The sensitivity achieved by large ring-laser gyroscopes will make it possible to detect faint relativistic effects related to the rotation of the Earth’s mass. This task requires a strict control of the ring cavity geometry (shape and orientation), which can be performed by a novel network of portable heterodyne interferometers, capable of measuring the absolute distance betweeen two retro-reflectors with a nominal accuracy better than 1nm. First steps have been taken towards the realization of this device and a starting prototype of distance gauge is under development and test.
Recently, we experimentally proved that, even in presence of strong decoherence, the bi—partite continuous variable entangled state generated by a sub—threshold type—II optical parametric oscillator (OPO) never disentangles. It keeps breaking the limits for some of the entanglement criteria. In this contribution, we extend our previous analysis by focusing on the behaviour, under decohenrece, of two entanglement measures: Logarithmic Negativity (LN) and Entanglement of Formation (EOF).
We report the first complete experimental reconstruction of the covariance matrix (CM) relative to a bi-partite
continuous variable (CV) entangled state outing a non degenerate optical parametric oscillator (OPO) below
threshold. The covariance matrix CM has been reconstructed following the method reported in V. D'Auria et
al., J. Opt. B 7, S750 (2005). The two entangled beams (signal (a) and idler (b)) are produced by a by a
continuous wave (CW) optical parametric oscillator (OPO) via type-II (same frequency but with orthogonal
polarizations), phase matching and working below threshold. Our experimental setup makes use of a single
homodyne detector and of a compact source of entangled beams. The quadratures values and other relevant
quantities are reconstructed by quantum tomography, without making any a priori assumption on the state under
CV entangled CW bright beams are experimentally generated by a Non-degenerate "triply resonant" Optical
Parametric Amplifier (NOPA) based on PPKTP type-II crystal below threshold. Operating the OPA at
frequency degeneracy makes the down-converted entangled beams able to be optically manipulated for generating
different combination of the entangled system. Particular care has been required by the triply resonance
condition essential to obtain a suffcient degree of entanglement. The triply resonance condition is pursued
by combining temperature phase-matching and crystal tilting in an optical cavity so to optimize the triple
resonance condition acting on different independent parameters. Exploiting the dependence of the bipartite
system covariance matrix on different combination of this continuous variable two-mode states it can be directly
measured using a single homodyne detector and few linear optical elements. The system is actually operating
at the University of Napoli-Quantum Optics Laboratory. Preliminary measurements show that CV entanglement
is present at the OPO output. This source of CV entanglement can be used for quantum communication
purposes. The technical tricks herein implemented are interesting also for realizing triply resonant CW OPOs
whose spectral properties and conversion efficiencies are better compared to single and double resonant devices.
The quality of entanglement is also improved thanks to the use of a single cavity instead of dual cavities often
employed in triply resonant devices. The measurement method can be applied to generic bipartite states made
of frequency degenerate but orthogonally polarized modes.
We have exploited pattern function quantum homodyne tomography (QHT) to enlight deviations from the Gaussian state for a squeezed vacuum field generated by a type-I below threshold OPO. This tomographic method allows to fully characterize the state without any a--priori assumption on its statistics. Applying pattern function QHT to the radiation outing a type-I below threshold OPO, we have measured photon number distributions different from those expected for a Gaussian field. Being the Wigner function and the quadrature marginal distribution Gaussian for a Gaussian field, the actual state has been analysed by looking at data variance and Kurtosis (0 for a Gaussian distribution) as a function of the quadrature angle θ. A clear deviation of the Kurtosis from zero has been found for different OPO cavities. This deviation increases as the threshold is approached.
Quantum tomography of cw homodyne detected field quadratures (QHT) is used for investigating quantum states of radiation fields. We apply QHT to the measurement of the transmittance of optical samples illuminated by vacuum squeezed radiation produced by a type-I degenerate OPO operating below threshold. The field reconstructed by QHT contains information about the transmittance of different absorbers illuminated by a very small number of photons.