300 research outputs found
Coupling single emitters to quantum plasmonic circuits
In recent years the controlled coupling of single photon emitters to
propagating surface plasmons has been intensely studied, which is fueled by the
prospect of a giant photonic non-linearity on a nano-scaled platform. In this
article we will review the recent progress on coupling single emitters to
nano-wires towards the construction of a new platform for strong light-matter
interaction. The control over such a platform might open new doors for quantum
information processing and quantum sensing at the nanoscale, and for the study
of fundamental physics in the ultra-strong coupling regime
Algebraic and algorithmic frameworks for optimized quantum measurements
Von Neumann projections are the main operations by which information can be
extracted from the quantum to the classical realm. They are however static
processes that do not adapt to the states they measure. Advances in the field
of adaptive measurement have shown that this limitation can be overcome by
"wrapping" the von Neumann projectors in a higher-dimensional circuit which
exploits the interplay between measurement outcomes and measurement settings.
Unfortunately, the design of adaptive measurement has often been ad hoc and
setup-specific. We shall here develop a unified framework for designing
optimized measurements. Our approach is two-fold: The first is algebraic and
formulates the problem of measurement as a simple matrix diagonalization
problem. The second is algorithmic and models the optimal interaction between
measurement outcomes and measurement settings as a cascaded network of
conditional probabilities. Finally, we demonstrate that several figures of
merit, such as Bell factors, can be improved by optimized measurements. This
leads us to the promising observation that measurement detectors which---taken
individually---have a low quantum efficiency can be be arranged into circuits
where, collectively, the limitations of inefficiency are compensated for
Continuous Variable Entanglement and Squeezing of Orbital Angular Momentum States
We report the first experimental characterization of the first-order
continuous variable orbital angular momentum states. Using a spatially
non-degenerate optical parametric oscillator (OPO) we produce quadrature
entanglement between the two first-order Laguerre-Gauss modes. The family of
OAM modes is mapped on an orbital Poincare sphere, and the modes position on
the sphere is spanned by the three orbital parameters. Using the non-degenerate
OPO we produce squeezing of these parameters, and as an illustration, we
reconstruct the "cigar-shaped" uncertainty volume on the orbital Poincare
sphere.Comment: 4 pages, 4 figure
Single-Quadrature Continuous-Variable Quantum Key Distribution
Most continuous-variable quantum key distribution schemes are based on the
Gaussian modulation of coherent states followed by continuous quadrature
detection using homodyne detectors. In all previous schemes, the Gaussian
modulation has been carried out in conjugate quadratures thus requiring two
independent modulators for their implementations. Here, we propose and
experimentally test a largely simplified scheme in which the Gaussian
modulation is performed in a single quadrature. The scheme is shown to be
asymptotically secure against collective attacks, and considers asymmetric
preparation and excess noise. A single-quadrature modulation approach renders
the need for a costly amplitude modulator unnecessary, and thus facilitates
commercialization of continuous-variable quantum key distribution.Comment: 13 pages, 7 figure
Continuous Variable Quantum Key Distribution with a Noisy Laser
Existing experimental implementations of continuous-variable quantum key
distribution require shot-noise limited operation, achieved with shot-noise
limited lasers. However, loosening this requirement on the laser source would
allow for cheaper, potentially integrated systems. Here, we implement a
theoretically proposed prepare-and-measure continuous-variable protocol and
experimentally demonstrate the robustness of it against preparation noise
stemming for instance from technical laser noise. Provided that direct
reconciliation techniques are used in the post-processing we show that for
small distances large amounts of preparation noise can be tolerated in contrast
to reverse reconciliation where the key rate quickly drops to zero. Our
experiment thereby demonstrates that quantum key distribution with
non-shot-noise limited laser diodes might be feasible.Comment: 10 pages, 6 figures. Corrected plots for reverse reconciliatio
Generation of Polarization Squeezing with Periodically Poled KTP at 1064 nm
We report the experimental demonstration of directly produced polarization
squeezing at 1064 nm from a type I optical parametric amplifier (OPA) based on
a periodically poled KTP crystal (PPKTP). The orthogonal polarization modes of
the polarization squeezed state are both defined by the OPA cavity mode, and
the birefringence induced by the PPKTP crystal is compensated for by a second,
but inactive, PPKTP crystal. Stokes parameter squeezing of 3.6 dB and anti
squeezing of 9.4 dB is observed.Comment: 4 pages, 2 figure
Assessments of macroscopicity for quantum optical states
With the slow but constant progress in the coherent control of quantum
systems, it is now possible to create large quantum superpositions. There has
therefore been an increased interest in quantifying any claims of
macroscopicity. We attempt here to motivate three criteria which we believe
should enter in the assessment of macroscopic quantumness: The number of
quantum fluctuation photons, the purity of the states, and the ease with which
the branches making up the state can be distinguished
Optimal cloning of coherent states by linear optics
We describe an optical scheme for optimal Gaussian n to m cloning of coherent
states. The scheme, which generalizes a recently demonstrated scheme for 1 to 2
cloning, involves only linear optical components and homodyne detection.Comment: 5 pages, 4 figures, presented at the 13th Central European Workshop
on Quantum Optics, May 23-27 2006, Vienna, Austria (Proceedings will be
published in "Acta Physica Hungarica"); reference added, Eq. (8) correcte
Tomography of a displacement photon counter for discrimination of single-rail optical qubits
We investigate the performance of a Kennedy receiver, which is known as a
beneficial tool in optical coherent communications, to the quantum state
discrimination of the two superpositions of vacuum and single photon states
corresponding to the eigenstates in the single-rail encoding of
photonic qubits. We experimentally characterize the Kennedy receiver in
vacuum-single photon two-dimensional space using quantum detector tomography
and evaluate the achievable discrimination error probability from the
reconstructed measurement operators. We furthermore derive the minimum error
rate obtainable with Gaussian transformations and homodyne detection. Our proof
of principle experiment shows that the Kennedy receiver can achieve a
discrimination error surpassing homodyne detection
Coupling of a Single Quantum Emitter to End-to-end Aligned Silver Nanowires
We report on the observation of coupling a single nitrogen vacancy (NV)
center in a nanodiamond crystal to a propagating plasmonic mode of silver
nanowires. The nanocrystal is placed either near to the apex of a single silver
nanowire or in the gap between two end-to-end aligned silver nanowires. We
observe an enhancement of the NV-centers' decay rate in both cases as a result
of the coupling to the plasmons. The devices are nano-assembled with a scanning
probe technique. Through simulations, we show that end-to-end aligned silver
nanowires can be used as a controllable splitter for emission from a dipole
emitter.Comment: 5 pages, 4 figure
- âŠ