500 research outputs found
Estimation of Spin-Spin Interaction by Weak Measurement Scheme
Precisely knowing an interaction Hamiltonian is crucial to realize quantum
information tasks, especially to experimentally demonstrate a quantum computer
and a quantum memory. We propose a scheme to experimentally evaluate the
spin-spin interaction for a two-qubit system by the weak measurement technique
initiated by Yakir Aharonov and his colleagues. Furthermore, we numerically
confirm our proposed scheme in a specific system of a nitrogen vacancy center
in diamond. This means that the weak measurement can also be taken as a
concrete example of the quantum process tomography.Comment: 4 pages, 1 table, 2 figures, to appear in Europhysics Letter
Asymptotic entanglement in 1D quantum walks with a time-dependent coined
Discrete-time quantum walk evolve by a unitary operator which involves two
operators a conditional shift in position space and a coin operator. This
operator entangles the coin and position degrees of freedom of the walker. In
this paper, we investigate the asymptotic behavior of the coin position
entanglement (CPE) for an inhomogeneous quantum walk which determined by two
orthogonal matrices in one-dimensional lattice. Free parameters of coin
operator together provide many conditions under which a measurement perform on
the coin state yield the value of entanglement on the resulting position
quantum state. We study the problem analytically for all values that two free
parameters of coin operator can take and the conditions under which
entanglement becomes maximal are sought.Comment: 23 pages, 4 figures, accepted for publication in IJMPB. arXiv admin
note: text overlap with arXiv:1001.5326 by other author
Quantum mechanics of time travel through post-selected teleportation
This paper discusses the quantum mechanics of closed-timelike curves (CTCs) and of other potential methods for time travel. We analyze a specific proposal for such quantum time travel, the quantum description of CTCs based on post-selected teleportation (P-CTCs). We compare the theory of P-CTCs to previously proposed quantum theories of time travel: the theory is inequivalent to Deutsch's theory of CTCs, but it is consistent with path-integral approaches (which are the best suited for analyzing quantum-field theory in curved space-time). We derive the dynamical equations that a chronology-respecting system interacting with a CTC will experience. We discuss the possibility of time travel in the absence of general-relativistic closed-timelike curves, and investigate the implications of P-CTCs for enhancing the power of computation.This paper discusses the quantum mechanics of closed-timelike curves (CTCs) and of other potential methods for time travel. We analyze a specific proposal for such quantum time travel, the quantum description of CTCs based on post-selected teleportation (P-CTCs). We compare the theory of P-CTCs to previously proposed quantum theories of time travel: the theory is inequivalent to Deutsch's theory of CTCs, but it is consistent with path-integral approaches (which are the best suited for analyzing quantum-field theory in curved space-time). We derive the dynamical equations that a chronology-respecting system interacting with a CTC will experience. We discuss the possibility of time travel in the absence of general-relativistic closed-timelike curves, and investigate the implications of P-CTCs for enhancing the power of computation
Thermodynamic formalism for dissipative quantum walks
We consider the dynamical properties of dissipative continuous-time quantum
walks on directed graphs. Using a large-deviation approach we construct a
thermodynamic formalism allowing us to define a dynamical order parameter, and
to identify transitions between dynamical regimes. For a particular class of
dissipative quantum walks we propose a quantum generalization of the the
classical PageRank vector, used to rank the importance of nodes in a directed
graph. We also provide an example where one can characterize the dynamical
transition from an effective classical random walk to a dissipative quantum
walk as a thermodynamic crossover between distinct dynamical regimes.Comment: 8 page
Optimal Covariant Measurement of Momentum on a Half Line in Quantum Mechanics
We cannot perform the projective measurement of a momentum on a half line
since it is not an observable. Nevertheless, we would like to obtain some
physical information of the momentum on a half line. We define an optimality
for measurement as minimizing the variance between an inferred outcome of the
measured system before a measuring process and a measurement outcome of the
probe system after the measuring process, restricting our attention to the
covariant measurement studied by Holevo. Extending the domain of the momentum
operator on a half line by introducing a two dimensional Hilbert space to be
tensored, we make it self-adjoint and explicitly construct a model Hamiltonian
for the measured and probe systems. By taking the partial trace over the newly
introduced Hilbert space, the optimal covariant positive operator valued
measure (POVM) of a momentum on a half line is reproduced. We physically
describe the measuring process to optimally evaluate the momentum of a particle
on a half line.Comment: 12 pages, 3 figure
Weak Values with Decoherence
The weak value of an observable is experimentally accessible by weak
measurements as theoretically analyzed by Aharonov et al. and recently
experimentally demonstrated. We introduce a weak operator associated with the
weak values and give a general framework of quantum operations to the W
operator in parallel with the Kraus representation of the completely positive
map for the density operator. The decoherence effect is also investigated in
terms of the weak measurement by a shift of a probe wave function of continuous
variable. As an application, we demonstrate how the geometric phase is affected
by the bit flip noise.Comment: 17 pages, 3 figure
Geometrical aspects of weak measurements and quantum erasers
We investigate the mechanism of weak measurement by using an interferometric
framework. In order to appropriately elucidate the interference effect that
occurs in weak measurement, we introduce an interferometer for particles with
internal degrees of freedom. It serves as a framework common to quantum eraser
and weak measurement. We demonstrate that the geometric phase, particularly the
Pancharatnam phase, results from the post-selection of the internal state, and
thereby the interference pattern is changed. It is revealed that the
extraordinary displacement of the probe wavepackets in weak measurement is
achieved owing to the Pancharatnam phase associated with post-selection.Comment: 11 pages, 4 figure
Wnt5a induces ROR1 to associate with 14-3-3ζ for enhanced chemotaxis and proliferation of chronic lymphocytic leukemia cells.
