47 research outputs found
The quest for three-color entanglement: experimental investigation of new multipartite quantum correlations
We experimentally investigate quadrature correlations between pump, signal,
and idler fields in an above-threshold optical parametric oscillator. We
observe new quantum correlations among the pump and signal or idler beams, as
well as among the pump and a combined quadrature of signal and idler beams. A
further investigation of unforeseen classical noise observed in this system is
presented, which hinders the observation of the recently predicted tripartite
entanglement. In spite of this noise, current results approach the limit
required to demonstrate three-color entanglement.Comment: 10 pages, 5 figures, submitted to Opt. Expres
Direct probing of the Wigner function by time-multiplexed detection of photon statistics
We investigate the capabilities of loss-tolerant quantum state
characterization using a photon-number resolving, time-multiplexed detector
(TMD). We employ the idea of probing the Wigner function point-by-point in
phase space via photon parity measurements and displacement operations,
replacing the conventional homodyne tomography. Our emphasis lies on
reconstructing the Wigner function of non-Gaussian Fock states with highly
negative values in a scheme that is based on a realistic experimental setup. In
order to establish the concept of loss-tolerance for state characterization we
show how losses can be decoupled from the impact of other experimental
imperfections, i.e. the non-unity transmittance of the displacement
beamsplitter and non-ideal mode overlap. We relate the experimentally
accessible parameters to effective ones that are needed for an optimised state
reconstruction. The feasibility of our approach is tested by Monte Carlo
simulations, which provide bounds resulting from statistical errors that are
due to limited data sets. Our results clearly show that high losses can be
accepted for a defined parameter range, and moreover, that (in contrast to
homodyne detection) mode mismatch results in a distinct signature, which can be
evaluated by analysing the photon number oscillations of the displaced Fock
states.Comment: 22 pages, 13 figures, published versio
Generation of Bright Two-Color Continuous Variable Entanglement
We present the first measurement of squeezed-state entanglement between the
twin beams produced in an Optical Parametric Oscillator (OPO) operating above
threshold. Besides the usual squeezing in the intensity difference between the
twin beams, we have measured squeezing in the sum of phase quadratures. Our
scheme enables us to measure such phase anti-correlations between fields of
different frequencies. In the present measurements, wavelengths differ by ~1
nm. Entanglement is demonstrated according to the Duan et al. criterion [Phys.
Rev. Lett. 84, 2722 (2000)] .
This experiment opens the way for new potential applications such as the
transfer of quantum information between different parts of the electromagnetic
spectrum.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Robustness of bipartite Gaussian entangled beams propagating in lossy channels
Subtle quantum properties offer exciting new prospects in optical
communications. Quantum entanglement enables the secure exchange of
cryptographic keys and the distribution of quantum information by
teleportation. Entangled bright beams of light attract increasing interest for
such tasks, since they enable the employment of well-established classical
communications techniques. However, quantum resources are fragile and undergo
decoherence by interaction with the environment. The unavoidable losses in the
communication channel can lead to a complete destruction of useful quantum
properties -- the so-called "entanglement sudden death". We investigate the
precise conditions under which this phenomenon takes place for the simplest
case of two light beams and demonstrate how to produce states which are robust
against losses. Our study sheds new light on the intriguing properties of
quantum entanglement and how they may be tamed for future applications.Comment: To be published - Nature Photonic
Disentanglement in Bipartite Continuous-Variable Systems
Entanglement in bipartite continuous-variable systems is investigated in the
presence of partial losses, such as those introduced by a realistic quantum
communication channel, e.g. by propagation in an optical fiber. We find that
entanglement can vanish completely for partial losses, in a situa- tion
reminiscent of so-called entanglement sudden death. Even states with extreme
squeezing may become separable after propagation in lossy channels. Having in
mind the potential applications of such entangled light beams to optical
communications, we investigate the conditions under which entanglement can
survive for all partial losses. Different loss scenarios are examined and we
derive criteria to test the robustness of entangled states. These criteria are
necessary and sufficient for Gaussian states. Our study provides a framework to
investigate the robustness of continuous-variable entanglement in more complex
multipartite systems.Comment: Phys. Rev. A (in press
Accessing the purity of a single photon by the width of the Hong-Ou-Mandel interference
We demonstrate a method to determine the spectral purity of single photons.
The technique is based on the Hong-Ou-Mandel (HOM) interference between a
single photon state and a suitably prepared coherent field. We show that the
temporal width of the HOM dip is not only related to reciprocal of the spectral
width but also to the underlying quantum coherence. Therefore, by measuring the
width of both the HOM dip and the spectrum one can directly quantify the degree
of spectral purity. The distinct advantage of our proposal is that it obviates
the need for perfect mode matching, since it does not rely on the visibility of
the interference. Our method is particularly useful for characterizing the
purity of heralded single photon states.Comment: Extended version, 16 pages, 9 figure
Extra phase noise from thermal fluctuations in nonlinear optical crystals
We show theoretically and experimentally that scattered light by thermal
phonons inside a second-order nonlinear crystal is the source of additional
phase noise observed in Optical Parametric Oscillators. This additional phase
noise reduces the quantum correlations and has hitherto hindered the direct
production of multipartite entanglement in a single nonlinear optical system.
We cooled the nonlinear crystal and observed a reduction of the extra noise.
Our treatment of this noise can be successfully applied to different systems in
the literature