489 research outputs found
Two methods for measuring Bell nonlocality via local unitary invariants of two-qubit systems in Hong-Ou-Mandel interferometers
We describe a direct method to experimentally determine local two-qubit
invariants by performing interferometric measurements on multiple copies of a
given two-qubit state. We use this framework to analyze two different kinds of
two-qubit invariants of Makhlin and Jing et. al. These invariants allow to
fully reconstruct any two-qubit state up to local unitaries. We demonstrate
that measuring 3 invariants is sufficient to find, e.g., the optimal Bell
inequality violation. These invariants can be measured with local or nonlocal
measurements. We show that the nonlocal strategy that follows from Makhlin's
invariants is more resource-efficient than local strategy following from the
invariants of Jing et al. To measure all of the Makhlin's invariants directly
one needs to use both two-qubit singlet and three-qubit W-state projections on
multiple copies of the two-qubit state. This problem is equivalent to a
cordinate system handness measurement. We demonstrate that these 3-qubit
measurements can be performed by utilizing Hong-Ou-Mandel interference which
gives significant speedup in comparison to the classical handness measurement.
Finally, we point to potential application of our results in quantum secret
sharing.Comment: 11 pages, 7 figures, 3 table
Optimal cloning of qubits given by arbitrary axisymmetric distribution on Bloch sphere
We find an optimal quantum cloning machine, which clones qubits of arbitrary
symmetrical distribution around the Bloch vector with the highest fidelity. The
process is referred to as phase-independent cloning in contrast to the standard
phase-covariant cloning for which an input qubit state is a priori better
known. We assume that the information about the input state is encoded in an
arbitrary axisymmetric distribution (phase function) on the Bloch sphere of the
cloned qubits. We find analytical expressions describing the optimal cloning
transformation and fidelity of the clones. As an illustration, we analyze
cloning of qubit state described by the von Mises-Fisher and Brosseau
distributions. Moreover, we show that the optimal phase-independent cloning
machine can be implemented by modifying the mirror phase-covariant cloning
machine for which quantum circuits are known.Comment: 8 pages, 6 figure
On 2D integro-differential systems. Stability and sensitivity analysis
In the paper a two-dimensional integro-differential system is considered.
Using some variational methods we give sufficient conditions for the existence
and uniqueness of a solution to the considered system. Moreover, we show that
the system is stable and robust
Experimental measurement of the collectibility of two-qubit states
We present a proof-of-principle experiment demonstrating measurement of the
collectibility, a nonlinear entanglement witness proposed by Rudnicki et al.
[Phys. Rev. Lett. 107, 150502 (2011)]. This entanglement witness works for both
mixed and pure two-qubit states. In the later case it can be used to measure
entanglement in terms of the negativity. We measured the collectibility for
three distinct classes of photonic polarization-encoded two-qubit states, i.e.,
maximally entangled, separable and maximally mixed states. We demonstrate that
the measurement procedure is feasible and robust against typical experimental
shortcomings such as imperfect two-photon indistinguishability.Comment: 7 pages, 4 figure
Discovery of four periodic methanol masers and updated light curve for a further one
We report the discovery of 6.7 GHz methanol maser periodic flares in four
massive star forming regions and the updated light curve for the known periodic
source G22.357+0.066. The observations were carried out with the Torun 32 m
radio telescope between June 2009 and April 2014. Flux density variations with
period of 120 to 245 d were detected for some or all spectral features. A
variability pattern with a fast rise and relatively slow fall on time-scale of
30-60 d dominated. A reverse pattern was observed for some features of
G22.357+0.066, while sinusoidal-like variations were detected in G25.411+0.105.
