1,940 research outputs found
Scaling laws for precision in quantum interferometry and bifurcation landscape of optimal state
Phase precision in optimal 2-channel quantum interferometry is studied in the
limit of large photon number , for losses occurring in either one or
both channels. For losses in one channel an optimal state undergoes an
intriguing sequence of local bifurcations as the losses or the number of
photons increase. We further show that fixing the loss paramater determines a
scale for quantum metrology -- a crossover value of the photon number
beyond which the supra-classical precision is progressively lost. For large
losses the optimal state also has a different structure from those considered
previously.Comment: 4 pages, 3 figures, v3 is modified in response to referee comment
Multi-Partite Entanglement Inequalities via Spin Vector Geometry
We introduce inequalities for multi-partite entanglement, derived from the
geometry of spin vectors. The criteria are constructed iteratively from cross
and dot products between the spins of individual subsystems, each of which may
have arbitrary dimension. For qubit ensembles the maximum violation for our
inequalities is larger than that for the Mermin-Klyshko Bell inequalities, and
the maximally violating states are different from Greenberger-Horne-Zeilinger
states. Our inequalities are violated by certain bound entangled states for
which no Bell-type violation has yet been found.Comment: 4 pages, 2 tables, 1 figure. A truncated version is published in
Physical Review Letters, volume 95 issue 18, 180402 (October 2005
Preferred Measurements: Optimality and Stability in Quantum Parameter Estimation
We explore precision in a measurement process incorporating pure probe
states, unitary dynamics and complete measurements via a simple formalism. The
concept of `information complement' is introduced. It undermines measurement
precision and its minimization reveals the system properties at an optimal
point. Maximally precise measurements can exhibit independence from the true
value of the estimated parameter, but demanding this severely restricts the
type of viable probe and dynamics, including the requirement that the
Hamiltonian be block-diagonal in a basis of preferred measurements. The
curvature of the information complement near a globally optimal point provides
a new quantification of measurement stability.Comment: 4 pages, 2 figures, in submission. Substantial Extension and
replacement of arXiv:0902.3260v1 in response to Referees' remark
Local and Global Distinguishability in Quantum Interferometry
A statistical distinguishability based on relative entropy characterises the
fitness of quantum states for phase estimation. This criterion is employed in
the context of a Mach-Zehnder interferometer and used to interpolate between
two regimes, of local and global phase distinguishability. The scaling of
distinguishability in these regimes with photon number is explored for various
quantum states. It emerges that local distinguishability is dependent on a
discrepancy between quantum and classical rotational energy. Our analysis
demonstrates that the Heisenberg limit is the true upper limit for local phase
sensitivity. Only the `NOON' states share this bound, but other states exhibit
a better trade-off when comparing local and global phase regimes.Comment: 4 pages, in submission, minor revision
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