Ivan Capraro

Iqueye, a single photon-counting photometer applied to the ESO new technology telescope

Context. A new extremely high speed photon-counting photometer, Iqueye, has been installed and tested at the New Technology Telescope, in La Silla. Aims. This instrument is the second prototype of a “quantum” photometer being developed for future Extremely Large Telescopes of 30–50 m aperture. Methods. Iqueye divides the telescope aperture into four portions, each feeding a single photon avalanche diode. The counts from the four channels are collected by a time-to-digital converter board, where each photon is appropriately time-tagged. Owing to a rubidium oscillator and a GPS receiver, an absolute rms timing accuracy better than 0.5 ns during one-hour observations is achieved. The system can sustain a count rate of up to 8 MHz uninterruptedly for an entire night of observation. Results. During five nights of observations, the system performed smoothly, and the observations of optical pulsar calibration targets provided excellent results.

Wave front active control by a digital-signal-processor-driven deformable membrane mirror

The correction of the beam wave front degradations, introduced by optical aberrations or by medium inhomogeneities, has a widespread relevance in several areas of optics. We describe here an adaptive optics system aimed to the optical optimization of the beam wave front in few seconds using an electrostatic deformable mirror and a controller based on a fixed-point digital signal processor. The correction system acts the wave front modification by a deformable mirror in closed loop with a genetic algorithm. We also studied the dynamics of the mirror for its use in beam wander correction due to atmospheric turbulence in free space classical and quantum optical communications.

AquEYE, a single photon counting photometer for astronomy

This paper describes the results obtained so far with AquEYE, a single photon counting, fixed aperture photometer for the Asiago 182 cm telescope. AquEYE has been conceived as a prototype of a truly ‘quantum’ photometer for future Extremely Large Telescopes of 30–50 m aperture. This prototype is characterized by four independent channels equipped with single photon avalanche diodes (SPADs) as detectors. The counts from the four channels are acquired by a TDC board which has a nominal 25 ps time tagging capability. Taking into account the 35 ps jitter in the SPAD itself, the overall precision of the time tags is of the order of 50 ps. The internal oscillator is locked to an external rubidium clock; a GPS pulse per second is collected by the TDC itself to obtain a UTC reference. The maximum photon count rate which the present system can sustain is 12 MHz.

Aqueye optical observations of the Crab Nebula pulsar

We observed the Crab pulsar in October 2008 at the Copernico Telescope in Asiago - Cima Ekar with the optical photon counter Aqueye (the Asiago Quantum Eye) which has the best temporal resolution and accuracy ever achieved in the optical domain (hundreds of picoseconds). Our goal was to perform a detailed analysis of the optical period and phase drift of the main peak of the Crab pulsar and compare it with the Jodrell Bank ephemerides. We determined the position of the main peak using the steepest zero of the cross-correlation function between the pulsar signal and an accurate optical template. The pulsar rotational period and period derivative have been measured with great accuracy using observations covering only a 2 day time interval. The error on the period is 1.7 ps, limited only by the statistical uncertainty. Both the rotational frequency and its first derivative are in agreement with those from the Jodrell Bank radio ephemerides archive. We also found evidence of the optical peak leading the radio one by ~230 microseconds. The distribution of phase-residuals of the whole dataset is slightly wider than that of a synthetic signal generated as a sequence of pulses distributed in time with the probability proportional to the pulse shape, such as the average count rate and background level are those of the Crab pulsar observed with Aqueye. The counting statistics and quality of the data allowed us to determine the pulsar period and period derivative with great accuracy in 2 days only. The time of arrival of the optical peak of the Crab pulsar leads the radio one in agreement with what recently reported in the literature. The distribution of the phase residuals can be approximated with a Gaussian and is consistent with being completely caused by photon noise (for the best data sets).

