Measurement-Assisted Quantum Communication in Spin Channels with Dephasing

We propose a protocol for countering the effects of dephasing in quantum state transfer over a noisy spin channel weakly coupled to the sender and receiver qubits. Our protocol, based on performing regular global measurements on the channel, significantly suppresses the nocuous environmental effects and offers much higher fidelities than the traditional no-measurement approach. Our proposal can also operate as a robust two-qubit entangling gate over distant spins. Our scheme counters any source of dephasing, including those for which the well established dynamical decoupling approach fails. Our protocol is probabilistic, given the intrinsic randomness in quantum measurements, but its success probability can be maximized by adequately tuning the rate of the measurements.Comment: 8 pages, 9 figure

Entanglement Transfer Through an Antiferromagnetic Spin Chain

We study the possibility of using an uniformly coupled finite antiferromagnetic spin-1/2 Heisenberg chain as a channel for transmitting entanglement. One member of a pair of maximally entangled spins is initially appended to one end of a chain in its ground state and the dynamical propagation of this entanglement to the other end is calculated. We show that compared to the analogous scheme with a ferromagnetic chain in its ground state, here the entanglement is transmitted faster, with less decay, with a much higher purity and as a narrow pulse form rising non-analytically from zero. Here non-zero temperatures and depolarizing environments are both found to be less destructive in comparison to the ferromagnetic case. The entanglement is found to propagate through the chain in a peculiar fashion whereby it hops to skip alternate sites.Comment: 5 pages, 5 figures. Modified version with more explanatio

Erosion behavior and scour risk of extremely coarse streambeds

Bridge scour, which is the erosion of soil and rock around bridge abutments and piers, is the principal cause of bridge failure in the United States and around the world. Previous investigations of scour have focused mostly on fine sediments such as sand, silt, and small gravel, because such alluvium underlies a majority of bridges. The erosion resistance of coarser sediments has received limited attention to date, even though they dominate many small to medium-size rivers in the northern tiers of the United States, Europe, and Asia. This study focuses on the scour behavior of extremely coarse particles (ECP), namely cobbles and boulders. A main objective of this research is to develop a relationship between critical (entrainment) velocity and grain size for sediment particles in the size range of 5 to 50 cm (2 to 20 in). This is accomplished by performing a limit analysis of sediments that exist within the stream beds of actual bridges. A basic premise is that the residual sediments reflect the maximum historic flow and velocity that has occurred over the life of the bridge. Thirty-five bridges in Northern New Jersey are initially screened for the study, and 12 bridges are selected for final analysis. Field visits are made to characterize the grain size distribution of the ECP sediments present at each site. Due to the extreme coarseness of the sediments, nontraditional methods are employed such as optical granulometry and statistical pebble counts. To assure geologic and hydrologic diversity for the data set, the sites span three of New Jersey’s physiographic provinces: Highlands, Valley and Ridge, and Piedmont. Hydraulic analyses are used to estimate the maximum velocity that the bridge has experienced during its lifetime. These are based on the maximum discharges measured at USGS gaging stations or computed with USGS StreamStats software. Final limit velocities range from 245 to 549 cm/sec (8 to 18 ft. /sec.). The limit analysis is performed by regression of the particle size and velocity data. This yields a nonlinear, exponential relationship between critical velocity and median particle size. The variance fraction associated with the data set is 0.642, indicating a reasonable fit. The limit analysis results are also compared with traditional sediment transport relationships, including Newton’s Law and Hjulstrøm’s envelope. Several applications of the limit analysis relationship are proposed and explored. The first is a method to assess the general scour risk for bridges underlain by ECP sediments. First, the median grain size of the bed sediments is measured. The corresponding limit velocity is then computed and compared with the desired scour design storm, e.g., Q100. If the design scour velocity exceeds the limit velocity, then the bridge is considered to have a higher scour risk. The limit analysis curve is also used to extend the useful range of the standard scour design equations, including the HEC-18 critical velocity and USACE EM 1601 riprap relations. Extrapolation of the limit results generally produces lower and more conservative critical velocities within the ECP size range than did the standard relations. Another application provides adjustment coefficients, which are useful for rapid photographic measurement of sediments (size and gradation) using WipFrag

