1,637 research outputs found
Macroscopicity of quantum superpositions on a one-parameter unitary path in Hilbert space
We analyze quantum states formed as superpositions of an initial pure product
state and its image under local unitary evolution, using two measurement-based
measures of superposition size: one based on the optimal quantum binary
distinguishability of the branches of the superposition and another based on
the ratio of the maximal quantum Fisher information of the superposition to
that of its branches, i.e., the relative metrological usefulness of the
superposition. A general formula for the effective sizes of these states
according to the branch distinguishability measure is obtained and applied to
superposition states of quantum harmonic oscillators composed of Gaussian
branches. Considering optimal distinguishability of pure states on a
time-evolution path leads naturally to a notion of distinguishability time that
generalizes the well known orthogonalization times of Mandelstam and Tamm and
Margolus and Levitin. We further show that the distinguishability time provides
a compact operational expression for the superposition size measure based on
the relative quantum Fisher information. By restricting the maximization
procedure in the definition of this measure to an appropriate algebra of
observables, we show that the superposition size of, e.g., N00N states and
hierarchical cat states, can scale linearly with the number of elementary
particles comprising the superposition state, implying precision scaling
inversely with the total number of photons when these states are employed as
probes in quantum parameter estimation of a 1-local Hamiltonian in this
algebra
Distinguishability times and asymmetry monotone-based quantum speed limits in the Bloch ball
For both unitary and open qubit dynamics, we compare asymmetry monotone-based
bounds on the minimal time required for an initial qubit state to evolve to a
final qubit state from which it is probabilistically distinguishable with fixed
minimal error probability (i.e., the minimal error distinguishability time).
For the case of unitary dynamics generated by a time-independent Hamiltonian,
we derive a necessary and sufficient condition on two asymmetry monotones that
guarantees that an arbitrary state of a two-level quantum system or a separable
state of two-level quantum systems will unitarily evolve to another state
from which it can be distinguished with a fixed minimal error probability
. This condition is used to order the set of qubit states
based on their distinguishability time, and to derive an optimal release time
for driven two-level systems such as those that occur, e.g., in the
Landau-Zener problem. For the case of non-unitary dynamics, we compare three
lower bounds to the distinguishability time, including a new type of lower
bound which is formulated in terms of the asymmetry of the uniformly
time-twirled initial system-plus-environment state with respect to the
generator of the Stinespring isometry corresponding to the dynamics,
specifically, in terms of ,
where .Comment: 13 pages, 4 figure
Continuous Measurement of a Non-Markovian Open Quantum System
Continuous quantum measurement is the backbone of various methods in quantum
control, quantum metrology, and quantum information. Here, we present a
generalized formulation of dispersive measurement of a complex quantum systems.
We describe the complex system as an open quantum system that is strongly
coupled to a non-Markovian environment, enabling the treatment of a broad
variety of natural or engineered complex systems. The system is monitored via a
probe resonator coupled to a broadband (Markovian) reservoir. Based on this
model, we derive a formalism of Stochastic Hierarchy Equations of Motion (SHEM)
describing the decoherence dynamics of the system conditioned on the
measurement record. Furthermore, we demonstrate a spectroscopy method based on
weak quantum measurement to reveal the non-Markovian nature of the environment,
which we term weak spectroscopy.Comment: Published version, the section on continuous state tomography will be
published in a separate manuscrip
Do We Need to Put God into Emotional Support?: A Comparison of Caucasians’ and African-Americans’ Evaluations of Religious versus Non-Religious Comforting Messages
The current study explored whether ethnicity influences young adults’ evaluations of two different sets of comforting messages: those in which concepts such as God, prayer, religion, and faith are woven into low, moderate, and high person-centered strategies (called ‘‘religious strategies’’) and those in which such concepts are not embedded (called ‘‘non-religious strategies’’) into the messages. One hundred ninety-seven college students (63% African-American; 37% Caucasian) rated the sensitivity and effectiveness of religious and non-religious comforting messages. Several significant differences were observed between Caucasians and African-Americans in their evaluations of these strategies. Findings are discussed in terms of their practical implications for ‘‘real world’’ comforting efforts as well as the theoretical significance they hold for the concept of person-centeredness
OCS in small para-hydrogen clusters: energetics and structure with N=1-8 complexed hydrogen molecules
We determine the structure and energetics of complexes of the linear OCS
molecule with small numbers of para-hydrogen molecules, N=1-8, using zero
temperature quantum Monte Carlo methods. Ground state calculations are carried
out with importance-sampled rigid body diffusion Monte Carlo (IS-RBDMC) and
excited state calculations with the projection operator imaginary time spectral
evolution (POITSE) methodology. The ground states are found to be highly
structured, with a gradual build up of two axial rings as N increases to 8.
Analysis of the azimuthal density correlations around the OCS molecule shows
that these rings are quite delocalized for small N values, but become strongly
localized for N \geq 5 . Excited state calculations are made for a range of
total cluster angular momentum values and the rotational energy levels fitted
to obtain effective rotational and distortion constants of the complexed OCS
molecule as a function of cluster size N. Detailed analysis of these
spectroscopic constants indicates that the complexes of OCS with para-hydrogen
have an unusually rich variation in dynamical behavior, with sizes N=1-2
showing near rigid behavior, sizes N=3-4 showing extremely floppy behavior, and
the larger sizes N=5-8 showing more rigid behavior again. The large values of
the distortion constant D obtained for N=3-4 are rationalized in terms of the
coupling between the OCS rotations and the "breathing" mode of the first,
partially filled ring of para-hydrogen molecules.Comment: 26 pages, 11 figures. accepted for publication in the Journal of
Chemical Physic
Editor\u27s Prelude
Founding and executive editor\u27s prelude to first issue of Nursing Communication
Collective Effects in Linear Spectroscopy of Dipole-Coupled Molecular Arrays
We present a consistent analysis of linear spectroscopy for arrays of nearest
neighbor dipole-coupled two-level molecules that reveals distinct signatures of
weak and strong coupling regimes separated for infinite size arrays by a
quantum critical point. In the weak coupling regime, the ground state of the
molecular array is disordered, but in the strong coupling regime it has
(anti)ferroelectric ordering. We show that multiple molecular excitations
(odd/even in weak/strong coupling regime) can be accessed directly from the
ground state. We analyze the scaling of absorption and emission with system
size and find that the oscillator strengths show enhanced superradiant behavior
in both ordered and disordered phases. As the coupling increases, the single
excitation oscillator strength rapidly exceeds the well known Heitler-London
value. In the strong coupling regime we show the existence of a unique spectral
transition with excitation energy that can be tuned by varying the system size
and that asymptotically approaches zero for large systems. The oscillator
strength for this transition scales quadratically with system size, showing an
anomalous one-photon superradiance. For systems of infinite size, we find a
novel, singular spectroscopic signature of the quantum phase transition between
disordered and ordered ground states. We outline how arrays of ultra cold
dipolar molecules trapped in an optical lattice can be used to access the
strong coupling regime and observe the anomalous superradiant effects
associated with this regime.Comment: 12 pages, 7 figures main tex
- …