arXiv subjects

Heng Fan

Publications and source records attributed to Heng Fan.

At least 307 records · Page 17Linked to original sources

arXiv2016

Maximally coherent states and coherence-preserving operations

We investigate the maximally coherent states to provide a refinement in quantifying coherence and give a measure-independent definition of the coherence-preserving operations. A maximally coherent state can be considered as the resource to create arbitrary quantum states of the same dimension by merely incoherent operations. We propose that only the maximally coherent states should achieve the maximal value for a coherence measure and use this condition as an additional criterion for coherence measures to obtain a refinement in quantifying coherence which excludes the invalid and inefficient coherence measures. Under this new criterion, we then give a measure-independent definition of the coherence-preserving operations, which play a similar role in quantifying coherence as that played by the local unitary operations in the scenario of studying entanglement.

quant-ph↗

arXiv2016

Nematic orders and nematicity-driven topological phase transition in FeSe

We investigate nematic states in both bulk FeSe and FeSe thin films. It is found that their band structures and signature features that were observed in a variety of experiments can be perfectly explained by introducing the $d$-wave nematic orders that are required to have contributions from all $t_{2g}$ d-orbitals, which contradicts the conventional wisdom that the nematicity is simply driven by the orbital degeneracy between the $d_{xz}$ and $d_{yz}$ orbitals. These orders can be generated by the Coulomb interaction between the nearest neighbor Fe sites. In the presence of spin-orbital couplings, we predict that the nematic order can drive a topological quantum phase transition through a band inversion at the $M$ point of the Brillouin zone to produce topologically protected edge states near the Fermi level. The prediction makes FeSe as a tunable system to integrate topological properties into high temperature superconductivity to realize Majorana related physics.

cond-mat.supr-con↗

arXiv2016

Influence of relativistic effects on satellite-based clock synchronization

Clock synchronization between the ground and satellites is a fundamental issue in future quantum telecommunication, navigation, and global positioning systems. Here, we propose a scheme of near-Earth orbit satellite-based quantum clock synchronization with atmospheric dispersion cancellation by taking into account the spacetime background of the Earth. Two frequency entangled pulses are employed to synchronize two clocks, one at a ground station and the other at a satellite. The time discrepancy of the two clocks is introduced into the pulses by moving mirrors and is extracted by measuring the coincidence rate of the pulses in the interferometer. We find that the pulses are distorted due to effects of gravity when they propagate between the Earth and the satellite, resulting in remarkably affected coincidence rates. We also find that the precision of the clock synchronization is sensitive to the source parameters and the altitude of the satellite. The scheme provides a solution for satellite-based quantum clock synchronization with high precision, which can be realized, in principle, with current technology.

quant-ph↗

arXiv2016

Steering Bell-diagonal states

We investigate the steerability of two-qubit Bell-diagonal states under projective measurements by the steering party. In the simplest nontrivial scenario of two projective measurements, we solve this problem completely by virtue of the connection between the steering problem and the joint-measurement problem. A necessary and sufficient criterion is derived together with a simple geometrical interpretation. Our study shows that a Bell-diagonal state is steerable by two projective measurements iff it violates the Clauser-Horne-Shimony-Holt (CHSH) inequality, in sharp contrast with the strict hierarchy expected between steering and Bell nonlocality. We also introduce a steering measure and clarify its connections with concurrence and the volume of the steering ellipsoid. In particular, we determine the maximal concurrence and ellipsoid volume of Bell-diagonal states that are not steerable by two projective measurements. Finally, we explore the steerability of Bell-diagonal states under three projective measurements. A simple sufficient criterion is derived, which can detect the steerability of many states that are not steerable by two projective measurements. Our study offers valuable insight on steering of Bell-diagonal states as well as the connections between entanglement, steering, and Bell nonlocality.