Many-body Localization Transition: Schmidt Gap, Entanglement Length & Scaling

Many-body localization has become an important phenomenon for illuminating a potential rift between non-equilibrium quantum systems and statistical mechanics. However, the nature of the transition between ergodic and localized phases in models displaying many-body localization is not yet well understood. Assuming that this is a continuous transition, analytic results show that the length scale should diverge with a critical exponent $\nu \ge 2$ in one dimensional systems. Interestingly, this is in stark contrast with all exact numerical studies which find $\nu \sim 1$. We introduce the Schmidt gap, new in this context, which scales near the transition with a exponent $\nu > 2$ compatible with the analytical bound. We attribute this to an insensitivity to certain finite size fluctuations, which remain significant in other quantities at the sizes accessible to exact numerical methods. Additionally, we find that a physical manifestation of the diverging length scale is apparent in the entanglement length computed using the logarithmic negativity between disjoint blocks.Comment: 8 pages, 7 figure

Entanglement Routers Using Macroscopic Singlets

We propose a mechanism where high entanglement between very distant boundary spins is generated by suddenly connecting two long Kondo spin chains. We show that this procedure provides an efficient way to route entanglement between multiple distant sites. We observe that the key features of the entanglement dynamics of the composite spin chain are remarkably well described using a simple model of two singlets, each formed by two spins. The proposed entanglement routing mechanism is a footprint of the emergence of a Kondo cloud in a Kondo system and can be engineered and observed in varied physical settings.Comment: 4 pages, 5 figures. Slightly longer than the published versio

Kondo Cloud Mediated Long Range Entanglement After Local Quench in a Spin Chain

We show that, in the gapless Kondo Regime, a single local quench at one end of a Kondo spin chain induces a fast and long lived oscillatory dynamics. This quickly establishes a high quality entanglement between the spins at the opposite ends of the chain. This entanglement is mediated by the Kondo Cloud, attains a constant high value independent of the length for large chains, and shows thermal robustness. In contrast, when the Kondo cloud is absent, e.g. in the gapped dimer regime, only finite size end to end effects can create some entanglement on a much longer time-scale for rather short chains. By decoupling one end of the chain during the dynamics one can distinguish between this end-end effect which vanishes, and the global Kondo cloud mediated entanglement, which persists. This quench approach paves the way to detect the elusive Kondo cloud through the entanglement between two individual spins. Our results show that non-perturbative cooperative phenomena from condensed matter may be exploited for quantum information.Comment: 4 pages, 4 figures. All comments are welcome

Spin State Transfer in Laterally Coupled Quantum Dot Chains with Disorders

Quantum dot arrays are a promising media for transferring quantum information between two distant points without resorting to mobile qubits. Here we study two most common disorders namely, hyperfine interaction and exchange coupling fluctuations, in quantum dot arrays and their effects on quantum communication through these chains. Our results show that the hyperfine interaction is more destructive than the exchange coupling fluctuations. The average optimal time for communication is not affected by any disorder in the system and our simulations show that anti-ferromagnetic chains are much more resistive than the ferromagnetic ones against both kind of disorders. Even when time modulation of a coupling and optimal control is employed to improve the transmission, the anti-ferromagnetic chain performs much better. We have assumed the quasi-static approximation for hyperfine interaction and time dependent fluctuations in the exchange couplings. Particularly, for studying exchange coupling fluctuations we have considered the static disorder, white noise and $1/f$ noise.Comment: 10 pages, 12 figures. Comments are welcome

Machine Learning Assisted Many-Body Entanglement Measurement

Entanglement not only plays a crucial role in quantum technologies, but is key to our understanding of quantum correlations in many-body systems. However, in an experiment, the only way of measuring entanglement in a generic mixed state is through reconstructive quantum tomography, requiring an exponential number of measurements in the system size. Here, we propose a machine learning assisted scheme to measure the entanglement between arbitrary subsystems of size $N_A$ and $N_B$, with $\mathcal{O}(N_A + N_B)$ measurements, and without any prior knowledge of the state. The method exploits a neural network to learn the unknown, non-linear function relating certain measurable moments and the logarithmic negativity. Our procedure will allow entanglement measurements in a wide variety of systems, including strongly interacting many body systems in both equilibrium and non-equilibrium regimes.Comment: 16 pages, 10 figures, including appendi

Global Control Methods for GHZ State Generation on 1-D Ising Chain

We discuss how to prepare an Ising chain in a GHZ state using a single global control field only. This model does not require the spins to be individually addressable and is applicable to quantum systems such as cold atoms in optical lattices, some liquid- or solid-state NMR experiments, and many nano-scale quantum structures. We show that GHZ states can always be reached asymptotically from certain easy-to-prepare initial states using adiabatic passage, and under certain conditions finite-time reachability can be ensured. To provide a reference useful for future experimental implementations three different control strategies to achieve the objective, adiabatic passage, Lyapunov control and optimal control are compared, and their advantages and disadvantages discussed, in particular in the presence of realistic imperfections such as imperfect initial state preparation, system inhomogeneity and dephasing.Comment: 13 pages, 11 figure