Local Quantum Coherence Research Articles

Coherence and realism in the Aharonov-Bohm effect

The Aharonov-Bohm effect is a fundamental topological phenomenon with a wide range of applications. It consists of a charge encircling a region with a magnetic flux in a superposition of wave packets having their relative phase affected by the flux. In this work we analyze this effect using an entropic measure known as realism, originally introduced as a quantifier of a system's degree of reality and mathematically related to notions of global and local quantum coherence. More precisely, we look for observables that lead to gauge-invariant realism associated with the charge before it completes its loop. We find that the realism of these operators has a sudden change when the line connecting the center of both wave packets crosses the solenoid. Moreover, we consider the case of a quantized magnetic-field source, pointing out similarities and differences between the two cases. Finally, we discuss some consequences of these results.

Deterministic controlled enhancement of local quantum coherence

We investigate assisted enhancement of quantum coherence in a bipartite setting with control and target systems, which converts the coherence of the control qubit into the enhanced coherence of the target qubit. We assume that only incoherent operations and measurements can be applied locally and classical information can be exchanged. In addition, the two subsystems are also coupled by a fixed Hamiltonian whose interaction strength can be controlled. This coupling does not generate any local coherence from incoherent input states. We show that in this setting a measurement and feed-forward based protocol can deterministically enhance the coherence of the target system while fully preserving its purity. The protocol can be iterated and several copies of the control state can be consumed to drive the target system arbitrarily close to a maximally coherent state. We experimentally demonstrate this protocol with a photonic setup and observe the enhancement of coherence for up to five iterations of the protocol.

Two-qubit Heisenberg XYZ dynamics of local quantum Fisher information, skew information coherence: Dyzaloshinskii–Moriya interaction and decoherence

We theoretically demonstrate how to achieve quantum entanglement control of a dissipative anisotropic two-qubit Heisenberg XYZ model. New dynamical features of local quantum coherence, local quantum Fisher information and Bures distance entanglement are achieved by controlling the initial state setting, DM interaction and decoherence. The proposed scheme allows for the generation of sudden changes in the entanglement. We illustrate the control by manipulating the local quantum coherence, Fisher information dynamics, and the sudden appearance of the disentanglement intervals. We further consider simultaneous changes in these behaviors for different values of the system parameters. These results provide helpful significance for understanding the dynamics of the dissipative anisotropic two-qubit Heisenberg XYZ model.

Experimental demonstration of the conversion of local and correlated Gaussian quantum coherence.

Quantum coherence plays an important role in quantum information processing. In this Letter, we experimentally demonstrate the conversion of local and correlated Gaussian quantum coherence in the process of converting two squeezed states into an entangled state. We also investigate the relationship among total, local, and correlated coherence and show that the total coherence of a two-mode Gaussian state is the sum of local quantum coherence of each mode and the correlated quantum coherence between two modes. Our results highlight the connection of different quantum coherence in a two-mode Gaussian system and provide references for potential application.

Local quantum uncertainty as a robust metric to characterize discord-like quantum correlations in subsets of the chromophores in photosynthetic light-harvesting complexes

Green sulfur bacteria is a photosynthetic organism whose light-harvesting complex accommodates a pigment-protein complex called Fenna-Matthews-Olson (FMO). The FMO complex sustains quantum coherence and quantum correlations between the electronic states of spatially separated pigment molecules as energy moves with nearly a 100% quantum efficiency to the reaction center. We present a method based on the quantum uncertainty associated to local measurements to quantify discord-like quantum correlations between two subsystems where one is a qubit and the other is a qudit. We implement the method by calculating local quantum uncertainty (LQU), concurrence, and coherence between subsystems of pure and mixed states represented by the eigenstates and by the thermal equilibrium state determined by the FMO Hamiltonian. Three partitions of the seven chromophores network define the subsystems: one chromophore with six chromophores, pairs of chromophores, and one chromophore with two chromophores. Implementation of the LQU approach allows us to characterize quantum correlations that had not been studied before, identify the most quantum correlated subsets of chromophores, and determine that, in the strongest associations of chromophores, the LQU is a monotonically increasing function of the coherence.