quant-ph↗

arXiv2016

Evolution equation for quantum coherence

The estimation of the decoherence process of an open quantum system is of both theoretical significance and experimental appealing. Practically, the decoherence can be easily estimated if the coherence evolution satisfies some simple relations. Based on the coherence quantification method, we prove a simple factorization relation for the $l_1$ norm measure of coherence, and analyze under which condition this relation holds. We also obtain a more general relation which applies to arbitrary $N$-qubit state, and determine a condition for the transformation matrix of the quantum channel which can support permanently freezing of the $l_1$ norm of coherence. These results simplify determination of a general decoherence dynamics to that the investigation of evolution about the representative probe state.

quant-ph↗

arXiv2016

Entanglement Rényi $α$-entropy

We study the entanglement Rényi $α$-entropy (ER$α$E) as the measure of entanglement. Instead of a single quantity in standard entanglement quantification for a quantum state by using the von Neumann entropy for the well-accepted entanglement of formation (EoF), the ER$α$E gives a continuous spectrum parametrized by variable $α$ as the entanglement measure, and it reduces to the standard EoF in the special case $α\rightarrow 1$. The ER$α$E provides more information in entanglement quantification, and can be used such as in determining the convertibility of entangled states by local operations and classical communication. A series of new results are obtained: (i) we can show that ER$α$E of two states, which can be mixed or pure, may be incomparable, in contrast to the fact that there always exists an order for EoF of two states; (ii) similar as the case of EoF, we study in a fully analytical way the ER$α$E for arbitrary two-qubit states, the Werner states and isotropic states in general d-dimension; (iii) we provide a proof of the previous conjecture for the analytical functional form of EoF of isotropic states in arbitrary d-dimension.

quant-ph↗

arXiv2016

Quantum coherence of steered states

Lying at the heart of quantum mechanics, coherence has recently been studied as a key resource in quantum information theory. Quantum steering, a fundamental notion originally considered by Schr{ö}dinger, has also recently received much attention. When Alice and Bob share a correlated quantum system, Alice can perform a local measurement to `steer' Bob's reduced state. We introduce the maximal steered coherence as a measure describing the extent to which steering can remotely create coherence; more precisely, we find the maximal coherence of Bob's steered state in the eigenbasis of his original reduced state, where maximization is performed over all positive-operator valued measurements for Alice. We prove that maximal steered coherence vanishes for quantum-classical states whilst reaching a maximum for pure entangled states with full Schmidt rank. Although invariant under local unitary operations, maximal steered coherence may be increased when Bob performs a channel. For a two-qubit state we find that Bob's channel can increase maximal steered coherence if and only if it is neither unital nor semi-classical, which coincides with the condition for increasing discord. Our results show that the power of steering for coherence generation, though related to discord, is distinct from existing measures of quantum correlation.

quant-ph↗

arXiv2016

Staggered FeSe Monolayer on SrTiO$_3$ (110) surface

We investigate the electronic and magnetic properties of FeSe monolayer on the anisotropic SrTiO$_3$ (110) surface. With compressive strain along $[1\bar{1}0]$ direction from the substrate, the monolayer FeSe possesses a staggered bipartite iron lattice with an height difference around 0.06Å~ along the out-plane direction. The staggering causes stronger magnetic frustration between the collinear and checkerboard antiferromagnetic orders, and the strain elongates one electron and two hole pockets along the strain direction and the remaining hole pocket along the orthogonal direction. The strain-induced band splitting at $Γ$ can also result in a band inversion to drive the system into a topologically nontrivial phase. The absence of strong superconducting suppression on the staggered lattice suggests that the superconducting pairings may be insensitive to the modification of interactions and hopping parameters between two Fe sublattices.

cond-mat.supr-con↗

arXiv2015

Genuine multipartite entanglement detection and lower bound of multipartite concurrence

The problems of genuine multipartite entanglement detection and classification are challenging. We show that a multipartite quantum state is genuine multipartite entangled if the multipartite concurrence is larger than certain quantities given by the number and the dimension of the subsystems. This result also provides a classification of various genuine multipartite entanglement. Then, we present a lower bound of the multipartite concurrence in terms of bipartite concurrences. While various operational approaches are available for providing lower bounds of bipartite concurrences, our results give an effective operational way to detect and classify the genuine multipartite entanglement. As applications, the genuine multipartite entanglement of tripartite systems is analyzed in detail.

