49 research outputs found
Experimental test of an entropic measurement uncertainty relation for arbitrary qubit observables
A tight information-theoretic measurement uncertainty relation is
experimentally tested with neutron spin-1/2 qubits. The noise associated to the
measurement of an observable is defined via conditional Shannon entropies and a
tradeoff relation between the noises for two arbitrary spin observables is
demonstrated. The optimal bound of this tradeoff is experimentally obtained for
various non-commuting spin observables. For some of these observables this
lower bound can be reached with projective measurements, but we observe that,
in other cases, the tradeoff is only saturated by general quantum measurements
(i.e., positive-operator valued measures), as predicted theoretically.Comment: 6 pages, 3 figure
Experimental Test of Entropic Noise-Disturbance Uncertainty Relations for Three-Outcome Qubit Measurements
Information-theoretic uncertainty relations formulate the joint
immeasurability of two non-commuting observables in terms of information
entropies. The trade-off of the accuracy in the outcome of two successive
measurements manifests in entropic noise-disturbance uncertainty relations.
Recent theoretical analysis predicts that projective measurements are not
optimal, with respect to the noise-disturbance trade-offs. Therefore the
results in our previous letter [PRL 115, 030401 (2015)] are outperformed by
general quantum measurements. Here, we experimentally test a tight
information-theoretic measurement uncertainty relation for three-outcome
positive-operator valued measures (POVM), using neutron spin-1/2 qubits. The
obtained results violate the lower bound for projective measurements as
theoretically predicted.Comment: 14 pages, 14 figure
Spin - Rotation Coupling Observed in Neutron Interferometry
Einstein's theory of general relativity and quantum theory form the two major
pillars of modern physics. However, certain inertial properties of a particle's
intrinsic spin are inconspicuous while the inertial properties of mass are well
known. Here, by performing a neutron interferometric experiment, we observe
phase shifts arising as a consequence of the spin's coupling with the angular
velocity of a rotating magnetic field. The resulting phase shifts linearly
depend on the frequency of the rotation of the magnetic field. Our results
agree well with the predictions derived from the Pauli - Schr\"odinger
equation
Neutron optical test of completeness of quantum root-mean-square errors
One of the major problems in quantum physics has been to generalize the
classical root-mean-square error to quantum measurements to obtain an error
measure satisfying both soundness (to vanish for any accurate measurements) and
completeness (to vanish only for accurate measurements). A noise-operator based
error measure has been commonly used for this purpose, but it has turned out
incomplete. Recently, Ozawa proposed a new definition for a noise-operator
based error measure to be both sound and complete. Here, we present a neutron
optical demonstration for the completeness of the new error measure for both
projective (or sharp) as well as generalized (or unsharp) measurements.Comment: 7 pages, 4 figures and Supplementary Informatio