Quantum transport and the phase space structure of the Wightman functions

We study the phase space structure of exact quantum Wightman functions in spatially homogeneous, temporally varying systems. In addition to the usual mass shells, the Wightman functions display additional coherence shells around zero frequency k0 = 0, which carry the information of the local quantum coherence of particle-antiparticle pairs. We find also other structures, which encode non-local correlations in time, and discuss their role and decoherence. We give a simple derivation of the cQPA formalism, a set of quantum transport equations, that can be used to study interacting systems including the local quantum coherence. We compute quantum currents created by a temporal change in a particle’s mass, comparing the exact Wightman function approach, the cQPA and the semiclassical methods. We find that the semiclassical approximation, which is fully encompassed by the cQPA, works surprisingly well even for very sharp temporal features. This is encouraging for the application of semiclassical methods in electroweak baryogenesis with strong phase transitions.

Sharing of nonlocal advantage of quantum coherence by sequential observers

Non-local Advantage of Quantum Coherence(NAQC) or steerability of local quantum coherence is a strong non-local resource based on coherence complementarity relations. In this work, we provide an upper bound on the number of observers who can independently steer the coherence of the observer in the other wing in a scenario where half of an entangled pair of spin-$\frac{1}{2}$ particles is shared between a single observer (Bob) in one wing and several observers (Alices) on the other, who can act sequentially and independently of each other. We consider one-parameter dichotomic POVMs for the Alices and mutually unbiased basis in which Bob measures coherence in case of the maximally entangled bipartite qubit state. We show that not more than two Alices can exhibit NAQC when $l_1$-norm of coherence measure is probed, whereas for two other measures of coherence, only one Alice can reveal NAQC within the same framework.

Local and intrinsic quantum coherence in critical systems

Local and intrinsic quantum coherence, as two contributions of total quantum coherence, have been introduced by Radhakrishnan et al. [C. Radhakrishnan, M. Parthasarathy, S. Jambulingam, and T. Byrnes, Phys. Rev. Lett. 116, 150504 (2016)]. In the present work we study the property of local, intrinsic, and total quantum coherence in critical systems such as the $XY$ model, extended $XY$ model, and Ashkin-Teller model to analyze their capability in identifying the second-order, infinite-order quantum phase transitions, as well as the topological quantum phase transitions. It is shown that not only the total coherence, but also the local and intrinsic coherence can spotlight the critical points efficiently. Moreover, the long-range properties of total and local coherence, and the short-range properties of intrinsic coherence are analyzed. The total and local coherence for spin-pairs farther than nearest neighbors can highlight the critical points while the intrinsic coherence is absent for long-distance spin-pairs.

Quantum coherence, uncertainty, nonlocal advantage of quantum coherence as indicators of quantum phase transition in the transverse Ising model

In this paper, we investigate the relation among local quantum coherence, quantum uncertainty, and the nonlocal advantage of quantum coherence based on skew information and quantum phase transition in the transverse Ising model by exploring the quantum renormalization group (QRG) method. The results reveal that the amount of the local quantum uncertainty is equal to the local quantum coherence corresponding to the local observable in the model, which can be generalized to a multipartite system. Moreover, the nonlocal advantage of quantum coherence is investigated, and we found that regardless of the value of the external magnet field and the number of QRG iterations, the quantum coherence of the subsystem was steerable, which is not only suitable for the two sites of the block, but also for the nearest-neighbor blocks in the long-ranged ferromagnetic phase. However, as the system becomes large enough, the quantum coherence of the subsystem is not steerable in the paramagnetic phase. Additionally, the QRG implementation of quantum coherence and uncertainty are effective and feasible to detect the quantum critical points associated with quantum phase transitions. We also make use of the QRG method to analyze the thermodynamic limit of the current model and the emergence of the nonanalytic and scaling behaviors of the nonlocal advantage of quantum coherence.

Probing quantum coherence, uncertainty, steerability of quantum coherence and quantum phase transition in the spin model

In this paper, we study the relation among quantum coherence, uncertainty, steerability of quantum coherence based on skew information and quantum phase transition in the spin model by employing quantum renormalization-group method. Interestingly, the results show that the value of the local quantum uncertainty is equal to the local quantum coherence corresponding to local observable $$\sigma _z$$?z in XXZ model, and unlikely in XY model, local quantum uncertainty is minimal optimization of the local quantum coherence over local observable $$\sigma _x$$?x and this proposition can be generalized to a multipartite system. Therefore, one can directly achieve quantum correlation measured by local quantum uncertainty and coherence by choosing different local observables $$\sigma _x$$?x, $$\sigma _z$$?z, corresponding to the XY model and XXZ model separately. Meanwhile, steerability of quantum coherence in XY and XXZ model is investigated systematically, and our results reveal that no matter what times the QRG iterations are carried out, the quantum coherence of the state of subsystem cannot be steerable, which can also be suitable for block---block steerability of local quantum coherence in both XY and XXZ models. On the other hand, we have illustrated that the quantum coherence and uncertainty measure can efficiently detect the quantum critical points associated with quantum phase transitions after several iterations of the renormalization. Moreover, the nonanalytic and scaling behaviors of steerability of local quantum coherence have been also taken into consideration.