quant-ph↗

arXiv2015

Evolution equation for geometric quantum correlation measures

A simple relation is established for the evolution equation of quantum information processing protocols such as quantum teleportation, remote state preparation, Bell-inequality violation and particularly dynamics of the geometric quantum correlation measures. This relation shows that when the system traverses the local quantum channel, various figures of merit of the quantum correlations for different protocols demonstrate a factorization decay behavior for dynamics. We identified the family of quantum states for different kinds of quantum channels under the action of which the relation holds. This relation simplifies the assessment of many quantum tasks.

quant-ph↗

arXiv2015

Quantum Criticality in the Biased Dicke Model

The biased Dicke model describes a system of biased two-level atoms coupled to a bosonic field, and is expected to produce new phenomena that are not present in the original Dicke model. In this paper, we study the critical properties of the biased Dicke model in the classical oscillator limits. For the finite-biased case in this limit, We present analytical results demonstrating that the excitation energy does not vanish for arbitrary coupling. This indicates that the second order phase transition is avoided in the biased Dicke model, which contrasts to the original Dicke model. We also analyze the squeezing and the entanglement in the ground state, and find that a finite bias will strongly modify their behaviors in the vicinity of the critical coupling point.

quant-ph↗

arXiv2015

Revisitation of superconductivity in K$_2$Cr$_3$As$_3$ based on the six-band model

We investigate the pairing symmetry of the newly synthesized quasi-one-dimension K$_2$Cr$_3$As$_3$ superconductor based on the six-band model. We adopt standard random-phase-approximation to study the Hubbard-Hund model of the system. Our results confirm the conclusions obtained from our previous three-band model: the triplet $p_z$ and $f_{y^3-3x^2y}$ wave pairings serve as the leading pairing symmetries in the weak and strong Hund's rule coupling regimes, respectively. For physically realistic parameters, the triplet $p_z$-wave pairing driven by ferromagnetic fluctuations is the leading pairing symmetry of the system. The consistency between the results of these two models suggests that the obtained $p_z$-wave pairing symmetry is physical and model-independent.

cond-mat.supr-con↗

arXiv2015

Thermalization of Topological Entropy after a Quantum Quench

In two spatial dimensions, topological order is robust for static deformations at zero temperature, while it is fragile at any finite temperature. How robust is topological order after a quantum quench? In this paper we show that topological order thermalizes under the unitary evolution after a quantum quench. If the quench preserves gauge symmetry, there is a residual topological entropy exactly like in the finite temperature case. We obtain this result by studying the time evolution of the topological 2-Rényi entropy in a fully analytical, exact way. These techniques can be then applied to systems with strong disorder to show whether a many-body localization phenomenon appears in topologically ordered systems.

cond-mat.str-el↗

arXiv2015

Phase diagram of quantum critical system via local convertibility of ground state

We investigate the relationship between two kinds of ground-state local convertibility and quantum phase transitions in XY model. The local operations and classical communications (LOCC) convertibility is examined by the majorization relations and the entanglement-assisted local operations and classical communications (E- LOCC) via Renyi entropy interception. In the phase diagram of XY model, LOCC convertibility and ELOCC convertibility of ground-states are presented and compared. It is shown that different phases in the phase dia- gram of XY model can have different LOCC or ELOCC convertibility, which can be used to detect the quantum phase transition. This study will enlighten extensive studies of quantum phase transitions from the perspective of local convertibility, e.g., finite-temperature phase transitions and other quantum many-body models.

quant-ph↗

arXiv2015

Experimental testing of entropic uncertainty relations with multiple measurements in pure diamond

