145 research outputs found
Collinear source of polarization-entangled photon pairs at non-degenerate wavelengths
We report on a simple but highly efficient source of polarization-entangled
photon pairs at non-degenerate wavelengths. The fully collinear configuration
of the source enables very high coupling efficiency into a single optical mode
and allows the use of long nonlinear crystals. With optimized dispersion
compensation it is possible to use a free-running laser diode as pump source
and to reach an entanglement fidelity of 99.4 % at rates as high as 27000
pairs/s per mW of pump power. This greatly enhances the practicality of the
source for applications in quantum communication and metrology.Comment: 4 pages, 4 figures, accepted in Applied Physics Letter
Impact of the slit geometry on the performance of wire-grid polarisers
Wire-grid polarisers are versatile and scalable components which can be
engineered to achieve small sizes and extremely high extinction ratios. Yet the
measured performances are always significantly below the predicted values
obtained from numerical simulations. Here we report on a detailed comparison
between theoretical and experimental performances. We show that the discrepancy
can be explained by the true shape of the plasmonic structures. Taking into
account the fabrication details, a new optimisation model enables us to achieve
excellent agreement with the observed response and to re-optimise the grating
parameters to ensure experimental extinction ratios well above 1,000 at 850 nm.Comment: 8 pages, 6 figure
Secure communication with single-photon two-qubit states
We propose a cryptographic scheme that is deterministic: Alice sends single
photons to Bob, and each and every photon detected supplies one key bit -- no
photon is wasted. This is in marked contrast to other schemes in which a random
process decides whether the next photon sent will contribute to the key or not.
The determinism is achieved by preparing the photons in two-qubit states,
rather than the one-qubit states used in conventional schemes. In particular,
we consider the realistic situation in which one qubit is the photon
polarization, the other a spatial alternative. Further, we show how one can
exploit the deterministic nature for direct secure communication, that is:
without the need for establishing a shared key first.Comment: 9 pages, 1 figure, 3 tables; final versio
Experimental Schmidt Decomposition and Entanglement Detection
We introduce an experimental procedure for the detection of quantum
entanglement of an unknown quantum state with as few measurements as possible.
The method requires neither a priori knowledge of the state nor a shared
reference frame between the observers. The scheme starts with local
measurements, possibly supplemented with suitable filtering, that can be
regarded as calibration. Consecutive correlation measurements enable detection
of the entanglement of the state. We utilize the fact that the calibration
stage essentially establishes the Schmidt decomposition for pure states.
Alternatively we develop a decision tree which reveals entanglement within few
steps. These methods are illustrated and verified experimentally for various
two-qubit entangled states.Comment: 6 pages, 7 figures, journal versio
Experimental Observation of Four-Photon Entangled Dicke State with High Fidelity
We present the experimental observation of the symmetric four-photon
entangled Dicke state with two excitations . A simple
experimental set-up allowed quantum state tomography yielding a fidelity as
high as . We study the entanglement persistency of the state
using novel witness operators and focus on the demonstration of a remarkable
property: depending on the orientation of a measurement on one photon, the
remaining three photons are projected into both inequivalent classes of genuine
tripartite entanglement, the GHZ and W class. Furthermore, we discuss possible
applications of in quantum communication.Comment: 5 pages, 3 figures, submitte
Linear Optics C-Phase gate made simple
Linear optics quantum logic gates are the best tool to generate multi-photon
entanglement. Simplifying a recent approach [Phys. Rev. A 65, 062324; Phys.
Rev. A 66, 024308] we were able to implement the conditional phase gate with
only one second order interference at a polarization dependent beam splitter,
thereby significantly increasing its stability. The improved quality of the
gate is evaluated by analysing its entangling capability and by performing full
process tomography. The achieved results ensure that this device is well suited
for implementation in various multi photon quantum information protocols.Comment: 5 pages, 4 figure
Analysis of a single-atom dipole trap
We describe a simple experimental technique which allows to store a single
Rubidium 87 atom in an optical dipole trap. Due to light-induced two-body
collisions during the loading stage of the trap the maximum number of captured
atoms is locked to one. This collisional blockade effect is confirmed by the
observation of photon anti-bunching in the detected fluorescence light. The
spectral properties of single photons emitted by the atom were studied with a
narrow-band scanning cavity. We find that the atomic fluorescence spectrum is
dominated by the spectral width of the exciting laser light field. In addition
we observe a spectral broadening of the atomic fluorescence light due to the
Doppler effect. This allows us to determine the mean kinetic energy of the
trapped atom corresponding to a temperature of 105 micro Kelvin. This simple
single-atom trap is the key element for the generation of atom-photon
entanglement required for future applications in quantum communication and a
first loophole-free test of Bell's inequality.Comment: Version 2; formula in equ. 3 correcte
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