Upgrade of Iqueye, a novel photon-counting photometer for the ESO New Technology Telescope

Iqueye is a novel extremely high speed photon-counting photometer for the European Southern Observatory New Technology Telescope in La Silla (Chile). Iqueye collects the light from the telescope through a few arcsec aperture, and splits it along four independent channels, each feeding a single photon avalanche diode. The produced count pulses are collected by a time-to-digital converter board and suitably time-tagged. Thanks to a rubidium oscillator and a GPS receiver, an absolute rms timing accuracy better than 0.5 ns during one-hour observations can be achieved by postprocessing the data. The system can sustain a count rate of up to 8 MHz uninterruptedly for an entire night of observation. After the first run in January 2009, some improvements have been evidenced and realized: a more practical mechanical structure, a better optimization of the optical design, an additional filter wheel per each channel, a fifth photon counting detector for monitoring the sky, a more interactive interface software. The updated Iqueye has been tested in December 2009, and the obtained results showed still better performance. As an example, the light curves of visible pulsars down to the 25th visible magnitude have been obtained in a few hours of observation.

Aqueye Plus: a very fast single photon counter for astronomical photometry to quantum limits equipped with an Optical Vortex coronagraph

In recent years, we developed two very high speed single photon photometers, Aqueye and Iqueye, as prototypes for “quantum” photometers for the Extremely Large Telescopes of the next decade. These instruments, based on single photon avalanche photodiodes and a 4-fold split-pupil concept, have been successfully used to obtain data of the highest quality on optical pulsars. Subsequently, we performed an attempt to utilize the Orbital Angular Momentum and associated Optical Vorticity to achieve high performance stellar coronagraphy. Presently, we are making a synergic effort in building Aqueye Plus, a new instrument for the 1.8 m telescope of the Asiago - Cima Ekar Observatory, which combines both functions, namely high speed simultaneous multicolor photon counting photometry and stellar coronagraphy. The innovative capability of Aqueye Plus is to take advantage of the two parallel outputs (NIM and TTL) of the four high time accuracy photon counting sensors. The NIM output preserves the best timing capability, while the TTL output drives a deformable 32-element mirror in a sort of quadrant detector to correct for defocus and tip/tilt aberrations of the stellar image on the phase mask discontinuity. This paper describes the Aqueye Plus instrument main characteristics and its foreseen performance.

The Crab pulsar seen with AquEYE at Asiago Cima Ekar observatory

We are developing fast photon-counter instruments to study the rapid variability of astrophysical sources by time tagging photon arrival times with unprecedented accuracy, making use of a Rubidium clock and GPS receiver. The first realization of such optical photon-counters, dubbed AquEYE (the Asiago Quantum Eye), was mounted in 2008 at the 182 cm Copernicus Observatory in Asiago. AquEYE observed the Crab pulsar several times and collected data of extraordinary quality that allowed us to perform accurate optical timing of the Crab pulsar and to study the pulse shape stability on a timescale from days to years with an excellent definition. Our results reinforce the evidence for decadal stability of the inclination angle between the spin and magnetic axis of the Crab pulsar. Future realizations of our instrument will make use of the Galileo Global Navigation Satellite System (GNSS) time signal.

Implementation of a Real Time High Level Protocol Software for Quantum Key Distribution

Quantum Key Distribution (QKD) guarantees the exchange of a perfect secure key between two parties exploiting the laws of quantum mechanics. Various QKD protocol have been proposed and all of them need a public communication in order to process the raw data and extract secure cryptographic keys. In this paper we present a real time JAVA implementation of the software needed to complete a quantum key distribution protocol. In particular we suggest a modified version of Cascade algorithm. An analysis of the problem is carried out by means of a Matlab simulator that we developed. The various steps, sifting, error correction and privacy amplification are discussed as well as the design choices including compatibility with various QKD protocols. A performances investigation with dedicated appliances is carried out in several operating conditions showing a strict relation between hardware and software implementation in terms of key rate.

Aqueye+: a wavefront sensorless adaptive optics system for narrow field coronagraphy

We have designed Aqueye+, an instrument for the Copernicus 182 cm Asiago Telescope, with two channels, one devoted to ultrafast photometry based on four single photon avalanche photodiodes, the second dedicated to stellar coronagraphy based on innovative optical vortex coronagraph system. The OVC requires a very good image quality, therefore an adaptive optic system AO was designed for the instrument. The peculiarity of this AO system is that there is no wavefront sensors, but the feedback for the deformable mirror is instead given by the photometric channel of Aqueye+.

Free Space Quantum Key Distribution System with Atmospheric Turbulence Mitigation by Active Deformable Mirror

Propagation through atmosphere is a major limitation in free space QKD implementations. Adaptive Optics can be a solution to this problem. This paper describes some results in this direction we obtained with our QKD setup.