactly the same manner as that of the staggered flux phase on square lattice. The critical pairing strength corresponding to the $π$-flux phase is found to be $Δ/χ=\sqrt{2}$. As a result of the geometric frustration between neighboring plaquette on honeycomb lattice, a direct generalization of the U(1) staggered flux pattern on square lattice to honeycomb lattice is impossible. Replacing it is the chiral d-wave state with $Z_{2}$ gauge structure. However, this $Z_{2}$ gauge structure is found to be ineffective after Gutzwiller projection and the system does not support topological degeneracy. The chiral d-wave RVB state is also found to be a rather good variational state for the Heisenberg model on honeycomb lattice. The spin correlation of the chiral d-wave state is found to be greatly enhanced as compared to the mean field prediction.

cond-mat.str-el↗