We report on the organization and realization of the Joint International Physics Summer School - Optics" devoted to High-School students. The idea of the School is to teach Physics through high-level experimental activities, suitably supported by introductory lectures and complemented by data analysis. The School is also open to the participation of a number of teachers, as an opportunity of refreshing their knowledge and increasing their experimental skills. Students and teachers are directly involved in the experimental activities. The aim of the activity is to stimulate students curiosity and interest and help them decide whether a future job career in Science could be suited for them. The School is organized in two weeks: the first in June-July in Como (Italy) at the Department of Science and High Technology and the second at the end of August in Olomouc (Czech Republic) at the Joint Laboratory of Optics. Two editions of the Summer School took place in 2013 and 2014 (overall 40 students and 3 teachers from Italy, 9 students from Czech Republic) and the third one will be in 2015. The first week of the School is devoted to introductory lectures (theoretical and experimental) to consolidate students' and teachers' knowledge of basic optics. The second week is devoted to several advanced experiments in linear, nonlinear, classical and quantum optics, performed in research laboratories. During the last day of the School, students are required to give a presentation of the results obtained during the experimental sessions.
We observed the spectral coherence structure of twin beam states generated by spontaneous parametric down-conversion by using a simple fiber spectrometer synchronized with a pulsed laser source. The recorded single-shot spectra exhibit a well-defined peak structure, where the peak wavelengths change values from shot to shot. We studied the number and the width of the peaks as a function of the different parameters in the experimental setup (pump power, iris size, iris distance from the BBO crystal). Moreover, we evaluated the number of modes in the intensity distribution of the light in different portions of the spectrum. The experimental results indicate that the number of modes from statistics and the number and width of the peaks evolve differently with the parameters.
The Kennedy-like receiver is a quasi-optimal discrimination scheme employed in binary phase-shift-keyed communication schemes with coherent states. In its standard configuration, it is based on the interference of the two signals encoding the message with a reference local oscillator and on the measurement by means of ON/OFF detectors. Here we demonstrate that, without interrupting the communication, it is possible to monitor the relative phase between the signals and the local oscillator by applying a Bayesian processing to the very data sample used to discriminate the signals at any shot. We show, both numerically and experimentally, that the minimum uncertainty in phase estimation can be achieved both with ON/OFF and photon-number resolving detectors. The performances of our phase-estimation method in the presence of either uniform phase noise or phase diffusion are also investigated and discussed.
It is well known that optical twin-beam states (TWB) generated by spontaneous parametric down-conversion (PDC) exhibit spatial and spectral correlations, which can appear in single-shot images obtained by using an imaging spectrometer to resolve emission angles and wavelengths simultaneously. By analyzing series of single-shot images recorded by an EMCCD camera at different powers of the pump beam, we studied the evolution of several quantities characterizing the generated TWB. In particular, we demonstrated that correlation widths in spectrum and space increase monotonically at low pump powers and then start decreasing at higher powers due to the onset of pump depletion. In a complementary way, the Fedorov ratio decreases and then increases again. At the same time, the number of modes evaluated from photon statistics follows a complementary behavior to correlation widths that can be interpreted in terms of the evolution of the number of Schmidt modes in the field.
We present the experimental investigation of the coherence properties of the light produced by parametric down conversion in the macroscopic regime, also including pump depletion. In particular, we compare the results obtained in very similar geometric conditions by using two nonlinear crystals having different lengths. We observe that the number of generated photons, the size of spatio-spectral coherence areas, and the number of modes in the photon number statistics exhibit a similar behavior in the two crystals as a function of pump mean power, even if we notice that the absolute values are different. The available theory of parametric down conversion cannot account for these differences.
The “LuNa” (La natura della Luce nella luce della Natura - The nature of Light in the light of Nature) Project is devoted to the experimental teaching of optics in the different school grades. The basic idea of the Project is that the history of optics and the debate about the nature of light are a meaningful example of how science proceeds in the development of a physical model. Moreover optical phenomena can be presented at different levels of complexity in order to be accessible to students of different age. At the core of the Project are several portable setups that support experimental and partially interactive lectures covering all the aspects of optical phenomena, from geometrical optics to single-photon interference passing through atmospheric optics, spectroscopy, holography and theory of perception. When possible, the setups are realized with simple and easy to find materials so as to be reproducible by teachers and students. Of course, for the most complicated setups (interferometers and holography) research materials are used. Each module is calibrated to fit teachers’ requirements either to be included in the curricula of their classes or to be used as an expansion of the optics program.
We present the characterization of a multi-pixel detector (SiPM, Hamamatsu) in the presence of dark-count
and cross-talk effects. Our description yields a self-consistent calibration of the device, based on the light
under investigation, which is used to evaluate shot-by-shot detected-photon numbers including dark-counts and
cross-talk. The analysis allows us to reliably reconstruct the detected-photons statistics of different light states
by taking into account the modifications introduced by detector features. Finally we quantify photon-number
correlations in bipartite states and use the data to produce conditional states: only if dark-count and cross-talk
effects can be neglected, the experimental results match theory.
By using hybrid photodetectors we exploited the photon-number correlations existing in bipartite optical states
to demonstrate the effect of multiple-photon subtraction on the generation of conditional states in the pulsed
regime. We operated on both classical and quantum Gaussian bipartite states in the mesoscopic regime without
background subtraction and corrections. The obtained conditional states are non-Gaussian in nature, thus particularly
useful for applications to Quantum Information. All the experimental results are in excellent agreement
with theoretical models.
We present an experimental scheme for the reconstruction of the Wigner function of optical states. The method
is based on direct intensity measurements by non-ideal photodetectors operated in the linear regime. We mix,
at a beam-splitter, the signal state with a set of coherent probes of known complex amplitudes, and measure the
probability distribution of the detected photons for each probe. The Wigner function is given by a suitable sum of
those probability distributions. For comparison, the same data are analyzed to obtain the number distributions
and the Wigner functions for photons.
Joint signal-idler photoelectron distributions of twin beams have been measured recently in two distinct regimes:
either the mean number of photon pairs per one pump pulse is lower that the number of independent modes or
vice versa. Exploiting a microscopic quantum theory for joint quasi-distributions in parametric down-conversion
based on the model of superposition of signal and noise we characterize properties of twin beams in terms of quasidistributions
using experimental data. In parallel to the microscopic model, joint signal-idler photon-number
distribution is reconstructed using the method of maximum likelihood. Negative values as well as oscillating
behavior in quantum region are characteristic for the joint signal-idler quasi-distributions of integrated intensities.
The larger the mean number of photon pairs per mode the weaker the quantum features are. However, they
survive even in the mesoscopic regime, i.e. when tens of photon pairs per mode are present on average. Also
the conditional and difference photon-number distributions are shown to be sub-Poissonian and sub-shot-noise,
respectively. Violation of classical inequalities for photon-number distributions is discussed.
We demonstrate, by direct measurement of the number of photons in signal and idler, that the twin-beam of light
produced by ps-pulsed spontaneous parametric downconversion is endowed with sub-shot-noise photon-number
correlations in a mesoscopic intensity regime (more than 1000 detected photons). The noise reduction, calculated
from the variance of the difference in the numbers of detected-photons, resulted to be 3.25 dB below the shot-noise
level. From experimental data we can recover joint photon-number distribution and a negative-valued
joint signal-idler quasi-distributions of integrated intensities, which demonstrates the nonclassical character of
the generated field.