Wnt5a can activate Rho GTPases in chronic lymphocytic leukemia (CLL) cells by inducing the recruitment of ARHGEF2 to ROR1. Mass spectrometry on immune precipitates of Wnt5a-activated ROR1 identified 14-3-3ζ, which was confirmed by co-immunoprecipitation. The capacity of Wnt5a to induce ROR1 to complex with 14-3-3ζ could be blocked in CLL cells by treatment with cirmtuzumab, a humanized mAb targeting ROR1. Silencing 14-3-3ζ via small interfering RNA impaired the capacity of Wnt5a to: (1) induce recruitment of ARHGEF2 to ROR1, (2) enhance in vitro exchange activity of ARHGEF2 and (3) induce activation of RhoA and Rac1 in CLL cells. Furthermore, CRISPR/Cas9 deletion of 14-3-3ζ in ROR1-negative CLL cell-line MEC1, and in MEC1 cells transfected to express ROR1 (MEC1-ROR1), demonstrated that 14-3-3ζ was necessary for the growth/engraftment advantage of MEC1-ROR1 over MEC1 cells. We identified a binding motif (RSPS857SAS) in ROR1 for 14-3-3ζ. Site-directed mutagenesis of ROR1 demonstrated that serine-857 was required for the recruitment of 14-3-3ζ and ARHGEF2 to ROR1, and activation of RhoA and Rac1. Collectively, this study reveals that 14-3-3ζ plays a critical role in Wnt5a/ROR1 signaling, leading to enhanced CLL migration and proliferation
Coined quantum walks on percolation graphs
Quantum walks, both discrete (coined) and continuous time, form the basis of
several quantum algorithms and have been used to model processes such as
transport in spin chains and quantum chemistry. The enhanced spreading and
mixing properties of quantum walks compared with their classical counterparts
have been well-studied on regular structures and also shown to be sensitive to
defects and imperfections in the lattice. As a simple example of a disordered
system, we consider percolation lattices, in which edges or sites are randomly
missing, interrupting the progress of the quantum walk. We use numerical
simulation to study the properties of coined quantum walks on these percolation
lattices in one and two dimensions. In one dimension (the line) we introduce a
simple notion of quantum tunneling and determine how this affects the
properties of the quantum walk as it spreads. On two-dimensional percolation
lattices, we show how the spreading rate varies from linear in the number of
steps down to zero, as the percolation probability decreases to the critical
point. This provides an example of fractional scaling in quantum walk dynamics.Comment: 25 pages, 14 figures; v2 expanded and improved presentation after
referee comments, added extra figur
Localization of the Grover walks on spidernets and free Meixner laws
A spidernet is a graph obtained by adding large cycles to an almost regular
tree and considered as an example having intermediate properties of lattices
and trees in the study of discrete-time quantum walks on graphs. We introduce
the Grover walk on a spidernet and its one-dimensional reduction. We derive an
integral representation of the -step transition amplitude in terms of the
free Meixner law which appears as the spectral distribution. As an application
we determine the class of spidernets which exhibit localization. Our method is
based on quantum probabilistic spectral analysis of graphs.Comment: 32 page
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