The complementarity relations of quantum coherence in quantum information processing

We establish two complementarity relations for the relative entropy of coherence in quantum information processing, i.e., quantum dense coding and teleportation. We first give an uncertainty-like expression relating local quantum coherence to the capacity of optimal dense coding for bipartite system. The relation can also be applied to the case of dense coding by using unital memoryless noisy quantum channels. Further, the relation between local quantum coherence and teleportation fidelity for two-qubit system is given.

Quantum coherence and quantum phase transitions.

We study the connections between local quantum coherence (LQC) based on Wigner-Yanase skew information and quantum phase transitions (QPTs). When applied on the one-dimensional Hubbard, XY spin chain with three-spin interaction, and Su-Schrieffer-Heeger models, the LQC and its derivatives are used successfully to detect different types of QPTs in these spin and fermionic systems. Furthermore, the LQC is effective as the quantum discord (QD) in detecting QPTs at finite temperatures, where the entanglement has lost its effectiveness. We also demonstrate that the LQC can exhibit different behaviors in many forms compared with the QD.

Quantum coherence and uncertainty in the anisotropic XY chain

We explore the local quantum coherence and the local quantum uncertainty, based on Wigner-Yanase skew information, in the ground state of the anisotropic spin-1/2 XY chain in transverse magnetic field. We show that the skew information, as a figure of merit, supplies the necessary information to reveal the occurrence of the second order phase transition and the completely factorized ground state in the XY model. Additionally, in the same context, we also discuss the usefulness of a simple experimentally friendly lower bound of local quantum coherence. Furthermore, we demonstrate how the connection between the appearance of non-analyticities in the local quantum uncertainty of the ground state and the quantum phase transitions does not hold in general, by providing explicit examples of the situation. Lastly, we discuss the ability of the local quantum coherence to accurately estimate the critical point of the phase transition, and investigate the robustness of the factorization phenomenon at low temperatures.

A new kind of electron-lattice interaction and the kinetics of breaking high-T c and low-T c superconductivity via large thermal energy fluctuations of lattice particles

A novel kind of a strong electron-lattice interaction in high-T c (HTSC's) and conventional low-T c (LTSC's)superconductors mediated by short-lived large energy fluctuations (SLEF's) of lattice atoms (ions) of lifetime Δτ≈10−13−10−12s, is considered and applied to the kinetics of thermal pair-breaking in the HTSC's and LTSC's. The transition from the superconducting (SC) state into a non-SC state at temperaturesT c in HTSC's or LTSC's is caused by a great number of SC pair-SLEF collisions each of which breaks the local quantum coherence and creates a local instability of the SC state. Quantum macroscopic percolation-like phenomena appear in HTSC's or LTSC's and destroy the SC state atT→T c when the mean distance between the simultaneously existing SLEF-induced local instabilities becomes of the order of the SC coherence length ζ. The transition temperaturesT c and pairing energies 2 Δ as well as coupling constants in HTSC's and LTSC's are calculated and linked with some material parameters (the elasticity modulus, Debye temperature, SC pair density, etc.) through a modification of our earlier proposed theory of SLEF's and electron-SLEF interactions (Phys. Rep.99, 237 (1983) and Phys. Rev. B33, 2983 (1986)). Quasi-2-dimensional properties of carriers in HTSC's and the 3-dimensional nature of SC electrons in LTSC's are taken into account. The obtained new exponential equations for 2 Δ have pre-exponential factor ℏωF≈ℏv F/d determined by the Fermi velocityv F and the interatomic distanced. The transition temperature shows only a weak, if at all, isotope in HTSC's and the conventional isotope effect in LTSC's, in agreement with observations. Numerical estimates ofT c and 2 Δ are in agreement with experimental data for both HTSC's and LTSC's. The comparison of HTSC and LTSC properties is discussed.