One unique feature of quantum mechanics is the Heisenberg uncertainty principle, which states that the outcomes of two incompatible measurements cannot simultaneously achieve arbitrary precision. In an information-theoretic context of quantum information, the uncertainty principle can be formulated as entropic uncertainty relations with two measurements for a quantum bit (qubit) in two-dimensional system. New entropic uncertainty relations are studied for a higher-dimensional quantum state with multiple measurements, the uncertainty bounds can be tighter than that expected from two measurements settings and cannot result from qubits system with or without a quantum memory. Here we report the first room-temperature experimental testing of the entropic uncertainty relations with three measurements in a natural three-dimensional solid-state system: the nitrogen-vacancy center in pure diamond. The experimental results confirm the entropic uncertainty relations for multiple measurements. Our result represents a more precise demonstrating of the fundamental uncertainty principle of quantum mechanics.

quant-ph↗

arXiv2015

Finite field calculations of static polarizabilities and hyperpolarizabilities of In$^{+}$ and Sr

The finite field calculations are performed for two heavy frequency-standard candidates In$^+$ and Sr. The progressive hierarchy of electron correlations is implemented by the relativistic coupled-cluster and configuration interaction methods combined with basis set of increasing size. The dipole polarizabilities, dipole hyperpolarizabilities, quadrupole moments, and quadrupole polarizabilities are recommended for the ground state 5s$^2$ $^1S_0$ and low-lying states 5s5p $^3P^{\rm o}_{0,1,2}$ of In$^+$ and Sr. Comparative study of the fully and scalar relativistic electron correlation calculations reveals the effect of the spin-orbit interaction on the dipole polarizabilities of In$^{+}$ and Sr. Finally, the blackbody-radiation shifts due to the dipole polarizability, dipole hyperpolarizability, and quadrupole polarizability are evaluated for the clock transition 5s$^2$ $^1S_0$ - 5s5p $^3P^{\rm o}_0$ of In$^+$ and Sr.

physics.atom-ph↗

arXiv2015

Triplet $p_z$-wave pairing in quasi one dimensional A$_2$Cr$_3$As$_3$ superconductors

We construct minimum effective models to investigate the pairing symmetry in the newly discovered quasi-one-dimensional superconductor K$_2$Cr$_3$As$_3$. We show that a minimum three-band model based on the $d_{z^2}$, $d_{xy}$ and $d_{x^2-y^2}$ orbitals of one Cr sublattice can capture the band structures near Fermi surfaces. In both weak and strong coupling limits, the standard random phase approximation (RPA) and mean-field solutions consistently yield the triplet $p_z$-wave pairing as the leading pairing symmetry for physically realistic parameters. The triplet pairing is driven by the ferromagnetic fluctuations within the sublattice. The gap function of the pairing state possesses line gap nodes on the $k_z=0$ plane on the Fermi surfaces.

cond-mat.supr-con↗

arXiv2015

Experimental consequences of $p_z$-wave spin triplet superconductivity in A$_2$Cr$_3$As$_3$

The experimental observable properties of the triplet $p_z$-wave pairing state, proposed by Wu {\em et al.} [arXiv:1503.06707] in quasi-one dimensional A$_2$Cr$_3$As$_3$ materials, are theoretically investigated. This pairing state is characterized by the line nodes on the $k_z=0$ plane on the Fermi surfaces. Based on the three-band tight binding model, we obtain the specific heat, superfluid density, Knight shift and spin relaxation rate and find that all these properties at low temperature ($T\ll T_c$) show powerlaw behaviors and are consistent available experiments. Particularly, the superfluid density determined by the $p_z$-wave pairing state in this quasi-one dimensional system is anisotropic: the in-plane superfluid density varies as $Δρ_{\parallel}\sim T$ but the out-plane one varies as $Δρ_{\perp}\sim T^3$ at low temperature. The anisotropic upper critical field reported in experiment is consistent with the $S_z=0$ (i.e., $(\uparrow\downarrow+\downarrow\uparrow)$) $p_z$-wave pairing state. We also suggest the phase-sensitive dc-SQUID measurements to pin down the triplet $p_z$-wave pairing state.

cond-mat.supr-con↗