23 research outputs found
How quantum correlations enhance prediction of complementary measurements
If there are correlations between two qubits then the results of the
measurement on one of them can help to predict measurement results on the other
one. It is an interesting question what can be predicted about the results of
two complementary projective measurements on the first qubit. To quantify these
predictions the complementary \emph{knowledge excesses} are used. A non-trivial
constraint restricting them is derived. For any mixed state and for arbitrary
measurements the knowledge excesses are bounded by a factor depending only on
the maximal violation of Bell's inequalities. This result is experimentally
verified on two-photon Werner states prepared by means of spontaneous
parametric down-conversion.Comment: 4 pages, 4 figure
Experimental implementation of the optimal linear-optical controlled phase gate
We report on the first experimental realization of optimal linear-optical
controlled phase gates for arbitrary phases. The realized scheme is entirely
flexible in that the phase shift can be tuned to any given value. All such
controlled phase gates are optimal in the sense that they operate at the
maximum possible success probabilities that are achievable within the framework
of any postselected linear-optical implementation. The quantum gate is
implemented using bulk optical elements and polarization encoding of qubit
states. We have experimentally explored the remarkable observation that the
optimum success probability is not monotone in the phase.Comment: 4 pages, 5 figures, 1 tabl
Compensating for Beamsplitter Asymmetries in Quantum Interference Experiments
The visibility of the quantum interference "dip" seen in the Hong-Ou-Mandel
experiment is optimized when a symmetric 50/50 beamsplitter is used in the
interferometer. Here we show that the reduction in visibility caused by an
asymmetric beamsplitter can be compensated by manipulating the polarization
states of the two input photons. We experimentally demonstrate this by using a
highly asymmetric 10/90 beamsplitter, and converting an initial dip visibility
of 22% to a compensated value of 99%.Comment: 3 pages, 4 figure
Several experimental realizations of symmetric phase-covariant quantum cloner of single-photon qubits
We compare several optical implementations of phase-covariant cloning
machines. The experiments are based on copying of the polarization state of a
single photon in bulk optics by special unbalanced beam splitter or by balanced
beam splitter accompanied by a state filtering. Also the all-fiber based setup
is discussed, where the information is encoded into spatial modes, i.e., the
photon can propagate through two optical fibers. Each of the four
implementations possesses some advantages and disadvantages that are discussed.Comment: 8 pages, 11 figure
Experimental asymmetric phase-covariant quantum cloning of polarization qubits
We report on two optical realizations of the asymmetric
phase-covariant cloning machines for polarization states of single photons. The
experimental setups combine two-photon interference and tunable polarization
filtering that enables us to control the asymmetry of the cloners. The first
scheme involves a special unbalanced bulk beam splitter exhibiting different
splitting ratios for vertical and horizontal polarizations, respectively. The
second implemented scheme consists of a balanced fiber coupler where photon
bunching occurs, followed by a free-space part with polarization filters. With
this later approach we were able to demonstrate very high cloning fidelities
which are above the universal cloning limit.Comment: 7 pages, 8 figure
Experimental linear-optical implementation of a multifunctional optimal qubit cloner
We present the first experimental implementation of a multifunctional device
for the optimal cloning of one to two qubits. Previous implementations have
always been designed to optimize the cloning procedure with respect to one
single type of a priori information about the cloned state. In contrast, our
"all-in-one" implementation is optimal for several prominent regimes such as
universal cloning, phase-covariant cloning, and also the first ever realized
mirror phase-covariant cloning, when the square of the expected value of
Pauli's Z operator is known in advance. In all these regimes the experimental
device yields clones with almost maximum achievable average fidelity (97.5% of
theoretical limit). Our device has a wide range of possible applications in
quantum information processing, especially in quantum communication. For
instance, one can use it for incoherent and coherent attacks against a variety
of cryptographic protocols, including the Bennett-Brassard 1984 protocol of
quantum key distribution through the Pauli damping channels. It can be also
applied as a state-dependent photon multiplier in practical quantum networks.Comment: 9 pages, 6 figures, accepted to Phys. Rev. A (Rapid Communications
Spatial and spectral properties of the pulsed second-harmonic generation in a PP-KTP waveguide
Spatial and spectral properties of the pulsed second harmonic generation in a
periodically-poled KTP waveguide exploiting simultaneously the first, second,
and third harmonics of periodic nonlinear modulation are analyzed. Experimental
results are interpreted using a model based on finite elements method.
Correlations between spatial and spectral properties of the fundamental and
second-harmonic fields are revealed. Individual nonlinear processes can be
exploited combining spatial and spectral filtering. Also the influence of
waveguide parameters to the second-harmonic spectra is addressed.Comment: 13 pages, 8 figure
Excitonic photoluminescence in symmetric coupled double quantum wells subject to an external electric field
The effect of an external electric field F on the excitonic photoluminescence
(PL) spectra of a symmetric coupled double quantum well (DQW) is investigated
both theoretically and experimentally. We show that the variational method in a
two-particle electron-hole wave function approximation gives a good agreement
with measurements of PL on a narrow DQW in a wide interval of F including
flat-band regime. The experimental data are presented for an MBE-grown DQW
consisting of two 5 nm wide GaAs wells, separated by a 4 monolayers (MLs) wide
pure AlAs central barrier, and sandwiched between Ga_{0.7}Al_{0.3}As layers.
The bias voltage is applied along the growth direction. Spatially direct and
indirect excitonic transitions are identified, and the radius of the exciton
and squeezing of the exciton in the growth direction are evaluated
variationally. The excitonic binding energies, recombination energies,
oscillator strengths, and relative intensities of the transitions as functions
of the applied field are calculated. Our analysis demonstrates that this simple
model is applicable in case of narrow DQWs not just for a qualitative
description of the PL peak positions but also for the estimation of their
individual shapes and intensities.Comment: 5 pages, 4 figures (accepted in Phys. Rev. B