A weak burst lasting ~520 d with the velocity drift of 0.24 km/s/yr occurred in
G22.357+0.066. For three sources for which high resolution maps are available,
we found that the features with periodic behaviour are separated by more than
500 au from those without any periodicity. This suggests that the maser flares
are not triggered by large-scale homogeneous variations in either the
background seed photon flux or the luminosity of the exciting source and a
mechanism which is able to produce local changes in the pumping conditions is
required.Comment: 12 pages, 15 figures. Accepted in MNRA
EVN observations of an OH maser burst in OH17.7-2.0
We have observed the OH 1612-MHz maser emission towards the proto-planetary
nebula candidate OH17.7-2.0 that underwent a very strong and unusual outburst
in 2003. Phase-referencing data were obtained with the EVN in order to localize
the outburst and to examine its possible causes. The majority of the emission
comes from an incomplete spherical shell with inner and outer radii of 220 and
850 mas, respectively. There is a strong evidence for maser components that
arise due to the interaction of a jet-like post-AGB outflow with the remnant
outer AGB shell. The most prominent signature of such an interaction is the
strongly bursting polarized emission near 73.3km/s coming from two unresolved
components of brightness temperature up to 10^11K located at the edge of the
biconal region 2500 AU from the central star. It is remarkable that this OH
biconal region is well-aligned with the polar outflow inferred from the
near-infrared image.Comment: 6 pages, 3 figures, Proceedings of the 8th European VLBI Network
Symposium, Torun, Poland, 26-29 Sep. 200
Scheme for a linear-optical controlled-phase gate with programmable phase shift
We present a linear-optical scheme for a controlled-phase gate with tunable
phase shift programmed by a qubit state. In contrast to all previous tunable
controlled-phase gates, the phase shift is not hard-coded into the optical
setup, but can be tuned to any value from 0 to pi by the state of a so-called
program qubit. Our setup is feasible with current level of technology using
only linear-optical components. We provide an experimental feasibility study to
assess the gate's implementability. We also discuss options for increasing the
success probability up to 1/12 which approaches the success probability of a
optimal non-programmable tunable controlled-phase gate.Comment: 7 pages, 3 figure
Direct method for measuring of purity, superfidelity, and subfidelity of photonic two-qubit mixed states
The Uhlmann-Jozsa fidelity (or, equivalently, the Bures distance) is a basic
concept of quantum communication and quantum information, which however is very
difficult to measure efficiently without recourse to quantum tomography. Here
we propose a direct experimental method to estimate the fidelity between two
unknown two-qubit mixed states via the measurement of the upper and lower
bounds of the fidelity, which are referred to as the superfidelity and
subfidelity, respectively. Our method enables a direct measurement of the
first- and second-order overlaps between two arbitrary two-qubit states. In
particular, the method can be applied to measure the purity (or linear entropy)
of a single two-qubit mixed state in a direct experiment. We also propose and
critically compare several experimental strategies for measuring the sub- and
superfidelities of polarization states of photons in various linear-optical
setups.Comment: 9 pages, 5 figure
State-dependent linear-optical qubit amplifier
We propose a linear-optical setup for heralded qubit amplification with
tunable output qubit fidelity. We study its success probability as a function
of output qubit fidelity showing that at the expense of lower fidelity, the
setup can considerably increase probability of successful operation. These
results are subsequently applied in a proposal for state dependent qubit
amplification. Similarly to state-dependent quantum cloning, the a priori
information about the input state allows to optimize the qubit amplification
procedure to obtain better fidelity versus success probability trade-off.Comment: 8 pages, 7 figure
Measuring distances in Hilbert space by many-particle interference
The measures of distances between points in a Hilbert space are one of the
basic theoretical concepts used to characterize properties of a quantum system
with respect to some etalon state. These are not only used in studying fidelity
of signal transmission and basic quantum phenomena but also applied in
measuring quantum correlations, and also in quantum machine learning. The
values of quantum distance measures are very difficult to determine without
completely reconstructing the state. Here we demonstrate an interferometric
approach to measuring distances between quantum states that in some cases can
outperform quantum state tomography. We propose a direct experimental method to
estimate such distance measures between two unknown two-qubit mixed states as
Uhlmann-Jozsa fidelity (or the Bures distance), the Hilbert-Schmidt distance,
and the trace distance. The fidelity is estimated via the measurement of the
upper and lower bounds of the fidelity, which are referred to as the
superfidelity and subfidelity, respectively. Our method is based on the
multiparticle interactions (i.e., interference) between copies of the unknown
pairs of qubits.Comment: 9 pages, 1 figure, 3 table
- …